Thermal stability characterization for practical use of quantum-dot based global optical sensor on anodized-aluminum

Sensors and Actuators B 185 (2013) 174–178

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Thermal stability characterization for practical use of quantum-dot based global optical sensor on anodized-aluminum Hirotaka Sakaue ∗ , Akihisa Aikawa Institute of Aeronautical Technology, Japan Aerospace Exploration Agency, 7-44-1 Jindaijihigashi, Chofu, Tokyo 182-8522, Japan

a r t i c l e

i n f o

Article history: Received 14 December 2012 Received in revised form 2 April 2013 Accepted 22 April 2013 Available online 30 April 2013 Keywords: Quantum dot Thermal stability Optical sensor Anodized aluminum Temperature-sensitive paint

a b s t r a c t A thermal stability of quantum-dot on an anodized aluminum support is presented for a practical use of this type of global optical sensor. The thermal stability is characterized by the luminescence output of the sensor. To apply this type of sensor for a global temperature measurement, the thermal stability is characterized in a wide temperature range from 100 to 450 K and in the time range from 0 to 1000 s. It is shown that the thermal stability is hold below 298 K. Above 315 K, a sudden decrease and recovery of the luminescent output is measured. It is found that the amount of decrease is proportional to the temperature. The maximum decrease in the intensity is 89% at 475 K after 1000 s. At 315 K, the intensity is recovered to the initial amount after 1000 s. A model describing the luminescence change is introduced based on the thermal degradation and recovery. From the time derivative of the model, the thermalstabilization time can be determined. The stabilization time exists above 315 K, which is related to the tolerance of the luminescence change due to the measurement purposes. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Semiconductor nanocrystals (quantum dots, QDs) have been paid great attention in recent years due to their size tunable and opto-electro properties [1–5]. QDs change their luminescent peaks due to their crystal size. They exhibit a strong and stable luminescence with a high quantum yield [6,7]. QDs have been applied in many fields for light-emitting diodes, pH, temperature detections, and QD lasers [2,3,8–13]. Walker et al. applied one of the QDs as a global temperature probe [9]. They used a polymer support of poly(lauryl methacrylate) to create a global temperature sensor. They characterized the temperature-dependent emission from 100 to 315 K. This type of sensor, called temperature-sensitive paint (TSP), has been widely used in aerospace measurements [14]. Conventional TSP uses a phosphorescent molecule as a temperature probe. This type of molecule has relatively a wide FWHM (full width at half maximum), which is roughly 100 nm. By applying a QD as a temperature probe, FWHM is narrower than that of the phosphorescent probes, which is roughly 40 nm [5,6,15]. The temperature detection range of a TSP is limited by the thermal stability of the temperature probe and supporting matrix. A conventional polymer as a TSP-supporting matrix shows a change in physical/chemical property around 375 K [14]. To extend the

∗ Corresponding author. Tel.: +81 50 3362 5299; fax: +81 422 40 3498. E-mail addresses: [email protected], [email protected] (H. Sakaue), [email protected] (A. Aikawa). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.04.101

detection range, Sakaue et al. used an anodized aluminum as a supporting matrix [10]. It provides a nano-porous structure created by an electro-chemical process of anodization. The melting point of anodized aluminum is at 930 K, which indicates us that the detection limit can be extended to this temperature. The temperature probe can be applied onto the porous surface by the dipping deposition method [16]. Sakaue et al. reported that anodized-aluminum TSP (AA-TSP) using a QD as a probe showed the temperaturedependent emission from 100 to 500 K [10]. As a practical use, Morita et al. captured the temperature distribution of an AA-TSP coated model to simulate an aerodynamic heating in hypersonic flow [17]. AA-TSP could measure the local temperature on the model surface, which was ranged from 350 to 400 K. The thermal degradation would occur on the surface that may result in an uncertainty of the temperature measurement. However, the stability effect was not discussed in their work. A thermal stability of a QD was discussed by Qu and Peng; a dissociation of the shell and a defect of the core cause the thermal instability [18]. Guo et al. reported that a shell structure of a QD influenced to the thermal stability [19]. A supporting matrix would greatly influence to the thermal stability of a QD based TSP, because the QD is mixed in the matrix or applied on the surface of the matrix. In this paper, we characterized the thermal stability of the QD based AA-TSP. For a practical use of QD based AA-TSP as a global temperature sensor, the thermal stability was characterized in a wide temperature range from 100 to 475 K as well as a long measurement duration from 0 to 1000 s. The change in the luminescent intensity was related to the rate of degradation with various

