Fabrication of Isolated Iron Nanowires

Abstract

Nanoscale interconnects are an important component of molecular electronics. Here we use X-ray spectromicroscopy techniques as well as scanning probe methods to explore the self-assembled growth of insulated iron nanowires as a potential means of supplying an earth abundant solution. The intrinsic anisotropy of a TiO₂(110) substrate directs the growth of micron length iron wires at elevated temperatures, with a strong metal-support interaction giving rise to ilmenite (FeTiO₃) encapsulation. Iron nanoparticles that decorate the nanowires display magnetic properties that suggest other possible applications.

Figure 1

Structural characterization of iron nanostructures prepared on TiO₂(110) at 1070 K. (A) STM image of an Fe nanowire (V_s = +1.0 V, I_t = 0.2 nA) recorded after deposition of 1 MLE Fe. (B) STM image of a pseudohexagonal Fe nanoisland (V_s = +2.9 V, I_t = 0.07 nA) recorded after deposition of 1 MLE Fe. (C) High-resolution image of the top surface of the Fe islands (V_s = +3.0 V, I_t = 0.14 nA). (D–F) XPEEM images of Fe nanowires recorded after deposition of 10 MLE Fe at 1070 K (hν = 708 eV, KE = 4 eV). All images have the same orientation with respect to the TiO₂(110) substrate.

Figure 2

The nature of the Ti species on the encapsulated nanowires and oxidation states of Ti and Fe associated with the nanowires and substrate. Ti L-edge XAS spectra of the Fe/TiO₂(110) system acquired from XPEEM images (KE = 4 eV), with sampling of areas corresponding to the bare substrate (A, red curve) and the Fe nanowires (B, blue curve). (C) Ti 2p XPS spectrum (hν = 650 eV) and (D) Fe 2p XPS spectrum (hν = 820 eV), obtained from the Fe nanowires supported on TiO₂(110) with bare substrate in-between.

Figure 3

Magnetic behavior of the nanowires and dots. XMCD-XPEEM (KE = 4 eV) images of Fe nanowires and nanodots (green circles in parts A and B) supported on TiO₂(110) at the Fe L₃ edge (hν = 708 eV). (A) XAS image and (B) XMCD-XPEEM image of the same 10 μm FOV. (C) XMCD image highlighting a few of the nanodots. (D) XAS image of the same area as part C, showing the morphology of the dots and wire (2 × 2 μm²). (E) Line profiles across two of the nanodots from the XMCD image in part C.

Figure 4

Fe L-edge XAS and XMCD spectra (KE = 4 eV) obtained from the Fe structures on TiO₂(110). (A) Integrated XAS (average of right-circular and left-circular spectra) taken from regions of the images in Figure 3A corresponding to the nanodots (green) and nanowires (orange). Spectra are normalized to the pre-edge region. The calculated Fe²⁺ XAS spectrum is shown in the lower panel as a dashed pink line. (B) Circularly polarized XAS (blue, red lines) and XMCD (dashed black line) spectra from the nanodots. Spectra are normalized to the edge step. The lower panel shows the calculated XMCD spectrum for Fe²⁺ in octahedral geometry (dashed pink line). (C) Circularly polarized XAS (blue, red lines) and XMCD (dashed black line) spectra from the nanowires. Spectra are normalized to the edge step. The lower panel shows the calculated XMCD spectrum for Fe²⁺ in octahedral geometry (dashed pink line).

Figure 5

Spin-polarized LEEM images of a typical Fe nanowire on TiO₂(110). The wire was prepared with the sample held at ∼1100 K. Parts A and B were acquired at room temperature with the electron beam polarization vector P // [110] with spin up and spin down, respectively. The resulting asymmetry image is displayed in part C. FOV = 10 μm, SV = 4.1 V.