Tip-sample Force Research Articles

Effects of tip‐sample forces and humidity on the imaging of DNA with a scanning force microscope

AbstractWe have investigated the effects of tip‐sample forces and relative humidity when using a scanning force microscope (SFM) to image DNA molecules adsorbed on fresh mica. As the force between the tip and the sample increases, the apparent height of the DNA molecules decreases. After being imaged with high forces, the DNA molecules recover partially in their apparent height, indicating that a plastic deformation of the DNA has been induced by the scanning tip. At low humidities, DNA molecules can be imaged with a force up to 150 nN during the scanning without obvious damages. At higher humidities, however, the DNA molecules can be dissected or swept away by the tip even at a tip‐sample force of 30 nN. The net force between the tip and the molecules is the vector sum of several forces, the dominant components of which are the elastic force due to the cantilever bending and the capillary force resulting from the water meniscus formed between the tip and the sample surface. When the relative humidity of the imaging environment is increased, the capillary force becomes stronger.

Contact imaging in the atomic force microscope using a higher order flexural mode combined with a new sensor

Using an atomic force microscope (AFM) with a silicon cantilever partially covered with a layer of zinc oxide (ZnO), we have imaged in the constant force mode by employing the ZnO as both a sensor and actuator. The cantilever deflection is determined by driving the ZnO at the second mechanical resonance while the tip is in contact with the sample. As the tip-sample force varies, the mechanical boundary condition of the oscillating cantilever is altered, and the ZnO electrical admittance is changed. Constant force is obtained by offsetting the ZnO drive so that the admittance remains constant. We have also used the ZnO as an actuator and sensor for imaging in the intermittent contact mode. In both modes, images produced by using the ZnO as a sensor are compared to images acquired with a piezoresistive sensor.

Atomic resolution imaging of InP(110) surface observed with ultrahigh vacuum atomic force microscope in noncontact mode

True atomic resolution imaging of the cleaved semi-insulating InP(110) surface was demonstrated using an ultrahigh vacuum atomic force microscope (AFM) in noncontact mode. The force gradient acting on the tip was detected by the frequency modulation method. The rectangular lattice could be clearly observed. The image contrast suddenly changed during the scan, which suggests that the noncontact AFM imaging is performed under the condition of nearly monoatomic tip–sample force interaction. Atomic defects have been clearly and reproducibly observed. These results suggest that noncontact AFM has the potential for true atomic-scale lateral resolution and is quite effective for atomic surface structure analysis in real space.

Driven nonlinear atomic force microscopy cantilevers: From noncontact to tapping modes of operation

A numerical model of the operation of an atomic force microscope with a driven cantilever is presented. This model takes into account the attractive van der Waals and repulsive indentation forces acting between tip and sample. The time-dependent displacement amplitude and phase of the tip oscillations, and the magnitude, duration, and sign of the short bursts of tip–sample force are derived. It is shown that the stiffness of the tip and sample materials is an important factor in determining the magnitude and duration of the tip–sample repulsive force and the magnitude of sample indentation. The model covers typical operating ranges of vibrating cantilever atomic force microscopes, from the noncontact to the tapping modes.

Study of plastic flow in ultrasmall Au contacts

Yielding properties of Au point contacts of nanometer-scale dimensions have been studied using a scanning tunneling microscope supplemented by a force sensor for measuring tip–sample forces. The contacts are made by indenting the tip typically 10 nm into the substrate, whereby an adhesion neck is formed. Three consecutive deformation phases of the neck can be identified during retraction of the tip: (1) buildup of tensile stress, (2) incomplete fracture, and (3) quasicontinuous plastic flow. Finally the neck breaks when a maximum of three to four atoms are left in the contact. In the plastic flow regime, the conductance and thus the contact area shrink exponentially with elongation of the neck. This provides strong evidence that plastic deformation occurs locally within five to six atomic layers. The latter is inferred from the decay constant of the exponential decrease of the cross section. The applied stress during plastic flow is initially of the order of 10 GPa and gradually increases to ≂20 GPa shortly before the neck breaks. From a fit to the data accounting for a surface force contribution, an intrinsic yield strength of the order of 6 GPa is obtained, which is more than one order of magnitude larger than the macroscopic yield strength of Au.

