Apex Of Atomic Force Microscope Tip Research Articles

Atomistic simulation of the measurement of mechanical properties of gold nanorods by AFM

Mechanical properties of nanoscale objects can be measured with an atomic force microscope (AFM) tip. However, the continuum models typically used to relate the force measured at a certain indentation depth to quantities such as the elastic modulus, may not be valid at such small scales, where the details of atomistic processes need to be taken into account. On the other hand, molecular dynamics (MD) simulations of nanoindentation, which can offer understanding at an atomistic level, are often performed on systems much smaller than the ones studied experimentally. Here, we present large scale MD simulations of the nanoindentation of single crystal and penta-twinned gold nanorod samples on a silicon substrate, with a spherical diamond AFM tip apex. Both the sample and tip sizes and geometries match commercially available products, potentially linking simulation and experiment. Different deformation mechanisms, involving the creation, migration and annihilation of dislocations are observed depending on the nanorod crystallographic structure and orientation. Using the Oliver-Pharr method, the Young’s moduli of the (100) terminated and (110) terminated single crystal nanorods, and the penta-twinned nanorod, have been determined to be 103 ± 2, 140 ± 4 and 108 ± 2 GPa, respectively, which is in good agreement with bending experiments performed on nanowires.

Probed adhesion force of living lung cells with a tip-modified atomic force microscope.

The mechanical properties of the extracellular matrix play an important role in bio-microenvironment activities. Herein, atomic force microscope (AFM) was used to measure the interaction between Au and Ag nanoparticle (NP) clusters on the surface of human fetal lung cells. Using (3-mercapto-propyl) triethoxysilane (MPTMS), NP clusters were grafted onto the apex of AFM tip, and then, the adhesion force between the tip and the cell was analyzed. The measured adhesion force increased from 92 pN for AFM tip to 332 pN for that modified with MPTMS. The increase is most probably contributed by the nonspecific interactions between the apex of the modified AFM tip and the surface of the cells. The adhesion forces between the surface of NPs clusters grafted AFM tip and that of lung cells were dramatically reduced as NPs clusters were replaced by MPTMS. For the former, as the Au NPs cluster was applied, the adhesion force reached to 122 pN, whereas it significantly augmented with the addition of the cluster's size and dimension on the AFM tip. For the case of Ag cluster grafted on AFM tip, its adhesion force with the surface of the cells significantly lowered and reduced to 56 pN. Presumably, the electrostatic or van der Waals force between the two surfaces results in the variation of measurements. It is also very likely that the cell-surface interactions are probably varied by the nature of the contact surfaces, like the force-distance of attraction. The result is significant for understanding the the nature of the interactions between the surface of NPs and the membrane of lung cells.

Selective and directed growth of silicon nanowires by tip-enhanced local electric field

We present a method to trigger highly selective and directed growth of individual silicon nanowires based on an electrically biased atomic force microscope (AFM) tip. The biased tip affects the nanowire growth behavior right from the initial stage. In particular, the locally intensified electric field at the AFM tip apex assists the dissociation of the precursor gas molecules and exerts augmented electrostatic force on the catalyst/NW assembly. Therefore, the electrically biased tip can be a candidate tool for the direct synthesis/integration of semiconductor nanomaterials on temperature-sensitive substrates.

Atomic friction at exposed and buried graphite step edges: Experiments and simulations

The surfaces of layered materials such as graphite exhibit step edges that affect friction. Step edges can be exposed, where the step occurs at the outmost layer, or buried, where the step is underneath another layer of material. Here, we study friction at exposed and buried step edges on graphite using an atomic force microscope (AFM) and complementary molecular dynamics simulations of the AFM tip apex. Exposed and buried steps exhibit distinct friction behavior, and the friction on either step is affected by the direction of sliding, i.e., moving up or down the step, and the bluntness of the tip. These trends are analyzing in terms of the trajectory of the AFM tip as it moves over the step, which is a convolution of the topography of the surface and the tip shape.

Three-Dimensional Atomic Force Microscopy: Interaction Force Vector by Direct Observation of Tip Trajectory

The prospect of a robust three dimensional atomic force microscope (AFM) holds significant promise in nanoscience. Yet, in conventional AFM, the tip-sample interaction force vector is not directly accessible. We scatter a focused laser directly off an AFM tip apex to rapidly and precisely measure the tapping tip trajectory in three dimensional space. This data also yields three dimensional cantilever spring constants, effective masses, and hence, the tip-sample interaction force components via Newton's second law. Significant lateral forces representing 49% and 13% of the normal force (Fz = 152 +/- 17 pN) were observed in common tapping mode conditions as a silicon tip intermittently contacted a glass substrate in aqueous solution; as a consequence, the direction of the force vector tilted considerably more than expected. When addressing the surface of a lipid bilayer, the behavior of the force components differed significantly from that observed on glass. This is attributed to the lateral mobility of the lipid membrane coupled with its elastic properties. Direct access to interaction components Fx, Fy, and Fz provides a more complete view of tip dynamics that underlie force microscope operation and can form the foundation of a three-dimensional AFM in a plurality of conditions.

