The emergence of surface patterns on the surfaces of compliant materials subject to plowing wear ... more The emergence of surface patterns on the surfaces of compliant materials subject to plowing wear is a complex problem which can be quantitatively characterized, e.g., on polymer surfaces scraped by an atomic force microscope (AFM) tip. Here we explore the applicability of a phenomenological model recently introduced to describe this phenomenon. Based on the competition between the viscoplastic indentation and the elastic shear stress caused by the tip, the model is able to reproduce the wavy features (ripples) observed when the tip is scanned along a series of parallel lines. For low values of the driving velocity v and the spacing b between scan lines, the existence of dotted areas formed by variously oriented pit alignments is observed. Moreover, coexistence of rippled with dotted domains is also observed at suitable parameter values. The formation process of the ripples is also described in detail. The amplitude, period, and orientation of these features are estimated numerically for different values of v and b parameters. We have also revisited the formation of the wavy patterns formed when a single line is scanned, and derived an equation which correctly describes their period and depth, and the static friction as well. This equation is not applicable when several lines are scanned one after the other and the ripples emerge as result of a cooperative process which involves the scanning of several lines.
We have performed AFM nanomanipulation experiments on triangular Au islands (with typical linear ... more We have performed AFM nanomanipulation experiments on triangular Au islands (with typical linear size of 25-80 nm) previously grown on a MoS 2 surface. These islands are found to move along preferential directions, independently of the angle of attack of the scanning probe. A comparison between molecular-dynamics simulations and atomically resolved STM images prove that these directions correspond to the zigzag alignments of the Mo and S atoms on the substrate. This is related to the observed systematic orientation of the islands, which is in turn a consequence of a sharp energy minimum as a function of each island's angular orientation. This directional-locked motion is entirely different from nanomanipulation involving disordered contact interfaces, where the direction of motion is determined by the island geometry and the scan pattern, and roto-translational motion is observed in arbitrary directions. Besides shedding light on the fundamental mechanisms of friction in the considered class of materials, our results could find important applications in the controlled positioning of metal nanoislands as electrodes for molecular electronics.
Surface nanostructures are increasingly more employed for controlled protein assembly on function... more Surface nanostructures are increasingly more employed for controlled protein assembly on functional nanodevices, in nanobiotechnology, and in nanobiomaterials. However, the mechanism and dynamics of how nanostructures induce order in the adsorbed protein assemblies are still enigmatic. Here, we use single-molecule mapping by accumulated probe trajectories and complementary atomic force microscopy to shed light on the dynamic of in situ assembly of human plasma fibrinogen (HPF) adsorbed on nanostructured polybutene-1 (PB-1) and nanostructured polyethylene (PE) surfaces. We found a distinct lateral heterogeneity of HPF-polymer nanostructure interface (surface occupancy, residence time, and diffusion coefficient) that allow identifying the interplay between protein topographical nanoconfinement, protein diffusion mechanism, and ordered protein self-assembly. The protein diffusion analysis revealed high-diffusion polarization without correlation to the anisotropic friction characteristic of the polymer surfaces. This suggests that HPF molecules confined on the nanosized PB-1 needle crystals and PE shish-kebab crystals, respectively, undergo partial detachment and diffuse via a Sansetsukon-like nanocrawling mechanism. This mechanism is based on the intrinsic flexibility of HPF in the coiled-coil regions. We conclude that nanostructured surfaces that encourage this characteristic surface mobility are more likely to lead to the formation of ordered protein assemblies and may be useful for advanced nanobiomaterials.
We report on the formation of regular surface ripples accompanying microscratching of vitreous si... more We report on the formation of regular surface ripples accompanying microscratching of vitreous silica using Berkovich diamond tips. The tips were pulled laterally along the surface with varying normal load of up to 30 mN and scan velocity of up to 500 μm/s. Post mortem AFM analysis and independent confirmation through laser scanning microscopy reveal the occurrence of regular surface ripples in the wear grooves. The average repetition distance of the ripples is in the sub-μm range and increases linearly with the scan velocity. Their corrugation is on the order of a few nm, increasing with the applied load, and well below the indentation depth (of a few hundred nm). The experimental results are interpreted assuming that the tip moves in a stick-slip fashion, while it is elastically driven along the surface. In this framework the static friction to be overcome in the stick phase corresponds to the lateral hardness of the material, whereas the kinetic friction slowing down the tip results from the plowing motion of the latter.