Study of yielding mechanics in nanometer-sized Au contacts

Yielding properties of Au point contacts of nanometer-scale dimensions have been studied using a scanning tunneling microscope supplemented by a force sensor for measuring tip–sample forces. The contacts are made by indenting the tip typically 10 nm into the substrate, whereby an adhesion neck is formed. Three consecutive deformation phases of the neck can be identified during retraction of the tip: (1) buildup of tensile stress, (2) incomplete fracture, and (3) quasicontinuous plastic flow. Finally the neck breaks when a maximum of three to four atoms are left in the contact. In the plastic flow regime, the conductance and thus the contact area shrink exponentially with elongation of the neck, suggesting that plastic deformation occurs locally within 5 to 6 atomic layers. The stress applied during plastic flow is initially of the order of 10 GPa and gradually increases to ≂ 20 GPa shortly before the neck breaks. Accounting for a surface force contribution, an intrinsic yield strength of the order of 5 to 8 GPa is obtained, which is more than one order of magnitude larger than the macroscopic yield strength of Au.

Nanometer-Scale Mechanism for the Constructive Modification of Cu Single Crystals and Alkanethiol Passivated Au(111) with an Atomic Force Microscope

In-situ atomic force microscopy (AFM) is used to enhance Cu electrodeposition on Cu and Au single crystal electrodes. On Cu surfaces, the enhanced deposition effect depends primarily on tip-sample force, crystallographic orientation, and solution pH. Enhanced Cu deposition is stronger on the (110) face of Cu than the (111) face which correlates with the reactivity of these lattices toward oxygen adlayer formation. For a specific orientation, enhanced Cu deposition becomes less pronounced with decreasing solution pH. These results are consistent with a modification mechanism in which partially passivating oxygen adlayers mediate both normal and enhanced Cu deposition. The AFM tip-sample force physically creates defects in these adlayers, thus forming active sites for Cu adsorption. The general applicability of this scheme is demonstrated on Au(111) passivated with self-assembled octadecanethiol nonolayers. 37 refs., 10 figs., 3 tabs.

Development of highly conductive cantilevers for atomic force microscopy point contact measurements

Several techniques for improving the electrical conductivity of micromachined silicon cantilevers for atomic force microscopy point contact measurements were investigated. The techniques studied included sputtering or evaporating thin layers of gold, platinum or silver onto the lower surface of the cantilever to create a conducting metal layer, and doping the cantilevers with phosphorus. It was found that the lowest resistance contacts to a gold surface can be made by the metal coated tips, which can make stable point contacts with resistances as low as 30 Ω at a tip sample force of 15 μN.

On the nature of nanometer-scale ring structures in the scanning tunneling microscopy images of tungsten diselenide WSe2

Scanning tunneling microscopy (SIM) measurements of tungsten diselenide at a large resistance gap show bright image imperfections on a nanometer scale. These bright spots are observed to be reversibly converted into ring structures as the resistance gap is decreased. This phenomenon was explained on the basis of the trapped electrons around donor dopant atoms and the tip-sample force interactions.

Atomic-scale metal adhesion investigated by scanning tunneling microscopy

The interaction of a sharply pointed metal tip with a metal surface is investigated both theoretically and experimentally. By resorting to an effective potential approach, the characteristics of tip-sample forces are analyzed systematically in terms of known theory of bulk metal adhesion. Experiments probing the short-range adhesion interaction by means of a combination of force gradient sensing with tunneling microscopy are described. It is found that the concepts based on adhesion at a macroscopic level are not generally applicable in describing the observed tip-sample force gradient characteristics. These characteristics can, however, be explained in a semiquantitative way using effective interactions determined from a cluster calculation. It is also shown that the chemical information obtained by probing short-range interactions can be used to identify adsorbates on metal surfaces.

In situ force calibration of high force constant atomic force microscope cantilevers

A miniature capacitive force sensor which fits in the sample position of an atomic force microscope (AFM) has been used to calibrate the force applied by the scanning tip during nanohardness measurements. The sensor is simple, physically robust, and easily fabricated from readily available materials. It can be operated with commonly available electronic instrumentation. The device described here is optimized for tip-sample forces between 10−4 and 10−2 N, a range useful for micrometer-scale mechanical modification of surfaces. With minor design changes, sensors of this general type should be capable of much greater sensitivity. The sensor is calibrated outside the AFM by the application of free weights. Calculated estimates of AFM cantilever force constants are difficult for all but the simplest geometries and may rely on uncertain values for certain critical dimensions and material properties. These estimates can thereby be replaced by a simple and direct measurement.

