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Vapour sensing properties of InP quantum dot luminescence
Sensors and Actuators B 162 (2012) 149–152
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Vapour sensing properties of InP quantum dot luminescence R. De Angelis a,∗ , M. Casalboni a , F. Hatami b , A. Ugur b , W.T. Masselink b , P. Prosposito a a b
Physics Department and INSTM, University of Rome, “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Rome, Italy Department of Physics, Humboldt University, Newtonstr. 15, D-12489 Berlin, Germany
a r t i c l e
i n f o
Article history: Received 4 November 2011 Accepted 15 December 2011 Available online 24 December 2011 Keywords: InP quantum dots Photoluminescence Vapour sensitivity Methanol
a b s t r a c t We investigated uncapped InP quantum dots grown epitaxially on InGaP buffer layer as an optically active element for chemical vapour detection. Near infrared luminescence has been studied as a function of the external environment. QD luminescence intensity changes rapidly and reversibly on exposure to methanol vapour while its spectral shape remains unchanged. For QDs about 45 nm average lateral size and 4–6 nm height, sensitivity to methanol vapour in the range 3.3 × 104 –7.2 × 103 ppm has been demonstrated. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The increasing demand for sensors able to monitor with high sensitivity the presence of hazardous chemicals, environmental pollutants in urban areas, and risky contaminants in foods has in recent years fostered the development of new materials [1–3] and detection strategies [4,5]. Semiconductor quantum dots (QDs) are natural candidates for promising vapour sensor devices due to the high effective surface area available for interaction with target chemicals, broad absorption bands, tuneable spectral emission as determined by their size and distribution [6,7], and enhanced energy transfer in coupled organic–inorganic systems due to their large transition dipole moments [4,5]. Luminescence chemical sensitivity has been demonstrated for II–VI cadmium (Cd) based colloidal semiconductor QDs by several research group [6–15]. The influence of the surface on QD photoluminescence (PL) can be understood in terms of trap states. The effect of these sates is usually avoided achieving passivation by covering QDs with a capping layer of a wider band-gap semiconductor. For uncapped QDs the photoluminescence intensity is lower due to the presence of non-radiative trap states. However it has been demonstrated that an interaction between QDs surface and some chemical species results in changes of the surface charge and/or in passivation of trap states affecting PL emission [6,8].
∗ Corresponding author. Tel.: +39 06 7259 4778; fax: +39 06 2023507. E-mail addresses: [email protected], [email protected] (R. De Angelis), [email protected] (M. Casalboni), [email protected] (F. Hatami), [email protected] (A. Ugur), [email protected] (W.T. Masselink), [email protected] (P. Prosposito). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.12.052
Nazzal et al. reported PL changes in colloidal CdSe QDs incorporated in PMMA thin films exposed to three amines (triethylamine, benzylamine and butylamine) [9]. In another work [10], the same group systematically studied the environmental effects on the photoluminescent behaviour of colloidal CdSe bare-core and CdSe/ZnS core–shell QDs embedded in polymer thin films under different gaseous environment (argon, oxygen, air, water vapour, and wet oxygen) and in different excitation conditions. The bare-core CdSe QDs displayed a higher PL yield and were more robust against photobleaching in an inert environment with respect to the core–shell quantum dots [10]. Selective detection of polar and non-polar solvents has been demonstrated for PMMA film incorporating CdSe nanocrystals of different size by means of a multivariate analysis [11]. A reversible decrease of QD PL has been reported for polymer-modified ZnS/CdSe QDs embedded in a sol–gel matrix when exposed to organic vapours (methanol, ethanol, chloroform, and acetone). In this case, partial selectivity has been achieved through variation of the host matrix composition and pattern recognition [12]. Hydrocarbon detection with surface functionalized CdSe incorporated in PMMA has been achieved for xylenes and toluene [13]. Xylenes was found to cause both PL enhancement and quenching depending on the gas concentration. Such behaviour has been ascribed to the competition between the PL enhancement that is an intrinsic optical property of QDs and film-wetting of the polymeric matrix at high exposure dose [14]. In order to retard the film wetting effect they use an anodic aluminium oxide (AAO) platform [15]. Literature is limited up to now to Cd based QDs which emission is tuneable in the visible range. Infrared emitting (IR) QDs are, however, also very attractive for many sensing purposes, for example in vivo biological tagging (in the NIR wavelength range
