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The near-field scanning thermal microscope
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Image of FIG. 1.
FIG. 1.

Heat flow in the NSThM (schematically). The tip holder, kept at ambient temperature , serves as source for the heat flow . This flow passes through the probe, the vacuum gap, and the sample, and finally reaches the sink provided by the cooling stage at temperature . The total temperature difference thus is . The measured thermovoltage is due to the much smaller temperature difference between the tip holder and the very end of the tip.

Image of FIG. 2.
FIG. 2.

Electrical setup of the NSThM. The STM gap voltage is applied to the sample, while the outer gold film of the probe is connected to ground. The thermovoltage is measured between the inner platinum wire and ground. Fluctuations of the gap voltage then do not cause noise on the thermovoltage signal.

Image of FIG. 3.
FIG. 3.

SEM image of a thermocouple after having been processed in the micropipette puller as described in the text. The resulting glass/wire system has been covered with a -thick gold film.

Image of FIG. 4.
FIG. 4.

Determination of the electrical resistance of the gold film and the platinum wire by measuring the change in voltage caused by a change in the tunneling current .

Image of FIG. 5.
FIG. 5.

(a) Microscope image of the foremost of a thermocouple tip. From such pictures, the cross sectional areas of the glass are determined, taking into account the apparent magnification of the wire by the glass. (b) Cumulative thermal resistance of the glas from the tip to a position along the shaft. Observe that the foremost contribute half of the total resistance.

Image of FIG. 6.
FIG. 6.

Relaxation of the inverted thermovoltage signal after suddenly retracting the tip from tunneling distance. The signal decays with a characteristic time of . The high initial level of the signal is caused by directly coupling the tip to the sample surface through a small particle.

Image of FIG. 7.
FIG. 7.

Thermovoltage delivered by a probe when its tip is dipped into a heated droplet of oil, the temperature of which is monitored by a commercial thermocouple. The temperature-dependent Seebeck coefficient is determined from a second-order fit to the data.

Image of FIG. 8.
FIG. 8.

(a) STM image and (b) scan along the line indicated in (a) of a gold surface, as recorded with the thermocouple probe acting as STM tip. The step height determined from (b) is close to the expected value for a Au(111) surface. The white spots visible in (a) result from deposited nanoparticles.

Image of FIG. 9.
FIG. 9.

Distance dependence of the inverted thermovoltage (left ordinate) and the tunneling current (right ordinate). Observe that a thermovoltage signal is detectable at distances an order of magnitude larger than those yielding a sizeable tunneling current.

Image of FIG. 10.
FIG. 10.

(a) STM topography and (b) NSThM thermography of a cooled gold surface, obtained in constant tunneling current mode. The line scan (c) has been made along the white line indicated in (a) and (b). A low thermovoltage signal corresponds to a lower temperature of the tip and thus to an increased heat transfer between probe and sample. Observe that here the thermovoltage image approximately is the inverted STM image, with a spatial resolution on the order of a few nanometers.


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Scitation: The near-field scanning thermal microscope