6.1 SUMMARY
Main motivation of this thesis work is to better probe the surface properties at the nanometer scale. Both of theoretical and experimental studies have demonstrated several improvements in development and application of AFM.
Chapter 2 and Chapter 3 demonstrate the calibration and optimization of sensitivity at normal direction for both of static and dynamic modes. First, for the static AFM mode as applied in liquid environment, a theoretical model has been developed for the detection sensitivity. The enhancement or degradation of AFM sensitivity in liquid environment as compared with sensitivity in air is determined by the coupling effects of laser spot size, spot location, and refractive indices of mediums which laser beam is transmitted through. In most of liquid applications, such sensitivity is enhanced. However, if low refractive index mediums such as gas are involved, the sensitivity may be degraded depending on the spot location and spot size. Based on the proposed model, the sensitivity as calibrated by force curve in air can be applied to predict the sensitivity in liquid. This is important for experiment designs and sensitivity improvement in advanced applications, including single molecule force, cell – cell interaction, interface & colloid force, confined liquid at nanoscale, et al.
However, the detection sensitivity from force curve in static mode is can not be applied as the sensitivity on dynamic AFM modes including tapping mode and higher – order mode. This is mainly due to that the optical – lever AFM is detecting the slope of the cantilever rather than its end deflection. The slope of the cantilever is further dependent on beam shape of the cantilever in vibration. Chapter 3 provides the detailed discussion on optimization and calibration of sensitivity in dynamic mode, which is completely affected by spot size, spot location, and tip – sample interaction in vibrating process. Higher – order AFM mode is more immune to the effect of tip – sample interaction and
has higher sensitivity and stability. This can partially explain the reason that higher – order AFM mode can provide more stable and accurate image. We established a method to calibrate the sensitivity of higher – order AFM mode.
Different from calibration of the sensitivity at normal direction, the sensitivity at lateral direction is difficulty to be implemented, in that there has not been any standard process to introduce exactly known displacement at lateral direction. The method in Chapter 4 applies a T – shape cantilever, with its tip offset by a distance from its main axis, in lateral contact scanning to benefit the simplification and accuracy of measurement of friction coefficient. We discussed the basic principle, algorithm, and its implementation on a chemical force microscopy.
In Chapter 5, the effects of hydrogen bombardment on surface properties of alkane thin films were studied. We discussed the excellence of applying AFM to characterize the bombardment process and relevant results can explain the principle of hydrogen bombardment. The study mainly focused on tapping mode, force modulation mode and HarmoniX mode for measuring stiffness of thin films before and after bombardment. Successful results demonstrate the induced cross-linking between alkane molecules by bombardment and the Young’s modulus of thin film is increased by 5 times as well as the smooth surface is retained.
6.2 Thesis Contributions
The contributions of this thesis are summarized as below:
z Based on a developed model, the sensitivity as calibrated by force curve in ambient can be applied for the sensitivity calibration in liquid environment, although they are different. To improve the detection sensitivity in biological applications, operation environment, laser spot size and laser spot location should be integrated for consideration.
z The optimization of the sensitivity in dynamic mode has been discussed, which is well beneficial for resolving details of AFM tip – sample interactions. AFM dynamic sensitivity is dependent on the significance of tip – sample interaction as compared to
the cantilever stiffness. An important conclusion has been obtained that, when laser spot is located at normalized position x/Leff =0.6, the sensitivity is same to the sensitivity from force curve and is least affected by the tip – sample interaction. It is critical for calibration of dynamic sensitivity at different resonance modes. The
6 . 0 /Leff =
x is also useful for calibration of AFM spring constant based on the
thermal method, which utilizes the spectrum information at the vicinity of tapping frequency.
z New nanotribology method based on T – shape cantilever has been developed to bypass the difficulty of calibrating lateral spring constant and lateral sensitivity. T – shape cantilever as applied in lateral scanning mode can decouple the effect of normal load from that of lateral load on the lateral signal. Two steps of individual calibrations save conventional efforts relying on complex accessories and procedures. It has been applied as chemical force microscopy to quantitatively distinguish chemical groups at nanoscale.
6.3 FUTURE WORK
AFM has been becoming one of most powerful tools to study phenomena at nanometer scale. The present work on AFM sensitivity can be applied to: (1) optimize and calibrate the sensitivity of AFM in applications, and therefore provide more accurate force information between tip and sample. We will apply these results in explanation of surface properties, with more focus on higher – order AFM mode; (2) optimize the design of nanomechanical cantilever sensors, which are being increasingly developed for measuring minute weights, pH value, hydrogen concentration, and so on. The relevant methodologies are also applicable for designing MEMS beam system as accelerator in aerospace and automotives.
Our lateral friction force microscopy based on T – shape cantilever is simple and accurate. It has been successfully demonstrated on chemical force microscopy to distinguish chemical groups. We are applying this method to study tip – sample interaction through modifying the cantilever tip with specified chemical group. Furthermore, with the
recently increasing interests on nanotribology, this method could be extended in lubricate coating on MEMS/NEMS devices, friction force to move nanotube on substrates, and tribological properties of nanowire / nanobelt / nanorod / nanosheet and even graphene.