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Photo-Induced Heating of Free-Standing Azo-Polymer Thin Films Monitored by Scanning Thermal Microscopy Sergey S. Kharintsev, Elena A. Chernykh, Alexandr I. Fishman, Semion K. Saikin, Alexander M. Alekseev, and Myakzyum Kh. Salakhov J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12658 • Publication Date (Web): 19 Jan 2017 Downloaded from http://pubs.acs.org on January 19, 2017
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The Journal of Physical Chemistry
Photo-Induced Heating of Free-Standing AzoPolymer Thin Films Monitored by Scanning Thermal Microscopy Sergey S. Kharintsev1,2*, Elena A. Chernykh1, Alexandr I. Fishman1 , Semion K. Saikin3,4, Alexander M. Alekseev5,6 and Myalzyum Kh. Salakhov1,2 1
Department of Optics and Nanophotonics, Institute of Physics, Kazan Federal University,
Kremlevskaya, 16, Kazan, 420008, Russia 2
Institute of Perspective Technologies, Tatarstan Academy of Sciences, Baumana, 20, Kazan,
420111, Russia 3
Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street
Cambridge, MA 02138, USA 4
Department of Theoretical Physics, Institute of Physics, Kazan Federal University,
Kremlevskaya, 16, Kazan, 420008, Russia 5
National Laboratory Astana, Nazarbayev University, Kabanbay batyr Ave., 53, Astana,
01000, Kazakhstan 6
STC NMST, Moscow Institute for Electronic Technology, Moscow, 124498, Russia
ABSTRACT
Despite numerous attempts to capture a temperature rise in azobenzene-functionalized polymer thin films exposed to laser irradiation, direct temperature measurements are paid no attention so far. Here, we characterize a photo-induced heating of free-standing thin films using nanoscale resolution scanning thermal microscopy. The polymer films under study are composed
of
epoxy-based
oligomers
with
chemically
attached
nitroazobenzene
chromophores. A temperature change of 1.7 K only is observed when an 800 nm thick film is 1 ACS Paragon Plus Environment
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subject to resonant 532 nm illumination with a modest intensity of 25 mW/cm2. A freestanding and glass substrate supported 20 nm azo-polymer films exhibit an anomalous depression of the glass transition temperature by approximately 80 K and 70K, respectively, that is probed with thermally assisted atomic force microscopy. Our results show that the photo-induced heating can negatively affect an ordered state of the azo-dyes within the polymer ultrathin film (< 100 nm) at room temperature.
INTRODUCTION Azobenzene-functionalized polymers have been the subject of intensive studies during the last decades owing to their nonlinear optical properties.1,2 A key peculiarity of these structures is the ability of azo-chromophores to get oriented in glassy environment under polarized light and dc electric field. The underlying trans-to-cis and back photoisomerization3-9 leads to photo-induced surface deformations,10-15 optical dichroism16 and birefringence.6,17-21 These effects have been widely used in different practical applications such as frequency conversion,22-24 optical switching,16 and storage25-27 to name a few. Multiple studies28-30 reported that photo-induced thermal effects are negligible to affect the formation of surface patterns and the intrinsic optical anisotropy due to polymer mass migration and side-chains molecular movements, respectively. Upon resonant 488 nm illumination with a modest intensity of 50 mW/cm2, temperature of bulk polymer changes only by about 5 K due to photo-induced heating, as demonstrated with numerical simulations27 and experimental studies.30 Physically, the azo-dye molecule, originally being in the trans state, absorbs a light quantum and transforms to an excited state. Further, it relaxes either directly back to the trans state or indirectly via a metastable cis state to the trans state. During the whole process the excess energy is converted to heat. Such a many-cycling mechanism results in photo-induced heating of the whole film. However, this temperature rise is not sufficient to affect the surface relief grating formation process. On the other hand, the 2 ACS Paragon Plus Environment
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heat accumulated within the film can impact the stability of the oriented state of the dipoles within glassy environment and, therefore, the optical anisotropy can be changed in time. A stability of the oriented state of polymers can be characterized by the glass transition temperature ( Tg ) which is strongly dependent on the film thickness h .31-35 Therefore, a study of glass transitions in polymer ultrathin films may have sufficient impact on the development of optical near-field memory. This effect becomes stronger for a free-standing polymer thin film, where both sides are unsupported.35 Interfacial interactions between the film and a substrate lead to a glass transition temperature rise compared to that for the free-standing film. In this paper, we demonstrate the photo-induced heating of a free-standing azo-polymer thin film using nanoscale scanning thermal microscopy (SThM). Glass transitions in the freestanding and glass substrate supported azo-polymer thin films are studied with thermally assisted atomic force microscopy (TA-AFM).
