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Abstract
Tip-enhanced Raman spectroscopy (TERS) has become a well-applied technique for nanospectroscopy, allowing for single molecule sensitivity with sub-nanometer spatial resolution. The demand for efficient, reproducible and cost-effective probes for TERS is increasing. Here we report on a new electrochemical etching protocol to fabricate TERS tips starting from 125 µm diameter gold wires in a reproducible way. The process is reliable (50% of the tips have radius of curvature <35 nm, 66% <80 nm), fast (less than 2 min) and 2.5 times cheaper than the etching of standard 250 µm diameter wires. The TERS performance of the tips is tested on dyes, pigments and biomolecules and enhancement factors higher than 105 are observed. TERS mapping with a spatial resolution of 5 nm is demonstrated.
Highlights
Tip-enhanced Raman spectroscopy (TERS) combines the chemical and structural information of Raman spectroscopy with the large signal gain provided by plasmonic resonances in metal tips and the high spatial resolution mapping offered by scanning probe microscopy [1,2,3,4,5]
TERS setups based on atomic force microscopy (AFM) [1,13], scanning tunneling microscopy (STM) [14] and shear-force microscopy (ShFM) [15] allow for chemical imaging of nanostructured materials, surfaces andmolecular layers with a spatial resolution of 4–10 nm in ambient conditions [15,16], and can even reach atomic-level sensitivity in ultrahigh vacuum (UHV) [17,18,19]
TERS features unique advantages as compared with scanning electron microscopy (SEM), scanning near-field Raman microscopy (Raman-SNOM) [26] and farfield nanoscopy [27,28]: (i) it is a label-free technique, i.e., it does not require sample pretreatment, (ii) it can be operated in ambient conditions, liquid environments, as well as in UHV and at low temperatures, (iii) it combines the surface morphology information with the chemical information, (iv) optical excitation powers are virtually unlimited, and (v) it can attain atomiclevel resolution
Summary
Tip-enhanced Raman spectroscopy (TERS) combines the chemical and structural information of Raman spectroscopy with the large signal gain provided by plasmonic resonances in metal tips and the high spatial resolution mapping offered by scanning probe microscopy [1,2,3,4,5]. A pre-etching step of the gold wire is performed at a voltage V1 = 5 VDC for a time t1 = 15–20 s (Figure 2b) and permits quick reduction of the wire diameter and, almost halves the overall tip production time. During this step the wire diameter at the meniscus is thinned at a rate δd/δt ≈ 3.5–4 μm/s and intense bubbling is observed. The x–y scan plane is orthogonal to the optical axis (z) of the objective
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