Near-Field Manipulation in a Scanning Tunneling Microscope Junction with Plasmonic Fabry-Pérot Tips

Near-field manipulation in plasmonic nanocavities can provide various applications in nanoscale science and technology. In particular, a gap plasmon in a scanning tunneling microscope (STM) junction is of key interest to nanoscale imaging and spectroscopy. Here we show that spectral features of a plasmonic STM junction can be manipulated by nanofabrication of Au tips using focused ion beam. An exemplary Fabry–Pérot type resonator of surface plasmons is demonstrated by producing the tip with a single groove on its shaft. Scanning tunneling luminescence spectra of the Fabry–Pérot tips exhibit spectral modulation resulting from interference between localized and propagating surface plasmon modes. In addition, the quality factor of the plasmonic Fabry–Pérot interference can be improved by optimizing the overall tip shape. Our approach paves the way for near-field imaging and spectroscopy with a high degree of accuracy.


FIB fabrication of the Au tips
The FIB fabrication of electrochemically-etched Au tips was performed an FEI Helios NanoLab G3 FIB-SEM DualBeam system and Ga ions were used. It provides Gallium ions with an acceleration voltage up to 30 kV and enables milling and deposition of structures with critical dimensions of less than 10 nm. The tips were mounted on a pre-tilted specimen holder and oriented towards the ion beam such that the axis of the tip is collinear with the ion beam.
We performed a multiple-step annular milling process with different parameters and a varying order, depending on the initial tip profile and the required sharpened length. Taking into account the re-deposition dynamics during ion milling and the fact, that the main re-deposition of removed material occurs underneath the ion beam incidence point at highly steep surfaces, we preferred an inner-to-outer-radius scan direction, i.e., from the apex downwards, in order to simultaneously remove re-deposited material.
Several tip fabrication procedures using FIB milling were already suggested before. 1,2 Here we introduce a similar procedure with a reverse milling sequence and different parameters.
We first start the tip fabrication with a low ion energy step at 5 kV in order to shape and smoothen the apex. Subsequently, we increase the inner and outer radii as well as the ion energy and beam current to obtain higher milling rates to only sharpen the shaft to the required length.
This step order aims to minimize damage and Gallium implantation into the apex and in close vicinity to it. Finally, we apply a low energy polishing step at 5 kV to the sharpened length of the shaft to reduce damage layer caused by 30 kV ions.
A key parameter in ion milling processes is the volume per dose value ΔV, i.e. the removed material volume per primary ion. It is material-specific and determines, at a certain ion energy, the milling time t for a required depth Z and a given beam current I: For the groove fabrication and in order to precisely control the groove depth Z and the milling time t, we set this value to 1.5 µm³/ nC and 0.42 µm³/ nC for Au and Ag, respectively, 3 and used, at 30 kV ion energy, the lowest ion beam current of ca. 7 pA to avoid excessive milling and thus damaging the apex. Figure S1 shows Multi-peak fitting analysis of the bias voltage-dependent STML spectra in Fig. 2c. It appears that a slight blue-shift is observed at long wavelengths (peak number 4 and 5). As shown below (Fig. S2), the peak positions remain almost identical in the currentdependent STML spectra where the tip-surface distance is also changed. Therefore, the blueshift in the voltage-dependent STML spectra may not be related to the gap distance. As seen in Fig. 1 (the voltage-dependent STML spectra for a non-groove tip), the applied voltage determines the quantum cutoff of the LSPR excitation and affects the overall spectral response.

Peak position analysis in the bias voltage-dependent STML spectra
The different spectral feature of the LSP may lead to different coupling to the propagating SPP modes.

Current dependence of the STML intensity for the grooved tip
The luminescence intensity at 615 nm for the 3-μm grooved tip (Fig. 2d) is plotted as a function of the tunneling current, indicating a linear dependence. Figure S2. Peak intensity at 614 nm of the STML spectra in Fig. 2d is plotted as a function of the tunneling current. The black line indicates the linear fitting result.

Numerical simulation of electromagnetic field distributions
Numerical simulations were performed to calculate the plasmonic response of the nanotip and STM-junction by solving the time-harmonic wave equation for the electric field within the RF-Module of COMSOL Multiphysics 5.3a. Simulations are performed in 3D for a groove distance of L = 3 µm (although the problem is fully radially symmetric, simulations needed to be performed in 3D as the point dipole excitation feature is not available in 2D radially symmetric models in COMSOL 5.3a). The structural parameters of the tip were taken from the SEM micrograph (Fig. 2b). The width of the simulation volume is 1.3 µm and the tip is truncated above the groove at 4 µm distance from the apex. Except for the 100 nm thick sample, which is chosen thick enough to not transmit any fields, the simulation volume is surrounded by perfectly matched layers to absorb all outgoing waves.