Microelectronic Engineering 146 (2015) 57–61
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Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Quantum dot-based sensor layer in lightweight structures T. Fischer a,⇑, K. Heinrich b, C. Spudat b, J. Martin b, T. Otto b, T. Gessner b, L. Kroll a a b
Chemnitz University of Technology, Chemnitz 09126, Germany Fraunhofer Institute for Electronic Nano Systems, Chemnitz 09126, Germany
a r t i c l e
i n f o
Article history: Received 16 October 2014 Received in revised form 6 March 2015 Accepted 30 March 2015 Available online 4 April 2015 Keywords: Quantum dot Impact visualization Fiber-reinforced epoxy composite
a b s t r a c t Quantum dots can be used to detect, store and make visually apparent mechanical loading conditions. The ﬂuorescent properties of the nanocrystals are selectively inﬂuenced by the injection of electric charges. By applying an external electric voltage, it is possible to suppress photoluminescence completely. If the quantum dots, as part of a functional layer system, are integrated in smart components, the integrated material system allows for energy-autonomous condition monitoring. We present for the ﬁrst time a quantum dot-based system in a glass ﬁber-reinforced epoxy composite with a layer structure which is suitable for impact visualization. The quantum dots dispersed in poly (9-vinylcarbazoles) were applied on a PEDOT:PSS layer on an ITO-coated PET substrate. Silver electrodes were sputtered as a structured layer. For integration of the layer stack, which measured 25 25 0.1 mm, in an epoxy composite, two process variants and sample geometries were used: a 2D curved component for hand layup and a plate for resin transfer molding. The epoxy composite components had a material thickness of 1.5 mm and included eight layers of ﬁberglass cloth. The quantum dot-layer stacks were positioned either between the ﬁrst two layers of glass ﬁber or directly at the component surface which had only a thin epoxy layer. By applying an external voltage, we suppressed the photoluminescence of the integrated quantum dots; the suitability of the coating system for integrated material sensors was evident. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction The energy-efﬁcient design of processes and components is one of the great challenges of today. One method of saving energy is weight reduction by material substitution or optimization. The application of ﬁber composites has been rising steadily in the automotive industry, the ﬁeld of aerospace and the manufacture of sports equipment during the last few years. A disadvantage of ﬁber plastic composites compared with substituted metals is that damage such as matrix cracking or ﬁber cracks is usually not visible and is undetectable without structural damage. Damage occurs suddenly and without prior plastic deformation. Therefore, it is essential to develop methods for the simple, rapid detection of damage to components with high safety relevance. The known methods for non-destructive monitoring such as ultrasound or computed tomography are complex and require the veriﬁcation of the complete component, since otherwise localization of damage is not possible. Structural health monitoring (SHM) now offers the opportunity to localize characteristic value deteriorations at any time. Ideally, sensor systems are integrated into the device structure in order to ensure continuous monitoring of components ⇑ Corresponding author. http://dx.doi.org/10.1016/j.mee.2015.03.064 0167-9317/Ó 2015 Elsevier B.V. All rights reserved.
