Journal of Colloid and Interface Science 445 (2015) 69–75
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Linear and nonlinear optical properties of functionalized CdSe quantum dots prepared by plasma sputtering and wet chemistry Christophe Humbert a, Abdellatif Dahi b, Laetitia Dalstein a, Bertrand Busson a, Marjorie Lismont b, Pierre Colson c, Laurent Dreesen b,⇑ a b c
Univ Paris-Sud, Laboratoire de Chimie-Physique, CNRS, Bâtiment 201P2, 91405 Orsay, France GRASP – Biophotonics, University of Liege, Institute of Physics, Allée du 6 août 17, 4000 Liège, Belgium GREEnMat – LCIS, University of Liege, Institute of Chemistry, Allée de la Chimie 3, 4000 Liège, Belgium
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 17 October 2014 Accepted 22 December 2014 Available online 31 December 2014 Keywords: Quantum dots Magnetron sputtering Optical properties Sum-frequency generation
a b s t r a c t We develop an innovative manufacturing process, based on radio-frequency magnetron sputtering (RFMS), to prepare neat CdSe quantum dots (QDs) on glass and silicon substrates and further chemically functionalize them. In order to validate the fabrication protocol, their optical properties are compared with those of QDs obtained from commercial solutions and deposited by wet chemistry on the substrates. Firstly, AFM measurements attest that nano-objects with a mean diameter around 13 nm are located on the substrate after RFMS treatment. Secondly, the UV–Vis absorption study of this deposited layer shows a specific optical absorption band, located at 550 nm, which is related to a discrete energy level of QDs. Thirdly, by using two-color sum-frequency generation (2C-SFG) nonlinear optical spectroscopy, we show experimentally the functionalization efficiency of the RFMS CdSe QDs layer with thiol derived molecules, which is not possible on the QDs layer prepared by wet chemistry due to the surfactant molecules from the native solution. Finally, 2C-SFG spectroscopy, performed at different visible wavelengths, highlights modifications of the vibration mode shape whatever the QDs deposition method, which is correlated to the discrete energy level of the QDs. Ó 2015 Elsevier Inc. All rights reserved.
1. Introduction
⇑ Corresponding author. E-mail address: [email protected] (L. Dreesen). http://dx.doi.org/10.1016/j.jcis.2014.12.075 0021-9797/Ó 2015 Elsevier Inc. All rights reserved.
Recent years have seen an increase in research works on quantum dots (QDs), i.e. semiconducting nanoparticles (NPs) characterized by a diameter around 10 nm and, consequently, exhibiting particular electronic and optical properties arising from quantum
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confinement [1]. Their specific electronic and optical properties can lead to new and interesting applications in various fields, such as catalysis, semiconductors industry, health care, pharmacology, medicine and biotechnologies [2–6]. For instance, QDs are used for fluorescence labeling instead of classical fluorophores due to their higher emission yield and stability. More particularly, this advantage is of great interest for FRET (Fluorescence Resonant Energy Transfer) technique and biosensor applications [7,8]. In the former case, QDs allow the study of molecular interaction with a better sensitivity while, in the latter case, they allow the detection of lower amounts of molecular species. Among the various II–VI semi-conductors, bulk CdSe is one of the most promising for these applications. The reason stems in its band gap located around 1.74 eV (710 nm) and therefore tunable in the visible spectral range if NPs are used instead of bulk semi-conductor [9]. Indeed, the nanometric size gives rise to localized energy levels whose energies depend on the QDs diameter [9]. Commercial CdSe QDs solutions are generally synthesized by techniques such as sol–gel and colloidal chemistries which require the use of toxic and volatile precursors and solvents [9–12]. Moreover, to prevent NPs aggregation and, therefore, the loss of their interesting optical properties, stabilizing agents or capping molecules are required. These generally complicate the functionalization process, especially when NPs need to be deposited on a substrate. As a consequence, the manufacturing of biosensors or molecular labels is compromised if this difficulty cannot be overcome. Physical vapor deposition (PVD) techniques such as radio-frequency magnetron sputtering (RFMS) do not suffer from the aforementioned limitations. Indeed, due to the physical nature of the process, the