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700–1300 nm in which living tissues has a penetration depth of about 5–10 cm) and integration with fibre optic communication (in the wavelength range 800–900 nm) [16]. In addition the toxicity of cadmium is well known while IR emitters phosphor (P) based QDs show a lower toxicity and therefore they represent a valuable alternative [2]. Epitaxial QDs fabricated on conventional semiconductor wafers such as GaAs are easily integrable with the well-developed semiconductor technology. Hence, epitaxial InP QDs emitting in the IR are promising for luminescence based sensors. In our previous work we described optical properties and carrier dynamics of InP QDs grown on both GaAs and GaP wafers [17–20]. Very recently, we have demonstrated the photoluminescence of uncapped epitaxial InP quantum dots on a GaAs substrate [21]. The emission wavelength of uncapped InP QDs ranges from 750 to 865 nm, depending on their size. In the present paper we describe the photoluminescence behaviour of uncapped epitaxial InP QDs grown on InGaP layer lattice-matched to GaAs in vacuum, pure N2 , and in different methanol/N2 mixtures. Dependence of the PL intensity as a function of methanol amount in the range 3.3 × 104 to 7.2 × 103 ppm has been measured. A reversible increase in the PL intensity proportional to the methanol concentration was measured that can be tentatively related to the passivation of surface trap states. 2. Experimental 2.1. Synthesis and characterization of epitaxial uncapped InP quantum dots QDs composed of InP on InGaP were grown using gas-source molecular beam epitaxy (GSMBE) in a RIBER 21T MBE system on (100) GaAs substrates. The lattice mismatch of 3.8% between InP and In0.48 Ga0.52 P (lattice matched to GaAs) drives the straininduced formation of QDs via the Stranski–Krastanow mechanism. After oxide desorption, a 100-nm thick GaAs buffer was grown at 550 ◦ C at a rate of 0.4 atomic monolayer per second (ML/s), followed by 250 nm In0.48 Ga0.52 P grown at 460 ◦ C. Subsequently, the substrate temperature was lowered to about 410 ◦ C and the InP deposited, resulting in the formation of the quantum dots, followed by cooling down the sample to 20 ◦ C. During the growth process, the surface of In0.48 Ga0.52 P shows a 2 × 1-reconstruction, as observed using reflection high-energy electron-diffraction (RHEED). The areal density and the size of QDs were controlled using the InP deposition rate and time. The InP deposition rate was 0.25 ML/s [19]. The structural properties of the samples and the composition of the InGaP layers were characterized using double-crystal X-ray diffractometry (DCXD) and atomic force microscopy (AFM). The results show lattice-matched In0.48 Ga0.52 P to GaAs with good crystal quality [22]. AFM was carried out using an ex situ Nanoscope IIIa in the tapping mode to image the surfaces of the samples. The investigated sample contains surface QDs with an average lateral size of about 45 nm and a height of 4–6 nm as shown in Fig. 1. 2.2. Epitaxial uncapped InP quantum dots photoluminescence and methanol sensing measurements Samples prepared as described in Section 2.1 have been used to test the dependence of the luminescence of epitaxial uncapped InP QDs on the surrounding gaseous environment. To this end, the sample was tightly mounted inside a sealed temperature-controlled chamber equipped with UV-grade fused silica optical windows allowing for laser excitation and luminescence collection. The photoluminescence was excited by the 458 nm line of a CW Ar+ laser with an excitation power density of 90 mW/cm2 held constant on sample. The laser power was continuously monitored with a power
Fig. 1. AFM image (2 m × 2 m) of uncapped InP QDs grown on InGaP/GaAs substrate.
metre which measures the light reflected by a beam splitter placed on the excitation beam. The excitation wavelength (458 nm) was selected considering the small penetration depth at this energy in order to avoid the effect of the GaAs substrate luminescence. PL was collected and analysed by a 25-cm monochromator (ARC SpectraPro – 300i) and a photomultiplier (Hamamatsu PM363610). Data were acquired using a lock-in technique (Stanford Research Systems – SR830 DSP lock in amplifier) and collected by two different homemade acquisition programs: the first one allows the full PL spectra in a selected wavelength range to be recorded, and the second one performs a time scan of the PL signal at a fixed wavelength. For the sensing measurements we used N2 as carrier gas for methanol vapour. Preliminary tests injecting controlled amount of pure N2 in the chamber were performed to check its effect on the luminescence and no significant response was observed compared to vacuum; neither in PL shape nor intensity changes. At standard temperature and pressure, methanol is a liquid solvent. We used anhydrous methanol 99.8+% of analytical-reagent grade from Sigma–Aldrich. Its saturated vapour (obtained by N2 bubbling at room temperature in a drexel bottle) was mixed with N2 in controlled ratio and then injected in the test chamber. The saturated concentration was estimated according to the empirical Antoine’s law (parameters from NIST data [23]). Gas mixtures prepared as described above were injected into the chamber by means of a graded syringe and the PL intensity change was followed for 300 s. Then, to restore the initial condition, the system was pumped to vacuum for the same time. Methanol concentrations within the range of 3.3 × 104 to 7.2 × 103 ppm with an error of ∼5% (due to the approximately 1 ◦ C thermal fluctuations of the bubbler during the experiments) were used in the experiments. All the measurements were performed with chamber and gases at room temperature (25 ◦ C) and the temperature was checked carefully since its effect on the PL intensity was relevant: e.g. a temperature increase of 1 ◦ C produces a decrease of ∼3% in the PL intensity. 3. Results and discussion Fig. 2 shows the PL spectra of the epitaxial uncapped InP QD sample studied in this work. We measured the PL spectra in
R. De Angelis et al. / Sensors and Actuators B 162 (2012) 149–152
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Fig. 4. PL intensity as a function of time cycling different amounts of methanol vapour inside the chamber. The methanol vapour concentrations corresponding to the rises in the figure are (from left to right): 3.3 × 104 ppm; 2.8 × 104 ppm; 2.3 × 104 ppm; 1.7 × 104 ppm; 1.2 × 104 ppm.