Analysis of Subsurface Imaging and Effect of Contact Elasticity in the Ultrasonic Force Microscope

We examined, both theoretically and experimentally, the characteristics of subsurface imaging with nanometer resolution and the effect of contact elasticity in the ultrasonic force microscope (UFM). In particular, the effect of the surface energy and effective elasticity on the maximum tip-sample force and the shift of the averaged tip-sample distance were examined. Furthermore, kink formation in the cantilever deflection (z a) against the ultrasonic frequency vibration (UFV) amplitude (a) characteristics was predicted. This model was used to explain experimental observations in UFM, such as the features of the measured z a(a) curve and the damping of the cantilever torsion vibration by the UFV. Moreover, the previously reported lateral ultrasonic force microscope image of subsurface features was explained by the response of subsurface edge dislocation to a large instantaneous force enhanced by the UFV.

Detailed experimental investigation of the barrier-height lowering and the tip-sample force gradient during STM operation in air.

Detailed experimental investigation of the barrier-height lowering and the tip-sample force gradient during STM operation in air.

In situ atomic force microscope study of lead underpotential deposition on gold (111): structural properties of the catalytically active phase

Atomic resolution atomic force microscope (AFM) images of Pb underpotential deposition on Au(111) electrodes are presented. Pb grows on the Au(111) surface by forming islands which are initially one Pb adatom high (0.30 nm) but which rapidly grow in height to 0.53 nm through association of the Pb with solvent or hydroxide. The Pb aggregates have a hexagonal close-packed atomic structure exhibiting an atom-atom spacing of 0.35±0.03 nm. The islands can be easily swept away by the AFM tip when the tip sample force is set at 80 nN, but Pb deposited at step edges is not swept away under these conditions. This suggests that the Pb (island)-Au interaction is relatively weak

Attaching molecules to surfaces for scanning probe microscopy

Attaching molecules to surfaces for scanning probe microscopy

Tip–sample forces in scanning probe microscopy in air and vacuum

Forces between a probe and sample have been measured in air and vacuum using a rocking beam force balance sensor capable of simultaneously obtaining both force and tunneling topographs. Force versus distance curve measurements demonstrate the effects of capillary forces in air but not in vacuum. Simultaneous scanning tunneling microscope (STM) imaging and force measurement in air show spatial force variations consistent with repulsive force tunneling. Our results support observations of large repulsive forces in STM occurring from surface contaminants. These results show how forces can affect image formation in both STM and scanning force microscopy.

Role of atomic force in tunneling-barrier measurements

Experimental measurements of the apparent barrier height as a function of tip-sample separation using a scanning tunneling microscope (with clean W tips and clean Si surfaces in ultra high-vacuum) show that the barrier height starts at 3.5 eV at large separations, increases to 4.8 eV at about 1.5 Å before the mechanical contact, and then drops to below 0.3 eV within a fraction of an ångström. At the distances encountered in scanning tunneling microscopy, forces between sample and tip can be significant. Using a simple model of this system including tip-sample forces leads to a calculated apparent barrier height which quantitatively reproduces the observed variation in apparent barrier height over the entire range of tip-sample separations.

Direct force measurement in scanning tunneling microscopy

A novel force measurement using a scanning tunneling microscope as a forced oscillator is described. Results obtained from tunneling between a tungsten tip and a graphite substrate show that a maximum tip-sample force about 10−6 N exists during the constant current mode of operation. These results are in agreement with a previous model where large contact areas insulated by contaminants between tip and substrate were suggested as a cause of large tip-sample interaction forces. This method can achieve a force sensitivity of 10−8 N and for conductive substrates provide a simple, versatile alternative to existing methods of atomic force microscopy.

Tunneling microscopy of NbSe2 in air

We have obtained atomic-resolution images of NbSe2 single crystals in air. Constant-height images clearly show the expected atomic structure, and can distinguish the two inequivalent halves of the unit cell. Constant-current images show an anomalously high atomic corrugation, associated with elastic deformation of the sample. Surface contamination probably plays an important role in transmitting the tip-sample forces. A larger-scale apparent buckling of the surface with a period of several times the atomic spacing is sometimes observed.