In Situ Observation of Atomic-Scale Growth of a NaCl Thin Crystal on Au(111) by Scanning Tunneling Microscopy

Abstract

We have investigated the growth of the sodium chloride (NaCl) bilayer on Au(111) by scanning tunneling microscopy (STM). In the low-NaCl exposure regime, not only monocrystalline but also polycrystalline NaCl monolayer islands formed simultaneously on the Au surface at room temperature. With an increasing amount of NaCl exposure, the Au(111) surface is mostly covered by NaCl bilayer islands. The dynamic growth processes of the NaCl bilayer on Au(111) were revealed by sequential STM imaging. It was clarified that the two-dimensional growth of a second NaCl layer proceeds in an edge-advanced manner, the growth direction of which is strongly affected by the crystallographic orientation of NaCl monolayer islands. The edges and corners of NaCl islands become sharp and stabilized at the end of the bilayer formation. The results strongly suggest that the formation of the NaCl bilayer involves the migration of preadsorbed NaCl molecules from the Au surface onto the NaCl monolayer.

One-Dimensional Periodic Buckling at a Symmetry-Incompatible Heterointerface of the NaCl(001) Monolayer on Ir(111)

“…Figure a shows a typical STM image of a monolayer NaCl island on the Ir(111) surface with a height of ∼250 pm, which is similar to that grown on other substrates such as Au, Cu, and Al. ,, The bright atomic contrasts in STM characterization are attributed to Cl atoms. ,, It is found that clear one-dimensional electronic modulations are observed in parallel patterns, which are distinct from the monolayer or bilayer NaCl(001) on other metal substrates. The zoom-in image (Figure b) exhibits a complicated “reconstruction” compared to the original square lattice of the NaCl(001) surface, which is reminiscent of the typical striped incommensurate phases observed in the metal–metal interface. − , Seeing through the [100] direction, the surface can be viewed as a combination of three kinds of striped features: two-row domains (type A, marked by red), one-row domains (type B, marked by yellow), and pristine domains (type C, marked by blue).…”

“…Figure 1a shows a typical STM image of a monolayer NaCl island on the Ir(111) surface with a height of ∼250 pm, which is similar to that grown on other substrates such as Au, Cu, and Al. 12,27,28 The bright atomic contrasts in STM characterization are attributed to Cl atoms. 11,27,29 It is found that clear onedimensional electronic modulations are observed in parallel patterns, which are distinct from the monolayer or bilayer NaCl(001) on other metal substrates.…”

Cinematographic Recording of a Metastable Floating Island in Two- and Three-Dimensional Crystal Growth

“…The events often occur too rapidly, beyond the spatiotemporal capability of available analytical methods, 11,12 including in situ transmission electron microscopy (TEM) and scanning probe microscopy whose resolution seldom reaches angstrom and millisecond (ms). 13 Having recently developed an experimental setup where a NaCl nanocrystal (NC) grows spontaneously in a closed carbon nanotube (CNT) containing a supply of NaCl ion pairs, 14 we performed operando imaging of two-dimensional (2-D) nucleation on a defect-free (100) surface of the NC using single-molecule atomic-resolution time-resolved electron microscopy (SMART-EM). 14−17 Here we can study the crystal growth from the side orthogonal to the direction of epitaxy.…”

“…The current consensus on the energetics of nucleation considers metastable states along an uphill path to a critical crystal nucleus (Figure a). , A cascade of intermediates emerge stochastically in the metastable states to finally generate a transient crystal nucleus that grows irreversibly into a crystal. − However, little has been reported on the dynamics of the metastable states at an atomistic level. The events often occur too rapidly, beyond the spatiotemporal capability of available analytical methods, , including in situ transmission electron microscopy (TEM) and scanning probe microscopy whose resolution seldom reaches angstrom and millisecond (ms) . Having recently developed an experimental setup where a NaCl nanocrystal (NC) grows spontaneously in a closed carbon nanotube (CNT) containing a supply of NaCl ion pairs, we performed operando imaging of two-dimensional (2-D) nucleation on a defect-free (100) surface of the NC using single-molecule atomic-resolution time-resolved electron microscopy (SMART-EM). − Here we can study the crystal growth from the side orthogonal to the direction of epitaxy. , We describe here the formation and dynamics of a metastable “floating island” (FI) as the first intermediate of two-dimensional epitaxy that has shorter interionic distances than those in the crystal (cf.…”

In Situ Growth and Bias-Dependent Modification of NaBr Ionic Layers on Ag(111)

“…With the increasing importance of nanoscale surface insulators in the study of intrinsic properties of atoms and molecules, ionic layers are most promising due to their high bandgap and low reactivity, which decouple interactions from the metal surfaces. , NaCl layers have already gotten much attention in that respect, e.g., for imaging individual molecular orbitals of adsorbed molecules, − their self-assembly, − isomerization, and their functionalities. , Recently, “on surface reaction” techniques have been widely used in scanning tunneling microscopy (STM), where insulating layers play an essential role in the synthesis of functional materials . The growth of several ionic layers on metal surfaces has been extensively studied by STM, AFM, and LEED. − Among them, NaCl is the most studied due to its stoichiometric growth into atomic layer islands. Most studies revealed that the NaCl islands grow in the form of a bilayer instead of a monolayer when deposited at room temperature to compensate for the ionic charge of the first layer. − However, the explanations for such a growth mode are still incomplete because layer formation is too fast at room temperature to be imaged directly.…”