Fig. 2. PL spectra of QDs before (blue line – full circles) and after (red line – full squares) injection of methanol vapour (3.3 × 104 ppm). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)
two different environmental conditions: vacuum (∼1 mbar) and a methanol vapour/N2 blend. In both conditions, the PL spectral shape is the same (with its maximum at 865 nm and a FWHM of 57 nm) but a more intense signal was observed in the methanol vapour/N2 atmosphere. In the figure, the effect of 3.3 × 104 ppm of methanol in N2 is shown. Under this condition an increase of the PL maximum IPL /IPL of about 20% has been estimated. Fig. 3 shows the time-dependent scan of the PL maximum for two cycle of vapour in/out of 3.3 × 104 ppm methanol. The response of the material is fully reversible. Indeed, methanol vapours determine an increase of the PL over a broad range of concentrations (see below) and the reversibility has been verified in all cases. The mechanism responsible for this behaviour has been already discussed for bulk III–V semiconductors [24] and II–VI core–shell quantum dots [8,14]. Polar adsorbates can reduce the effect of intrinsic surface states when their local electric field partially offsets the fields induced by surface trap states. In particular, QDs surface trap states, responsible for non-radiative decay, can be thus inhibited by the physisorption of suitable analytes. This effect results in an amplitude enhancement as those shown in Fig. 3. This hypothesis requires further studies to be fully verified, however the
Fig. 3. Two-cycle time PL variation at fixed detection wavelength ( = 865 nm) for 3.3 × 104 ppm methanol vapour in N2 .
evidence that the spectral shape of the PL remains unchanged even for high methanol concentration supports the assignment of the PL enhancement to intrinsic trap state passivation. Moreover our QDs were uncapped and presumably the number of uncoordinated QD surface sites generating surface trap states is higher would be the case in capped QDs. The effect of the methanol molecules can be regarded as a saturation of such states resulting in an increase in the overall PL intensity. In Fig. 4 the effect of different concentrations of methanol vapour on the PL peak intensity is shown. We started with high concentrations of the solvent, namely 3.3 × 104 ppm, and gradually the amount was reduced to the lowest concentration tested 7.2 × 103 ppm. Vapour injection produces a pressure increase in the chamber to atmosphere pressure that can be considered instantaneous (less than 1 s). We followed the PL rise for 300 s and then the system was pumped out. Within about 15 s the chamber pressure reaches the initial value of 1 mbar while the PL signal decreases to the previous value, as before the vapour input, with a longer time. The rise/fall dynamics of the signal can be related to the adsorption/desorption time of the analyte on the surface of QDs. At the end of an entire cycle (T = 600 s) the PL signal shows a lower value compared to that measured before the cycle (see Fig. 4). This is due to a slight decrease of the luminescence intensity under continuous excitation with laser light. Such behaviour was observed also without vapour. The overall decrease during a measurement cycle is of the order of 2% of the luminescence intensity and it is reversible in dark. We ascribe it to the small heating of the sample due to the continuous lighting, since we verified that the PL intensity is lower for higher temperatures. In any case, such a behaviour does not affect the sensing capability of the QDs: the increase of the PL signal (IPL /IPL ) related to a certain amount of methanol does not depend on signal intensity before the injection. Fig. 5 shows the increase of the PL signal (IPL /IPL ) as a function of the concentration of methanol vapour. A detailed analysis of the data shows the existence of a linear relationship in the concentration range between about 7.2 × 103 and 1.7 × 104 ppm (correlation coefficient higher than 0.997). For higher concentrations, the QDs are still sensitive but the response is no longer linear, due to the limited number of sites available for methanol adsorption on QD surface. A final important note regards the stability of our material. We performed tests on these samples on a period of approximately one year (with samples stored in air at room temperature): no degradation of the luminescence signal was observed and the sensing capability of the material was preserved. Moreover, a methanol sensitivity of epitaxial uncapped InP QDs grown on InGaP layer has been verified also for other samples with QDs of different sizes. In these cases the emission band varies with QD size as expected [21]:
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R. De Angelis et al. / Sensors and Actuators B 162 (2012) 149–152
Fig. 5. (Blue line – blue circle) PL change (%) as a function of methanol concentration. The plot displays a linear behaviour up to 1.7 × 104 ppm. (Black line) Linear fit to the experimental data. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)
the smaller the size, the higher the energy. The effect of methanol is quite similar for each sample: the PL spectral shape remains unchanged while its intensity increases with methanol concentration. 4. Conclusion In this paper we have demonstrated the luminescence sensitivity to methanol vapour of epitaxial uncapped InP QDs grown on InGaP layer lattice matched to GaAs. The intensity of QD PL undergoes a reversible and reproducible increase in response to changes of the local concentration of methanol over a broad range (from 3.3 × 104 ppm to 7.2 × 103 ppm). We ascribe this behaviour to the electron donor ability of methanol in the passivation of the non-radiative trap states on the QD surfaces. Although the detection limit demonstrated in the current results is quite high, this preliminary investigation opens the possibility to exploit this system as a sensing material for volatile organic compounds in integrated optical sensor device (lab-on-a-chip) merging gas-sensor and semiconductor technology. Acknowledgments We are indebted to Dr. Chiara La Storia for the precious technical cooperation and with Prof. Roberto Pizzoferrato for the many stimulating discussions and suggestions in the early stage of this study. This work was supported in part by the Italian CARIPLO foundation through the project number 2010–0525. References [1] R.C. Somers, M.G. Bawendi, D.G. Nocera, Chem. Soc. Rev. 36 (2007) 579. [2] U. Resch-Genger, M. Grabolle, S. Cavaliere-Jaricot, R. Nitschke, T. Nann, Nat. Methods 5 (2008) 763. [3] M.F. Frasco, N. Chaniotakis, Sensors 9 (2009) 7266. [4] I.L. Medintz, A.R. Clapp, H. Mattoussi, E.R. Goldman, B. Fisher, J.M. Mauro, Nat. Mater. 2 (2003) 630. [5] A.R. Clapp, I.L. Medintz, H. Mattoussi, ChemPhysChem 7 (2006) 47. [6] X. Wang, L. Qu, J. Zhang, X. Peng, M. Xiao, Nano Lett. 3 (2003) 1103.
