[Paper Review] Near-field optical investigation of Ni clusters inside single-walled carbon nanotubes on the nanometer scale

Gergely Németh, Dániel Datz+8 more|arXiv (Cornell University)|Aug 21, 2018

Carbon Nanotubes in Composites24 references3 citations

TL;DR

This study uses scattering-type scanning near-field optical microscopy (s-SNOM) to detect sub-1000-atom nickel nanoclusters inside single-walled carbon nanotubes with nanoscale resolution. The method achieves high sensitivity to metallic clusters via free-carrier absorption, detecting clusters as small as ~600 Ni atoms, outperforming magnetic force microscopy in locating these nanostructures due to stronger optical contrast from charge carriers.

ABSTRACT

We used scattering-type scanning near-field optical microscopy (s-SNOM) to characterize nickel nanoclusters grown inside single-walled carbon nanotubes (SWCNT). The nanotubes were filled with Ni(II) acetylacetonate and the molecules were transformed into nickel clusters via annealing. The metal clusters give high local contrast enhancement in near-field phase maps caused by the excitation of free charge carriers. The near-field contrast was simulated using the finite dipole model, approximating the clusters with elliptical nanoparticles. Compared to magnetic force microscopy, s-SNOM appears much more sensitive to localize metal clusters inside carbon nanotubes. We estimate the detection threshold to be ~600 Ni atoms.

Motivation & Objective

• To detect and characterize sub-1000-atom nickel nanoclusters inside single-walled carbon nanotubes (SWCNTs) with nanoscale resolution.
• To evaluate the sensitivity of s-SNOM for identifying metallic phases in low-dimensional nanostructures.
• To compare s-SNOM with magnetic force microscopy (MFM) in detecting small Ni clusters inside SWCNTs.
• To validate the finite dipole model for simulating near-field optical contrast from metallic nanoparticles.

Proposed method

• Scattering-type scanning near-field optical microscopy (s-SNOM) was employed using a 980 cm⁻¹ quantum cascade laser to excite infrared optical responses.
• A metal-coated atomic force microscope tip served as a nano-antenna to enhance the local electric field and collect near-field scattered light.
• Near-field scattered light was demodulated at higher harmonics of the tip oscillation frequency using pseudo-heterodyne detection in a Michelson interferometer.
• The finite dipole model (FDM) was applied to simulate phase contrast, approximating Ni clusters as elliptical nanoparticles.
• Sample preparation involved filling SWCNTs with Ni(II) acetylacetonate followed by vacuum annealing at 700 °C for 2 hours to form Ni clusters.
• Complementary characterization included attenuated total reflection (ATR) spectroscopy, transmission electron microscopy (TEM), and magnetic force microscopy (MFM).

Experimental results

Research questions

• RQ1 Can s-SNOM detect metallic nickel nanoclusters inside SWCNTs with sub-1000-atom sensitivity?
• RQ2 How does the near-field optical contrast of Ni clusters compare to that of non-metallic or non-conductive species in SWCNTs?
• RQ3 Why does s-SNOM outperform MFM in detecting small Ni clusters despite their strong magnetic properties?
• RQ4 To what extent does the finite dipole model accurately predict the near-field phase contrast of nanoscale metallic inclusions?

Key findings

• s-SNOM detected nickel nanoclusters with a minimum threshold of approximately 600 Ni atoms, demonstrating high sensitivity to metallic phases.
• The measured third-harmonic phase contrast of the brightest spots was φO3 = 0.139 ± 0.01 rad, closely matching the simulated value of φ ≈ 0.142 rad.
• Magnetic force microscopy failed to detect any signal from Ni clusters, even at lift heights up to 130 nm, indicating poor sensitivity for such small magnetic dipoles.
• The finite dipole model successfully predicted the near-field phase contrast, confirming its reliability for simulating optical responses of small metallic nanoparticles.
• High optical contrast in s-SNOM maps clearly identified Ni clusters along nanotube bundles, consistent with TEM observations of inhomogeneous cluster distribution.
• The study confirms that free-carrier absorption in metallic clusters generates strong near-field contrast, enabling reliable localization even in complex nanostructures.