Lateral Force Microscopy Images Research Articles

Synthesis and Characterization of Boron Nitride Thin Films Deposited by High-Power Impulse Reactive Magnetron Sputtering

In the present research, hexagonal boron nitride (h-BN) films were deposited by reactive high-power impulse magnetron sputtering (HiPIMS) of the pure boron target. Nitrogen was used as both a sputtering gas and a reactive gas. It was shown that, using only nitrogen gas, hexagonal-boron-phase thin films were synthesized successfully. The deposition temperature, time, and nitrogen gas flow effects were studied. It was found that an increase in deposition temperature resulted in hydrogen desorption, less intensive hydrogen-bond-related luminescence features in the Raman spectra of the films, and increased h-BN crystallite size. Increases in deposition time affect crystallites, which form larger conglomerates, with size decreases. The conglomerates’ size and surface roughness increase with increases in both time and temperature. An increase in the nitrogen flow was beneficial for a significant reduction in the carbon amount in the h-BN films and the appearance of the h-BN-related features in the lateral force microscopy images.

Verification of modulation mechanism of the interfacial dipole effect by changing the stacking sequence of monatomic layers in perovskite oxides

The magnitude of the dipole effect detected by the cutoff energy measurement was modulated for SrTiO3 (STO)/LaAlO3 (LAO)/SrRuO3 (SRO)/STO (001) subs. epitaxial stacks by ultrathin SrAlOx (SAO) insertion prior to LAO deposition. The difference in frictional forces on the LAO surfaces between the stacks with and without SAO insertion was clearly detected by lateral force microscopy (LFM), which indicated the change in dominating surface atomic layers. From the statistical analysis of LFM images, the SAO insertion was found to improve the lateral uniformity of the stacking sequence of charged monatomic layers in epitaxial LAO along the c-axis [(LaO)+ and (AlO2)−], taking account of the inevitable correlation between the surface-terminating atoms with the stacking sequence of the charged monatomic layers in epitaxial LAO. These results are consistently explainable with the proposed model that the interface dipole effect along the c-axis of the perovskite epitaxial stack is determined by the stacking sequence of charged monatomic layers of LAO.

Ultrafast Impact Superspreading on Superamphiphilic Silicon Surfaces for Effective Thermal Management.

Controllable impact spreading behavior is critical for effective thermal management of spray cooling. However, splash and retraction are common problems on hydrophobic (HPB) and hydrophilic (HPL) surfaces. Herein, by regulation of surface wettability, we report a controllable ultrafast impact superspreading behavior (superspreading time of ∼3.0 ms) without splash and retraction on superamphiphilic (SAPL) silicon surfaces. Analysis of dynamic wetting processes combined with observation of lateral force microscopy images on SAPL surfaces reveals the existence of a precursor film at the spreading edge induced by heterogeneous surface wettability at nanoscale. Further study indicates that the inhibition of splash results from the high liquid flux in precursor film, which suppresses the interposition of air at the spreading edge. The reduction of Laplace forces owing to the presence of precursor film inhibits retraction at the spreading frontier. Taking advantage of this impact superspreading behavior on SAPL surfaces, effective heat dissipation is demonstrated, offering uniform and high heat flux for the spray cooling process.

Atomic-Scale Friction Study by EC-AFM: Underpotential Deposition (UPD) of Ag on I-Modified Au(111) and Its Tip Penetration

The underpotential deposition (UPD) of Ag on an I-modified Au(111) surface is examined in perchloric acid solution by electrochemical lateral force microscopy (LFM). The transitions of ordered structures are observed as potential is swept and normal load changes. At low normal load, the structural changes with potential indicate that the iodine structure is very sensitive to the coverage of silver. At potentials positive of the 1st Ag UPD peak, the iodine forms a ( × R30°) structure (θ I = 0.33) on Au(111). The structural transition to a 1 × 1 structure observed as the normal load increases demonstrates that the AFM tip penetrates the iodine adlayer. As the potential passes the 1st Ag UPD peak, the iodine forms a p(3 × 3) (θ I = 0.44) structure and also silver seems to form a p(3 × 3) (θ Ag = 0.44) structure, as suggested by LFM images at high load. At even higher load, also this Ag layer is penetrated and the 1 × 1 structure of the substrate is observed. At potentials slightly negative of the 2nd Ag UPD peak, even though the iodine structure is maintained as a p(3 × 3) (θ I = 0.44) structure, the atomic corrugations observed at high normal load indicate that silver completes a monolayer. The large-scale images clearly show that an Ag bilayer is formed at the 3rd Ag UPD peak. When the Ag bilayer completes, the iodine forms a ( × R30°) (θ I = 0.33).

