NC-AFM

Most NC-AFMs use a frequency modulation (FM) teclmique where the cantilever is mounted on a piezo and serves as the resonant element in an oscillator circuit [101. 102]. The frequency of the oscillator output is instantaneously modulated by variations in the force gradient acting between the cantilever tip and the sample. This teclmique typically employs oscillation amplitudes in excess of 20 mn peak to peak. Associated with this teclmique, two different imaging methods are currently in use namely, fixed excitation and fixed amplitude. 

Weisenhorn A L and Hansma P K 1989 Forces in atomic force microscopy in air and water Appl. Phys. Lett. 54 2651 Proceedings from the First internationai Conference on NC-AFM Appl. Surf. Sc/. 140 243... 

Non-contact atomic force microscopy (NC-AFM) Magnetic force microscopy (MFM)... 

All the STM results from our group presented in this chapter employed the variable temperature STM, with tips made by electrochemical etching of tungsten wire. For noncontact AFM (NC-AFM), we employ commercial conducting silicon cantilevers with force constants of approximately 2-14 rn 1 and resonant frequencies of approximately 60-350kHz (Nanosensors and Mikromasch). The NC-AFM images we present here were recorded in collaboration with Professor Onishi at Kobe University and employed a UHV JEOL (JSPM-4500A) microscope. 

Figure 8.7 Simultaneously recorded 80A x 150A STM and NC-AFM images of Ti02(l 1 0). (a) A current map represents the STM image. Because the tip is oscillating during NC-AFM measurements, the time-averaged current is recorded. The current map is color contoured so that OHb appear as broad, red spots and Ob-vacs appear as...

High-resolution STM and NC-AFM images have appeared both at elevated pressures and in liquids, and one would expect a great deal of attention to be focused in this direction over the coming years. An area of particular topical interest is the interface between light harvesting surfaces (e.g., Ti02) and liquid water in connection with photocatalysis. 

Juszczak, L. (2003). Surface of triticale starch granules - NC-AFM observations. Electronic J. Polish Agric. Univ. Food Sci. TechnoL, 6. 

Fig. 20. Comparison among CM-AFM, NC-AFM and DFM-AFM lithographies (a) the topographic image and (b) the surface potential one after CM-AFM lithography (c) the topographic image and (d) the surface potential one after NC-AFM lithography and (e) the topographic image and (f) the surface potential one after DFM-AFM lithography.

Tanner RE, Sasahara A, Liang Y, Altman El, Onishi H (2002) Formic acid adsorption on anatase TiO (001)-(l X 4) thin films studied by NC-AFM and STM. 1 Phys Chem B 106 8211... 

Figure 18.1 Surface structure ofTi02(l 10) characterized by STM and non-contact atomic force microscope (NC-AFM). (a) An empty state STM image (7.3 x 7.3 nm2, Vs = 1.2 V, It = 0.15 nA) of a TiO2(110) surface at RT. (b) A structure model of a clean Ti02(l 10) surface. The fivefold coordinated Ti atoms are visualized, (c) A perspective view of a TiO2(110) surface with oxygen defect.

For all nc-AFM measurements, a Kelvin probe force microscopy (KPFM) feedback controller was additionally activated for simultaneous topographic imaging [19]. In order to compensate for electrically or electronically induced artefacts, an ac voltage was applied between tip and sample and used in combination with lock-in techniques and a feedback controller to compensate for the contact potential difference (CPD) between tip and sample. With this method, nc-AFM is assiued to image the sample topography without any artefacts originating from different local surface potentials [20]. 

Figure 5.10 displays the X-ray pattern of the DCNDBQT thin film prepared by vacuum deposition. For this investigation the same sample was used, which was measured before by nc-AFM under UFTV conditions. 

Fig. 6 Above Schematic view of steps involved in the self-assembly process in the comonomer mixture (DELT and L-tyrosine) and its enzymatic copolymerization. States 1 and 11 represents the species we visualized in this study, where = m (a 1 1 ratio of comonomers). State 11 is an hypothetical structure of the final copolymer formed. Below NC-AFM visualization (vertical and 3-D views) of samples following the 1 1 ratio comonomers (DELT and L-tyrosine) copolymerized over a 24h reaction and equilibrium period, followed by immersion of the gold coated mica a 20 xm x 20 xm area and b 5 p.m X 5 p.m area. This figure was reprinted with permission from [59]...

Proceedings from the First International Conference on NC-AFM Appl. Surf. Sci. 140 243... 

Non-contact mode (NC-AFM) Intermittent contact mode (TM-AFM) Lateral force mode Magnetic force Thermal scanning... 

NC-AFM belongs to a family of AC modes, which refers to the use of an oscillating cantilever. A stiff cantilever is oscillated in the effective regime, meaning that the tip is quite close to the sample, but not touching it (non-contact). 

In NC-AFM, the stiff cantilever oscillates near the surface of the sample at a frequency of 50 000-500 000 cps. The tip has no contact with the sample. The cantilever is held 5-10 nm away from the surface, within the region of the force distance curve where the long-range van der Waals forces are dominant [7]. In this mode of operation, the tip is responding to a force between the tip and the sample and can be several orders of magnitude lower than the force in contact mode. 

The non-contact mode AFM was developed by Martin et al. [14]. It profiles a surface in a different fashion than the contact AFM. NC-AFM is desirable in studying the membrane surface, because synthetic membranes are mostly made of polymers, which make the surface soft [15,16]. Stiff cantilevers are used in NC-AFM studies because the soft cantilevers can be pulled into contact with the sample surface. However, the use of stiffer cantilevers reduces the change in cantilever deflection and vi-... 

Prepare the cantilever Etched silicon Ccintilever substrates are generally used for NC-AFM or TM-AFM, and silicon nitride cantilevers are used for C-AFM. In both cases, the cantilever probe should be inspected under the microscope when being used for first time. Use the sharp-pointed tweezers to remove the cantilever substrate from the container. Grasp the sides of the substrate, away from the lever and probe tip. Be very careful about avoiding any contact with the probe lever, since it will immediately snap off. Silicon is very brittle. 

NC-AFM is better than C-AFM for imaging small pores such as those in ultrafiltration and nanofiltration membranes. The reason for this is that the diameter of the cantilever tip apex is greater than the pore diameter. When the tip is passed over the small pore, the tip cannot penetrate into the pore, and there will not be a great change in cantilever deflection. However, TM-AFM is more successful at measiuing the pore size and nodule size on the membrane surface. 

Molecular-scale imaging in FFM [46, 81], PFM-AFM [82], noncontact atomic force microscopy (nc-AFM) [83-88] and anisotropy in friction and molecular stick-shp motion [89] have been discussed. Other important issues in CFM are chiral discrimination [90-92] and calibration of AFM cantilever spring constants [67,93, 94] and tip radii [39, 95]. 

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