use of precursors and solvents is not required. Moreover, the method is performed under vacuum and with high purity targets resulting in low contamination of the deposited sample. Controlling the sputtering parameters, such as the pressure in the vacuum chamber, also allows the tuning of the NPs size and therefore of their optical properties [13]. RFMS was previously successfully used for the deposition of TiO2, Ge, Si, ZnO, GaAs, CuInSe2 and CdS QDs [13–22]. RFMS was also applied to the deposition of CdSe thin film and CdSe quantum dots embedded in SiO2 or in an organic matrix [23,24]. However, to the best of our knowledge, there is no work devoted to the preparation of layers of clean CdSe QDs on a solid surface using RFMS. As a consequence, the functionalization of such QDs layer by chemical species has also not been reported. To reach that goal, a characterization tool sensitive at the molecular layer level is needed. We therefore used two-color sum-frequency generation (2C-SFG) spectroscopy, a surface-sensitive nonlinear optical vibrational spectroscopy [25], to probe at the nanoscale and molecular scale the electronic and chemical properties of functionalized CdSe QDs as well as their potential coupling. This technique is, at present, successfully used to probe the surface vibrational properties of functionalized metal nanoparticles as shown in works mostly related to gold spherical (17 nm diameter) nanoparticles (AuNPs) [26]. Furthermore, it has been demonstrated that sum-frequency generation (SFG) can assign the geometrical orientation of organic ligands adsorbed on CdSe QDs synthesized by wet chemistry [27]. In the present paper, we firstly show with AFM and UV–Vis measurements that the RFMS deposition process of CdSe QDs is efficient on silicon and glass. Then, 2C-SFG spectroscopy is used to prove that thiol adsorption chemistry is directly and easily feasible on CdSe QDs prepared by RFMS. Finally, we use 2C-SFG spectroscopy to highlight a possible doubly resonant sum-frequency generation process (DRSFG), i.e. a coupling of the vibrational properties of an adsorbed molecular layer and the QDs electronic properties. The results obtained on RFMS samples are compared with the ones obtained by wet chemistry using commercial QDs solutions.
2. Materials and methods 2.1. CdSe quantum dots deposition and functionalization Glass (borosilicate) and Si (p-type, orientation: 0 0 1, resistivity: 4.6 X cm) substrates were utilized. Both were carefully rinsed in ultrasonic baths of acetone, ethanol and water before their use. Two methods of QDs deposition were used: radio-frequency magnetron sputtering (RFMS) and wet chemistry. In the former case, the distance between the target, CdSe (Ampere Industrie; diameter 33 mm; thickness 3 mm; purity 99.99%), and the substrate (Si or glass) was set at 15 cm. The base pressure of the plasma chamber was 10 4 Pa. The CdSe target was sputtered with RF plasma (15 W) of argon (99.99%) at 20 Pa. The deposition was performed at room temperature during 7.5 min for the samples analyzed here. When samples were prepared by wet chemistry, the substrates (Si or glass) were immersed in a 10 3 M CdSe quantum dots commercial solution in toluene (Ref. 662607, Lumidot™ CdSe 590, Sigma–Aldrich) during 72 h, then thoroughly rinsed with high purity water and dried. The Lumidot™ CdSe 590 QDs, named L590 throughout the paper, exhibit absorption and fluorescence bands at 570 and 590 nm, respectively [28]. For comparison, the QDs expected diameter size including the capping shell is around 8 nm as mentioned by the manufacturer [29]. For their functionalization, RFMS QDs substrates were immersed in a Dodecanethiol (DDT 98%, Sigma–Aldrich, 10 3 M) or in Thiophenol (TP 99%, Sigma–Aldrich, 10 3 M) solutions in ethanol during 18 h. Samples prepared by wet chemistry were immersed in Thiophenol solution (10 3 M) in ethanol only or used directly after the deposition of the commercial QDs. All samples were thoroughly rinsed, first in ethanol, then in high purity water (18 MX). Finally, they were dried under nitrogen flow before their characterization. 2.2. AFM AFM imaging was performed with a commercial STM/AFM (Multimode AFM equipped with a Nanoscope IIIa electronics, Digital Instruments, Santa-Barbara, CA) operating in the Tapping-Mode™ (TM-AFM). The AFM images were performed using a cantilever from BudgetSensors with a 75 kHz resonance frequency, a spring constant of 42 N/m and an apex radius of 7 nm as nominal values [30]. The background slope was resolved using first or second order polynomial functions. No further filtering was performed. 