The Optics Laboratory
Group of Hans Hallen, Physics Department, North Carolina State University
The split tip consists of a sharpened optical fiber with two electrically isolated electrodes that run to opposite edges of the end of it. It can provide an enormously large electric field near the tip, which can be used to orient molecules in oriented-along-the-surface-molecular-deposition (as opposed to SAMs, which are oriented perpendicular to the surface), or it can be used in a type of capacitance microscopy that does not make any assumptions about the sample: it can even be insulating. Typical scanning capacitance microscopy uses the sample as one electrode, so if it has nanoscale features, complex modeling is required to extract useful information. Here, only a nanoscale volume between the ends of the electrodes is sampled, so the sample complexity does not matter.
Since the electrodes are supported by an optical fiber, sharpened as a near-field microscope tip, light can be easily inserted directly into the region sampled by the two electrodes. This enables photoconductivity measurements at the nanoscale and is the source of UV light to remove 'leaving' groups from the aligned molecules and make them insoluble during the oriented molecular deposition process.
An introduction to the split tip and molecular orientation can be found here , the calculations for orientation and field strength finite element modeling in , and fabrication means and a demonstration of in-plane molecular orientation in . The use as a scanning conductivity or scanning capacitance microscope can be found in Bev Clark's Ph.D. dissertation at NC State University (available online) and in forthcoming papers.
Examples of split tips can be seen in Fig. 1. Tapered optical fibers can be made by either heating the fiber locally with a CO2 laser and pulling, or by chemical etching, both of which are described in the NSOM tutorial section of this web site. We have tip holders that clamp ten fibers at once for etching, electrode deposition, which is done in a vacuum system on a rotating stage with stops for accurate 180 degree rotation between depositions, and storage so that the electrode orientation is known. When making a traditional NSOM probe, it is coated all the way around, so stresses in the deposited metal layer are held by the film itself. For a split tip, the sample-film adhesion must support this stress. It cannot support much. Therefore, the stress must be minimized. It can be done by interrupted deposition, allowing the tip to cool regularly so deposition occurs near room temperature, or (as modeled in the paper), a liquid nitrogen reservoir in the vacuum can be connected to the tip holder via the shafts and clamped copper braids, and the cold carried through the holder and tops to the deposition area. At a relatively rapid deposition rate, as is needed for Al evaporations to have clean films, the temperature at the probe tips remains close to room temperature, so the thermally evaporated films have little stress.
The other trick to make split tips is electrically isolating the electrodes. Theoretically, the metal film thickness should approach zero at the sides of the tip. Surface roughness, evaporant source size, imperfect rotation, and diffusion can disturb this. We use gold or aluminum electrodes, and the work for two different reasons. Gold tends to diffuse rapidly on not-atomically-clean surfaces. Its high surface energy makes it want to ball up, which it does except on freshly discharge cleaned surfaces or freshly deposited metal layers. This balling up where thin breaks the electrical contacts between the electrodes. Note the grains in Fig 1. Aluminum evaporates into flatter films, but oxidizes easily. We either oxidize in vacuum or later to break the contacts by oxidizing through the thin parts that might connect them. A split tip might emerge from the evaporator shorted, but be fine a week later.
Figure 1. Examples of split tip probes viewed by SEM (a) up close with the aperture at the end in sight, (b) the vertical shank. The tapers are about 1/2 mm long from a 120 micron diameter fiber. We make them both by a heat-and-pull or chemical-etching means.
The electric field produced locally by the split tip can be calculated with finite element methods. What is striking is its strength. It is larger than can be produced without breakdown (arcing) in the far-field. Why? Because with a single volt across a 10 nm gap, the field is 1e8 V/m, about 100 times stronger than one can produce in the far-field. It can't break down air or surface contamination though, since the 1 eV maximum energy is not enough to knock an electron off another molecule and start an avalanche. Far field methods must use very high voltages, so fluctuations can allow very energetic electrons to be produced. These can start an avalanche that leads to breakdown and arcing. Figure 2 shows the field magnitudes at levels as the simple estimate above predicts.
Figure 2. The electric field of a split-tip probe with one side of the probe was set to ground, and the other to a 1 volt potential.
The large electric field can align molecules, even with the presence of other aligned molecules reducing the local fields. UV applied just the (aligned) molecules under the tip sets just the nanoscale dot of molecules to the sample. The tip can then be moved and the process repeated to produce dots or lines or areas. To check alignment, it is useful to do a large region so that polarization in a conventional microscope can be used. Such a result is shown in Fig. 3, showing alignment. A large area of the surface was covered in several 4-micron-square scans, with exposures from a Photon Systems HeAg laser at 224.3 nm wavelength. Exposures varied from an average of 5.8 to 50 pulses per point, where the points were spaced by 40 nm, approximately the optical resolution of the probe. Each pulse was ~80 microseconds long and of 3 microJoules of energy. The pulse rate of the laser was 20 Hz. The size of the exposed region is consistent with deposition at all exposure times.
Figure 3. The region of molecular deposition is imaged with polarized microscopy. (a) Crossed polarizer configuration, the deposited region is bright since the oriented molecules have rotated the polarization. (b) with the analyzer removed. The deposited region is darker due to absorption. The bars are 20 microns long.
More info is in the papers.
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