[7] B.J. Walker, V. Bulovi, M.G. Bawendi, Nano Lett. 10 (2010) 3995. [8] S. Jeong, M. Achermann, J. Nanda, S. Ivanov, V.I. Klimov, J.A. Hollingsworth, J. Am. Chem. Soc. 127 (2005) 10126. [9] A.Y. Nazzal, L. Qu, X. Peng, M. Xiao, Nano Lett. 3 (2003) 819. [10] A.Y. Nazzal, X. Wang, L. Qu, W. Hu, Y. Wang, X. Peng, M. Xiao, J. Phys. Chem. B 108 (2004) 5507. [11] R.A. Potyrailo, A.M. Leach, Appl. Phys. Lett. 88 (2006) 134110. [12] M. Hasani, A.M. Coto-García, J.M. Costa-Fernández, A. Sanz-Medel, Sens. Actuators B 144 (2010) 198. [13] O.V. Vassiltsova, Z. Zhao, M.A. Petrukhina, M.A. Carpenter, Sens. Actuators B 123 (2007) 522. [14] Z. Zhao, M. Arrandale, O.V. Vassiltsova, M.A. Petrukhina, M.A. Carpenter, Sens. Actuators B 141 (2009) 26. [15] Z. Zhao, T.M. Dansereau, M.A. Petrukhina, M.A. Carpenter, Appl. Phys. Lett. 97 (2010) 113105. [16] E.H. Sargent, Adv. Mater. 5 (2005) 515. ˜ C. Kristukat, F. Hatami, S. Dreßler, W.T. Masselink, C. Thomsen, Phys. [17] A.R. Goni, Rev. B 67 (2003) 75306. [18] F. Hatami, W.T. Masselink, L. Schrottke, J.W. Tomm, V. Talalaev, C. Kristukat, ˜ Phys. Rev. B 67 (2003) 85306. A.R. Goni, [19] A. Ugur, F. Hatami, W.T. Masselink, A.N. Vamivakas, L. Lombez, M. Atature, Appl. Phys. Lett. 93 (2008) 143111. [20] A. Ugur, F. Hatami, A.N. Vamivakas, L. Lombez, M. Atatüre, K. Volz, W.T. Masselink, Appl. Phys. Lett. 97 (2010) 253113. [21] K. Hestroffer, J.W. Tomm, A. Ugur, R. Braun, F. Hatami, E-MRS 2011, Symposium G, Nice, France (2011). [22] A. Ugur, F. Hatami, M. Schmidbauer, M. Hanke, W.T. Masselink, J. Appl. Phys. 105 (2009) 124308. [23] D. Ambrose, C.H.S. Sprake, J. Chem. Thermodyn. 2 (1970) 631. [24] F. Seker, K. Meeker, T.F. Kuech, A.B. Ellis, Chem. Rev. 100 (2000) 2505.