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Fig. 1. Schematic description of dipping deposition method.

temperatures. A model describing the behavior of the thermal stability is also discussed. 2. Experimental

spectrum from 560 to 640 nm. We defined the reference intensity, Iref , as the initial I at the equilibrium. The temperature was varied from 100 to 475 K. For each temperature, two AA-TSP samples were used to minimize the uncertainty caused by a small change in the preparation procedures.

2.1. Materials 3. Results and discussions

2.2. Experimental setup Fig. 2 schematically describes our characterization setup, consisting of the temperature-controlled chamber, illumination source, and a spectrometer. The temperature can be controlled from 100 to 500 K. The chamber was filled with dry air at 100 kPa. An AA-TSP sample was placed on the test section in the chamber. The illumination was given by a 407 nm laser (NEO ARK, DPS-5001). A spectrometer (Oceanoptics, USB4000) was connected to an optical fiber with a 500 nm high-pass filter in front of the fiber end. It acquired the luminescent spectrum of AA-TSPs. The temperature should be equilibrium between the test section in the chamber and an AA-TSP sample to discuss the thermal stability. Thermocouples were placed on the test section and the AA-TSP surface to detect the equilibrium. We acquired an AA-TSP spectrum under the equilibrium. The spectrum was acquired every 1 s, and the illumination was given to the AA-TSP sample during the spectral acquisition for 0.5 s. The illumination was not continuously given to the AA-TSP sample to minimize the photo-degradation. The luminescent intensity, I, was determined by an integration of the AA-TSP

thermocouple/ thermometer

spectrometer

407 nm laser 500nm High-Pass filter

3.1. Luminescence measurement from 100 to 475 K Fig. 3 shows temperature spectra of AA-TSP, ranging from 100 to 475 K. Each spectrum was normalized by the luminescent peak at 100 K of 580 nm. As temperatures increased, the luminescent spectra decreased. A red-shift of the luminescent peak occurred with increasing temperature [10]. A maximum change of at least 20 nm was observed. Fig. 4 shows the luminescent intensity, I, related to the measurement time under the temperatures from 100 to 475 K. A standard deviation was shown as an error bar. The intensity was normalized by Iref at each temperature. The data points were shown every 4 s within the time range from the initial time to 40 s. After this time range, the data points were shown every 50 s. For the temperature from 100 to 298 K, we can see that I was thermally stable throughout the acquisition time from 0 to 1000 s (Table 1). Here, the decrease was shown as ∼0% in Table 1. We can see that there are two stages involved for the results above 315 K as shown in Fig. 4. The first stage was a sudden decrease of I. That can be seen from the initial acquisition time up to the time around 40 s. The second stage was a recovery or relatively small decrease of I. That can be seen after the first stage up to 1000 s, where the change of I was fairly stabilized except for I at 475 K. For the temperature above 315 K, the amount of decrease was proportional to the temperature at the first stage (Table 1). For the temperature at 315 K, 350 K, and 375 K, we can see a recovery of I during the second stage. At 1000 s, I at 315 K

1

normalized spectrum

A temperature probe of QD was applied on an anodizedaluminum surface by the dipping deposition method [16]. We used a QD of birch yellow from Evident Technologies (ED-C11-TOL0580). We call this QD as QDBY in our paper. QDBY was delivered in toluene as a product. The dipping deposition method requires a temperature probe of QDBY , a solvent, and an anodized-aluminum coating. The application procedure is schematically shown in Fig. 1. QDBY was dissolved in chloroform [10]. The concentration of 15 ␮M was adjusted to create these solutions or mixtures.

temperature controller 0 540

heating/cooling unit AA-TSP

insulation space

Fig. 2. Schematic of AA-TSP characterization setup.

560

580 600 620 wavelength (nm)

640

660

Fig. 3. The temperature spectra of AA-TSP at temperature ranges from 100 to 475 K. Each spectrum was normalized by the luminescent peak at 100 K.

