The cumulenic linear C₅ and its coupling-reaction products

Area of Science:

• Materials Science
• Nanotechnology
• Chemistry

Background:

• Linear carbon allotropes (C_n) are elusive sp-hybridized structures with debated configurations.
• Interest in C_n stems from their unique properties and potential applications.

Purpose of the Study:

• To synthesize uncapped linear carbon chains, specifically C₅.
• To investigate the formation of longer carbon chains via tip-induced coupling.
• To characterize the structures and electronic properties of synthesized carbon chains.

Main Methods:

• Tip-induced dehalogenation and ring-opening of C₅Br₆ to synthesize linear C₅.
• Atomic force microscopy (AFM) for structural confirmation.
• Tip-induced coupling of C₅ to form longer chains (C₁₀, C₁₅).
• Synthesis of various carbon chains (C₉-C₂₃) using higher voltage pulses on C₅Br₆ and C₆Br₆.
• Scanning tunneling spectroscopy (STS) to probe electronic transport gaps.

Main Results:

• Successful synthesis of uncapped linear C₅ with a confirmed cumulenic structure.
• Demonstration of tip-induced coupling to create longer carbon chains.
• Even-numbered chains (e.g., C₁₀, C₁₄, C₁₈) exhibit polyynic structures due to Peierls transition.
• Odd-numbered chains show varied structures: C₉ is cumulene-like, longer chains are hybrid.
• STS revealed smaller transport gaps in odd-numbered chains compared to even-numbered ones, with gaps decreasing as chain length increases.

Conclusions:

• This study provides a method for synthesizing and controlling the length of linear carbon chains.
• The distinct structural and electronic properties of odd- and even-numbered carbon chains are elucidated.
• Findings align with Peierls theory, offering insights into the fundamental behavior of sp-hybridized carbon allotropes.