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I. Falcón Casas and W. Kautek
3.3.4 Non-contact Heat Transfer Between a Hot Tip and a
Substrate
A hot tip can transfer heat to the substrate by conduction and radiation. Air molecules
acquire kinetic energy via collisions due to the tip temperature and hit the substrate
below the tip. Heat flux density between a silicon tip at a temperature of 800 K and
a substrate can reach values up to 0.5 MW m
−2 K
−1 [75]. In this study, radiation
contribution was neglected compared to air conduction.
However, radiation heat transfer must be considered. Near-field thermal radiation
can be strongly enhanced when the tip and/or substrate materials support a plasmon
excitation [76]. The thermal near-field radiation between nanostructures has been
studied for different geometries [76, 77]. High values of near-field radiation heat
transfer coefficient up to 1.0 MW m
−2 K
−1 were found [78]. The thermal near-field
radiation between a heated silicon oxide micro-sphere and a vanadium oxide film
substrate was measured in vacuum [79]. The micro-sphere temperature was varied
between 100 and 200
◦ C, and conductances slightly above 10
3 W m
−2 K
−1 were
found at a sphere-substrate distance of about 20 nm.
3.4 Experimental Results
3.4.1 Continuous Wave Lasers
There are numerous examples using continuous wave (cw) lasers as light sources for
aNFOL. The spatial distribution of the near-field tip-enhancement has been determined by observing the surface topography of a polymer after CW laser irradiation
[80]. A nanofilm of a positive photosensitive polymer (PMMA) mixed with a chromophore (Dispersed Red 1, DR1) was deposited on a glass substrate by spin coating.
A cw Nd:VYO 4 diode laser (λ = 532 nm) irradiated a Co-coated silicon tip working
in SFM tapping mode. The tip was positioned a few nanometers above the surface
and 1 mW average power was applied to the tip during 20 s with a laser spot of
100 μm. Fringes with a period of about half of the laser wavelength were found on
the polymer surface probably due to far-field interference. A nanobump of 40 nm
width and few nanometers height was observed at the center of the circular fringes,
indicating the presence of a singularity in the field distribution at the tip apex. Similar
results were found using a cw fibre laser (808 nm) and a Pt tip [81].
Using a different approach, near-field tip-enhancement has been also exploited to
pattern aluminium thin films by photo-thermally induced corrosion in water [82]. A
cw Nd:YVO 4 diode laser with linear p-polarization illuminated a glass slide from the
bottom in total reflection configuration. The surface of the glass was coated with a
20 nm thick aluminium film. The laser beam was focused on the aluminium surface
and surface plasmons were excited leading to an enhanced near-field on the metal
surface. Due to the localized nature of the near-field, only material close to the tip
I. Falcón Casas and W. Kautek
3.3.4 Non-contact Heat Transfer Between a Hot Tip and a
Substrate
A hot tip can transfer heat to the substrate by conduction and radiation. Air molecules
acquire kinetic energy via collisions due to the tip temperature and hit the substrate
below the tip. Heat flux density between a silicon tip at a temperature of 800 K and
a substrate can reach values up to 0.5 MW m
−2 K
−1 [75]. In this study, radiation
contribution was neglected compared to air conduction.
However, radiation heat transfer must be considered. Near-field thermal radiation
can be strongly enhanced when the tip and/or substrate materials support a plasmon
excitation [76]. The thermal near-field radiation between nanostructures has been
studied for different geometries [76, 77]. High values of near-field radiation heat
transfer coefficient up to 1.0 MW m
−2 K
−1 were found [78]. The thermal near-field
radiation between a heated silicon oxide micro-sphere and a vanadium oxide film
substrate was measured in vacuum [79]. The micro-sphere temperature was varied
between 100 and 200
◦ C, and conductances slightly above 10
3 W m
−2 K
−1 were
found at a sphere-substrate distance of about 20 nm.
3.4 Experimental Results
3.4.1 Continuous Wave Lasers
There are numerous examples using continuous wave (cw) lasers as light sources for
aNFOL. The spatial distribution of the near-field tip-enhancement has been determined by observing the surface topography of a polymer after CW laser irradiation
[80]. A nanofilm of a positive photosensitive polymer (PMMA) mixed with a chromophore (Dispersed Red 1, DR1) was deposited on a glass substrate by spin coating.
A cw Nd:VYO 4 diode laser (λ = 532 nm) irradiated a Co-coated silicon tip working
in SFM tapping mode. The tip was positioned a few nanometers above the surface
and 1 mW average power was applied to the tip during 20 s with a laser spot of
100 μm. Fringes with a period of about half of the laser wavelength were found on
the polymer surface probably due to far-field interference. A nanobump of 40 nm
width and few nanometers height was observed at the center of the circular fringes,
indicating the presence of a singularity in the field distribution at the tip apex. Similar
results were found using a cw fibre laser (808 nm) and a Pt tip [81].
Using a different approach, near-field tip-enhancement has been also exploited to
pattern aluminium thin films by photo-thermally induced corrosion in water [82]. A
cw Nd:YVO 4 diode laser with linear p-polarization illuminated a glass slide from the
bottom in total reflection configuration. The surface of the glass was coated with a
20 nm thick aluminium film. The laser beam was focused on the aluminium surface
and surface plasmons were excited leading to an enhanced near-field on the metal
surface. Due to the localized nature of the near-field, only material close to the tip