Non-contact Atomic Force Microscopy Study Research Articles

Phase-selective in-plane heteroepitaxial growth of H-phase CrSe2

Phase engineering of two-dimensional transition metal dichalcogenides (2D-TMDs) offers opportunities for exploring unique phase-specific properties and achieving new desired functionalities. Here, we report a phase-selective in-plane heteroepitaxial method to grow semiconducting H-phase CrSe2. The lattice-matched MoSe2 nanoribbons are utilized as the in-plane heteroepitaxial template to seed the growth of H-phase CrSe2 with the formation of MoSe2-CrSe2 heterostructures. Scanning tunneling microscopy and non-contact atomic force microscopy studies reveal the atomically sharp heterostructure interfaces and the characteristic defects of mirror twin boundaries emerging in the H-phase CrSe2 monolayers. The type-I straddling band alignments with band bending at the heterostructure interfaces are directly visualized with atomic precision. The mirror twin boundaries in the H-phase CrSe2 exhibit the Tomonaga-Luttinger liquid behavior in the confined one-dimensional electronic system. Our work provides a promising strategy for phase engineering of 2D TMDs, thereby promoting the property research and device applications of specific phases.

On-Surface Synthesis of Polyphenylene Wires Comprising Rigid Aliphatic Bicyclo[1.1.1]Pentane Isolator Units.

Bicyclo[1.1.1]pentane (BCP) motifs are of growing importance to the pharmaceutical industry as sp3 -rich bioisosteres of benzene rings and as molecular building blocks in materials science. Herein we explore the behavior of 1,3-disubstituted BCP moieties on metal surfaces by combining low-temperature scanning tunneling microscopy / non-contact atomic force microscopy studies with density functional theory modeling. We examine the configuration of individual BCP-containing precursors on Au(111), their supramolecular assembly and thermally activated dehalogenative coupling reactions, affording polymeric chains with incorporated electronically isolating units. Our studies not only provide the first sub-molecular insights of the BCP scaffold behavior on surfaces, but also extend the potential application of BCP derivatives towards integration in custom-designed surface architectures.

On-surface polyarylene synthesis by cycloaromatization of isopropyl substituents

Immobilization of organic molecules on metal surfaces and their coupling via thermally induced C–C bond formation is an important technique in organic and polymer synthesis. Using this approach, insoluble and reactive carbon nanostructures can be synthesized and the reactions monitored in situ using scanning probe microscopy methods. The diversity of conceivable products, however, is limited by the number and variety of known on-surface reactions. Here, we introduce the on-surface synthesis of polyarylenes by intermolecular oxidative coupling of isopropyl substituents of arenes. This [3+3] dimerization reaction forms a new phenylene ring and can be regarded as a formal cycloaromatization. The synthetic value of this reaction is proved by the synthesis of polyarylenes and co-polyarylenes, which we demonstrate by synthesizing poly(2,7-pyrenylene-1,4-phenylene). Scanning tunnelling microscopy and non-contact atomic force microscopy studies, complemented by density functional theory calculations, offer mechanistic insight into the on-surface cycloaromatization reaction. On-surface methods can be used to synthesize organic molecules, polymers and nanomaterials, however, the diversity of conceivable products is limited by the number of known on-surface reactions. Now, a phenylene ring-forming reaction on a gold surface by intermolecular oxidative coupling of isopropyl substituents on arenes is reported. The reaction is probed using bond-resolved imaging and computational modelling.

Molecular Self-Assembly in a Poorly Screened Environment: F4TCNQ on Graphene/BN.

We report a scanning tunneling microscopy and noncontact atomic force microscopy study of close-packed 2D islands of tetrafluorotetracyanoquinodimethane (F4TCNQ) molecules at the surface of a graphene layer supported by boron nitride. While F4TCNQ molecules are known to form cohesive 3D solids, the intermolecular interactions that are attractive for F4TCNQ in 3D are repulsive in 2D. Our experimental observation of cohesive molecular behavior for F4TCNQ on graphene is thus unexpected. This self-assembly behavior can be explained by a novel solid formation mechanism that occurs when charged molecules are placed in a poorly screened environment. As negatively charged molecules coalesce, the local work function increases, causing electrons to flow into the coalescing molecular island and increase its cohesive binding energy.