Accurate Atomic-Scale Imaging of Two-Dimensional Lattices Using Atomic Force Microscopy in Ambient Conditions.

To facilitate the rapid development of van der Waals materials and heterostructures, scanning probe methods capable of nondestructively visualizing atomic lattices and moiré superlattices are highly desirable. Lateral force microscopy (LFM), which measures nanoscale friction based on the commonly available atomic force microscopy (AFM), can be used for imaging a wide range of two-dimensional (2D) materials, but imaging atomic lattices using this technique is difficult. Here, we examined a number of the common challenges encountered in LFM experiments and presented a universal protocol for obtaining reliable atomic-scale images of 2D materials under ambient environment. By studying a series of LFM images of graphene and transition metal dichalcogenides (TMDs), we have found that the accuracy and the contrast of atomic-scale images critically depended on several scanning parameters including the scan size and the scan rate. We applied this protocol to investigate the atomic structure of the ripped and self-folded edges of graphene and have found that these edges were mostly in the armchair direction. This finding is consistent with the results of several simulations results. Our study will guide the extensive effort on assembly and characterization of new 2D materials and heterostructures.

Chemical bond imaging using torsional and flexural higher eigenmodes of qPlus sensors.

Non-contact atomic force microscopy (AFM) with CO-functionalized tips allows visualization of the chemical structure of adsorbed molecules and identify individual inter- and intramolecular bonds. This technique enables in-depth studies of on-surface reactions and self-assembly processes. Herein, we analyze the suitability of qPlus sensors, which are commonly used for such studies, for the application of modern multifrequency AFM techniques. Two different qPlus sensors were tested for submolecular resolution imaging via actuating torsional and flexural higher eigenmodes and via bimodal AFM. The torsional eigenmode of one of our sensors is perfectly suited for performing lateral force microscopy (LFM) with single bond resolution. The obtained LFM images agree well with images from the literature, which were scanned with customized qPlus sensors that were specifically designed for LFM. The advantage of using a torsional eigenmode is that the same molecule can be imaged either with a vertically or laterally oscillating tip without replacing the sensor simply by actuating a different eigenmode. Submolecular resolution is also achieved by actuating the 2nd flexural eigenmode of our second sensor. In this case, we observe particular contrast features that only appear in the AFM images of the 2nd flexural eigenmode but not for the fundamental eigenmode. With complementary laser Doppler vibrometry measurements and AFM simulations we can rationalize that these contrast features are caused by a diagonal (i.e. in-phase vertical and lateral) oscillation of the AFM tip.

Direct curvature measurement of the compartments in bamboo-shaped multi-walled carbon nanotubes via scanning probe microscopy

Bamboo-shaped multi-walled carbon nanotubes (BS-MWCNTs) have compartmented structures inherently obtained during their catalytic growth, and the curvature of the compartmented structure is known to be determined by the morphology of the metal catalysts. In this study, the inside curvature of the BS-MWCNTs was directly measured through scanning probe microscopy (SPM). The surface of the compartment structures of BS-MWCNTs has discontinuous graphene layers and different frictional force levels depending on the curvature direction. That of the inside curvature can be directly observed through tribological analysis by adding and subtracting the lateral force microscopy images obtained on opposite sides along the axial direction of the BS-MWCNT (diameter of 500 nm). This tells us the direction of the inside curvature of the BS-MWCNT, which was also confirmed by identifying the growth direction of the BS-MWCNTs via scanning electron microscopy. Our demonstration implies that SPM can give the same insight into the structural characterization of nanomaterials that is relatively inexpensive and more user-friendly than currently used methods.

The influence of thiolate readsorption on the quality of mixed monolayers formed through an electrochemcial method.

