Conducting Tip Atomic Force Microscopy

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Overview

Atomic Force Microscopy (AFM)is a well established process for visualizing ultrafine surface characteristics. In normal AFM scanning mode a fine needle is drawn very near a surface and is gently bent by the various atomic forces. The conducting tip gives you the chance to measure electrical conductivity at discrete locations and then correlate these measurement with the surface scan that reveals the shape.

see wikipedia:Atomic_force_microscope

Operation

In C-AFM a metal-coated cantilever is moved back and forth across a sample’s surface. The vertical deflection of the cantilever is measured by monitoring the deflection of a laser beam reflected off the back of the cantilever, giving a topographic map of the surface. By applying a voltage to the tip and measuring the current flow we generate corresponding maps of sample topography and electrical properties. It is also possible to obtain current-voltage curves at a single point with an area of ~20 nm2.

This video is also available on YouTube:

AFM Video, Part 1]

Part 2

Significance

This is of particular interest to the field of photonics research because the structure of thin coatings has a huge effect on the performance of devices.

Application example from Alex Veneman at U of A.:

Indium Tin Oxide (ITO) is the most commonly used anode in Organic Photovoltaics (OPVs) due to its optical transparency and relatively high electrical conductivity. ITO is an imperfect electrode, and electron transfer between the ITO and adjacent organic layers is hampered by heterogeneous coverage of surface contaminants and the fact that the oxide itself is most likely a heterogeneous mixture of phases with varying electrical properties.

ITO for organic Light-Emitting Diodes (OLEDs) and OPVs is often treated by methods such as detergent or solvent cleaning, oxygen plasma or ozone cleaning and/or coating with poly(3,4-ethylenedioxythiophene) heavily doped with poly(styrenesulfonic acid) (PEDOT:PSS). The effect of these modifications at the nanoscopic level is still not fully understood, and although the effect of current-voltage properties of the devices has been studied, a working model describing their physical effects at the relevant length scales are lacking. In this work we use Conducting-Probe Atomic Force Microscopy (C-AFM) to study these surface modifications at the nanometer length scale, and compare these results to current-voltage data for macroscopic OPVs.

Our results indicate that PEDOT:PSS is a ‘band-aid’ fix for the deeper problem of heterogeneity of the ITO surface. PEDOT:PSS electrically wires over ‘dead’ spots on the ITO, making an electrically uniform electrode, but it also introduces another energy barrier to the device that increases the diode quality factor and thus decreases fill factor. We also find that aggressive acid etching of the ITO surface results in increased homogeneity, and much improved repeatability in the manufacture of devices.

These C-AFM images demonstrate the electrical heterogeneity of the ITO surface. This lack of uniformity is due to carbonaceous impurities and hydroxide species contaminating the surface. Additionally it is unclear whether the ITO is composed of a single or multiple phases of varying electrical activity. Modification of the ITO surface can increase the electrical activity of the film by removing contaminant species and possible changing the relative ratio of phases present on the surface.
Each semi-log plot on the left shows current-voltage curves at several different ~20 nm2 areas on the same organic film. The data indicate that the electrical heterogeneity of the ITO affects the current flowing through the above Copper Phthalocyanine layer (top row). The addition of a PEDOT:PSS mediator layer (bottom row) makes the electrode electrically uniform by allowing current to pass over any insulating regions on the ITO surface. The PEDOT:PSS/CuPc interface is also rectifying in such a manner as to collect photocurrent and suppress dark current. It should also be noted that the acid etch produces a surface that is already very uniform and is actually hindered by the addition of the PEDOT:PSS layer (right column).

Links

Lab activities using AFM

AFM animations