Image formation and contrast inversion in noncontact atomic force microscopy imaging of oxidized Cu(110) surfaces

Joint experimental and theoretical investigation of image formation in noncontact atomic force microscopy (NC-AFM) of the $c$(6 $\ifmmode\times\else\texttimes\fi{}$ 2) and $p$(2 $\ifmmode\times\else\texttimes\fi{}$ 1) phases of the Cu(110):O surface is presented. We proposed previously that the $c$(6 $\ifmmode\times\else\texttimes\fi{}$ 2) reconstruction of the Cu(110):O surface may serve as a reference system allowing chemical identification of the tip apex atom during the course of NC-AFM experiments. The identification is possible due to the fact that two most likely possible contrasts that could be observed after intermittent contacts of the tip with the surface were found which can be attributed to the tip being either terminated by Cu or O atoms. In this paper the idea of exploiting the Cu(110):O surface in NC-AFM studies is further developed. Specifically, (i) we show that there must be an image contrast inversion when the $c$(6 $\ifmmode\times\else\texttimes\fi{}$ 2) surface reconstruction is scanned depending on the tip-surface distance: at the usual imaging conditions, at tip-sample distances of 2--5 \AA{}, the previously reported contrast is observed; however, an opposite contrast is observed for larger separations with one of the two tip terminations. (ii) We study in detail also the image contrast formation of the $p(2\ifmmode\times\else\texttimes\fi{}1)$ surface, which is another common surface reconstruction, and show that tip identification for it is not possible. (iii) Finally, we discuss here possible effects of the actual tip atomic structure on the NC-AFM image. In particular, we show that the O-terminated tip will remain such even after picking up a Cu atom from the surface, experimentally a frequently observed process. Hence this type of the tip modification would only affect secondary features in the image.

Non-contact atomic force microscopy study of hydroxyl groups on the spinel MgAl2O4(100) surface

Atom-resolved non-contact atomic force microscopy (NC-AFM) studies of the magnesium aluminate (MgAl2O4) surface have revealed that, contrary to expectations, the (100) surface is terminated by an aluminum and oxygen layer. Theoretical studies have suggested that hydrogen plays a strong role in stabilizing this surface through the formation of surface hydroxyl groups, but the previous studies did not discuss in depth the possible H configurations, the diffusion behaviour of hydrogen atoms and how the signature of adsorbed H is reflected in atom-resolved NC-AFM images. In this work, we combine first principles calculations with simulated and experimental NC-AFM images to investigate the role of hydrogen on the MgAl2O4(100) surface. By means of surface energy calculations based on density functional theory, we show that the presence of hydrogen adsorbed on the surface as hydroxyl groups is strongly predicted by surface stability considerations at all relevant partial pressures of H2 and O2. We then address the question of how such adsorbed hydrogen atoms are reflected in simulated NC-AFM images for the most stable surface hydroxyl groups, and compare with experimental atom-resolved NC-AFM data. In the appendices we provide details of the methods used to simulate NC-AFM using first principles methods and a virtual AFM.

Molecular Self-Assembly of Enantiopure Heptahelicene-2-Carboxylic Acid on Calcite (101̅4)

Chirality can have a decisive influence on the molecular structure formation upon self-assembly on surfaces. In this paper, we study the structures formed by enantiopure (M)-heptahelicene-2-carboxylic acid ((M)-[7]HCA) on the calcite (101̅4) cleavage plane under ultrahigh vacuum conditions. Previous noncontact atomic force microscopy studies have revealed that the racemic mixture of (M)-[7]HCA and (P)-[7]HCA (1:1) self-assembles into well-defined molecular double rows that are oriented along the calcite [011̅0] direction. Here, we investigate the enantiopure (M)-[7]HCA compound, resulting in distinctly different molecular structures upon deposition onto calcite (101̅4). In sharp contrast to the racemate, the (M)-[7]HCA enantiomer forms molecular islands with a (2 × 3) superstructure. Comparison of the results presented here for the enantiopure compound with the results previously obtained from the racemate indicates that heterochiral recognition is responsible for the formation of the unidirectional double row structures formed by the racemate.