2.3. UV–Vis spectroscopy The UV–Vis transmission spectra on the glass substrates were recorded using a QE6500 Spectrometer from Ocean Optics and a deuterium tungsten-halogen light source. For the characterization of the CdSe QDs solutions, we used a cell with a 1 cm optical path. On solid substrates, the cell was removed and replaced by the glass samples. The UV–Vis linear absorbance spectra were recorded using a CARY 5000 spectrophotometer (Agilent) equipped with a specular reflectance accessory (UV–Vis VeeMax, Pike) set at 55° incidence angle for the light beam. The goal was to be in similar conditions as for the incident visible beam used in nonlinear SFG spectroscopy as detailed further. Each absorbance curve was obtained by differential measurements between the corresponding bare silicon wafer used as a reference and the two samples covered with QDs. 2.4. Sum-frequency generation vibrational spectroscopy The experimental setup used to perform nonlinear optical vibrational spectroscopy is detailed elsewhere [31]. Briefly, it is
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currently based on a Nd:YVO4 laser source at 1064 nm wavelength. After the shaping of its temporal structure and power amplification, it is sent through a rotating LiNBO3 crystal for one part to obtain an IR beam tunable in the 2.6–4.2 lm range. The other part, after conversion to a UV beam (355 nm) through b-Barium Borate (BBO) and Lithium Triborate (LBO) crystals, is sent through a rotating BBO crystal to obtain a visible tunable beam in the 440–710 nm range (i.e. from blue to red). A ppp-polarization scheme is established for the three beams (SFG, visible, IR). The IR and visible beams are mixed at the same point of the probed sample with incidence angles of 65° and 55° with respect to the z-normal direction to the sample surface, respectively (see Fig. S1 in supplementary information). As mentioned earlier, the visible beam has the same incidence angle as for the UV–Visible specular reflectance measurements in order to probe the QDs electronic properties in similar conditions for linear and nonlinear optical spectroscopy. Furthermore, as SFG is a spatial and temporal coherent process, this spectroscopy follows the energy (⁄xSFG = ⁄xvis + ⁄xir) and momentum (⁄kSFG = ⁄kvis + ⁄kir) conservation rules [25]. Two-color SFG measurements are performed on both samples in the same conditions by selecting 13 visible wavelengths (442, 488, 500, 514, 532, 550, 575, 590, 610, 650, 670, 690 and 710 nm) and by tuning the IR beam in the 3.3–3.6 lm spectral range. Therefore, SFG photons are collected by photomultipliers after spatial and spectral filtering through a monochromator. SFG sample spectra are normalized by the power of the IR and visible beams in order to compensate for laser fluctuations. SFG photons are generated by the enhancement of the nonlinear second order susceptibility of the probed interface as explained in literature [25]. In these conditions, it is possible to probe the QDs surface chemistry and to highlight the eventual coupling between the vibrational properties of the adsorbed molecules and the QDs electronic properties.
3. Results and discussion 3.1. AFM measurements AFM was used to analyze the topography and roughness of a bare silicon substrate (Fig. 1a), a silicon substrate covered by QDs deposited by wet chemistry (Fig. 1c and d) and a QDs-covered silicon substrate using RFMS (Fig. 1e and f). The comparison of Fig. 1a with 1c and e clearly attests the deposition of layers of CdSe QDs whatever the deposition method. For the sample prepared by wet chemistry, Fig. 1c and d shows that the CdSe QDs agglomerate forming clusters on the Si substrate. For the QDs prepared by RFMS, Fig. 1e and f highlight a relatively homogenous coverage of the surface by the QDs. Using a standard image analysis program (Image J), we evaluate the surface coverage by the nanoparticles at 25–30% of the overall surface assuming circular shape [32]. The surface roughness was calculated over 3 images (2.5 2.5 lm2). The measured root-mean-square roughnesses, defined as the average height deviations taken from the mean plane, are equal to 0.7, 6.25 and 3.7 nm for bare, wet chemistry and RFMS covered Si, respectively, confirming the deposition of a QDs layer. The higher value of the roughness associated to the sample prepared by wet chemistry is explained by the formation of clusters on the surface. Considering Fig. 1f, we measured the apparent NPs diameter mean value over 100 NPs. The histogram of the diameter distribution is reported in Fig. 1b and shows the diameter mean value is equal to 39 nm (assuming a Gaussian distribution) which is an overestimation due to the tip-particle interaction [33]. In order to correct this effect, we applied the tip size deconvolution process as explained in Ref. [33] and obtained a mean diameter value around 13 nm. Although slightly higher than the diameter (8 ± 0.8 nm with
