Distribution and thickness of the surface contaminations on STM tungsten tips, studied by AES/SEM and ARXPS

Abstract

Abstract. The combination of Auger electron spectroscopy (AES), scanning electron microscopy (SEM) and angle resolved X-ray photoelectron spectroscopy (ARXPS) has been applied to the analysis of the distribution of elements at the surface region of electrochemically etched tungsten tips and the determination of the thickness of a layer with oxygen and carbon contamination. Auger line profiling revealed a homogeneous distribution of oxygen and significant enrichment of carbon on the W tip between 0 and 1.5/~m from the top. The thickness of the contamination layer on various W materials, electrochemically etched, was found to be 1.35 __+ 0.15 nm as measured using ARXPS, and was estimated to be about 1-3 nm as measured by AES.Key words: AES, sputtering, STM, XPS.One of the important aspects of scanning tunneling microscopy (STM) operation is the reliable formation of tunneling probe tips. Characterization of the surface contaminations on STM tips is helpful for a preliminary selection of good performance tips [1]. It is well known, that the oxides and graphite layers can influence the tunneling current by changing the effective resistance of tip surface layer I-2, 3]. In the light of these results it was interesting to investigate the thickness of these layers.Transmission electron microscopy (TEM) studies indicated that the most widely applied tungsten tips, prepared by electrochemical etching, were covered by thin (1-5 nm thick) polycrystalline tungsten oxides 1-3, 4] and carbon or graphite [1][2][3][4][5]

STM Tip Catalyzed Adsorption of Thiol Molecules at the Nanometer Scale

“…Transmission electron microscopy (TEM) studies indicated that the tip was covered by thin (1−5 nm thick) polycrystalline tungsten oxides , and carbon or graphite layers. Auger electron spectroscopy (AES) combined with Ar + sputter profiling revealed a 1−3 nm thick oxide and carbon layer on W tips, and a W4f X-ray photoelectron spectroscopy (XPS) spectrum of etched W single crystal showed that the surface was comprised of tungsten carbide (WC) and tungsten trioxide (WO 3 ) …”

Hybrid scanning transmission electron/scanning tunnelling microscope system for the preparation and investigation of biomolecules

“…First experiments were carried out with W and Au tips. Electrochemically etched W tips provide apex radii of a few nanometres (Hacker et al, 1992), but they are covered with a thin oxide layer (Beigelsen et al, 1987;Lisowski et al, 1992), inducing unstable tunnelling currents, requiring frequent tip changes. For the vacuum hybrid STEM/STM this becomes a time-consuming task.…”