In this work we compare DC double pass versus AC single pass Electrostatic Force Microscopy (EFM) methods. We found that the later approach is not only faster than the former but also reduces the noise level significantly. Moreover, we also found that the AC single pass method enhances the dielectric contrast between both components of the system when compared to the DC double pass EFM method. In addition, we found no significant change in the lateral resolution for these two methods.

Introduction

In recent years a great effort has been done to improve the imaging techniques using Atomic Force Microscope (AFM) especially, Electrostatic Force Microscope (EFM). Despite the continuing interest and progress in high-resolution imaging using EFM, its practical value is strongly related to compositional imaging [1-4 and references therein]. More recently it has been also used to study the relaxation properties of polymers [5-8 and references therein]. Under ambient conditions EFM is most often used in the double pass mode [6, 7]. In the first pass a conducting tip scan the sample in order to obtain the topography. In the lift mode, a dc or ac signal is applied to the conducting probe, which is positioned 10-20 nm above the sample. During this second scan the long-range electrostatic force is sensed by the probe, while guiding the probe along the topography obtained during the first pass. AC-EFM, or the so called broadband nano-dielectric spectroscopy (nano-BDS) or local dielectric spectroscopy (LDS) has been recently implemented to measure dynamic relaxation in insulator films in both frequency modulation FM mode [6, 8] and amplitude modulation AM mode [7]. In many cases this method of separating the mechanical and electrostatic forces is rather useful; however, measurements of the electrostatic force in non-contact mode limit their sensitivity and, particularly, spatial resolution. Moreover, the double pass method is time consuming due to the two scans needed at each line. Therefore, it may be advantageous to check the capabilities of single pass EFM (SP-EFM) [9] under ambient conditions.

In this work we compare both techniques namely double pass and single pass EFM for the study of the phase structure of poly (styrene) / poly (vinyl acetate) (PS/PVAc) polymer blends.

Blends made of polystyrene (PS) and poly vinyl acetate (PVAc) are among such blends in which the dielectric properties of both components vary significantly. Here we mapped the dielectric contrast of the sample using both EFM methods in addition to the more standard topography and AFM phase images.

Experimental Technique

Sample
A polymer blend made of PS (Mw = 70,950) and PVAc (Mw = 83,000) in the ratio 75/25 was prepared in toluene. This solution was then spin coated on a metallic disk to get a thin film of about 200 nm. Details of sample preparation are given elsewhere [1]. This polymer system is particularly suitable for using EFM technique because of the high dielectric contrasts between both components.

M

The whole EFM work was accomplished by using a Veeco di Multimode V AFM from Bruker, with a NanoScope V Controller. The topography (height) images of the sample were obtained by performing standard tapping mode AFM and the dielectric contrast images were obtained by using EFM with both double and single pass methods. In double pass EFM method, during the first pass, the topographic image was acquired in normal tapping mode. During the second pass the conductive AFM probe was raised above the sample to a fixed distance (approximately 20 nm) and re-scanned the surface. A dc bias voltage of 3V was applied between the probe and AFM sample holder. The change in resonant frequency of the cantilever (Δf₀) was recorded while the tip follows the previously recorded topography at a constant tip-sample separation (z). The dielectric permittivity is related to the measured Δf0 through the capacitance, according to the equation (1) (see fig.3), where k_c, is the spring constant of the cantilever and C the tip-sample capacitance.

In single pass method, both the topo­graphy and dielectric contrast mapping were obtained during the main scan itself. In this method an additional ac signal was applied to the tip. The response of the cantilever was then taken from the photodiode and given to an external lock-in amplifier (LIA). The second harmonic of the cantilever response gives the information about the dielectric properties of the materials under investigation. The LIA analyzes this second harmonic and the imaginary part of this signal is mapped along with the height and the mechanical phase. In this way we simultaneously plot the topographical features and dielectric contrast map of the sample. In this case the dielectric permittivity is related to the measured displacement in the photodiode through the complex capacitance [7] according to the equation (2) (see fig.4), where ω is the frequency of the AC signal applied to the cantilever, k_c the spring constant of the cantilever and χ is the proportionality constant expressed in nm/V.

Results and Discussions

Figure 1 shows the dielectric contrast mapping of the blend acquired by using the double pass method at 60ºC. The PVAc component is well separated from the PS matrix forming an island like structure with an average size of 1 µm [1]. A profile is drawn across the PVAc islands. Since the static dielectric constant of PVAc at this temperature is about three times higher than that of PS, the frequency shift of the cantilever resonance frequency (Δf₀) is significantly higher (negative value) on the islands than on the matrix.

Figure 2 shows a dielectric contrast image obtained by the SP-EFM method, i.e. by mapping the imaginary part of the second harmonic signal. The profile drawn across the PVAc islands shows again that both polymers are well segregated. Even though the separation between the component polymers are clear in both images (fig. 1 and 2), the contrast between components is higher in the image captured by the SP-EFM technique. This is likely to be because in SP-EFM the tip is closer to the sample and therefore the signal to noise ratio is higher. From the profiles drawn across the PVAc islands for both images, it is clear that the first profile (fig. 1), is noisier than the second one (fig. 2). This suggests that the SP-EFM technique not only enhances the contrast but also reduces the noise level compared to double pass EFM. Above all, the time required to get a complete image is halved in single pass EFM, because in double pass EFM each line is scanned twice. Furthermore, in single pass method the contrast can be enhanced by tuning the frequency applied to the cantilever in addition to changing the temperature of the sample.

Finally, we quantified the lateral resolution of the images obtained in both ways and found that they are almost the same. In both methods, lateral resolution is estimated to be between 12-13 nm. It is well known that, the force gradient measurements (double pass DC-EFM) have a better lateral resolution than that of the force measurements (single pass AC-EFM). But in the later case, since the tip is closer to the sample, the lateral resolution is enhanced. So these two effects compensate each other to give almost the same lateral resolution in both cases.

Conclusion

Imaging samples by using single pass EFM opens a new method of compositional imaging. This method offers a less noisy and faster way of imaging with an enhanced contrast between the components. The contrast can be improved by tuning the frequency applied to the cantilever and by changing the temperature of the sample.

Acknowledgements
The authors acknowledge the financial support provided by European Union (INFRA-2010-1-1-30 ref: 262348), the Spanish Ministry of Education (code: MAT2007-63681), M. M. acknowledges the Ph.D. grant from the University of the Basque Country (UPV/EHU), Spain.