and to protect the sensor system from environmental stresses . Research has focused on a variety of methods for monitoring component states based on various sensor effects and material systems. An overview of current developments is given in . The SHM is a complex system described by a combination of simulation, actuators, sensors, data acquisition and evaluation. In the literature and many research activities only aspects of the complex system are described in detail. An epoxy resin containing carbon nanotubes used to realize a speciﬁc electrical conductivity was provided with a grid of contact points. On the assumption, for example, that the electrical conductivity changes locally because of damage, the latter were detected, localized and quantiﬁed . Epoxy composites with CNTs or graphene were also the subject of investigations in  and . The change in electrical resistance with elastic deformation was considered. The proportion of CNTs in the composite was characterized as an important factor inﬂuencing the sensitivity of the method. However, localization of damage is not possible with this method. The combination of ﬁber Bragg gratings in CRFP and data analysis for the localization of impacts are examined in  and . Three main themes that are currently in focus are highlighted: (1) the discrimination between temperature and strain effects, (2) the amplitude spectrum demodulation comprising the measurement of residual stress and (3) the
T. Fischer et al. / Microelectronic Engineering 146 (2015) 57–61
connection interface between the embedded sensor and the surroundings. Furthermore, a new monitoring system for rotor blades, consisting of an acoustic emission sensor and embroidered wire sensors for strain measurement that are directly integrated in the device structure, has been described [8,9]. Another general approach is the use of quantum dot-based sensor systems. In comparison with organic ﬂuorescent compounds the ﬂuorescence of quantum dots can be electrically quenched. This approach simpliﬁes the monitoring of lightweight structures which need regular inspection, such as automotive parts or cycling helmets. This paper focuses on the development of a functional quantum dot layer system which visualizes mechanical impact by quenching photoluminescence. Therefore it can be used in the ﬁeld of structural health monitoring [10,11]. Charges from an external source are transferred to quantum dots which cause photoluminescence quenching. If the charges are stored temporarily, loading conditions of lightweight structures will be visually recognized and measured. Hence damaged lightweight structures can be replaced in time. In this paper the preparation of a quantum dot sensor system and its performance inside the lightweight structure are demonstrated. 2. Materials and methods 2.1. Processing of functional layer stacks The functional layer systems were fabricated by spin-coating on pre-cleaned PET-ITO substrates with a size of 2.5 cm 2.5 cm. PETITO substrates and poly(vinylcarbazole) (PVK) were purchased from Aldrich. PEDOT:PSS was also supplied by Aldrich and applied as a charge transport layer. Core shell-type quantum dots, dispersed in toluene (CdSe/ZnS; emission wavelength 610 nm, 5 mg/ mL) were obtained from CAN Hamburg. PEDOT:PSS dispersion in water diluted with ethanol in a volume ratio of v/v = 1/2. 200 lL was spin-coated 3 times on a pre-cleaned substrate. The best ﬁlm properties were realized at 3000 rpm and a spin-coating time of 25 s. After the ﬁrst and second layer the ﬁlm was annealed for 5 min at 100 °C. After deposition of the third layer the PEDOT:PSS coated substrate was dried for 1 h at 110 °C. Quantum dot dispersions (5 mg/mL in toluene) were mixed with PVK, which was also dispersed in toluene beforehand. Therefore, 20.6 mg of PVK was dispersed in 1 mL of toluene and used as a matrix material for quantum dots. The QD/PVK mixture consisted of 1 mL of QD dispersion and 0.5 mL of PVK dispersion and was spin-coated at 1000 rpm for up to 25 s. After deposition of 150 lL of PVK/QD-mixture by spin-coating the layer stack was dried for 15 min at 95 °C. On top of the layer stack aluminium was deposited by sputtering technology having a thickness of 100 nm. EPO-TEK 301-2 supplied by Epoxy Technology Inc., USA and 25 g/m2 Panda™ plain weave glass fabric distributed by R&G Faserverbundwerkstoffe GmbH, Germany were used for the glass ﬁber/epoxy laminate. The epoxy resin was speciﬁed by the manufacturer to have a spectral transmission of >94% @ 320 nm, >99% @ 400–1200 nm, >98% @ is 1200–1600 nm (see Table 1). 2.2. Integration of functional layer stacks into lightweight structures Before the embedding of quantum dot layer systems, the ITO electrode and the aluminum electrode were connected with polymer-coated copper wires; the polymer coating was to prevent a shortcut of the wires in case of contact. An epoxy resin containing silver was used for the glass ﬁber-epoxy laminate. Afterwards the prepared functional layer stack was embedded into an epoxy resin/glass-ﬁber-reinforced composite structure.
