Yan Jun Li

The stray capacitance effect in Kelvin probe force microscopy using FM, AM and heterodyne AM modes

The effect of stray capacitance on potential measurements was investigated using Kelvin probe force microscopy (KPFM) at room temperature under ultra-high vacuum (UHV). The stray capacitance effect was explored in three modes, including frequency modulation (FM), amplitude modulation (AM) and heterodyne amplitude modulation (heterodyne AM). We showed theoretically that the distance-dependence of the modulated electrostatic force in AM-KPFM is significantly weaker than in FM- and heterodyne AM-KPFMs and that the stray capacitance of the cantilever, which seriously influences the potential measurements in AM-KPFM, was almost completely eliminated in FM- and heterodyne AM-KPFMs. We experimentally confirmed that the contact potential difference (CPD) in AM-KPFM, which compensates the electrostatic force between the tip and the surface, was significantly larger than in FM- and heterodyne AM-KPFMs due to the stray capacitance effect. We also compared the atomic scale corrugations in the local contact potential difference (LCPD) among the three modes on the surface of Si(111)-7 × 7 finding that the LCPD corrugation in AM-KPFM was significantly weaker than in FM- and heterodyne AM-KPFMs under low AC bias voltage conditions. The very weak LCPD corrugation in AM-KPFM was attributed to the artefact induced by topographic feedback.

Elimination of instabilities in phase shift curves in phase-modulation atomic force microscopy in constant-amplitude mode

The authors propose phase-modulation atomic force microscopy (PM-AFM) in constant-amplitude mode using automatic gain control to prevent the instabilities of cantilever dynamics. Under the condition that the driving frequency is set to the resonant frequency of the cantilever, phase shift curve in constant-amplitude mode shows no discontinuity, which resembles a typical behavior of the frequency shift curve in frequency-modulation AFM. They demonstrate that PM-AFM in constant-amplitude mode can clearly resolve phase-separated structures on polymer blend film without instability. These results indicate that PM-AFM in constant-amplitude mode is more stable than that in constant-excitation mode.

High potential sensitivity in heterodyne amplitude-modulation Kelvin probe force microscopy

A surface potential measurement method using amplitude-modulation and heterodyne techniques is proposed. The effect of the stray capacitance between a cantilever and a sample in Kelvin probe force microscopy and the electrostatic force spectroscopy measurements are almost completely removed, because the distance (z) dependence of the modulated electrostatic force increases from 1/z to1/z2. This method improves the sensitivity of short range forces and reduces the surface potential measurement crosstalk that is induced by topographic feedback. This method has the advantage of high potential sensitivity due to the high cantilever Q value under vacuum. Quantitative surface potential measurements are demonstrated.

High-sensitivity force detection by phase-modulation atomic force microscopy

Performance of phase-modulation atomic force microscopy (PM-AFM) is investigated to achieve high force sensitivity and high-resolution imaging. PM-AFM detects the phase change of the oscillating cantilever relative to the excitation signal and uses it as the feedback signal. We compare the force sensitivity of PM-AFM with that of amplitude-modulation AFM (AM-AFM) theoretically as well as experimentally. We show that PM-AFM has a better signal-to-noise ratio than AM-AFM. We demonstrate that the electrostatic force images obtained by PM-AFM have clearer contrast than those obtained by AM-AFM.

Multifrequency high-speed phase-modulation atomic force microscopy in liquids.

We have developed a new technique, called multifrequency high-speed phase-modulation atomic force microscopy (PM-AFM) in constant-amplitude (CA) mode based on the simultaneous excitation of the first two flexural modes of a cantilever. By performing a theoretical investigation, we have found that this technique enables the simultaneous imaging of the surface topography, energy dissipation and elasticity (nonlinear mapping) of materials. We experimentally demonstrated high-speed imaging at a scan speed of 5 frames/s for a polystyrene (PS) and polyisobutylene (PIB) polymer-blend thin-film surface in water.

