Recrystallization of tungsten wire for fabrication of sharp and stable nanoprobe and field-emitter tips

Electron-bombarded ⟨110⟩-oriented tungsten tips for stable tunneling electron emission

A clean tungsten (W) tip apex with a robust atomic plane is required for producing a stable tunneling electron emission under strong electric fields. Because a tip apex fabricated from a wire by aqueous chemical etching is covered by impurity layers, heating treatment in ultra-high vacuum is experimentally known to be necessary. However, strong heating frequently melts the tip apex and causes unstable electron emissions. We investigated quantitatively the tip apex and found a useful method to prepare a tip with stable tunneling electron emissions by controlling electron-bombardment heating power. Careful characterizations of the tip structures were performed with combinations of using field emission I-V curves, scanning electron microscopy, X-ray diffraction (transmitted Debye-Scherrer and Laue) with micro-parabola capillary, field ion microscopy, and field emission microscopy. Tips were chemically etched from (1) polycrystalline W wires (grain size ∼1000 nm) and (2) long-time heated W wires (grain size larger than 1 mm). Heating by 10-40 W (10 s) was found to be good enough to remove oxide layers and produced stable electron emission; however, around 60 W (10 s) heating was threshold power to increase the tip radius, typically +10 ± 5 nm (onset of melting). Further, the grain size of ∼1000 nm was necessary to obtain a conical shape tip apex.

In situ TEM characterization of shear-stress-induced interlayer sliding in the cross section view of molybdenum disulfide

The experimental study of interlayer sliding at the nanoscale in layered solids has been limited thus far by the incapability of mechanical and imaging probes to simultaneously access sliding interfaces and overcome through mechanical stimulus the van der Waals and Coulombic interactions holding the layers in place. For this purpose, straightforward methods were developed to achieve interlayer sliding in molybdenum disulfide (MoS2) while under observation within a transmission electron microscope. A method to manipulate, tear, and slide free-standing atomic layers of MoS2 is demonstrated by electrostatically coupling it to an oxidized tungsten probe attached to a micromanipulator at a bias above ±7 V. A first-principles model of a MoS2 bilayer polarized by a normal electric field of 5 V/nm, emanating from the tip, demonstrates the appearance of a periodic negative sliding potential energy barrier when the layers slide into the out-of-registry stacking configuration, hinting at electrostatic gating as a means of modifying the interlayer tribology to facilitate shear exfoliation. A method to shear focused ion beam prepared MoS2 cross section samples using a nanoindenter force sensor is also demonstrated, allowing both the observation and force measurement of its interlayer dynamics during shear-induced sliding. From this experiment, the zero normal load shear strength of MoS2 can be directly obtained: 25.3 ± 0.6 MPa. These capabilities enable the site-specific mechanical testing of dry lubricant-based nanoelectromechanical devices and can lead to a better understanding of the atomic mechanisms from which the interlayer tribology of layered materials originates.

Note: electrochemical etching of silver tips in concentrated sulfuric acid

Sharp metal tips have many uses, including in scanning probe microscopy. Silver is a particularly interesting metal for plasmonic enhancement, e.g., in tip-enhanced Raman spectroscopy, however few methods for the production of silver tips have been explored. A simple and reliable one step dc electrochemical method for the production of sharp silver tips in concentrated H2SO4 is presented. The optimal conditions are 10 V cell voltage and 99% sulfuric acid for tip radii below 100 nm. A LabView program was written to control the cut-off of the circuit to within a millisecond to avoid blunting the tips after drop-off.

Implementation of atomically defined field ion microscopy tips in scanning probe microscopy

The field ion microscope (FIM) can be used to characterize the atomic configuration of the apices of sharp tips. These tips are well suited for scanning probe microscope (SPM) use since they predetermine the SPM resolution and the electronic structure for spectroscopy. A protocol is proposed for preserving the atomic structure of the tip apex from etching due to gas impurities during the period of transfer from the FIM to the SPM, and estimations are made regarding the time limitations of such an experiment due to contamination with ultra-high vacuum rest gases. While avoiding any current setpoint overshoot to preserve the tip integrity, we present results from approaches of atomically defined tungsten tips to the tunneling regime with Au(111), HOPG (highly oriented pyrolytic graphite) and Si(111) surfaces at room temperature. We conclude from these experiments that adatom mobility and physisorbed gas on the sample surface limit the choice of surfaces for which the tip integrity is preserved in tunneling experiments at room temperature. The atomic structure of FIM tip apices is unchanged only after tunneling to the highly reactive Si(111) surface.

Refined tip preparation by electrochemical etching and ultrahigh vacuum treatment to obtain atomically sharp tips for scanning tunneling microscope and atomic force microscope

A modification of the common electrochemical etching setup is presented. The described method reproducibly yields sharp tungsten tips for usage in the scanning tunneling microscope and tuning fork atomic force microscope. In situ treatment under ultrahigh vacuum (p ≤10(-10) mbar) conditions for cleaning and fine sharpening with minimal blunting is described. The structure of the microscopic apex of these tips is atomically resolved with field ion microscopy and cross checked with field emission.