Lateral Force Microscopy (LFM) was used to probe the quality of binary mixed monolayers formed on planar polycrystalline gold through an electrochemical method. In the approach, portions of a self-assembled monolayer (SAM) composed of 2-aminoethanethiol (AET) were removed from the Au(111) surface facets by selective reductive desorption which maintained undisrupted regions of AET elsewhere on the polycrystalline surface. Monolayer voids created by this method were backfilled with 11-mercaptoundecanoic acid (MUA) and the interface characterized with ex situ LFM. This produced images with domains of high and low friction corresponding to isolated zones of MUA and AET respectively. Reverse sequence mixed monolayers were also prepared with MUA as the starting layer and rendered LFM images that mirrored the AET based layers. This demonstrates flexibility of the electrochemical method to produce heterogeneous binary SAMs, and to further probe the quality of mixed monolayers, a number of experimental conditions including desorption time, electrode configuration, and initial incubation period were studied. AET/MUA layers that produced the most enhanced LFM images were formed on a planar electrode that was vertically submerged into the electrolyte while maintaining a selective desorption potential for 5 min before backfilling with MUA. This condition allowed for the effective diffusion of AET away from the interface and created well-defined monolayer voids for backfilling. At desorption times lower than 1 min, some of the AET molecules that remained near the interface would readsorb onto the surface and interfere with the backfilling process thereby creating lower contrast LFM images. Structural features of these layers were independent of initial incubation time (10 min and 16 h); however, the contrast between domains was improved when using AET layers formed over a longer incubation period. Interestingly, the contrast was significantly reduced when mixed layers were created on electrodes set in a hanging meniscus with the electrolyte. Here, electrochemical evidence pointed to prolonged readsorption of thiolates creating less well-defined voids for backfilling, and the event was most pronounced for MUA based layers.

Fabrication of Well-Defined Microdomains Composed of Aldehyde- and Carboxy-Terminated Self-Assembled Monolayers

Coplanar microdomains consisting of two organosilane self-assembled monolayers (SAMs) terminated with chemically reactive functional groups (e.g., aldehyde-[CHO] and carboxy-[COOH] groups) were prepared on Si substrates covered with native oxide (SiO2/Si). A SAM of triethoxysilylundecanal (TESUD) molecules was prepared by chemical vapor deposition on the SiO2/Si surfaces. A few of these samples were exposed to 172 nm vacuum-ultraviolet (UV) radiation for 5-60 min at 10(5) Pa. Various experimental techniques including water-contact angle measurements, ellipsometry, attenuated total reflection Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and atomic force microscopy confirmed that CHO terminal groups of the SAM have been photochemically converted to COOH groups. Based on this result, site-selective photoirradiation of the TESUD-SAM with the same radiation source was performed using a photomask. Lateral force microscopy images revealed that well-ordered microstructures of 5 x 5 microm2 square-shaped features are formed on the substrate. The difference in chemical reactivity was characterized by fluorescence microscopy using specific bonding between biotin hydrazide and streptavidin-conjugated quantum dots (QDs). The fluorescence microscopy images revealed that biotin hydrazide immobilized site-selectively on the masked CHO-terminated regions and not on the vacuum-UV irradiated regions. However, the latter regions showed chemical reactivity to biotin hydrazide after activation treatments using N-hydroxysuccinimide and 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide hydrochloride, and well-defined QDs patterns can be obtained within these regions. This suggests that the surface CHO groups of the vacuum-UV irradiated regions were transformed to COOH groups, and the activation treatments lead to the formation of active N-hydroxysuccinimidyl esters, which are reactive toward amine groups of biotin hydrazide.

Effect of Surface Roughness on Dip-Pen Nanolithography

In the present work, five gold thin films with various surface roughnesses were prepared by sputtering and the influence of the surface roughness of gold substrate on dip-pen nanolithography (DPN) was studied using 1-octadecanethiol (ODT) and poly(vinylidene fluoride-trifluorethylene) [P(VDFTrFE)] as inks. It was shown that surface roughness influences both the contrast in lateral force microscopy (LFM) images and the transport rate of ink. Surfaces with less roughness give good contrast in LFM images, while rough surfaces give poor contrast. The transport rate of ink increases as the roughness decreases; however, the extent of the influence is strongly ink-dependent.

