Conducting probe-AFM

Since the deviation of device characteristics may come from the local heterogeneity of metal/molecule interfaces, the local characterization of the molecular structure and interfaces is necessary to learn how to alter processes and materials in order to achieve high yield, stable process condition, and low deviations of device characteristics. A few researchers have proposed the use of the scanning probe microscopic technique [conducting probe AFM, surface potential... 

Fig. 9 a Schematic illustration of the measurement with a conducting-probe AFM. b Relationship between resistance and DNA length for poly(G)-poly(C) (dark marks) for poly(A)-poly(T) (empty marks). The exponential fitting plots of the data are also shown, c Typical I-V curves of poly(dG)-poly(dC), the linear ohmic behaviors on I lOO nm at the repeat measurement of five samples, d Rectifying curves of poly(dG)-poly(dC) at L=100 nm (from [68], with permission Copyright 2000 by the American Institute of Physics)... 

This section describes a variant of SPM where condncing probes are used to measure the current-voltage (J-V) relationship and resistance (conductance) of conducting materials. Conducting probe AFM (CP-AFM) is also called conducting AFM, current-sensing AFM [11,126], or scanning resistance microscopy [127,128]. 

Schematic diagram of AFM with conducting-probe AFM option.

Conducting probe atomic force microscopy (CP-AFM), in which a SAM is formed on a Au metal surface, and a Au coated AFM tip is used to contact the top of the SAM (a different procedure than that above) has been used to measure conductance of alkanethiolate monolayers.42 No NDR effects were seen. 

In this example (Section 7.5.18) ATM rather than STM was used. Whereas STM depends on electron transfer between the specimen and the tip of the probe, AFM depends only on mechanical forces and is independent of the conductance of the specimen, and this may be an advantage for alloys of A1 with oxides that show poor conductance. In other experiments AFM images of an Al-Cu alloy immersed in 1M HC1 were recorded. After 24 hr new pits were formed and the ones formed earlier have grown. After 6 hr the sample was severely damaged and the surface is very rough. 

The key factor to all CP-AFM measurements is contact resistance of the conducting probes. Reported conducting probes include heavily doped Si tips and conventional Si or SijN4 tips coated with metal (e.g., Ag, Au, Pt, NbN) or B-doped diamond films. Thomson and Moreland provided a detailed investigation of Ag-, Au-, and Pt-coated tips and doped Si tips and achieved five orders of magnitude lower contact resistance to Au surfaces with metal-coated probes than with doped... 

Figure 4.3.28 demonstrates that the gate-modulated conductance of the crystals can be recorded with CP-AFM methodology. Another key result is shown in Figure 4.3.29. After the conducting probe was placed at points 1-7 labeled in Figure 4.3.29a, the /o-Vo characteristics were recorded at each point. From the linear portion of the /o-Vo traces, the differential resistances dVjy/dlj ) were determined as a function of Vq.

AFM also provides information on the lateral distribution of the surface potential over a sample. The term surface potential in this context means the potential difference between the sample and a conducting probe that is positioned close to the sample. This is so-called Kelvin probe force microscopy (KFM) [46], which offers a possibility of distinguishing between regions with different chemical natures or compositions. For example. 

AFM is similar to STM in many aspects, but a fundamental difference between them is that STM measures the tunneling current, while AFM probes the force. So unhke STM, which requires a conductive surface or small adsorbates on a conductive surface, AFM can provide high-resolution images of both conductive and insulating surfaces. This is important for imaging large biological molecules or... 

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