Simultaneous observation of surface topography and elasticity at atomic scale by multifrequency frequency modulation atomic force microscopya)

The authors integrated the frequency modulation (FM) technique into multifrequency atomic force microscopy (AFM). Based on theoretical considerations, simultaneous excitation of the cantilever oscillation at the first and second flexural modes allows us to acquire the surface topography and surface elasticity simultaneously. The authors performed multifrequency FM-AFM observation using a tungsten-coated silicon cantilever on a Ge(001) surface exhibiting a dimer structure at room temperature. The topography and the elasticity of the surface were successfully obtained at the atomic scale. The authors found that the dimer atoms around a missing dimer defect have higher elasticity than the other dimer atoms. This suggests that stiffer atomic bonding of the dimer atoms occurred as a result of the additional tensile strain field from the defect. Therefore, the multifrequency FM-AFM described in the present study is expected to be useful for the investigation of the surface elasticity at the atomic scale.

High-Speed Phase-Modulation Atomic Force Microscopy in Constant-Amplitude Mode Capable of Simultaneous Measurement of Topography and Energy Dissipation

We have developed high-speed phase-modulation atomic force microscopy (PM-AFM) in a constant-amplitude (CA) mode. Using this imaging mode, we have theoretically demonstrated that energy dissipation due to tip–sample interaction can be obtained from the excitation amplitude of a cantilever. Moreover, we have found that the photothermal excitation method is better than the acoustic excitation method for cantilever oscillation in liquids. For the first time, we have demonstrated that a homebuilt high-speed PM-AFM in the CA mode has the capability to simultaneously measure the topography and energy dissipation with a material-specific contrast for a PS/PIB polymer-blend film.

Development of atomic force microscope with wide-band magnetic excitation for study of soft matter dynamics

In order to probe dynamical properties of mesoscopic soft matter systems such as polymers, structured liquid, etc., a new atomic force microscopy apparatus with a wide-band magnetic cantilever excitation system was developed. Constant-current driving of an electromagnet up to 1 MHz was implemented with a closed-loop driver circuit. Transfer function of a commercial cantilever attached with a magnetic particle was measured in a frequency range of 1-1000 kHz in distilled water. Effects of the laser spot position, distribution of the force exerted on the cantilever, and difference in the detection scheme on the obtained transfer function are discussed in comparison with theoretical predictions by other research groups. A preliminary result of viscoelasticity spectrum measurement of a single dextran chain is shown and is compared with a recent theoretical calculation.

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

A sharp probe tip with atomic scale stability is essential and desirable for noncontact atomic 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 room temperature in order to investigate the influence of the tip apex structure on the NC-AFM images. By using the Si cantilever, we first obtained four types of image at the atomic scale which can be explained assuming a dimer structure on the tip apex. On the other hand, the home-made tungsten coated tip, which has atomic scale stability and high electric conductivity, imaged the so-called ordered c(4 x 2) structure without any artifacts. The tungsten coated cantilever was found to have significantly higher performance for NC-AFM studies at the atomic scale than the Si cantilever.

Photoresponse of Titanium Dioxide Surface on Atomic Scale: Site for Visible Light Absorption

The local photoresponse of a rutile TiO2(110) surface was visualized on the atomic scale by scanning tunneling microscopy (STM). Almost all the areas on the (110) surface responded to the illumination of 325 nm ultraviolet (UV) light, in accordance with the minimum bulk band-gap (3.2 eV) electron transition, with steps and (1×2) ridge areas exhibiting higher response intensities. When the same surface was illuminated with blue light at 442 nm, which is well below the minimum bulk band-gap transition energy, most of the surface areas were dormant, but certain areas at step regions and (1×2) ridge structures responded to the blue light. These sites are associated with oxygen deficiency on the surface, which may give a clue to the possibility of the surface engineering of a titania photocatalyst to be employed under visible light.