EXPERIMENTAL Figure 1(a) shows schematically an experimental setup for measuring temperature of the azo-polymer thin film exposed to a collimated laser beam. Thermal measurements were performed with the SThM setup (AU040 unit, NT-MDT) using a specialized SThM probe (it is further referred to as a sensor). A tip apex of the sensor is a NiCr/Pd resistor with a curvature radius of < 100 nm. An instrumental error is on the order of 0.1 oC . A SThM cantilever is brought into the film illuminated with resonant laser light. A thermal fingerprint is produced by raster scanning the SThM cantilever over the film surface exposed to the 532 nm excitation with an intensity of 25 mW/cm2. A tick-shaped response comes from a structure of the resistive tip apex in which the current of 0.7 mA flows (Fig. 1(b)). It means that the photo-heated film is probed with the “hot” tip apex. The collimated laser beam of diameter 5 mm illuminates the isotropic (non-oriented) 800 thick film at normal incidence.
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In our experiment we used a well-established nonlinear optical organic material – an epoxy-based oligomer containing hydroxyl groups with covalently attached nitroazobenzene chromophores (it is further referred to as CFAO), which exhibits a strong quadratic susceptibility of 62 pm/V.36,37 A solution was prepared by dissolving 5% wt CFAO compound in 1 ml cyclohexanone. Then, the thin film was produced by means of spin coating 1 µl CFAO solution onto a KBr substrate at a rotation rate of 12 000 rpm for 3 min to reach 800 nm in thickness. The film was then floated off onto a water surface, and picked up onto a glass substrate with a drilled hole of 5 mm in diameter. In order to eliminate remaining solvents and to make the thin film isotropic the latter was annealed at the glass transition temperature of 130 ◦C and 10 mbar for 2 h. A chemical structure of this molecule and absorbance of the non-oriented azo-polymer thin film are given in Fig. 1(c).
Fig. 1 (a) A schematic illustration of our experimental setup for measuring temperature of a free-standing thin film, (b) a temperature map when a scanning SThM cantilever is located over a laser spot, (c) a plot of absorbance vs. wavelength. Inset: a chemical structure of the CFAO polymer.
Glass transition temperature of the azo-polymer thin film is determined with a TA-AFM technique. For this purpose, we used the AFM unit (SF005NTF, NT-MDT) equipped with a 4 ACS Paragon Plus Environment
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10 nm curvature radius AFM silicon cantilever with a resonant frequency of 150 kHz and a force constant of 5 N/m (NSG01, NT-MDT). Thermal drifts of a scanning x-y stage did not exceed 10 nm per hour at all temperatures. A thermo stage (SU045NTF, NT-MDT) with an accuracy of 0.05°C was utilized to preset temperature of the CFAO thin film within a range of 15-150°C.