Biographies Roberta De Angelis received her MS (cum laude) in physics from “Tor Vergata” University, Rome, Italy in September 2010. In November 2010 she joined the NeMO – New Material For Optolectronic research group (University of Rome “Tor Vergata”) headed by Prof. M. Casalboni as a Ph.D. student in physics. She is working under the supervision of Dr. Paolo Prosposito and Prof. M. Casalboni on the line of investigation “Luminescence chemical sensors based on quantum dots”. Mauro Casalboni degree in physics at the University of Rome (1978), Researcher (1983), Associated Professor (1995) at the Universities of Rome Tor Vergata and Camerino. Since 2004 he is full professor at the University of Rome Tor Vergata. His scientific activity is in field of optics and optical properties of materials. Prof. Casalboni was coordinator of many national based projects and was European coordinator of STREP project in the framework of VI FP named ODEON on electro-optic devices based on innovative hybrid and polymeric materials. He is author of more than 100 international publications and some industrial patents. Fariba Hatami 2002 PhD in physics from Humboldt-Universität zu Berlin. 2003–2005 Alexander von Humboldt (Feoder Lynen) research fellow, Stanford University, Department of Electrical Engineering. 2005-present Senior scientist at Department of Physics, Humboldt-Universität zu Berlin. Asli Ugur is a Ph.D. student at physics department at Humboldt University, Berlin, Germany. She received her M.Sc. degree in 2006 and her B.Sc. degree in physics in 2004, from Technical University of Munich and Izmir Institute of Technology at Turkey, respectively. Since 2007, she is a research scientist at Humboldt University and has performed research in the fields of InP-InGaP quantum dots for quantum information. William Ted Masselink received the Ph.D. degree in 1986 from the University of Illinois in Urbana-Champaign for work in semiconductor heterostructure physics and engineering. From 1986 to 1994 he was a research staff member at the IBM T.J. Watson Research Center in Yorktown Heights, NY. Since 1994 he is professor of physics at the Humboldt University in Berlin, Germany. He has co-authored about 12 distinct patent disclosures including those for the widely used AlGaAs/InGaAs pseudomorphic field-effect transistor (pHEMT) and an advanced quantum-cascade laser design. He has authored or co-authored about 200 refereed publications. Paolo Prosposito received the M.Sc. in physics and the Ph.D. in physics from the University of Rome Tor Vergata, in 1992 and 1995, respectively. In 1997/98 he was visiting fellow at the Laboratory for Physical Chemistry of the University of Amsterdam. From 1998 to 2003 he was research fellow of the National Institute for Matter Physics (INFM). Since 2004 he is assistant professor at the Physics Department of the University of Rome Tor Vergata. He is author of more than 50 papers in refereed journals.
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Vapour sensing properties of InP quantum dot luminescence R. De Angelis a,∗ , M. Casalboni a , F. Hatami b , A. Ugur b , W.T. Masselink b , P. Prosposito a a b
Physics Department and INSTM, University of Rome, “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Rome, Italy Department of Physics, Humboldt University, Newtonstr. 15, D-12489 Berlin, Germany
a r t i c l e
i n f o
Article history: Received 4 November 2011 Accepted 15 December 2011 Available online 24 December 2011 Keywords: InP quantum dots Photoluminescence Vapour sensitivity Methanol
a b s t r a c t We investigated uncapped InP quantum dots grown epitaxially on InGaP buffer layer as an optically active element for chemical vapour detection. Near infrared luminescence has been studied as a function of the external environment. QD luminescence intensity changes rapidly and reversibly on exposure to methanol vapour while its spectral shape remains unchanged. For QDs about 45 nm average lateral size and 4–6 nm height, sensitivity to methanol vapour in the range 3.3 × 104 –7.2 × 103 ppm has been demonstrated. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The increasing demand for sensors able to monitor with high sensitivity the presence of hazardous chemicals, environmental pollutants in urban areas, and risky contaminants in foods has in recent years fostered the development of new materials [1–3] and detection strategies [4,5]. Semiconductor quantum dots (QDs) are natural candidates for promising vapour sensor devices due to the high effective surface area available for interaction with target chemicals, broad absorption bands, tuneable spectral emission as determined by their size and distribution [6,7], and enhanced energy transfer in coupled organic–inorganic systems due to their large transition dipole moments [4,5]. Luminescence chemical sensitivity has been demonstrated for II–VI cadmium (Cd) based colloidal semiconductor QDs by several research group [6–15]. The influence of the surface on QD photoluminescence (PL) can be understood in terms of trap states. The effect of these sates is usually avoided achieving passivation by covering QDs with a capping layer of a wider band-gap semiconductor. For uncapped QDs the photoluminescence intensity is lower due to the presence of non-radiative trap states. However it has been demonstrated that an interaction between QDs surface and some chemical species results in changes of the surface charge and/or in passivation of trap states affecting PL emission [6,8].