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the first stage the second stage

100 K – 298 K 315 K 350 K

100

I/Iref (%)

80 60

375 K

40

425 K

20 475 K 0 0

200

400

600 Time (s)

800

1000

Fig. 4. The luminescent intensity related to the acquisition time at temperatures from 100 to 475 K.

recovered to the original amount (Table 1). However, I at 350 K and I at 375 K were reduced from the original amounts at 1000 s. At 425 K, I seem to be thermally stable after the first stage. This can be found from Table 1 that the change in I at the end of the first and the second stages were only 1%. At 475 K, I kept decreased. Even though a recovery was seen, the final I was decreased from the original amount above 350 K. For the second stage, the amount of decrease was proportional to the temperature above 350 K (Table 1). The maximum decrease of I was seen at 475 K after 1000 s, which was reduced by 89% from its original amount.

3.2. Modeling: thermal stability A model describing the luminescence change was introduced in Eq. (1). We made a physical interpretation that the ZnS-capped shell of QD is unstable with increase of temperature [18]. If the shell is completely destroyed, QD is no longer luminescing, which results in the decrease of I. Some of unstable shells become stable that result in a recovery of I. Both factors would occur at the same time during the present experiment. To describe this interpretation, we introduced two factors in the first and the second stages: thermal degradation and recovery. The decrease of I would be caused by the thermal degradation factor, while the increase of I would be caused by the thermal recovery factor. The former would be dominant at the first stage, and the latter would be apparent at the second stage. The first term in the right-hand side of Eq. (1) describes the thermal degradation factor, while the second term describes the recovery factor. The thermal degradation is described as an

Table 1 The decrease of the luminescent intensity from the intensity at the initial time. A mean with a standard deviation was shown as an uncertainty. The decrease was shown as ∼0% if the amount was less than 1%. Temperature (K)

End of the first stage: 40 (s)

End of the second stage: 1000 (s)

100 150 200 250 273 298 315 350 375 425 475

∼0% ∼0% ∼0% ∼0% ∼0% ∼0% 11 ± 1% 28 ± 2% 51 ± 5% 65 ± 1% 76 ± 1%

∼0% ∼0% ∼0% ∼0% ∼0% ∼0% ∼0% 6 ± 4% 39 ± 1% 66 ± 2% 89 ± 1%

Table 2 The characterization times of the thermal degradation and thermal recovery,  td and  tr as well as the coefficients, Ctd , Ctr , and C0 . The time scales are shown as >10,000 s if  td and  tr are greater than 10,000 s. Temperature (K)

Ctd

 td (s)

Ctr

100 150 200 250 273 298 315 350 375 425 475

0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.3 0.6 0.6 0.7

>10,000 >10,000 >10,000 >10,000 >10,000 >10,000 10 10 9 6 6

0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.3 0.2 1.0 −15

 tr (s) >10,000 >10,000 >10,000 >10,000 >10,000 >10,000 256 357 110 >10,000 >10,000

C0 1.0 1.0 1.0 1.0 1.0 1.0 0.9 0.7 0.4 0.4 0.3

exponential decay, while the thermal recovery factor is described as an exponential rise. Both factors are a function of time, t. I = Ctd e−t/td + Ctr (1 − e−t/td ) + C0 Iref

(1)

The characterization times for the thermal degradation and recovery are shown as  td and  tr . Here, Ctd and Ctr are weight coefficients with a constant C0 . These can be determined by fitting the model to the present results. At t = 0, I is unity. Then, coefficients Ctd and C0 satisfy Eq. (2). Ctd + C0 = 1

(2)

Table 2 shows  td and  tr as well as the coefficients at the measured temperatures. The model in Eq. (1) is fitted to the present results shown in Fig. 4 at each temperature. We can see that the model fits fairly well to the experimental results. That can be seen from the coefficients, which satisfy Eq. (2). If the characterization time is small, it denotes that the process occurs rapidly, and vice versa. When the characterization time was greater than 10,000 s, it was shown as >10,000 s. In this case, for the time range of 1000 s, it can be considered that the process hardly occurred. Below 298 K, both of the characterization times were all greater than 10,000 s. The results tell us that the luminescent intensity was not decreased or recovered; thermal stability was hold below 298 K. At the temperatures 315 K, 350 K, and 375 K, the thermal degradation occurred faster than the thermal recovery. There was roughly one order of magnitude difference in  td and  tr . Even though  tr was over 10,000 s at 425 K, Ctr was unity. This suggests that the thermal recovery factor was an important factor. Overall, I showed a pseudo-stable intensity. At 475 K, the thermal recovery hardly occurred for the time range of 1000 s. A negative value of Ctr tells us that the process was not a recovery but degradation. Overall, I kept decreasing with time. The thermal degradation coefficient, Ctd , increased with temperature. This tells us that the thermal degradation factor is proportional to the temperature. We can discuss the thermal stability of AA-TSP from the model introduced. By taking a time derivative of the model, we can define that AA-TSP is thermally stable if the time derivative is within a tolerant amount (Eq. (3)). d(I/Iref ) dt