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the surface ligand capping shell) of commercial CdSe QDs (Lumidot™ CdSe 590, Sigma–Aldrich) exhibiting similar optical absorption and used hereafter, this value attest the deposition of nano sized regular objects on the substrate. The growth mechanisms will be discussed in detail as a function of the RFMS parameters in a forthcoming paper where it will be shown that the QDs adopt dome-shaped structures similar to the ones reported by Samavati et al. for germanium quantum dots [18] as depicted in Fig. S1. In the present paper, we focus our attention on the optical properties of the sample in the linear and nonlinear regimes as on the possibility to functionalise them or not. 3.2. UV–Vis measurements on glass substrates The UV–Vis absorption spectra are reported in Fig. 2a–c for QDs deposited on glass substrates by RFMS, wet chemistry, and QDs in solution (solvent: toluene), respectively. Each spectrum exhibits an absorption band at 570 nm. The differences in the amplitudes of the absorption bands reflect the number of QDs probed by the UV–Vis beam, higher in solution and depending on the surface coverage for the glass samples. The origin of the bands lies in the localized energy states resulting from the nanometer size of the deposited objects, their semi-conducting nature and their separation [1]. The similarities in the spectral features between the measurements in solution and after deposition (from solution), and the samples manufactured by RFMS technique confirm the success of QDs layer deposition on the glass substrates. 3.3. UV–Vis measurements on silicon substrates The UV–Vis reflection spectra are shown in Fig. 3a and b for the QDs prepared on Si by RFMS and wet chemistry, respectively. Fig. 3a is characterized by two bands centered at 550 and 640 nm while Fig. 3b is dominated by features located around 560 and 630 nm. They are pointed out by arrows in Fig. 3. The lower wavelengths bands are similar to the ones observed on the QDs layer fabricated on glass and are therefore assigned to the QDs localized absorption energy level. They attest the deposition of a QDs layer on the Si substrate. The origin of the higher wavelength features remains unclear for the moment but is probably related to the substrate. Indeed, they are only observed for the deposition on Si substrates but not on glass ones. The full widths at half maximum of the lower wavelength (FWHM) features are typically 20 nm and 50 nm for the samples prepared by wet chemistry and RFMS, respectively. They reflect the nanoparticles size dispersion, higher for the RFMS sample as mentioned in the AFM section. The observation of bands related to the localized absorption energy level is a strong indication that the QDs are isolated from each other due to the quantum confinement [1]. In other words, for the sample prepared by wet chemistry, it means the capping molecules (Hexadecylamine, as enlightened by SFG spectroscopy in Section 3.4) prevent a direct contact in the agglomerates shown in Fig. 1c and d. For the layer deposited by RFMS, it means no CdSe film is present on the surface otherwise a continuous decrease of the UV–Vis absorbance with wavelength would have been observed instead of well resolved bands. 3.4. SFG measurements at fixed visible wavelength We use thiophenol (TP, C6H5SH) molecules to study the efficiency of the functionalization process of the QDs prepared by RFMS with respect to wet chemistry. Indeed, thiol molecules easily adsorb on various surfaces, including CdSe, and their spectral characteristics can be probed using sum-frequency generation spectroscopy [34–36]. Moreover, TP is characterized by a vibrational SFG resonance around 3060 cm 1 which is out of the CH2 and
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Fig. 1. AFM images of a bare (a) silicon substate, a CdSe layer prepared by wet chemistry on silicon (c, d) and a CdSe layer prepared by RFMS on silicon (e, f). (b) Is the histogram of the apparent diameters distribution of sample prepared by RFMS. a, c and e stand for 2.5 lm 2.5 lm images while d and f stand for 500 nm 500 nm images.