Langmuir : the ACS journal of surfaces and colloids, Mar 13, 2018
We report high-resolution surface morphology and friction force maps of polycrystalline organic t... more We report high-resolution surface morphology and friction force maps of polycrystalline organic thin films derived by deposition of the n-type perylene diimide semiconductor PDI8-CN. We show that the in-plane molecular arrangement into ordered, cofacial slip-stacked rows results in a largely anisotropic surface structure, with a characteristic sawtooth corrugation of a few Ångstroms wavelength and height. Load-controlled experiments reveal different types of friction contrast between the alternating sloped and stepped regions, with transitions from atomic-scale dissipative stick-slip to smooth sliding with ultralow friction within the surface unit cell. Notably, such a rich phenomenology is captured under ambient conditions. We demonstrate that friction contrast is well reproduced by numerical simulations assuming a reduced corrugation of the tip-molecule potential nearby the step edges. We propose that the side alkyl chains pack into a compact low-surface-energy overlayer, and fric...
Friction force microscopy (FFM) in aqueous environments has recently proven to be a very effectiv... more Friction force microscopy (FFM) in aqueous environments has recently proven to be a very effective method for lattice-resolution imaging of crystal surfaces. Here we demonstrate the use of ethanol for similar measurements on water-soluble materials. Lattice resolved frictional stick-slip traces of a cleaved NaCl(100) surface submerged in ethanol are compared with previous obtained FFM results in ultrahigh vacuum (UHV). We use the Prandtl-Tomlinson framework to estimate the amplitude of the corrugation potential and the contact stiffness. The surface potential amplitude scales with the applied normal loads are in good agreement with data obtained for NaCl measured under UHV conditions, but demonstrates deviations from the ideal periodic potential given by the Prandtl-Tomlinson model. An additional finding is that the use of ethanol allows us to explore higher load ranges without detectable evidence of surface wear. The contact stiffness does not vary significantly with the normal loa...
We present a series of numerical simulations on the friction-anisotropy behavior and stick-slip d... more We present a series of numerical simulations on the friction-anisotropy behavior and stick-slip dynamics of a point mass in the framework of a 2d Prandtl-Tomlinson model. Results for three representative surface lattice are shown: square, hexagonal and honeycomb. Curves for scan angle dependence of static friction force, and kinetic one at T = 0 K and T = 300 K are shown. Friction force maps are computed at different directions.
We have performed molecular dynamics simulations of nanomanipulation experiments on short single-... more We have performed molecular dynamics simulations of nanomanipulation experiments on short single-stranded DNA chains elastically driven on a graphene surface. After a brief transient, reproducible stick-slip cycles are observed on chains made by ten units of thymine, cytosine, adhenine and guanine. The cycles have the periodicity of the graphene substrate, and take place via an intermediate stage, appearing as a dip in the sawtooth variations of lateral force recorded while the chains are manipulated. Guanine present remarkable differences from the other bases, since a lower number of nucleotides are prone to stick to the substrate in this case. Nevertheless, the magnitudes of static friction and lateral stiffness are similar for all chains (~30 pN and 0.7 N/m per adsorbed nucleotide respectively).
In this work we present friction-force microscopy (FFM) lattice-resolved images acquired on the (... more In this work we present friction-force microscopy (FFM) lattice-resolved images acquired on the (100) facet of the semiconductor organic oligomer (2Z,2 Z) − 3,3-(1,4-phenylene)bis(2-(4-butoxyphenyl)acrylonitrile) (β-DBDCS) crystal in water at room temperature. Stick-slip contrast, lateral contact stiffness, and friction forces are found to depend strongly on the sliding direction due to the anisotropic packing of the molecular chains forming the crystal surface along the [010] and [001] directions. The anisotropy also causes the maximum value of the normal force applicable before wearing to increase by a factor of 3 when the scan is performed along the [001] direction on the (100) face. Altogether, our results contribute to achieving a better understanding of the molecular origin of friction anisotropy on soft crystalline surfaces, which has been often hypothesized but rarely investigated in the literature.