∗ Corresponding author. Tel.: +39 06 7259 4778; fax: +39 06 2023507. E-mail addresses: [email protected], [email protected] (R. De Angelis), [email protected] (M. Casalboni), [email protected] (F. Hatami), [email protected] (A. Ugur), [email protected] (W.T. Masselink), [email protected] (P. Prosposito). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.12.052
Nazzal et al. reported PL changes in colloidal CdSe QDs incorporated in PMMA thin films exposed to three amines (triethylamine, benzylamine and butylamine) [9]. In another work [10], the same group systematically studied the environmental effects on the photoluminescent behaviour of colloidal CdSe bare-core and CdSe/ZnS core–shell QDs embedded in polymer thin films under different gaseous environment (argon, oxygen, air, water vapour, and wet oxygen) and in different excitation conditions. The bare-core CdSe QDs displayed a higher PL yield and were more robust against photobleaching in an inert environment with respect to the core–shell quantum dots [10]. Selective detection of polar and non-polar solvents has been demonstrated for PMMA film incorporating CdSe nanocrystals of different size by means of a multivariate analysis [11]. A reversible decrease of QD PL has been reported for polymer-modified ZnS/CdSe QDs embedded in a sol–gel matrix when exposed to organic vapours (methanol, ethanol, chloroform, and acetone). In this case, partial selectivity has been achieved through variation of the host matrix composition and pattern recognition [12]. Hydrocarbon detection with surface functionalized CdSe incorporated in PMMA has been achieved for xylenes and toluene [13]. Xylenes was found to cause both PL enhancement and quenching depending on the gas concentration. Such behaviour has been ascribed to the competition between the PL enhancement that is an intrinsic optical property of QDs and film-wetting of the polymeric matrix at high exposure dose [14]. In order to retard the film wetting effect they use an anodic aluminium oxide (AAO) platform [15]. Literature is limited up to now to Cd based QDs which emission is tuneable in the visible range. Infrared emitting (IR) QDs are, however, also very attractive for many sensing purposes, for example in vivo biological tagging (in the NIR wavelength range
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R. De Angelis et al. / Sensors and Actuators B 162 (2012) 149–152
700–1300 nm in which living tissues has a penetration depth of about 5–10 cm) and integration with fibre optic communication (in the wavelength range 800–900 nm) [16]. In addition the toxicity of cadmium is well known while IR emitters phosphor (P) based QDs show a lower toxicity and therefore they represent a valuable alternative [2]. Epitaxial QDs fabricated on conventional semiconductor wafers such as GaAs are easily integrable with the well-developed semiconductor technology. Hence, epitaxial InP QDs emitting in the IR are promising for luminescence based sensors. In our previous work we described optical properties and carrier dynamics of InP QDs grown on both GaAs and GaP wafers [17–20]. Very recently, we have demonstrated the photoluminescence of uncapped epitaxial InP quantum dots on a GaAs substrate [21]. The emission wavelength of uncapped InP QDs ranges from 750 to 865 nm, depending on their size. In the present paper we describe the photoluminescence behaviour of uncapped epitaxial InP QDs grown on InGaP layer lattice-matched to GaAs in vacuum, pure N2 , and in different methanol/N2 mixtures. Dependence of the PL intensity as a function of methanol amount in the range 3.3 × 104 to 7.2 × 103 ppm has been measured. A reversible increase in the PL intensity proportional to the methanol concentration was measured that can be tentatively related to the passivation of surface trap states. 2. Experimental 2.1. Synthesis and characterization of epitaxial uncapped InP quantum dots QDs composed of InP on InGaP were grown using gas-source molecular beam epitaxy (GSMBE) in a RIBER 21T MBE system on (100) GaAs substrates. The lattice mismatch of 3.8% between InP and In0.48 Ga0.52 P (lattice matched to GaAs) drives the straininduced formation of QDs via the Stranski–Krastanow mechanism. After oxide desorption, a 100-nm thick GaAs buffer was grown at 550 ◦ C at a rate of 0.4 atomic monolayer per second (ML/s), followed by 250 nm In0.48 Ga0.52 P grown at 460 ◦ C. Subsequently, the substrate temperature was lowered to about 410 ◦ C and the InP deposited, resulting in the formation of the quantum dots, followed by cooling down the sample to 20 ◦ C. During the growth process, the surface of In0.48 Ga0.52 P shows a 2 × 1-reconstruction, as observed using reflection high-energy electron-diffraction (RHEED). The areal density and the size of QDs were controlled using the InP deposition rate and time. The InP deposition rate was 0.25 ML/s [19]. The structural properties of the samples and the composition of the InGaP layers were characterized using double-crystal X-ray diffractometry (DCXD) and atomic force microscopy (AFM). The results show lattice-matched In0.48 Ga0.52 P to GaAs with good crystal quality [22]. AFM was carried out using an ex situ Nanoscope IIIa in the tapping mode to image the surfaces of the samples. The investigated sample contains surface QDs with an average lateral size of about 45 nm and a height of 4–6 nm as shown in Fig. 1. 2.2. Epitaxial uncapped InP quantum dots photoluminescence and methanol sensing measurements Samples prepared as described in Section 2.1 have been used to test the dependence of the luminescence of epitaxial uncapped InP QDs on the surrounding gaseous environment. To this end, the sample was tightly mounted inside a sealed temperature-controlled chamber equipped with UV-grade fused silica optical windows allowing for laser excitation and luminescence collection. The photoluminescence was excited by the 458 nm line of a CW Ar+ laser with an excitation power density of 90 mW/cm2 held constant on sample. The laser power was continuously monitored with a power
Fig. 1. AFM image (2 m × 2 m) of uncapped InP QDs grown on InGaP/GaAs substrate.
