[Paper Review] Enhanced field emission from multiwall carbon nanotube films by secondary growth

Christian Klinke, Erik Delvigne+2 more|arXiv (Cornell University)|Apr 28, 2006

Carbon Nanotubes in Composites25 references30 citations

TL;DR

This study demonstrates that growing secondary multiwall carbon nanotubes (MWCNTs) on the sidewalls of primary CNT films via secondary chemical vapor deposition (CVD) significantly enhances field emission performance. Using nickel or ferritin as catalysts, secondary nanotubes form on primary nanotubes, increasing the effective emitter density and improving emission current and homogeneity without altering field enhancement factors, due to more emitters being oriented toward the anode.

ABSTRACT

We have studied nickel, gold, and ferritin coatings on catalytically grown multiwall carbon nanotubes, as well as the generation of secondary nanotubes by resubmitting the decorated nanotubes to the chemical vapor deposition process. Nickel layers sputtered on nanotubes show a stronger interaction with the nanotube walls than gold coatings. At ambient temperature this results in a metal film that is more homogeneous for Ni than for Au. Surface mass transport at elevated temperatures leads to a transformation of the coating to nanoscale clusters on the nanotube surface. The resulting Au clusters are sphere-like with a very small contact area with the nanotube whereas the Ni clusters are stretched along the tube axis and have a large contact area. Secondary nanotubes were established by growing nanotubes directly on the walls of primary nanotubes. Thin Ni layers or ferritin served as catalyst. We compared the field emission properties of samples with and without secondary nanotubes. The presence of secondary nanotubes enhances the field emission substantially.

Motivation & Objective

• To improve field emission efficiency and homogeneity in carbon nanotube films for applications in flat panel displays and electron sources.
• To investigate the role of metal catalysts (Ni vs. Au) in decorating primary MWCNTs and their thermal evolution behavior.
• To develop a method for growing secondary nanotubes directly on the sidewalls of primary nanotubes to increase effective emitter density.
• To compare field emission performance between primary CNT films and those with secondary nanotubes grown via Ni or ferritin catalysis.
• To evaluate the impact of secondary nanotube morphology and distribution on field emission characteristics.

Proposed method

• Synthesized primary MWCNTs using ferritin or ferric nitrate as catalysts via chemical vapor deposition (CVD) at 660 °C with C2H2.
• Sputtered nominal 5–15 nm nickel or deposited gold coatings on primary CNTs for catalyst seeding.
• Used ferritin solution to coat CNTs for secondary nucleation, followed by re-CVD to grow secondary nanotubes.
• Performed in-situ transmission electron microscopy (TEM) with resistive heating to observe metal cluster evolution and nanotube growth.
• Conducted field emission measurements using a 125 µm gap between CNT cathode and stainless steel anode, with current measured via Keithley 237 source-measure unit.
• Compared I–V characteristics, turn-on field (Eto), and threshold field (Eth) for samples with and without secondary nanotubes.

Experimental results

Research questions

• RQ1 How does the interaction between Ni and Au coatings and MWCNTs differ at ambient and elevated temperatures?
• RQ2 What is the effect of secondary nanotube growth on field emission current density and homogeneity?
• RQ3 Why does secondary nanotube growth improve field emission if field enhancement factors do not increase?
• RQ4 How do catalyst type (Ni vs. ferritin) and nanotube morphology affect the uniformity and performance of secondary nanotube growth?
• RQ5 To what extent does the diameter of primary nanotubes influence field emission when secondary nanotubes are present?

Key findings

• Nickel coatings on MWCNTs form a more homogeneous film at room temperature than gold due to stronger interaction with the nanotube surface.
• Upon heating to 660 °C, nickel forms elongated clusters with large contact area along the tube axis, while gold forms spherical clusters with minimal contact area.
• Secondary nanotubes grew directly on the sidewalls of primary MWCNTs using Ni or ferritin as catalysts, with lengths of 50–100 nm.
• Field emission from samples with secondary nanotubes showed a significant improvement: for ferritin-catalyzed primary CNTs, Eth decreased from 7.9 V/µm to 6.6 V/µm.
• For ferric nitrate-catalyzed primary CNTs, Eth decreased from >8 V/µm to 6.7 V/µm, and Eto decreased from 5.2 V/µm to 5.0 V/µm after secondary growth.
• The improvement in field emission is attributed to an increased number of effective emitters due to secondary nanotubes on the sidewalls, not to enhanced field emission factors.