Atomic-Scale Structure and Stability of the31×31R9°Surface ofAl2O3(0001)

Through the interplay of noncontact atomic force microscopy studies and density functional theory calculations, an atomistic model for the Al2O3(0001)-square root(31) x square root(31)R9 degrees surface reconstruction is revealed. The surface is found to consist of an Al adlayer on the Al2O3 substrate, and the driving force for the formation of the reconstruction is related to a detailed balance between strain in the adlayer and the preference for Al atoms to be located on distinct substrate sites.

The influence of a Si cantilever tip with/without tungsten coating on noncontact atomicforce microscopy imaging of a Ge(001) surface

A sharp probe tip with atomic scale stability is essential and desirable for noncontactatomic force microscopy (NC-AFM) studies at the atomic scale. We observed a Ge(001)surface using both a Si cantilever and a tungsten coated Si cantilever at roomtemperature in order to investigate the influence of the tip apex structure on theNC-AFM images. By using the Si cantilever, we first obtained four types of image atthe atomic scale which can be explained assuming a dimer structure on the tipapex. On the other hand, the home-made tungsten coated tip, which has atomicscale stability and high electric conductivity, imaged the so-called ordered c(4 × 2) structure without any artifacts. The tungsten coated cantilever was found to havesignificantly higher performance for NC-AFM studies at the atomic scale than the Sicantilever.

Non-contact atomic force microscopy studies of (2 × 4) InP(0 0 1) surface

A sputter-cleaned indium-rich (2 × 4) InP(0 0 1) surface was investigated by non-contact scanning atomic force microscopy (NCAFM). Atomically-resolved images of the surface exhibit two different patterns. The patterns can be interpreted within the mixed dimer model of (2 × 4) reconstructed InP(0 0 1) surface. It is shown that due to contrast formation mechanism in NCAFM the features resolved are in close correspondence to scanning tunnelling microscopy (STM) data. Due to chemical interaction a P-terminated tip gives the image similar to an empty-state STM image, whereas an In-terminated tip gives the image resembling a filled-state STM one. Moreover, it is shown that due to dipole–dipole interaction, NCAFM can be sensitive to orientation of In-P dimers.

Non-contact atomic force microscopy study of atomic manipulation on an insulator surfaceby nanoindentation

Experimental results on vertical manipulation on an insulator surface using non-contactatomic force microscopy are presented. Cleaved ionic KCl(100) single crystal isused as an insulator surface. With the nanoindentation method used, the verticalmanipulation of a single atom in an ionic crystal surface is more difficult than in asemiconductor surface. Therefore, in many cases, more than one surface atom ismanipulated while, in rare cases, single-atom manipulation is successfully performed.Lateral manipulation of a vacancy has occasionally succeeded on the KCl(100)surface. We have presumed that the lateral manipulation was induced by pulling.

Frequency-Modulated Non-contact Atomic Force Microscopy Study of Heat-Treated Oxide Surface with Organic Molecules

The morphology of the (100) surface of single-crystalline SrTiO3 was studied using frequency-modulated non-contact atomic force microscopy (FM-NCAFM) at a temperature of 100 K. After appropriate etching and heating treatments, unit-cell steps with atomically flat terraces have been successfully obtained. These surfaces were found to be quite stable under ultrahigh vacuum conditions as long as the temperature was maintained below 600 K. A specially tailored porphyrin-based molecule with carboxyphenyl substituent was dispersed on the substrates, and strong suppression of their mobility on the surface was found by FM-NCAFM observations.

Conformation Control and High-Resolution Noncontact Atomic Force Microscopy Study of Porphyrin Derivatives on the Substrates

To investigate the conformational features of individual organic molecules on the substrates, we synthesized 5-(4-methylthiophenyl)-10, 15, 20-tris(3,5-di-t-butylphenyl) porphyrin (MSTBPP) molecules. The conformation of MSTBPP molecules dispersed on an Au(111) surface was investigated using Frequency Modulation Non-contact Atomic Force Microscopy (FM-NCAFM) under UHV condition with nano-meter spatial resolution. It was found that the methylthiophenyl group of MSTBPP molecules enhances the attractive interaction between the molecule and the substrate, which works as an anchor to fix their relative position to the substrate. This function dominates their conformation and mobility on the substrate. These findings lead to various possible strategies to control the conformation of molecules on the substrate, which is one of the important keys to realize the supra-molecular devices.