The formation of nanometer-scale patterns while scratching a KBr(001) surface with a scanning for... more The formation of nanometer-scale patterns while scratching a KBr(001) surface with a scanning force microscope in ultrahigh vacuum is reported. Wear of single atomic layers has been observed when the microscope tip is repeatedly scanned across a line. The initially flat surface is rearranged in a quasiperiodic pattern of mounds and pits. The distance between the pits is about 40 nm when normal forces of a few nanonewtons are applied, and it slowly increases with the load. If a square area is scanned, a pattern of ripples is formed. These features can be interpreted within an erosion process induced by a periodic increase of the strain produced by the scanning tip.
Physical Review B Condensed Matter and Materials Physics, 2010
The angular dependence of the lateral forces acting on an atomically sharp tip slowly pulled by a... more The angular dependence of the lateral forces acting on an atomically sharp tip slowly pulled by an elastic spring along a crystal surface with square symmetry is investigated in the framework of a separable two-dimensional tip-surface interaction potential. In the stick-slip regime kinetic friction is proportional to (cosphi+|sinphi|) , phi being the angle between the scan direction and a particular symmetry axis. For a high enough normal force, static friction is proportional to 1/cosphi , whereas for intermediate loads it shows a phi -dependent spread of possible values. Continuous sliding with ultralow friction sets in below a load-dependent corrugation amplitude. Numerical simulations help interpret those analytic results in terms of the zigzag motion of a friction force microscope tip sliding on the (001) surface of a rocksalt crystal. The influence of the offset between the start of a scan and the center of the corresponding unit cell, in particular, for scans along directions is also elucidated. The predicted ratio of kinetic friction along the and directions agrees best with values measured on alkali halides with similar cation and anion radii. This ratio, as well as the angular dependence of the static friction may be used to determine fine details of the lateral tip-sample interaction.
Novel phenomena accompanying atomic-scale friction are studied on NaCl(001) by the combination of... more Novel phenomena accompanying atomic-scale friction are studied on NaCl(001) by the combination of quasistatic lateral force measurements with dynamic measurements of contact resonance frequencies. For loads up to a few nN the flexural resonance is tracked by a phase-locked-loop by the use of small oscillation amplitude (50 pm). The contact resonance varies during the stick stages, which demonstrates that the dynamic measurement provides additional information about small changes of the stressed contact. Improved sensitivity is also observed across atomic-scale defects which are clearly observed in the contact frequency channel. The low lateral contact stiffness inferred from the observed torsional resonance agrees well with that deduced from the quasistatic measurements and strongly suggests that the contact is atomic-sized.
ABSTRACT The growth of ordered molecular monolayers on insulating surfaces is often hampered by t... more ABSTRACT The growth of ordered molecular monolayers on insulating surfaces is often hampered by the strength molecule-molecule compared to surface-molecule interactions, resulting in molecule clusters aggregated at step edges rather than ordered flat films. We have succeeded in employing the enhanced interaction at edges of nanometer-scale structures to confine molecules into flat islands. Monolayer-deep pits with atomically straight edges were produced on a KBr surface by an electron-irradiation induced desorption process. Phtalocyanin and perylene derived molecules were evaporated onto the surface and imaged by means of high resolution force microscopy. Molecules were found to be trapped in the pits and the structure of the islands could be resolved. We will discuss possible mechanisms for the trapping and show how dynamic force microscopy can even provide information about mechanical dissipation in such molecular islands.
The sliding of a sharp nanotip on graphene completely immersed in water is investigated by molecu... more The sliding of a sharp nanotip on graphene completely immersed in water is investigated by molecular dynamics (MD) and atomic force microscopy. MD simulations predict that the atomic-scale stick-slip is almost identical to the one found in ultra-high vacuum. Furthermore, they show that water plays a purely stochastic role on sliding (solid-to-solid) friction. These observations are substantiated by friction measurements on graphene grown on Cu and Ni, where, oppositely to operation in air, lattice resolution is readily achieved. Our results promote friction force microscopy in water as a robust alternative to ultra-high vacuum measurements.