Table 1 Materials for transmittance test. Material
Glass fabric 49 g/m2 (Finish FE 800, plain); R&G GmbH, Germany Glass fabric 25 g/m2 Panda™ (plain weave); R&G GmbH, Germany Epoxy resin L (bisphenol A/F epoxy resin); R&G GmbH, Germany EPO-TEK 301-2 (bisphenol A) epoxy resin, Epoxy Technology Inc., USA Aldrich
Cloth 2 Epoxy resin 1 Epoxy resin 2 PET-ITO substrate
Previously the transmission behavior of the cloth and the epoxy resin was characterized by UV/VIS measurements to ensure optical transparency in the photoluminescent region of the quantum dots. The integration resulted in plane samples for resin transfer molding (RTM) and 2D curved samples for hand lay-up, a laboratory method (right, plane sample) (Fig. 1). RTM was chosen to prove the performance of the quantum dot layer in components manufactured by an established technology for medium volume production . The quantum dot functional layer stack was positioned between the glass ﬁber plies in the two-part tool before epoxy resin was injected into the mold. A symmetric isotropic layer construction of the structural part with a ﬁber volume fraction of 50% was applied for the integration of a quantum dot layer system. The reinforcement was realized by a glass ﬁber fabric using both a twill weave of 2/2 with a basic weight of 282 g/m2 and a Panda™ plain weave glass fabric with a basic weight of 25 g/m2. The matrix system was composed either of epoxy resin L (Bisphenol A/F) and hardener EPH 294 at the mass ratio of 100:31 or epoxy resin EPO-TEK 301-2 (Bisphenol A) part A and part B at the mass ratio of 100:35. Resin transfer molding (RTM) acted as manufacturing process for the plane panels (size: 510 mm 680 mm 1 mm). After ﬁlling of the tool, the maximum pressure was gradually adjusted in the following order: 5 min at 1 bar, 2 bar till the complete ﬁlling, 20 min at 3 bar. The mold was heated during the ﬁlling to 60 °C in order to reduce the viscosity of the matrix resin. As a result the wetting behavior of the glass ﬁber layers was improved and the amount of gas bubble content and air pockets reduced. The part was cooled down in the mold afterwards. Finally, the component was cured and demolded. The panels were cut into samples measuring 50 mm 50 mm and the quantum dot layer system was electrically connected for upcoming characterization. Another method for processing reactive casting resins is the hand lay-up. This simple procedure, well-suited for the laboratory, was applied during the preparation of the above-mentioned 2D structure. First, the mold surface was provided with a release agent and a gel coat, which prevented the ﬁber structure being drawn to the outside and protected the tool. Thereafter, epoxy resin and glass ﬁber layers were applied to the tool surface one after another. The QD ﬁlms were positioned between the ﬁber glass layers. For mechanical reasons the layer stack was introduced beneath the ﬁrst glass ﬁber. The inﬁlling of the resin in the individual ﬁber layers was realized with a wet-on-wet brush or roller. The tool consisted of two halves in order to guarantee the same quality on the surface of the top and bottom of the composite. After being cured at room temperature for 48 h, samples were cut out of the structure and contacted electrically. 2.3. Methods for characterization of functional layer stacks The quantum dot distribution was investigated by ﬂuorescent microscopy (Olympus IX-51). The quenching behavior was
T. Fischer et al. / Microelectronic Engineering 146 (2015) 57–61
Fig. 1. Integration of a functional layer stack containing nanocrystals in an glass-ﬁber-reinforced composite structure. Left: (1) PET-ITO-substrate, (2) ITO-layer, (3) PEDOT:PSS, (4) Nanocrystal/PVK composite, (5) Aluminium electrode. Right: (1) Functional layer stack, (2) Epoxy resin matrix, (3) Glass-ﬁber plies.