Atomic force microscopic studies on erythrocytes from an evolutionary perspective.

We examined atomic force microscopy (AFM) and lateral force microscopy (LFM) images of human, avian, reptilian, amphibian, and piscine erythrocytes to determine whether the general pattern of erythrocyte membrane architecture has been largely conserved in the course of phylogenetic evolution or relatively minor modifications have taken place. The general pattern of the cell surface structure is indeed very similar among the phyla examined. The surface features include a number of blebs or globular structures and hole-like depressions. Such features are particularly clear in fish (Heteropneustes sp.), in which globular blebs are arranged in tiers around the depressions. The same pattern is found in the other phyla, although the sizes of the blebs and depressions vary. The depressions are approximately 340 and approximately 100 nm in diameter in chickens and fish, respectively, and are smaller in other phyla. The images of human erythrocytes presented here show holes more clearly than the images obtained by Zhang et al. (Scanning Electron Microsc., 1995; 9:981-989), who showed for the first time the highly uneven surface of these cells. The globules range in size from approximately 50-150 nm in diameter. These nanostructures have a width of approximately 333-1,000 atoms, assuming that the average dimension of an atom is 1.5 A. The size range of the holes is approximately 40-432 nm (equivalent to a width of approximately 266-2880 atoms). LFM images, which take into account the lateral component of the force, represent the variation of surface friction (roughness) on the erythrocyte surface. This is very clear in the toad images, which show well-ordered strata that have not been revealed in ordinary AFM images.

Tribological behavior of alumina doped zinc oxide films grown by pulsed laser deposition

Zinc oxide, a well-known piezoelectric material, has become the subject of tribological investigations. This research describes the synthesis and tribological evaluation of alumina doped zinc oxide [ZnO(Al2O3]) films grown in vacuum by a pulsed laser deposition (PLD) technique using hot pressed ZnO–5 wt % Al2O3 targets. For comparison, pure ZnO films were grown under identical PLD conditions. The films were characterized by scanning electron microscopy, energy dispersive x-ray spectroscopy, x-ray diffraction, and x-ray photoelectron spectroscopy. Friction measurements were made using a ball-on-disk tribometer. Nanotribological studies were conducted on wear scars of ZnO and ZnO(Al2O3) films by atomic and lateral force microscopy. Both ZnO and ZnO(Al2O3) films were crystalline, with a strong (002) texture. The friction coefficient of the ZnO(Al2O3) film (μ=0.15) was considerably less than that of the pure ZnO film (μ=0.34). Wear scars on doped ZnO films were relatively smooth and, unlike in the case of pure ZnO films, no cracks were observed. Lateral force microscopy images of wear scars on ZnO(Al2O3) films showed dark contrast regions indicating the presence of a slippery phase. No such phase contrast was observed in the case of pure PLD ZnO film grown in vacuum.

Lateral Force Microscopy Investigation of Bacteriorhodopsin Adsorption onto Mixed Self-Assembled Monolayers

The chemically distinctive domains of mixed self-assembled monolayers (SAMs) were investigated by lateral force microscopy (LFM). Mixed SAMs were formed onto the silver coated Si-wafers by soaking them into the ethanolic solution of 11-aminoundecanoic acid (H2N(CH2)10COOH) and undecanoic acid (CH3(CH2)9COOH). Based on the LFM images, it was found that the distinctive domains were formed due to the different functional groups. The prepared mixed SAMs were used as a template for the adsorpton of bacteriorhodopsin (bR). It is concluded that site-directed adsorption of the protein molecules would be possible by using the mixed SAMs containing different functional groups.