metre which measures the light reflected by a beam splitter placed on the excitation beam. The excitation wavelength (458 nm) was selected considering the small penetration depth at this energy in order to avoid the effect of the GaAs substrate luminescence. PL was collected and analysed by a 25-cm monochromator (ARC SpectraPro – 300i) and a photomultiplier (Hamamatsu PM363610). Data were acquired using a lock-in technique (Stanford Research Systems – SR830 DSP lock in amplifier) and collected by two different homemade acquisition programs: the first one allows the full PL spectra in a selected wavelength range to be recorded, and the second one performs a time scan of the PL signal at a fixed wavelength. For the sensing measurements we used N2 as carrier gas for methanol vapour. Preliminary tests injecting controlled amount of pure N2 in the chamber were performed to check its effect on the luminescence and no significant response was observed compared to vacuum; neither in PL shape nor intensity changes. At standard temperature and pressure, methanol is a liquid solvent. We used anhydrous methanol 99.8+% of analytical-reagent grade from Sigma–Aldrich. Its saturated vapour (obtained by N2 bubbling at room temperature in a drexel bottle) was mixed with N2 in controlled ratio and then injected in the test chamber. The saturated concentration was estimated according to the empirical Antoine’s law (parameters from NIST data [23]). Gas mixtures prepared as described above were injected into the chamber by means of a graded syringe and the PL intensity change was followed for 300 s. Then, to restore the initial condition, the system was pumped to vacuum for the same time. Methanol concentrations within the range of 3.3 × 104 to 7.2 × 103 ppm with an error of ∼5% (due to the approximately 1 ◦ C thermal fluctuations of the bubbler during the experiments) were used in the experiments. All the measurements were performed with chamber and gases at room temperature (25 ◦ C) and the temperature was checked carefully since its effect on the PL intensity was relevant: e.g. a temperature increase of 1 ◦ C produces a decrease of ∼3% in the PL intensity. 3. Results and discussion Fig. 2 shows the PL spectra of the epitaxial uncapped InP QD sample studied in this work. We measured the PL spectra in
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Fig. 4. PL intensity as a function of time cycling different amounts of methanol vapour inside the chamber. The methanol vapour concentrations corresponding to the rises in the figure are (from left to right): 3.3 × 104 ppm; 2.8 × 104 ppm; 2.3 × 104 ppm; 1.7 × 104 ppm; 1.2 × 104 ppm.
Fig. 2. PL spectra of QDs before (blue line – full circles) and after (red line – full squares) injection of methanol vapour (3.3 × 104 ppm). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)
two different environmental conditions: vacuum (∼1 mbar) and a methanol vapour/N2 blend. In both conditions, the PL spectral shape is the same (with its maximum at 865 nm and a FWHM of 57 nm) but a more intense signal was observed in the methanol vapour/N2 atmosphere. In the figure, the effect of 3.3 × 104 ppm of methanol in N2 is shown. Under this condition an increase of the PL maximum IPL /IPL of about 20% has been estimated. Fig. 3 shows the time-dependent scan of the PL maximum for two cycle of vapour in/out of 3.3 × 104 ppm methanol. The response of the material is fully reversible. Indeed, methanol vapours determine an increase of the PL over a broad range of concentrations (see below) and the reversibility has been verified in all cases. The mechanism responsible for this behaviour has been already discussed for bulk III–V semiconductors [24] and II–VI core–shell quantum dots [8,14]. Polar adsorbates can reduce the effect of intrinsic surface states when their local electric field partially offsets the fields induced by surface trap states. In particular, QDs surface trap states, responsible for non-radiative decay, can be thus inhibited by the physisorption of suitable analytes. This effect results in an amplitude enhancement as those shown in Fig. 3. This hypothesis requires further studies to be fully verified, however the
Fig. 3. Two-cycle time PL variation at fixed detection wavelength ( = 865 nm) for 3.3 × 104 ppm methanol vapour in N2 .
evidence that the spectral shape of the PL remains unchanged even for high methanol concentration supports the assignment of the PL enhancement to intrinsic trap state passivation. Moreover our QDs were uncapped and presumably the number of uncoordinated QD surface sites generating surface trap states is higher would be the case in capped QDs. The effect of the methanol molecules can be regarded as a saturation of such states resulting in an increase in the overall PL intensity. In Fig. 4 the effect of different concentrations of methanol vapour on the PL peak intensity is shown. We started with high concentrations of the solvent, namely 3.3 × 104 ppm, and gradually the amount was reduced to the lowest concentration tested 7.2 × 103 ppm. Vapour injection produces a pressure increase in the chamber to atmosphere pressure that can be considered instantaneous (less than 1 s). We followed the PL rise for 300 s and then the system was pumped out. Within about 15 s the chamber pressure reaches the initial value of 1 mbar while the PL signal decreases to the previous value, as before the vapour input, with a longer time. The rise/fall dynamics of the signal can be related to the adsorption/desorption time of the analyte on the surface of QDs. At the end of an entire cycle (T = 600 s) the PL signal shows a lower value compared to that measured before the cycle (see Fig. 4). This is due to a slight decrease of the luminescence intensity under continuous excitation with laser light. Such behaviour was observed also without vapour. The overall decrease during a measurement cycle is of the order of 2% of the luminescence intensity and it is reversible in dark. We ascribe it to the small heating of the sample due to the continuous lighting, since we verified that the PL intensity is lower for higher temperatures. In any case, such a behaviour does not affect the sensing capability of the QDs: the increase of the PL