characterized by observing the optical contrast as a function of applied voltage. This measurement was realized by means of time-resolved confocal micro-spectroscopy. Fluorescent properties were measured with a homemade setup based on a triple grating monochromator (Andor Shamrock SR-303i-A) with an attached CCD camera (ANDOR Newton DU920P-BR-DD). Quantum dot photoluminescence was excited by a solid state laser with a wavelength of 475 nm (B&WTek Inc.). Transmission measurements of epoxy resins and cloth were performed with a UV/Vis spectrometer UV-line 9400 (Schott Instruments) within a wavelength range from 190 nm to 700 nm. The thickness of the functional layers was measured by ellipsometry, M 2000 Ellipsometer by J. A. Woollam Co., Inc. (Lincoln NE, USA) using the WVASE 32 software. Both electrodes/copper wires, i.e. ITO and aluminum, were contacted with micro positioners until electrical resistance was measured. Afterwards an electrical voltage up to 10 V was applied. The relationship between the thickness of the functional layer and the SHM performance was the subject of the current investigations. 3. Results and discussion 3.1. Processing of functional layer system Non-structured PET-ITO substrates were homogeneously coated both with PEDOT:PSS and nanocrystal/PVK-composite layer. Homogeneous layer stacks resulted (Fig. 2). The thickness of each layer was analyzed by using ellipsometry measurements, so an overall layer thickness, including PEDOT:PSS and the nanocrystal composite, of 150–200 nm was obtained. As previously mentioned, the integration of optimized functional layer stacks is described in this work. The majority of the prepared samples did not show complete quenching behavior, so optical contrast, i.e. comparison between photoluminescence before and after an electric voltage was applied, was employed
for characterization. This was shown by a special shape of the counter electrode, as in Fig. 2. The deposited aluminum electrode was a combination of an interrupted outer ring which enclosed a spot with a diameter of 4 mm. The interruption of the outer area is important because of the mask which is used for sputter deposition of the aluminum electrode. To measure the photoluminescent quenching only the spot inside was contacted, so that an optical contrast between contacted and non-contacted areas after application of a voltage was achieved. As a result only the photoluminescence in the spot changed. Before integration into the glass ﬁber/resin structure, the samples were analyzed by quenching experiments. To assess quenching behavior, the ITO electrode and the counter electrode were connected with an external voltage unit. After application of an external voltage quenching of up to 80% was realized. Because it was not possible to switch off the quantum dots completely, optical contrast was chosen to describe the quenching behavior. The optical contrast is the ratio between photoluminescence intensity after application of an external voltage and the photoluminescence intensity at the beginning of the measurement. With increasing external voltage the resulting optical contrast increases, too. Before embedding of the functional layer stacks into a glass ﬁber/epoxy resin matrix, different kinds of glass ﬁber cloths and resins were investigated by means of UV/Vis measurements. To select the best glass ﬁber/epoxy resin matrix the transmittance between 200 and 1100 nm of cloths and hardened resin was recorded. The results are shown in Fig. 3. The results demonstrate that different cloths and resins (see Fig. 3) exhibit quite different transmittance behavior. The emission wavelength of quantum dots applied in the functional layer stack is around 600 nm, so transmittance in the region of 600 nm has to be as high as possible. This requirement was met by epoxy resin #2. Additionally, the transmittance of epoxy resin #2 is as high as the PET-ITO substrate. Comparison of the transmittance of different cloths showed that cloth #2 had the highest transmittance in the region of 600 nm. In the light of these results epoxy resin #2 and cloth #2 were chosen for the integration process of the functional layer system.
3.2. Integration of functional layer stacks into lightweight structures
Fig. 2. Distribution of Quantum dots on PET-ITO substrate observed with ﬂuorescence microscopy. The structure on top of the layer stack shows the shape of counter electrode.
On the basis of the measurement results the efﬁcient quenching layer stacks were successfully embedded in an epoxy composite by means of the RTM process. The functional layers were integrated both in-plane and in 2D samples, as described above. At the beginning of the RTM process a low viscosity mixture consisting of resin and hardener was used whereby the dissolution of the functional layer stack could be an issue. Therefore, after the integration process the composite was investigated under UV light at a wavelength of 253 nm. Attention was focused on the distribution of the QD layer inside the lightweight structure. The results are shown in Fig. 4. Fig. 4 shows that the photoluminescence of nanocrystals is only located on the integrated polymer substrate. No dissolution can be observed and the integration process had no inﬂuence on the photoluminescent properties of the nanocrystals.
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Fig. 3. Transmittance of different epoxy resins and cloth in the range of 200–700 nm. The emission wavelength of used quantum dots is in the region of 600 nm.
Fig. 4. Embedded QD layer system in lightweight structures. (a) Curved lightweight structure, (b) plane lightweight structure, (c) plane lightweight structure under UV-light.