=−

Ctd −t/ Ctr −t/tr td + e e td tr

(3)

The tolerance should be determined depending on the temperature measurement. For example, if measurement duration is on the order of hundred seconds and the luminescent intensity should be kept within ±1%, the tolerance is on the order of ±0.01%/s. If the measurement duration is on the order of seconds and the luminescent intensity should be kept within ±1%, the tolerance is on the order of ±1%/s. Fig. 5(a) and (b) shows the time derivative at the first and second stages. We can see that the derivatives were

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of 100 to 475 K and the time range of 0–1000 s. The experimental results showed that the thermal stability was hold below 298 K; the change in the luminescence was minimal. It was found that above 315 K, a sudden decrease of the luminescent intensity was seen in the time range from the initial time to 40 s. After this time range, a recovery or a small decrease of the intensity was seen. The amount of decrease in the intensity was proportional to the temperature. The maximum decrease of 89% in the intensity was measured at 475 K after 1000 s. At 315 K, the intensity was recovered to the initial amount after 1000 s. The amount of decrease was proportional to the temperature. A model was introduced to describe the luminescence change based on the thermal degradation and recovery. A time derivative of the model was used to determine the thermal stability of the quantum-dot on anodized aluminum. Depending on the tolerance, which can be set by the measurement purposes, the stabilization time was determined. Below 298 K, the luminescence was stable throughout. Above 315 K, the stabilization time existed related to the tolerance; larger the tolerance gave the shorter the stabilization time. References

Fig. 5. The time derivatives of the luminescent intensity at temperatures from 100 to 475 K. (a) The first stage and (b) overall time span. Table 3 The thermal-stabilization time at defined tolerances (%/s). Temperature (K)

100–298 315 350 375 425 475

Thermal stability (s) Tolerance: ±0.01 (%/s)

Tolerance: ±0.05 (%/s)

Tolerance: ±0.5 (%/s)

0 435 695 295 41 NA

0 20 116 119 32 36

0 9 17 21 19 20

negative and approached to zero at the first stage (Fig. 5(a)). At the end of the first stage, AA-TSP was thermally pseudo-stable. The derivatives were positive in the second stage (Fig. 5(b)). The amount of the time derivatives in the second stage was smaller than that in the first stage. We can determine the thermal stability from the present results based on the tolerance. Say that a tolerance is ±0.01%/s, the thermal stability was provided in the second stage except for the temperatures at 475 K (Table 3). At 475 K, the thermal stability could not be hold within this tolerance. If we increase the tolerance, say ±0.05%/s, the thermal stability can be provided in the intermediate of the first and second stages (Table 3). If the tolerance can be allowed for a larger amount, say ±0.5%/s, the thermal stability can be provided in the first stage (Table 3). 4. Conclusions A thermal stability of quantum-dot on an anodized aluminum support was studied for a practical use in the temperature range

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Biographies Hirotaka Sakaue received his M.S. and Ph.D. degrees at School of Aeronautics and Astronautics, Purdue University, in 1999 and 2003, respectively. At present, he works at Japan Aerospace Exploration Agency (JAXA) as a researcher. His research interests are the interdisciplinary studies on fluid dynamics and chemistry.

Akihisa Aikawa is a degree-seeking student at JAXA. He received his B.A. degree in Law and B.S. degree in Mechanical Engineering at Sophia University in 2008 and 2011, respectively. He received his M.S. degree in Maritime Engineering at Graduate School of Engineering, Kyushu University in 2013. He is currently pursuing a Ph.D. program at School of Aeronautics and Astronautics, Purdue University. His research interests are developments of luminescent sensors and their applications to Ocean Engineering.