Lateral force microscopy study of functionalized self-assembled monolayer surfaces

Patterned self-assembly of functionalized alkanethiols on the gold surface on a submicron scale was achieved by microcontact printing with elastomeric poly(dimethylsiloxane) (PDMS) stamps. Atomic force microscopy (AFM) and lateral force microscopy (LFM) were used to examine the topography and properties of the self-assembled monolayer (SAM) surface containing Br and OH groups. Au-coated probe tips were modified with alkanethiolates of such functional groups as CH 3, NHCOCH 3 and NHCOCF 3. LFM images were examined to elucidate the adhesive interaction between modified probe tips and SAM surfaces. AFM images scanned by normal and modified probe tips clearly show that the height difference of the alternating lines on the exposed surface is attributed to the size differences of Br and OH groups. With an NHCOCH 3 probe the image contrast of the lines due to the friction is almost indistinguishable or inverted in the LFM image. LFM image by a normal and NHCOCF 3 probes exhibit the same contrast as topography in AFM images. Therefore, the specific functional group in this system could be recognized by the comparison of the adhesive interaction between tip and sample surfaces.

Self-Assembly of Anthraquinone-2-carboxylic Acid on Silver: Fourier Transform Infrared Spectroscopy, Ellipsometry, Quartz Crystal Microbalance, and Atomic Force Microscopy Study

The adsorption of anthraquinone-2-carboxylic acid (AQ-2-COOH) in ethanol on a silver surface has been investigated by reflection−absorption infrared (RAIR) spectroscopy, ellipsometry, quartz crystal microbalance (QCM), and atomic force microscopy (AFM). The RAIR spectral data were found to be consistent with those gathered by other experimental means. Specifically, from the RAIR spectral data it was concluded that the molecule chemisorbs on silver as carboxylate, after deprotonation, with its two oxygen atoms bound symmetrically to the metal substrate. The molecular plane was determined to be tilted by 40° from the surface normal. The ellipsometric thickness of the monolayer, 10.6 ± 1.2 Å, agreed well with that predicted from the RAIR data, 10.4 Å. QCM data suggested that the self-assembling process could be described in terms of the Langmuir adsorption model, providing the value of the free energy of adsorption at −21.8 kJ/mol. The limiting surface coverage was determined from the QCM data to be 2.5 × 10-10 mol/cm2, corresponding to the area per adsorbate to be 65 Å2. The latter value was quite close to that predicted from the RAIR spectral data, 66 Å2/molecule. Neither a periodic nor a molecularly resolved image could be obtained under a contact AFM measurement, but the lateral force microscopy (LFM) images revealed that the anthraquinone moieties were close-packed on the silver surface, with the rings assembled parallel to one another to form a brick like architecture. The area per adsorbate estimated from the LFM images, 62 ± 5 Å2, was also consistent with that estimated from the QCM and RAIR data.

Reactivity of gypsum faces according to the relative humidity by scanning force microscopy

This article reports the experimental observation of the stability of the different faces of calcium sulphate dihydrate (gypsum CaSO 4.2H 2O) according to the relative humidity. Scanning Force Microscopy experiments were carried out with a view to discerning the topography of the surfaces, the chemical compositional domains, and in an attempt to evaluate the friction and viscoelastic properties of the surface. Our results indicate that the (010) face of gypsum is hydrophilic and very reactive contrary to the less hydrophilic (120) and (1̄01) faces which remain stable depending on the relative humidity. It is clear from our results that a precipitation like process can be induced by the SFM tip. The dissolution of this precipitate depends on the amount of water in the meniscus between the probe and the surface. For the purpose of comparison, the cleaved surfaces of natural anhydrite (CaSO 4), calcite (CaCO 3), and muscovite mica were also observed. The Lateral Force Microscopy images provided the undisputed evidence that the process of dehydration of the (010) face actually occurs with a change of the elastic constant of the crystal.

Computer Simulation of the AFM/LFM Imaging Process: Hexagonal versus Honeycomb Structure on Graphite

Although the atoms in cleavage planes of graphite are arranged in a honeycomb structure, it is well known from experimental work that atomic force microscopy (AFM) yields a hexagonal structure, a phenomenon that has not been understood so far. Here, computer simulations of the atomic-scale imaging process on graphite by AFM are reported, showing that this behaviour can be explained within a simple model of elastic tip–sample interaction. Both the topographic (AFM) images and the friction force or lateral force microscopy (LFM) images were simulated as a function of the scanning direction relative to the graphite lattice and as a function of the cantilever force constant. The scan distortions and the skipped area due to the AFM/LFM imaging process were evaluated. Simulations were performed both in the presence and in the absence of atomic-scale stick–slip processes. It is shown that neither stick–slip processes nor an inequivalence of the A- and B-sites of graphite is necessary to generate a hexagonal AFM image when scanning an atomic honeycomb structure. Rather, the simulations demonstrate that due to the two-dimensional elastic lateral displacement of the cantilever, the potential maxima—which correspond to the positions of the honeycomb lattice—are avoided by the scanning path of the tip apex, resulting in a hexagonal structure of the AFM and LFM images.© 1997 John Wiley & Sons, Ltd.