The emergence of surface patterns on the surfaces of compliant materials subject to plowing wear ... more The emergence of surface patterns on the surfaces of compliant materials subject to plowing wear is a complex problem which can be quantitatively characterized, e.g., on polymer surfaces scraped by an atomic force microscope (AFM) tip. Here we explore the applicability of a phenomenological model recently introduced to describe this phenomenon. Based on the competition between the viscoplastic indentation and the elastic shear stress caused by the tip, the model is able to reproduce the wavy features (ripples) observed when the tip is scanned along a series of parallel lines. For low values of the driving velocity v and the spacing b between scan lines, the existence of dotted areas formed by variously oriented pit alignments is observed. Moreover, coexistence of rippled with dotted domains is also observed at suitable parameter values. The formation process of the ripples is also described in detail. The amplitude, period, and orientation of these features are estimated numerically for different values of v and b parameters. We have also revisited the formation of the wavy patterns formed when a single line is scanned, and derived an equation which correctly describes their period and depth, and the static friction as well. This equation is not applicable when several lines are scanned one after the other and the ripples emerge as result of a cooperative process which involves the scanning of several lines.
We have performed AFM nanomanipulation experiments on triangular Au islands (with typical linear ... more We have performed AFM nanomanipulation experiments on triangular Au islands (with typical linear size of 25-80 nm) previously grown on a MoS 2 surface. These islands are found to move along preferential directions, independently of the angle of attack of the scanning probe. A comparison between molecular-dynamics simulations and atomically resolved STM images prove that these directions correspond to the zigzag alignments of the Mo and S atoms on the substrate. This is related to the observed systematic orientation of the islands, which is in turn a consequence of a sharp energy minimum as a function of each island's angular orientation. This directional-locked motion is entirely different from nanomanipulation involving disordered contact interfaces, where the direction of motion is determined by the island geometry and the scan pattern, and roto-translational motion is observed in arbitrary directions. Besides shedding light on the fundamental mechanisms of friction in the considered class of materials, our results could find important applications in the controlled positioning of metal nanoislands as electrodes for molecular electronics.
Surface nanostructures are increasingly more employed for controlled protein assembly on function... more Surface nanostructures are increasingly more employed for controlled protein assembly on functional nanodevices, in nanobiotechnology, and in nanobiomaterials. However, the mechanism and dynamics of how nanostructures induce order in the adsorbed protein assemblies are still enigmatic. Here, we use single-molecule mapping by accumulated probe trajectories and complementary atomic force microscopy to shed light on the dynamic of in situ assembly of human plasma fibrinogen (HPF) adsorbed on nanostructured polybutene-1 (PB-1) and nanostructured polyethylene (PE) surfaces. We found a distinct lateral heterogeneity of HPF-polymer nanostructure interface (surface occupancy, residence time, and diffusion coefficient) that allow identifying the interplay between protein topographical nanoconfinement, protein diffusion mechanism, and ordered protein self-assembly. The protein diffusion analysis revealed high-diffusion polarization without correlation to the anisotropic friction characteristic of the polymer surfaces. This suggests that HPF molecules confined on the nanosized PB-1 needle crystals and PE shish-kebab crystals, respectively, undergo partial detachment and diffuse via a Sansetsukon-like nanocrawling mechanism. This mechanism is based on the intrinsic flexibility of HPF in the coiled-coil regions. We conclude that nanostructured surfaces that encourage this characteristic surface mobility are more likely to lead to the formation of ordered protein assemblies and may be useful for advanced nanobiomaterials.
We report on the formation of regular surface ripples accompanying microscratching of vitreous si... more We report on the formation of regular surface ripples accompanying microscratching of vitreous silica using Berkovich diamond tips. The tips were pulled laterally along the surface with varying normal load of up to 30 mN and scan velocity of up to 500 μm/s. Post mortem AFM analysis and independent confirmation through laser scanning microscopy reveal the occurrence of regular surface ripples in the wear grooves. The average repetition distance of the ripples is in the sub-μm range and increases linearly with the scan velocity. Their corrugation is on the order of a few nm, increasing with the applied load, and well below the indentation depth (of a few hundred nm). The experimental results are interpreted assuming that the tip moves in a stick-slip fashion, while it is elastically driven along the surface. In this framework the static friction to be overcome in the stick phase corresponds to the lateral hardness of the material, whereas the kinetic friction slowing down the tip results from the plowing motion of the latter.