CH3 spectral range [36]. The SFG measurements on the TP/wet chemistry QDs/Si and TP/RFMS QDs/Si systems are reported on Fig. 4. The spectra are both acquired with the incident visible laser wavelength set at 575 nm. For IR wavelengths between 3000 and 3150 cm 1, no SFG vibrational feature is observed in the former spectrum while a band, located at 3063 cm 1, appears on the RFMS related curve. It is assigned to the specific CH stretching vibration mode of the TP molecule and therefore attests its grafting on the QDs prepared by RFMS [36]. This observation confirms that there are no polluting species on the substrate during the RFMS process. In the 2800–3000 cm 1 range, no vibration mode is observed for the sample prepared by RFMS while 5 vibration modes characterize the wet chemistry one. This spectral region can therefore be fitted, as explained in the supporting information (Formula S1 and S2, Table S1), using 5 vibration modes located at 2862, 2887, 2928, 2952 and 2975 cm 1. These ones are typical of symmetric
and asymmetric CH stretching vibration modes of the methyl and methylene groups of alkyl chains [35]. While a toluene (C7H8) contribution to the CH3 vibration cannot be excluded, they origin lies more probably in the capping hexadecylamine (HDA, CH3(CH2)14CH2NH2) molecules, used to prevent QDs aggregation in solution [29]. These chemical entities seems therefore not replaced by thiophenol during the functionalization process. As a matter of fact, we highlight a key problem when commercial QDs are used for deposition on a substrate: the replacement of the adsorbed stabilizing species by the molecules of interest. This problem is well-know and is solved by different procedures: multiple washing of the QDs, ligands exchange, use of conjugation chemistry [37]. The efficiencies of the different techniques are variable and these ones obviously complicate the preparation process. On the contrary, RFMS QDs samples are neat (at the exit of the vacuum PVD chamber) and therefore more easily chemically modified
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Fig. 4. SFG spectra of the QDs deposited by RFMS (circles) and wet chemistry (triangles) on Si substrates and functionalized by thiophenol molecules. Continuous lines are fits to the experimental data.
3.5. SFG measurements at various visible wavelengths
Fig. 2. UV–Vis transmission spectra of CdSe quantum dots deposited on glass substrates by RFMS (a), wet chemistry (b) and in solution (c). The numbers in parenthesis are the spectrum intensity multiplying factors. The zero of each curve is indicated by dashes on the right axis.
Fig. 3. UV–Vis reflection spectra of CdSe quantum dots deposited on Si substrates by RFMS (a) and by wet chemistry (b). The zero of each curve is indicated by dashes on the right axis.
by molecules possessing a strong affinity for CdSe such as thiol ones. This is really interesting for the development of biological sensors where the QDs surface needs to be easily and softly modified by probe molecules designed to specifically bind target species in solution.
As shown in Paragraphs 3.2 and 3.3, RFMS and wet chemistry deposited QDs are both characterized by a visible absorption band which is related to a discrete energy level. It has also been shown that the SFG signal of adsorbed species may be enhanced by selecting a visible laser wavelength matching a particular visible feature such as a molecular visible energy level [38,39] or a plasmon resonance [26,36]. In both cases, the signal increase and shape modifications are due to a coupling between the electronic properties of the molecule or the substrate and the vibrational properties of the absorbed species [26,36,38,39]. Now, we would like to assess the possibility of a similar coupling between QDs and adsorbed species which has not been reported till now. We therefore immersed RFMS QDs (on Si substrates) in Dodecanethiol (DDT, CH3(CH2)11SH) solutions and acquired SFG spectra at various visible laser wavelengths ranging between 442 and 710 nm. We used DDT instead of TP to functionalize the RFMS QDs sample because DDT gives a stable adsorbed layer, its alkane chain allows the formation of more ordered monolayer and the molecule gives well characterized SFG resonances [35]. The SFG spectra are shown in Fig. 5 for the RFMS (left panel) and the wet chemistry (right panel) samples, respectively. All the spectra are fitted, as explained in the supporting information (Formula S1 and S2), using 5 vibration modes located at 2862, 2887, 2928, 2952 and 2975 cm 1 (see Tables S2 and S3 of the supporting information for the values of the parameters giving the best fits) which are particularly distinguishable on the 575 nm curves. As explained in the previous paragraph, they are due to the HDA vibration modes for the sample prepared by wet chemistry. For the RFMS sample, these molecules being absent in the manufacturing process, the vibrational features are attributed to DDT. It is worth noting that the similar chemical nature of HDA and DDT in the alkane skeleton implies the presence of similar vibrational features in the 2800–3000 cm 1 spectral range. Anyway, our measurements of Fig. 5(left) attest the formation of DDT layer on the QDs surfaces and confirm the interest of thiol function as anchoring group. What is surprising in Fig. 5 for both samples is the evolution of the vibrational shapes with the visible wavelength. A theoretical model of the SFG signal evolution with the visible wavelength is out of the paper scope because it requires the knowledge of the CdSe QDs nonlinear susceptibility which is unknown at present. Nevertheless, we fit the SFG spectral features using the procedure described in the supporting information. We only provide here a qualitative discussion of the observed features (see Tables S2 and
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Fig. 5. 2C-SFG spectra of the QDs deposited by RFMS (left) and wet chemistry (right) on Si substrates. RFMS QDs are functionalized by DDT molecules. Continuous lines are fits to the experimental data. The zero of each curve is indicated by dashes on the left and right axes.
