Enhanced resolution imaging of ultrathin ZnO layers on Ag(111) by multiple hydrogen molecules in a scanning tunneling microscope junction

Abstract

Molecular hydrogen in a scanning tunneling microscope (STM) junction has been found to enhance the lateral spatial resolution of the STM imaging, referred to as scanning tunneling hydrogen microscopy (STHM). Here we report atomic resolution imaging of 2-and 3-monolayer (ML) thick ZnO layers epitaxially grown on Ag(111) using STHM. The enhanced resolution can be obtained at a relatively large tip to surface distance and resolves a more defective structure exhibiting dislocation defects for 3-ML-thick ZnO than for 2 ML. In order to elucidate the enhanced imaging mechanism, the electric and mechanical properties of the hydrogen molecular junction (HMJ) are investigated by a combination of STM and atomic force microscopy. It is found that the HMJ shows multiple kinklike features in the tip to surface distance dependence of the conductance and frequency shift curves, which are absent in a hydrogen-free junction. Based on a simple modeling, we propose that the junction contains several hydrogen molecules and sequential squeezing of the molecules out of the junction results in the kinklike features in the conductance and frequency shift curves. The model also qualitatively reproduces the enhanced resolution image of the ZnO films.

Resolving the Correlation between Tip-Enhanced Resonance Raman Scattering and Local Electronic States with 1 nm Resolution

“…The 2-ML ZnO has a (0001) orientation with a flat geometry like hexagonal boron nitride, whereas it relaxes to the wurtzite structure in thicker ZnO layers . Previous studies suggest that this structural change occurs (partially) already at 3 ML. − Figure c shows TERRS spectra obtained using an Au tip on 2-ML ZnO with 633 nm excitation at different laser fluences, exhibiting the characteristic vibrational peaks. No Raman signal of the ZnO layer is observed in the far-field spectrum (see Supporting Information).…”

Atomic-scale quantum sensing based on the ultrafast coherence of an H 2 molecule in an STM cavity

“…With a H 2 molecule weakly trapped inside the STM cavity, the spatial resolution in constant-current topography can be greatly enhanced ( 22 ). A topographic image of an incommensurate Cu 2 N island grown on Cu(100) ( 32 ) is shown in Fig.…”

“…Relative to other quantum sensors such as NV centers in diamond, the H 2 coherent sensor in the STM cavity provides simultaneous atomic-scale spatial and femtosecond temporal resolutions with GHz energy discrimination. H 2 molecules have been found to be trapped over a variety of surfaces, atoms, and molecules ( 22 , 24 , 27 ). THz pump-probe measurements with a H 2 molecule in the STM cavity can be used to probe the electrostatic field and potential energy surface of the sample.…”

“…Hydrogen molecules adsorb on a variety of materials, including metal surfaces (21), insulating layers (22)(23)(24)(25)(26), and single molecules (27,28). To describe a H 2 molecule in the tunnel junction, a TLS in a double-well potential has been widely adopted (29,30) in which the H 2 molecule switches between two different adsorption configurations (31) (Fig.…”

Dramatic Enhancement of Tip-Enhanced Raman Scattering Mediated by Atomic Point Contact Formation

“…Figure a displays an STM image of ultrathin ZnO films epitaxially grown on the Ag(111) surface (see the Supporting Information for experimetal details). The hexagonal protrusions represent the Moiré pattern resulting from the lattice mismatch between the ZnO film and the surface. , Figure b illustrates the experiment, where TERS is recorded as a function of the relative displacement of the tip–sample distance (Δ z ) under continuous-wave laser excitation with wavelength λ ext . We used an electrochemically etched Ag tip further sharpened by focused ion beam (FIB) milling (Figure c and d), which enables control of localized surface plasmons in the STM junction …”