PHOTO-INDUCED HEATING PROBED WITH SCANNING THERMAL MICROSCOPY Figure 2 demonstrates temperature changes in the free-standing CFAO film under resonant (532 nm) and non-resonant (632.8 nm) linearly polarized laser irradiation with modest intensity of 2.5 ÷ 25 mW/cm2. In order to take into account the photo-induced heating of the sensor we first illuminate it without the film with two excitation wavelengths. As follows from the figure, noticeable changes in the net temperature of 0.3 ± 0.1 K (632.8 nm, dark
red
curve)
Fig. 2. Photo-induced heating of a sensor with and without the CFAO thin film exposed to a laser light with the wavelength of 532 nm and 632.8 nm. A light intensity, expressed in mW/cm2, is indicated in the figure. 5 ACS Paragon Plus Environment
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and 0.4 ± 0.1 K (532 nm, dark green curve) are observed for the intensity of 25.5 mW/cm2. Thus, the heating of the SThM probe must be taken into account in net thermal measurements of the film in question. Alternatively, the temperature of the film can be measured with the SThM probe that is positioned beyond the diffraction-limited laser spot. By solving the heat equation ∂T ∂t − α ∇ 2T = Q ( α is a thermal diffusivity, Q is a distributed heat source), provided that the thermal conductivity of the film is available, the temperature at the center of the focal spot can be readily reconstructed. By landing the SThM probe into the free-standing CFAO film under illumination we observe an abrupt depression of temperature by a magnitude of 1 K when contacted. This is stipulated by a heat transfer from the “hot” sensor to the sample. A minimal intensity needed to increase the temperature by 0.1 K is on the order of 2.5 mW/cm2 and 10.1 mW/cm2 for the 532 nm and 632.8 nm excitation wavelength, respectively. At the intensity of 25.5 mW/cm2 the net change in temperature reaches 0.2 ± 0.1 K (632.8 nm) and 1.7 ± 0.1 K (532 nm) (Fig. 2). On the other hand, the extinction coefficients for two wavelengths, α532 nm = 1.3 µm −1 and
α 632.8 nm = 0.06 µm −1 ,20 differ by approximately 20 times. This mismatch can be explained by photobleaching due to the orientation of the pull-push azo-dyes perpendicular to the polarization of the resonant light. In other words, the photo-isomerization depletes the film with the photo-active azo-dyes. By this cause, there is likely an optimal thickness of the film that provides the maximal heating. For sub-100 nm films the heating is retarded due to the photo-orientation of the azo-dyes and, therefore, decreasing a number of the absorbing chromophores; whereas for thicker films (greater 10 µm in magnitude) the whole laser irradiation is absorbed by the bulk of the film (see Fig. 1(a)) and we register scattered heat passed through the upper layers. It should be noticing that the stronger light localization the thinner film can be used. 6 ACS Paragon Plus Environment
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In a case of supported CFAO films, in our experiments we used glass substrates, changes in temperature of the film were too small to be registered. In such a system, we have observed a thermal response from the substrate rather than the film itself, since the thermal conductivity of glass (0.8 W/mK) is 4 times higher than the azo-dyes (0.17 W/mK).38 In the experiment, a total temperature excess is independent on the excitation wavelength of incident irradiation. Indeed, due to both the interfacial interaction and the higher heat capacity of the substrate (1.56 × 106 J Km 3 ) compared to that for air ( 1300 J Km 3 ) photo-induced heat inside the CFAO film is immediately delivered into the glass substrate. Photo-induced heating of azo-polymers is critically dependent on the orientation of the azo-dyes in glassy environment, if all azo-molecules are perpendicular to the light polarization then the film is not heated. This means that a temperature profile of the film is not straightforwardly governed by the Fourier equation and orientation effects of the absorbing pigments should be necessarily taken into account. Such an ordered (centrosymmetric or non-centrosymmetric) state is very sensitive to the thickness-dependent glass transition temperature Tg ( h ) .32,39-41 In particular, this effect becomes appreciable for sub-100 nm thick polymer films. A significant role here is played by a supporting substrate which is able to control Tg within a wide range.35
DETERMINATION OF GLASS TRANSITION TEMPERATURE WITH THERMALLY ASSISTED ATOMIC FORCE MICROSCOPY Nowadays, there is a lot of the instruments for measuring glass transitions of bulk amorphous and liquid crystalline polymers, which are reviewed in detail in Refs.31,42-45 Unfortunately, most of them suffer from a low sensitivity of a measured quantity while probing a tiny amount of a substance. TA-AFM is a promising technique, which provides indirect access to intrinsic glass transitions through a mechanical behavior of the AFM cantilever with nanometer accuracy.46-48 Despite the fact that this instrument is widely used in 7 ACS Paragon Plus Environment
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various practical applications, herein, we suggest, to our knowledge, a new approach based on the measurement of a phase of the AFM cantilever for monitoring Tg . The use of the semi-contact AFM method to monitor glass transitions in polymeric systems is of great advantage, since a phase of the oscillating AFM cantilever allows us to reliably probe heterogeneous surfaces of a specimen in question. In a linear approximation, the phase shift ϕ of the AFM cantilever is defined by the following relationship:49
sin ϕ =
f nf Anf f f Af
+
QEdis , πkAf Anf
(1)
where Anf and A f are amplitudes of non-free and free oscillations, f nf and f f are frequencies of non-free and free oscillations, Q is a quality factor, k is a force constant, and
Edis is energy loss during one period due to a sample-tip interaction. A phase contrast is a delicate tool able to sense a tiny change in physical and chemical properties of the film surface, for example, adhesion, friction, viscoelasticity, etc. This is mainly due to energy loss Edis of the AFM cantilever that is commonly related to the change in free Gibbs energy of the film surface: ∆G = ∆H − T ∆S , where ∆H and ∆S are changes in enthalpy and entropy, respectively. At the glass transition the film performs the work p∆V ( p is a pressure) caused by the film expansion/compression when heated or cooled.