signal (IPL /IPL ) related to a certain amount of methanol does not depend on signal intensity before the injection. Fig. 5 shows the increase of the PL signal (IPL /IPL ) as a function of the concentration of methanol vapour. A detailed analysis of the data shows the existence of a linear relationship in the concentration range between about 7.2 × 103 and 1.7 × 104 ppm (correlation coefficient higher than 0.997). For higher concentrations, the QDs are still sensitive but the response is no longer linear, due to the limited number of sites available for methanol adsorption on QD surface. A final important note regards the stability of our material. We performed tests on these samples on a period of approximately one year (with samples stored in air at room temperature): no degradation of the luminescence signal was observed and the sensing capability of the material was preserved. Moreover, a methanol sensitivity of epitaxial uncapped InP QDs grown on InGaP layer has been verified also for other samples with QDs of different sizes. In these cases the emission band varies with QD size as expected [21]:
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Fig. 5. (Blue line – blue circle) PL change (%) as a function of methanol concentration. The plot displays a linear behaviour up to 1.7 × 104 ppm. (Black line) Linear fit to the experimental data. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)
the smaller the size, the higher the energy. The effect of methanol is quite similar for each sample: the PL spectral shape remains unchanged while its intensity increases with methanol concentration. 4. Conclusion In this paper we have demonstrated the luminescence sensitivity to methanol vapour of epitaxial uncapped InP QDs grown on InGaP layer lattice matched to GaAs. The intensity of QD PL undergoes a reversible and reproducible increase in response to changes of the local concentration of methanol over a broad range (from 3.3 × 104 ppm to 7.2 × 103 ppm). We ascribe this behaviour to the electron donor ability of methanol in the passivation of the non-radiative trap states on the QD surfaces. Although the detection limit demonstrated in the current results is quite high, this preliminary investigation opens the possibility to exploit this system as a sensing material for volatile organic compounds in integrated optical sensor device (lab-on-a-chip) merging gas-sensor and semiconductor technology. Acknowledgments We are indebted to Dr. Chiara La Storia for the precious technical cooperation and with Prof. Roberto Pizzoferrato for the many stimulating discussions and suggestions in the early stage of this study. This work was supported in part by the Italian CARIPLO foundation through the project number 2010–0525. References [1] R.C. Somers, M.G. Bawendi, D.G. Nocera, Chem. Soc. Rev. 36 (2007) 579. [2] U. Resch-Genger, M. Grabolle, S. Cavaliere-Jaricot, R. Nitschke, T. Nann, Nat. Methods 5 (2008) 763. [3] M.F. Frasco, N. Chaniotakis, Sensors 9 (2009) 7266. [4] I.L. Medintz, A.R. Clapp, H. Mattoussi, E.R. Goldman, B. Fisher, J.M. Mauro, Nat. Mater. 2 (2003) 630. [5] A.R. Clapp, I.L. Medintz, H. Mattoussi, ChemPhysChem 7 (2006) 47. [6] X. Wang, L. Qu, J. Zhang, X. Peng, M. Xiao, Nano Lett. 3 (2003) 1103.
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Biographies Roberta De Angelis received her MS (cum laude) in physics from “Tor Vergata” University, Rome, Italy in September 2010. In November 2010 she joined the NeMO – New Material For Optolectronic research group (University of Rome “Tor Vergata”) headed by Prof. M. Casalboni as a Ph.D. student in physics. She is working under the supervision of Dr. Paolo Prosposito and Prof. M. Casalboni on the line of investigation “Luminescence chemical sensors based on quantum dots”. Mauro Casalboni degree in physics at the University of Rome (1978), Researcher (1983), Associated Professor (1995) at the Universities of Rome Tor Vergata and Camerino. Since 2004 he is full professor at the University of Rome Tor Vergata. His scientific activity is in field of optics and optical properties of materials. Prof. Casalboni was coordinator of many national based projects and was European coordinator of STREP project in the framework of VI FP named ODEON on electro-optic devices based on innovative hybrid and polymeric materials. He is author of more than 100 international publications and some industrial patents. Fariba Hatami 2002 PhD in physics from Humboldt-Universität zu Berlin. 2003–2005 Alexander von Humboldt (Feoder Lynen) research fellow, Stanford University, Department of Electrical Engineering. 2005-present Senior scientist at Department of Physics, Humboldt-Universität zu Berlin. Asli Ugur is a Ph.D. student at physics department at Humboldt University, Berlin, Germany. She received her M.Sc. degree in 2006 and her B.Sc. degree in physics in 2004, from Technical University of Munich and Izmir Institute of Technology at Turkey, respectively. Since 2007, she is a research scientist at Humboldt University and has performed research in the fields of InP-InGaP quantum dots for quantum information. William Ted Masselink received the Ph.D. degree in 1986 from the University of Illinois in Urbana-Champaign for work in semiconductor heterostructure physics and engineering. From 1986 to 1994 he was a research staff member at the IBM T.J. Watson Research Center in Yorktown Heights, NY. Since 1994 he is professor of physics at the Humboldt University in Berlin, Germany. He has co-authored about 12 distinct patent disclosures including those for the widely used AlGaAs/InGaAs pseudomorphic field-effect transistor (pHEMT) and an advanced quantum-cascade laser design. He has authored or co-authored about 200 refereed publications. Paolo Prosposito received the M.Sc. in physics and the Ph.D. in physics from the University of Rome Tor Vergata, in 1992 and 1995, respectively. In 1997/98 he was visiting fellow at the Laboratory for Physical Chemistry of the University of Amsterdam. From 1998 to 2003 he was research fellow of the National Institute for Matter Physics (INFM). Since 2004 he is assistant professor at the Physics Department of the University of Rome Tor Vergata. He is author of more than 50 papers in refereed journals.