Noncontact atomic force microscopy studies of ultrathin films of amorphous solid water deposited on Au(111)

Noncontact atomic force microscopy was used to study the morphological changes of an ultrathin amorphous solid water (ASW) film as a function of deposition temperature, annealing temperature, and annealing time. ASW deposited at 80 or 108 K on Au(111) formed truncated hemispherical clusters of increasing size during annealing at 134 K; these clusters were inferred to be crystalline. The number of nuclei present at the outer surface of the film after deposition was greater for higher deposition temperature. For lower cluster densities, depletion of the ASW film around the clusters was observed when the clusters became larger and dendritic growth was observed when the apparent cluster footprint radius exceeded 100 nm.

Non-contact atomic force microscopy study of the Sn/Si(1 1 1) mosaic phase

We have analyzed the height dependence of the Si and Sn atoms on the 1/6 monolayer (ML) Sn/Si(1 1 1)-(√3 × √3) R30° surface, also known as Sn/Si(1 1 1) mosaic phase, by means of non-contact atomic force microscopy (NC-AFM) technique. By preparing samples in which the Sn/Si(1 1 1) mosaic phase and the Si(1 1 1)-(7 × 7) surfaces coexist, and taking account of the proportion of bright and dim contrast atoms when comparing NC-AFM images of both surfaces, we have been able to discriminate between the two atom species forming the mosaic phase. Additionally, we have found a pronounced variation of the height of the Si adatoms with the number of first neighboring Sn adatoms in the Sn/Si(1 1 1) mosaic phase.

Noncontact atomic force microscopy study of copper-phthalocyanines: Submolecular-scale contrasts in topography and energy dissipation

Copper-phthalocyanine (CuPc) thin films on MoS2 surfaces were investigated by noncontact atomic force microscopy (NC-AFM). Submolecular resolution was successfully obtained in both topographic and dissipation images of CuPc monolayers. For topographic contrasts, the influence of short-range chemical interactions is particularly considered while the dissipation contrasts are discussed in relation to the tip-induced molecular fluctuations. Molecularly-resolved NC-AFM image was also obtained on CuPc multilayer, which revealed the structural difference between the monolayer and multilayer surfaces. The energy dissipation measured on these surfaces showed distinctive difference reflecting the different structural stabilities in the films. Furthermore, local surface modification of CuPc monolayer was demonstrated by NC-AFM. This is a direct evidence for the existence of energy transfer from the vibrating cantilever to the molecules through dissipative tip–sample interactions.

Arabinogalactan from the Western larch tree: A new, purified and highly water-soluble polysaccharide-based protecting agent for maintaining precious metal nanoparticles in colloidal suspension

TEM and non-contact atomic force microscopy studies conducted on unprotected and protected platinum, palladium and silver nanoparticle suspensions provide evidence for the protective ability of a newly available polysaccharide known as arabinogalactan. The arabinogalactan offers advantages over conventionally used gum arabic in being highly water soluble, capable of being prepared in higher purity and not settling out insoluble material over time. Nanoparticle suspensions prepared in the presence of this protecting agent show a change in overall particle morphology and reduction in the necklace like aggregation typical of unprotected particles on the TEM grids. It is believed that the protective action exerted by the arabinogalactan on the metal nanoparticles arises out of its unique spheroidal structure adopted in aqueous solution which is thought to sequester individual metal particles by virtue of surface interactions between oxide functionalities on the colloid and hydroxyl groups on the arabinogalactan.

Pseudo-non-contact AFM imaging?

Recent non-contact atomic force microscopy studies have demonstrated that imaging of single atom defects is possible. However, the imaging mechanism was unclear. Long-range forces of attraction, which are normally associated with non-contact mode, are not known to produce sufficient lateral resolution to image atoms. In this study, we suggest a mechanism that could be responsible for the resolution achieved. We use realistic interatomic interaction parameters to do numerical simulations. These simulations are in good agreement with experimental data. As a result, we are able to `separate' the attractive and repulsive forces acting between the AFM tip and the sample surface. Calculations indicate that the force responsible for image contrast in the experimental studies mentioned above, is in most cases the repulsive contact force, and not the long-range attractive force. We check our conclusions against a variety of interatomic interaction parameters and our results remain valid for any reasonable set of such parameters, including the power law of the attractive potential N<9.