Computer Simulation of the AFM/LFM Imaging Process: Hexagonal versus Honeycomb Structure on Graphite

Although the atoms in cleavage planes of graphite are arranged in a honeycomb structure, it is well known from experimental work that atomic force microscopy (AFM) yields a hexagonal structure, a phenomenon that has not been understood so far. Here, computer simulations of the atomic-scale imaging process on graphite by AFM are reported, showing that this behaviour can be explained within a simple model of elastic tip-sample interaction. Both the topographic (AFM) images and the friction force or lateral force microscopy (LFM) images were simulated as a function of the scanning direction relative to the graphite lattice and as a function of the cantilever force constant. The scan distortions and the skipped area due to the AFM/LFM imaging process were evaluated. Simulations were performed both in the presence and in the absence of atomic-scale stick-slip processes. It is shown that neither stick-slip processes nor an inequivalence of the A- and B-sites of graphite is necessary to generate a hexagonal AFM image when scanning an atomic honeycomb structure. Rather, the simulations demonstrate that due to the two-dimensional elastic lateral displacement of the cantilever, the potential maxima-which correspond to the positions of the honeycomb lattice-are avoided by the scanning path of the tip apex, resulting in a hexagonal structure of the AFM and LFM images.

Modelling of the scan process in lateral force microscopy

A model for the simulation of the profiling process of a scanning force microscope tip scanning a sample surface is introduced. Starting from the equations of motion, complete lateral force microscopy images as well as individual scan lines can be calculated. The model is applied to the constant-force mode of a lateral force microscope using a model potential for the tip-sample interaction which has the translational symmetry of a MoS 2(001) surface. Comparison with recent experimental data shows good agreement. Subsequent analysis of the tip movement demonstrates the characteristic two-dimensional stick-slip behavior of the tip. In addition, the influence of the scan speed on the measured lateral forces is discussed.

Nanometer-Scale Patterning of Surfaces Using Self-Assembly Chemistry. 2. Preparation, Characterization, and Electrochemical Behavior of Two-Component Organothiol Monolayers on Gold Surfaces

We report a study of the physical and electrochemical properties of two-component self-assembled monolayers (SAMs) composed of both electroactive (4-aminothiophenol, 4-ATP) and electroinactive (n-octadecanethiol, ODT) species. In all of the experiments reported here, relatively short (3 h) assembly times were used to prepare the mixed SAMs. We have characterized the macroscopic composition and the microscopic structure of these SAMs using Auger electron spectroscopy (AES), coulometry, grazing angle Fourier transform infrared spectroscopy, and lateral force microscopy (LFM). The adsorption isotherms determined by AES and coulometry show significant deviation from Langmuirian behavior and are suggestive of phase separation. LFM images obtained at three points near the critical region of the isotherm ([4-ATP]/[ODT] ∼ 4) indicate that these two-component SAMs display complex phase behavior: At relatively low 4-ATP coverages, the surface consists of small islands of 4-ATP imbedded in an ordered film of ODT. At higher coverages of 4-ATP, however, we find evidence of both separation into distinct phases and mixing of the two components. In a second series of experiments, we demonstrate that phase domains of 4-ATP are electroactive and can be used to carry out localized electrochemistry. That is, the islands of 4-ATP, which are randomly distributed in the ODT matrix, behave as an array of ultramicroelectrodes. Surface-confined 4-ATP molecules can be used to nucleate the growth of polyaniline selectively from the phase separated domains of 4-ATP. We find that if a 4-ATP/ODT mixed monolayer is oxidized in the presence of aniline, nanometer scale polyaniline features are formed. The size and distribution of these features have been characterized by AFM and can be controlled through a combination of monolayer composition and the concentration of aniline in solution.