Three-Dimensional Atomic Force Microscopy: Interaction Force Vector by Direct Observation of Tip Trajectory

The prospect of a robust three-dimensional atomic force microscope (AFM) holds significant promise in nanoscience. Yet, in conventional AFM, the tip-sample interaction force vector is not directly accessible. We scatter a focused laser directly off an AFM tip apex to rapidly and precisely measure the tapping tip trajectory in three-dimensional space. This data also yields three-dimensional cantilever spring constants, effective masses, and hence, the tip-sample interaction force components via Newton's second law. Significant lateral forces representing 49 and 13% of the normal force (Fz = 152 ± 17 pN) were observed in common tapping mode conditions as a silicon tip intermittently contacted a glass substrate in aqueous solution; as a consequence, the direction of the force vector tilted considerably more than expected. When addressing the surface of a lipid bilayer, the behavior of the force components differed significantly from that observed on glass. This is attributed to the lateral mobility of the lipid membrane coupled with its elastic properties. Direct access to interaction components Fx, Fy, and Fz provides a more complete view of tip dynamics that underlie force microscope operation and can form the foundation of a three-dimensional AFM in a plurality of conditions.

Multiregional Hybrid Method and Its Application: Formation of an Atomic Protrusion at an Atomic Force Microscope Tip Apex

We present a multiregional hybrid scheme which incorporates first-principles (FP), tight-binding (TB), and molecular mechanical calculations. The key to this hybrid scheme is to find an explicit description of the FP-TB region coupling. We apply it to the atomic structure of a clean silicon atomic force microscope tip, and find the formation of a distinct atomic protrusion at the tip apex. The present study gives the reason why the atomic protrusion exits at the apex despite the fact that the atomic geometry of the very end of the tip is practically uncontrollable in the tip preparation.

Nanoscale piezoelectric coefficient measurements in ionic conducting ferroelectrics

In this work the piezoresponse mode of the atomic force microscope has been applied for piezoelectric coefficient measurements in nanometer scale in high conductive RbTiOPO4 and KTiOPO4 ferroelectric crystals with specifically tailored domain configurations. A strong dependence of the amplitude and phase contrast between oppositely polarized domains on the frequency of the measuring alternate voltage was observed, and allowed the finding of the optimal conditions for piezoelectric coefficient measurements. A theoretical method, taking into account the inhomogeneity of the electric field under the atomic force microscope tip apex, the screening of the applied electric field, and the elastic clamping of the piezoelectrically excited region by the surrounding matrix has been developed for obtaining d33 in ferroelectrics with high ionic conductivity.

Effect of an Atomic Scale Protrusion on a Tip Surface on Molecular Stick-Slip Motion and Friction Anisotropy in Friction Force Microscopy

A molecular dynamics (MD) method is used to simulate the molecular stick-slip motion and the friction anisotropy observed experimentally between an atomic force microscope (AFM) tip and an ordered monolayer of n-alkane chains which tilt in one of six equivalent stable directions. A slider with a single atomic scale protrusion, connected to an external force control unit via three orthogonal springs, is used to model the AFM tip apex with cantilever springs under feedback regulation of the applied normal force. Although there is almost no interfacial commensurability between the tip atomic lattice and the sample molecular lattice, molecular lattice-resolved images are observed due to molecular scale stick-slip motion when the size of the protrusion is comparable to the molecular lattice constant. The present MD simulation can provide an explanation of why we can see a molecular lattice in contact AFM.

Simulations of friction anisotropy on ordered organic monolayer

A method of molecular dynamics is used to investigate friction anisotropy observed on a hexagonally packed organic monolayer of straight-chain molecules, which tilt in a specific direction. A rigid gold slider with a single atomic protuberance is used as a model of a typical atomic force microscope tip apex. The friction anisotropy is observed at 50 K, which is below the melting point of rotation around a long axis of the molecule. The anisotropic frictional behavior is that sliding in directions normal to the direction of the collective tilt of the molecules results in the maximum friction force. The origin of the anisotropy is attributed to anisotropy in lateral compliance in the monolayer.