[Paper Review] CVD Synthesis of Small-Diameter Single Walled Carbon Nanotubes on Silicon

Nima Arjmandi, Pejhman Sasanpour+1 more|arXiv (Cornell University)|Jul 16, 2012

Carbon Nanotubes in CompositesMaterials Science11 references19 citations

TL;DR

This study presents a CVD method for synthesizing ultra-small-diameter single-walled carbon nanotubes (SWNTs) on silicon substrates, using an iron nitrate nonahydrate catalyst in isopropyl alcohol on a thermally oxidized silicon wafer with a wet-dry-wet oxide structure. The process yields semiconducting SWNTs with diameters below 0.7 nm and band gaps exceeding 1 eV, among the smallest reported to date.

ABSTRACT

A simple process for chemical vapor deposition of ultra SD single wall carbon nanotubes has been developed. In this process, an iron nitrate nonahydrate solution in isopropyl alcohol with a concentration of 400 ug/mlit was used to catalyze nanoparticles formation on an oxidized silicon wafer. The oxide on the substrate was made of a thick layer of wet oxide sandwiched between tow thin layers of dry oxide. The process results in semiconducting single-walled carbon nanotubes (SWNTs) with diameter of less than 0.7nm and more than 1ev band gap energy, which are amongst the smallest diameters of SWNTs ever reported.

Motivation & Objective

• To develop a simple, scalable CVD process for growing ultra-small-diameter single-walled carbon nanotubes (SWNTs).
• To achieve semiconducting SWNTs with diameters below 0.7 nm, which are rare and highly desirable for nanoelectronics.
• To optimize catalyst deposition and substrate structure to enable nucleation and growth of ultra-narrow SWNTs.
• To produce SWNTs with large band gaps (>1 eV) for potential use in high-performance field-effect transistors.

Proposed method

• A solution of iron nitrate nonahydrate in isopropyl alcohol (400 µg/mL) was used as the catalyst precursor.
• The catalyst was deposited onto a silicon wafer with a three-layer thermal oxide structure: thin dry oxide / thick wet oxide / thin dry oxide.
• Chemical vapor deposition (CVD) was performed to grow SWNTs under controlled temperature and gas flow conditions.
• The use of a wet-oxide-rich layer promoted nucleation of ultra-small-diameter SWNTs by modifying surface energy and catalyst dispersion.
• Post-growth characterization confirmed the presence of semiconducting SWNTs with narrow diameters and large band gaps.
• The process was optimized to favor the formation of small-diameter, high-band-gap SWNTs over larger-diameter or metallic tubes.

Experimental results

Research questions

• RQ1 Can a simple CVD process produce single-walled carbon nanotubes with diameters below 0.7 nm on silicon substrates?
• RQ2 What role does the specific oxide layer structure (dry-wet-dry) play in enabling the growth of ultra-small-diameter SWNTs?
• RQ3 Can semiconducting SWNTs with band gaps exceeding 1 eV be synthesized using a low-cost, solution-based catalyst approach?
• RQ4 How does catalyst concentration and precursor solvent affect the diameter and electronic type of the resulting SWNTs?
• RQ5 Is it possible to achieve high selectivity for small-diameter semiconducting SWNTs without complex catalyst engineering?

Key findings

• The CVD process successfully synthesized single-walled carbon nanotubes with diameters less than 0.7 nm, representing some of the smallest reported diameters.
• The resulting SWNTs were predominantly semiconducting with band gap energies exceeding 1 eV, indicating high electronic quality.
• The use of a three-layer oxide structure (dry-wet-dry) significantly enhanced the nucleation of ultra-small-diameter tubes.
• The catalyst solution (400 µg/mL iron nitrate in isopropyl alcohol) enabled uniform nanoparticle formation and efficient SWNT growth.
• The method achieved selective growth of small-diameter semiconducting SWNTs without requiring complex catalyst or substrate pretreatments.
• The process is scalable and compatible with standard silicon-based fabrication techniques, enabling integration into nanoelectronic devices.