Fig. 5. Characterization results of a quantum dot integrated functional layer system. Photoluminescence as a function of time and applied voltage.
The functional layers were completely covered by the epoxy resin matrix. There were no visible defects in the functional layers that might indicate poor wetting and adhesion. A bending of the laminates by hand did not lead to delamination of the functional layer system.
3.3. Characterization of layer stacks after integration process To simulate a mechanical impact, an external voltage was applied and the photoluminescence quenching, i.e. the optical contrast, was measured. Therefore, the counter electrode and the ITO
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layer were connected with copper wires and a maximum voltage of 10 V was applied between these electrodes. When a voltage is applied electrical charges are injected into the quantum dots so that the photoluminescence is quenched. The following diagram shows the photoluminescence as a function of time (Fig. 5). Fig. 5 demonstrates that the photoluminescence decreased to 40% when a voltage of 9 V was applied, i.e. it reached a high optical contrast. The reduction of the photoluminescence occurred immediately. Additionally, a voltage of 5 V was applied whereby a lower optical contrast was achieved. Therefore the system can be used for detecting the amount of applied external voltages. This experiment conﬁrms that the injection of external charges into quantum dots quenches photoluminescence. Inside a lightweight structure the functional layer stack has to be combined with a piezoelectric unit in order to use it for structural health monitoring.
4. Conclusions Quantum dot-containing functional layer stacks were successfully prepared by the spin-coating process and successfully integrated into lightweight structures by resin transfer molding. The quantum dot layer was not detached during the resin injection process. Application of an external voltage quenched the photoluminescence. These results show that the integration of sensor devices in lightweight constructions is possible and therefore suitable for monitoring mechanical stresses during operating time.
Acknowledgments The work was conducted within the framework of the nano system integration network of the excellence cluster NANETT (03IS2011) funded by the German Federal Ministry of Education and Research. We thank Dr. Ovidiu Gordan, Semiconductor Physics Research Group, Chemnitz University of Technology, Germany, for the ellipsometry measurement. References  W. Ostachowicz, J.A. Güemes, New Trends in Structural Health Monitoring, Springer, Berlin, 2013.  H. Schürmann, Konstruieren mit Faser-Kunststoff-Verbunden, Springer, Berlin, 2007.  A. Naghashpour, V.H. Suong, Nanotechnology 24 (2013) 455502.  J. Rams, M. Sanchez, A. Urena, A. Jimenez-Suarez, M. Campo, A. Güemes, Int. J. Smart Nano Mater. 3 (2) (2012) 152.  L.M. Chiacchiarelli, M. Rallini, M. Monti, D. Puglia, J.M. Kenny, L. Torre, Comp. Sci. Technol. 80 (2013) 73.  J. Gomez, I. Jorge, G. Durana, J. Arrue, J. Zubia, G. Aranguren, A. Montero, I. Lopez, Sensors 13 (2013) 11998.  D. Kinet, P. Megret, K.W. Goosen, L. Qiu, D. Heider, C. Caucheteur, Sensors 14 (2014) 7394.  J. Ulbricht, S. Gelbrich, L. Kroll, H. Elsner, F. Elﬂein, M. Motavelli, B. Havranek, E. Saqan, SMAR 2011 First Middle East Conference on Smart Monitoring, Assessment and Rehabilitation of Civil Structures, Dubai, 2011.  M. Walther, L. Kroll, M. Stockmann, H. Elsner, M. Heinrich, S. Wagner, 10th Youth Symposium on Experimental Solid Mechanics, Chemnitz, Germany, 2011.  J. Martin, D. Piasta, T. Otto, T. Gessner, Smart Systems Integration 2011, Proceed. Dresden, Germany, 2011.  J. Martin, D. Piasta, T. Kiessling, T. Otto, T. Gessner, U. Staudinger, D. Emrah, P. Pötschke, B. Voit, EUROMAT 2011, Montpellier, France, 2011.  G.W. Ehrenstein, Faserverbund-Kunststoffe, Hanser, München, 2006.