224
S. Napolitano
To ensure mechanical stability and to allow an easier manipulation of the whole
sample, the first conductive layer is thermally evaporated onto a thick boronsilicate glass plate, that was previously cleaned in baths of commonly used solvents
(e.g., acetone, isopropanol). The material usually chosen for the conductive layer is
aluminum, which quickly oxides in contact with air (oxide layer thickness ≈3 nm).
Aluminum is preferred to other metals with better electric properties, because it is
cheaper, it can be directly evaporated onto glass and its roughness can be easily
tuned [31], while Au having a lower wettability requires the previous deposition of
Cr. Furthermore, Al forms sharper interfaces with polymers, while other metals, as
Au and Ni, do diffuse inside the organic layer [32]. Regardless of the metal chosen for
evaporation, high purity is required. The presence of contaminants, e.g., other metals,
might increase the resistivity of the conductive layer, which results in a known electronic artifact affecting the high-frequency response of confined systems. Because
of the reduced distance between electrodes, these samples are characterized by a
high capacitance, which for thin films easily reach the nF range; combining the large
capacity, C, values with a nonzero resistivity of the (non-pure) metallic layers gives
rise to an RC circuit, with characteristic time up to few ms, which limits the investigation of dielectric properties at high frequencies. In the case of thin polymer films,
as C decreases with the inverse of film thickness, a bad control of the metallic can
evaporation can yield to a smaller and smaller frequency range where the measured
dielectric function is not affected by this electronic artifact.
The dielectric medium, in this case, the thin polymer film, is deposited via spincoating, a technique widely used to cover small to large flat surfaces. Solutions of
the polymer in a good solvent are poured on a plate spinning at several thousand
rotations per minute; within a short time (<1 min), the abrupt expulsion of solvent
upon spinning yields vitrification of the polymer layer. Annealing above T g allows
evaporation of the solvent an eases reduction in mechanical stresses arising from film
formation [33]. To allow application of an electric-field (E-field), a second metallic
layer is deposited on top of the dielectric medium.
Depending on the polymer and solvent pair, the thin polymer film could contain a
relevant crystalline volume fraction. To obtain noncrystalline samples, amorphization
is possible by quenching the nanocapacitors from a temperature above the melting
transition down to sufficiently low temperatures, ideally below the glass transition
temperature, T g . It should be noted that, despite the possibility of degradation upon
processing of thin layers above the melting temperature, T m , amorphizing thin films
is easier than obtaining noncrystalline bulk melts, because of the most efficient heat
transfer (reduced volume) and the drop in T m upon confinement.
Dielectric measurements on thin films are performed as in the case of bulk
samples, by applying a relatively small E-field (<10
6 V/m) to the (nano) capacitor
and measuring the current flow as a function of the frequency of the field. Differently than in the case of micron-thick samples, the E-field is applied without a direct
mechanical contact on the nanocondensator, because application of pressure would
induce a shortcut. Peaks in the frequency domain are associated with molecular
processes taking place at a timescale corresponding to the inverse of the frequency
of the maximum [34].
S. Napolitano
To ensure mechanical stability and to allow an easier manipulation of the whole
sample, the first conductive layer is thermally evaporated onto a thick boronsilicate glass plate, that was previously cleaned in baths of commonly used solvents
(e.g., acetone, isopropanol). The material usually chosen for the conductive layer is
aluminum, which quickly oxides in contact with air (oxide layer thickness ≈3 nm).
Aluminum is preferred to other metals with better electric properties, because it is
cheaper, it can be directly evaporated onto glass and its roughness can be easily
tuned [31], while Au having a lower wettability requires the previous deposition of
Cr. Furthermore, Al forms sharper interfaces with polymers, while other metals, as
Au and Ni, do diffuse inside the organic layer [32]. Regardless of the metal chosen for
evaporation, high purity is required. The presence of contaminants, e.g., other metals,
might increase the resistivity of the conductive layer, which results in a known electronic artifact affecting the high-frequency response of confined systems. Because
of the reduced distance between electrodes, these samples are characterized by a
high capacitance, which for thin films easily reach the nF range; combining the large
capacity, C, values with a nonzero resistivity of the (non-pure) metallic layers gives
rise to an RC circuit, with characteristic time up to few ms, which limits the investigation of dielectric properties at high frequencies. In the case of thin polymer films,
as C decreases with the inverse of film thickness, a bad control of the metallic can
evaporation can yield to a smaller and smaller frequency range where the measured
dielectric function is not affected by this electronic artifact.
The dielectric medium, in this case, the thin polymer film, is deposited via spincoating, a technique widely used to cover small to large flat surfaces. Solutions of
the polymer in a good solvent are poured on a plate spinning at several thousand
rotations per minute; within a short time (<1 min), the abrupt expulsion of solvent
upon spinning yields vitrification of the polymer layer. Annealing above T g allows
evaporation of the solvent an eases reduction in mechanical stresses arising from film
formation [33]. To allow application of an electric-field (E-field), a second metallic
layer is deposited on top of the dielectric medium.
Depending on the polymer and solvent pair, the thin polymer film could contain a
relevant crystalline volume fraction. To obtain noncrystalline samples, amorphization
is possible by quenching the nanocapacitors from a temperature above the melting
transition down to sufficiently low temperatures, ideally below the glass transition
temperature, T g . It should be noted that, despite the possibility of degradation upon
processing of thin layers above the melting temperature, T m , amorphizing thin films
is easier than obtaining noncrystalline bulk melts, because of the most efficient heat
transfer (reduced volume) and the drop in T m upon confinement.
Dielectric measurements on thin films are performed as in the case of bulk
samples, by applying a relatively small E-field (<10
6 V/m) to the (nano) capacitor
and measuring the current flow as a function of the frequency of the field. Differently than in the case of micron-thick samples, the E-field is applied without a direct
mechanical contact on the nanocondensator, because application of pressure would
induce a shortcut. Peaks in the frequency domain are associated with molecular
processes taking place at a timescale corresponding to the inverse of the frequency
of the maximum [34].