S3 of the supporting information for the values of the parameters giving the best fits). For the sample fabricated by RFMS, the resonances evolve from a derivative (Fano) shape at 442 nm toward a peak shape at 575 nm for finally recovering a Fano shape when the visible laser wavelength reaches 710 nm. The resonances amplitudes do not evolve significantly with the visible laser wavelength (see Table S3 of the supporting information for more details). For the sample prepared by wet chemistry, the spectral features are more drastically modified with the visible laser wavelength. For wavelengths between 442 and 575 nm, the resonances are also characterized by a Fano shape. They are well peak-shaped for wavelengths between 575 and 610 nm and a significant amplitude increase is also noticed. The resonances recover a Fano shape after 610 nm. It is worth noting that for the higher visible wavelengths the amplitude of the 2975 cm 1 vibration mode strongly increases. We correlate the above behavior to the electronic properties of the QDs layers and more precisely to the wavelength position and width of the main absorption band. Indeed, the shape is modified when the visible wavelength lies in the main absorption band centered around 550 and 560 nm for the RFMS and wet chemistry layers, respectively. The absorbance feature being narrower for the QDs prepared by wet chemistry with respect to RFMS (20 nm vs. 50 nm, see Fig. 3b), the SFG signal undergoes spectral changes in a more limited wavelength range for this sample. The particular behavior of the highest wavenumber vibrational mode at 690 and 710 nm observed in Fig. 5, right panel, is indicative of a DRSFG process [38]. Indeed, the SFG wavelength is around 570 nm in both cases which is close to the maximum of the associated QDs main absorption band on Si. Systematic SFG measurements at higher incident visible wavelengths (>710 nm) should be performed to validate quantitatively this effect but, at this stage, experimental limitations in our SFG setup have to be overcome to reach that goal in future works.
4. Conclusion We showed, using AFM and UV–Vis spectroscopies, the possibility to deposit CdSe quantum dots on glass and silicon substrates using radio-frequency magnetron sputtering technology. The electronic properties are similar to the ones obtained on QDs prepared by wet chemistry using commercial quantum dots solution. The efficiency of the functionalization process by thiol molecules was evaluated using sum-frequency generation spectroscopy. While surfactant/capping molecules prevented adsorption on the QDs surface prepared by wet chemistry, the nonlinear measurements showed the presence of the thiol molecules on the QDs fabricated by RFMS. This result is really important for the development of devices, such as biosensors, where a precise and versatile control of the surface properties is required. Finally, with 2C-SFG measurements, we highlighted drastic modification of the SFG optical response when it is energy-correlated to the QDs discrete energy absorption level. Acknowledgments Research leading to these results has received funding from the PHC Tournesol Franco-Belge (Project N°22541VD) and was supported by the Région Île-de-France in the framework of the funding program C’Nano IdF under Grant agreement CREMOSOFT. This work was also supported by the ‘‘Fonds de la Recherche Scientifique – FRS’’, the University of Liège (Projects N°: D-11/10 and FSRC-14/03) and by ‘‘Wallonie-Bruxelles International’’ (PHC Tournesol 2010 and 2011). We thank Dr. C. Volcke for his help in the AFM interpretation and Prof. Rudi Cloots for the access to AFM facilities (University of Liege – Microscopy Research and Teaching Support Unit (CAREl)).
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Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2014.12.075.