Langmuir : the ACS journal of surfaces and colloids, Mar 13, 2018
We report high-resolution surface morphology and friction force maps of polycrystalline organic t... more We report high-resolution surface morphology and friction force maps of polycrystalline organic thin films derived by deposition of the n-type perylene diimide semiconductor PDI8-CN. We show that the in-plane molecular arrangement into ordered, cofacial slip-stacked rows results in a largely anisotropic surface structure, with a characteristic sawtooth corrugation of a few Ångstroms wavelength and height. Load-controlled experiments reveal different types of friction contrast between the alternating sloped and stepped regions, with transitions from atomic-scale dissipative stick-slip to smooth sliding with ultralow friction within the surface unit cell. Notably, such a rich phenomenology is captured under ambient conditions. We demonstrate that friction contrast is well reproduced by numerical simulations assuming a reduced corrugation of the tip-molecule potential nearby the step edges. We propose that the side alkyl chains pack into a compact low-surface-energy overlayer, and fric...
Friction force microscopy (FFM) in aqueous environments has recently proven to be a very effectiv... more Friction force microscopy (FFM) in aqueous environments has recently proven to be a very effective method for lattice-resolution imaging of crystal surfaces. Here we demonstrate the use of ethanol for similar measurements on water-soluble materials. Lattice resolved frictional stick-slip traces of a cleaved NaCl(100) surface submerged in ethanol are compared with previous obtained FFM results in ultrahigh vacuum (UHV). We use the Prandtl-Tomlinson framework to estimate the amplitude of the corrugation potential and the contact stiffness. The surface potential amplitude scales with the applied normal loads are in good agreement with data obtained for NaCl measured under UHV conditions, but demonstrates deviations from the ideal periodic potential given by the Prandtl-Tomlinson model. An additional finding is that the use of ethanol allows us to explore higher load ranges without detectable evidence of surface wear. The contact stiffness does not vary significantly with the normal loa...
We present a series of numerical simulations on the friction-anisotropy behavior and stick-slip d... more We present a series of numerical simulations on the friction-anisotropy behavior and stick-slip dynamics of a point mass in the framework of a 2d Prandtl-Tomlinson model. Results for three representative surface lattice are shown: square, hexagonal and honeycomb. Curves for scan angle dependence of static friction force, and kinetic one at T = 0 K and T = 300 K are shown. Friction force maps are computed at different directions.
We have performed molecular dynamics simulations of nanomanipulation experiments on short single-... more We have performed molecular dynamics simulations of nanomanipulation experiments on short single-stranded DNA chains elastically driven on a graphene surface. After a brief transient, reproducible stick-slip cycles are observed on chains made by ten units of thymine, cytosine, adhenine and guanine. The cycles have the periodicity of the graphene substrate, and take place via an intermediate stage, appearing as a dip in the sawtooth variations of lateral force recorded while the chains are manipulated. Guanine present remarkable differences from the other bases, since a lower number of nucleotides are prone to stick to the substrate in this case. Nevertheless, the magnitudes of static friction and lateral stiffness are similar for all chains (~30 pN and 0.7 N/m per adsorbed nucleotide respectively).
In this work we present friction-force microscopy (FFM) lattice-resolved images acquired on the (... more In this work we present friction-force microscopy (FFM) lattice-resolved images acquired on the (100) facet of the semiconductor organic oligomer (2Z,2 Z) − 3,3-(1,4-phenylene)bis(2-(4-butoxyphenyl)acrylonitrile) (β-DBDCS) crystal in water at room temperature. Stick-slip contrast, lateral contact stiffness, and friction forces are found to depend strongly on the sliding direction due to the anisotropic packing of the molecular chains forming the crystal surface along the [010] and [001] directions. The anisotropy also causes the maximum value of the normal force applicable before wearing to increase by a factor of 3 when the scan is performed along the [001] direction on the (100) face. Altogether, our results contribute to achieving a better understanding of the molecular origin of friction anisotropy on soft crystalline surfaces, which has been often hypothesized but rarely investigated in the literature.