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Fig. 3. The phase of the AFM cantilever vs. temperature for 15 nm and 150 nm thick CFAO films while heating and cooling (a), a plot of glass transition temperature vs. thickness for both the free-standing and glass substrate supported CFAO film.
These changes are caused by the increased mobility of the main chain backbones. Since
Edis ~ ∆G , the phase, defined by Eq. (1), undergoes a reproducible jump when it passes the glass transition region in both directions – heating (red curve) and cooling (blue curve), as pictured in Fig. 3(a). With the method, we define Tg as a temperature at which a reproducible bump occurs on the temperature-dependent phase curves. However, with decreasing the film thickness a number of such matches increases. Fig. 3(a) indicates two characteristic temperatures of 39 oC and 46 oC for the 15 nm film. It is difficult to say whether they correspond to intrinsic glass transitions or measurement errors. To avoid misunderstanding we should expand the confidence intervals. A monotonic growth or falling of the phase is stipulated by changes in the temperature-dependent force constant k (T ) and, therefore, the frequency f nf . Upon heating/cooling the sample within a wide range of temperatures, the AFM cantilever undergoes a thermal expansion/compression and, thus, the measurements become ambiguous because of the unstable feedback. In other words, the probe is out of contact with the sample when its free amplitude becomes smaller than a set-point amplitude due to changes in the resonant frequency during heating/cooling. To avoid this, we heat the polymer thin film within the limits of 20 oC at a linear thermal rate of 12 oC/min. This stepwise regime permits the probe to be in a stable contact with the sample during thermal measurements. One of the possible reasons of a higher noise level in the experimental data obtained for thinner films is worse mechanical stability of samples, since free-standing films act as membranes. The glass transition temperature is unambiguously visualized for the thick (>100 nm) CFAO film and it is in a good agreement with the results reported in Ref.36 Fig. 3(b) shows a 9 ACS Paragon Plus Environment
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plot of the glass transition temperature vs. the thickness of the free-standing and glass supported CFAO thin film. As compared to the earlier published results,35,45,50,51 we observe an anomalous depression of Tg down to 62 oC for the supported 20 nm thick film, whereas the free-standing film with the same thickness shows the glass transition temperature of 50 o
C. A little difference between them is likely related to the presence of the azo-dyes having a
large dipole moment of 12 Debay20 that are predominately in-plane oriented. This leads, in turn, to the increase of the potential energy of the oriented or partly oriented film that is produced with spin-coating while preparing. With increasing the thickness the depolarization effect (average dipole moment tends to zero)51 takes place and the influence of the substrate is enhanced. On the other hand, the azo-dyes themselves can play a role of intrinsic molecular cage. In our opinion, this is the main reason why Tg (h) changes insignificantly between the free-standing and supported film. The experimental data for the supported film is −γ
satisfactorily fitted with a function of Tg (h) = Tg (bulk )[1 − (h h0 ) ] (where Tg (bulk ) = 130 oC , h0 = 12 nm and γ = 1.2 ; a blue dashed curve in Fig.3(b)) with a correlation coefficient of 0.99, as expected in correspondence with Refs.32,33 However, this fitting function yields the lower correlation of 0.88 for the free standing film ( Tg (bulk ) = 130 oC , h0 = 3.8 nm and
γ = 0.5 ; a dark red dot-dashed curve in Fig.3(b)). The best fitting is achieved with an exponential function of
Tg (h) = Tg (bulk )[1 − a exp(− h h0 )]
(where
Tg (bulk ) = 130 oC ,
a = 0.79 and h0 = 69 nm ; a red dashed curve) that provides the correlation coefficient of 0.97. This behavior is explained by a fast disordering of dipole moments in glassy environment with increasing the film thickness. A considerable discrepancy between the curves is observed for the film thickness of