The formation of nanometer-scale patterns while scratching a KBr(001) surface with a scanning for... more The formation of nanometer-scale patterns while scratching a KBr(001) surface with a scanning force microscope in ultrahigh vacuum is reported. Wear of single atomic layers has been observed when the microscope tip is repeatedly scanned across a line. The initially flat surface is rearranged in a quasiperiodic pattern of mounds and pits. The distance between the pits is about 40 nm when normal forces of a few nanonewtons are applied, and it slowly increases with the load. If a square area is scanned, a pattern of ripples is formed. These features can be interpreted within an erosion process induced by a periodic increase of the strain produced by the scanning tip.
Physical Review B Condensed Matter and Materials Physics, 2010
The angular dependence of the lateral forces acting on an atomically sharp tip slowly pulled by a... more The angular dependence of the lateral forces acting on an atomically sharp tip slowly pulled by an elastic spring along a crystal surface with square symmetry is investigated in the framework of a separable two-dimensional tip-surface interaction potential. In the stick-slip regime kinetic friction is proportional to (cosphi+|sinphi|) , phi being the angle between the scan direction and a particular symmetry axis. For a high enough normal force, static friction is proportional to 1/cosphi , whereas for intermediate loads it shows a phi -dependent spread of possible values. Continuous sliding with ultralow friction sets in below a load-dependent corrugation amplitude. Numerical simulations help interpret those analytic results in terms of the zigzag motion of a friction force microscope tip sliding on the (001) surface of a rocksalt crystal. The influence of the offset between the start of a scan and the center of the corresponding unit cell, in particular, for scans along directions is also elucidated. The predicted ratio of kinetic friction along the and directions agrees best with values measured on alkali halides with similar cation and anion radii. This ratio, as well as the angular dependence of the static friction may be used to determine fine details of the lateral tip-sample interaction.
Novel phenomena accompanying atomic-scale friction are studied on NaCl(001) by the combination of... more Novel phenomena accompanying atomic-scale friction are studied on NaCl(001) by the combination of quasistatic lateral force measurements with dynamic measurements of contact resonance frequencies. For loads up to a few nN the flexural resonance is tracked by a phase-locked-loop by the use of small oscillation amplitude (50 pm). The contact resonance varies during the stick stages, which demonstrates that the dynamic measurement provides additional information about small changes of the stressed contact. Improved sensitivity is also observed across atomic-scale defects which are clearly observed in the contact frequency channel. The low lateral contact stiffness inferred from the observed torsional resonance agrees well with that deduced from the quasistatic measurements and strongly suggests that the contact is atomic-sized.
ABSTRACT The growth of ordered molecular monolayers on insulating surfaces is often hampered by t... more ABSTRACT The growth of ordered molecular monolayers on insulating surfaces is often hampered by the strength molecule-molecule compared to surface-molecule interactions, resulting in molecule clusters aggregated at step edges rather than ordered flat films. We have succeeded in employing the enhanced interaction at edges of nanometer-scale structures to confine molecules into flat islands. Monolayer-deep pits with atomically straight edges were produced on a KBr surface by an electron-irradiation induced desorption process. Phtalocyanin and perylene derived molecules were evaporated onto the surface and imaged by means of high resolution force microscopy. Molecules were found to be trapped in the pits and the structure of the islands could be resolved. We will discuss possible mechanisms for the trapping and show how dynamic force microscopy can even provide information about mechanical dissipation in such molecular islands.
The sliding of a sharp nanotip on graphene completely immersed in water is investigated by molecu... more The sliding of a sharp nanotip on graphene completely immersed in water is investigated by molecular dynamics (MD) and atomic force microscopy. MD simulations predict that the atomic-scale stick-slip is almost identical to the one found in ultra-high vacuum. Furthermore, they show that water plays a purely stochastic role on sliding (solid-to-solid) friction. These observations are substantiated by friction measurements on graphene grown on Cu and Ni, where, oppositely to operation in air, lattice resolution is readily achieved. Our results promote friction force microscopy in water as a robust alternative to ultra-high vacuum measurements.
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Papers by Enrico Gnecco