References [1] L. Jacak, P. Hawrylak, A. Wojs, Quantum Dots (Nanoscience and Nanotechnology), Springer, Heidelberg, Germany, 1998. [2] X. Michalet, F.F. Pinaud, L.A. Bentolila, J.M. Tsay, S. Doose, J.J. Li, G. Sundaresan, A.M. Wu, S.S. Gambhir, S. Weiss, Science 307 (2005) 538. [3] P. Juzenas, W. Chen, Y.-P. Sun, M.A.N. Coelho, R. Generalov, N. Generalova, I.L. Christensen, Adv. Drug Delivery Rev. 60 (2008) 1600. [4] Y. Lu, J. Liu, Wiley Interdiscip. Rev. Nanomed. Nanobiotechnol. 1 (2009) 35. [5] P. Zrazhevskiv, M. Sena, X. Gao, Chem. Soc. Rev. 39 (2010) 4326. [6] Q. Dai, C.E. Duty, M.Z. Hu, Small 6 (2010) 1577. [7] W.R. Algar, U.J. Krull, Anal. Bioanal. Chem. 391 (2008) 1609. [8] U. Resch-Genger, M. Grabolle, S. Cavaliere-Jaricot, R. Nitschke, T. Nann, Nat. Methods 5 (2008) 763. [9] V. Biju, T. Itoh, A. Anas, A. Sujith, M. Ishikawa, Anal. Bioanal. Chem. 391 (2008) 24690. [10] M.D. Regulacio, M.-Y. Han, Acc. Chem. Res. 43 (2010) 621. [11] O. Obonyo, E. Fisher, M. Edwards, D. Douroumis, Crit. Rev. Biotechnol. 30 (2010) 283. [12] K. Grieve, P. Mulvaney, F. Grieser, Curr. Opin. Colloid Interface Sci. 5 (2000) 168. [13] L. Dreesen, F. Cecchet, S. Lucas, Plasma Process. Polym. 6 (2009) S849. [14] M. Shiratani, G. Uchida, H.W. Seo, D. Ichida, K. Koga, N. Itagaki, K. Kamataki, Adv. Mater. Res. 783–786 (2014) 2022. [15] A Samavati, Z. Othaman, S.K. Ghoshal, M.R. Dousti, J. Luminesc. 154 (2014) 51. [16] F. Aldaw Idrees, S. Sakrani, Z. Othaman, Adv. Mater. Res. 795 (2013) 721. [17] J.H. Sun, H.C. Kang, TSF 518 (2010) 6522. [18] A. Samavati, Z. Othaman, S.K. Ghoshal, M.R. Dousti, R.J. Amjad, Int. J. Mol. Sci. Chin. Phys. Lett. 29 (2012) 118101.
75
[19] S. Lee, D.W. Shin, W.M. Kim, B.-K. Cheong, T.S. Lee, K.S. Lee, S. Cho, TSF 514 (2006) 296. [20] K.-K. Kim, N. Koguchi, Y.-W. Ok, T.-Y. Seong, S.-J. Park, Appl. Phys. Lett. 19 (2004) 3810. [21] S. Huang, Z. Dai, F. Qu, L. Zhang, X. Zhu, Nanotechnology 13 (2002) 691. [22] A.G. Rolo, O. Conde, M.J.M. Gomes, TSF 318 (1998) 108. [23] S. Levichev, A. Chahboun, A.G. Rolo, O. Conde, M.J.M. Gomes, Thin Solid Films 517 (2009) 2538. [24] E. Campos-González, P. Rodríguez-Fragoso, G. Gonzalez de la Cruz, J. SantoyoSalazar, O. Zelaya-Angel, J. Cryst. Growth 338 (2012) 251. [25] C. Humbert, A. Tadjeddine, B. Busson, J. Phys. Chem. Lett. 2 (2011) 2770. [26] C. Humbert, O. Pluchery, E. Lacaze, A. Tadjeddine, B. Busson, PCCP 14 (2012) 280. [27] M.T. Frederick, J.L. Achtyl, K.E. Knowles, E.A. Weiss, F.M. Geiger, J. Am. Chem. Soc. 133 (2011) 7476. [28] Lumidot™ CdSe 590, (accessed June 2014). [29] Lumidot™ CdSe-6 quantum dot nanoparticles kit, (accessed June 2014). [30] AFM probe, (accessed December 2014). [31] C. Humbert, B. Busson, C. Six, A. Gayral, M. Gruselle, F. Villain, A. Tadjeddine, J. Electroanal. Chem. 621 (2008) 314. [32] M. Haidopoulos, F. Mirabella, M. Horgnies, C. Volcke, P.A. Thiry, P. Rouxhet, J.-J. Pireaux, J. Microsc. 228 (2007) 227. [33] M. Rasa, B.W.M. Kuipers, A.P. Philipse, J. Colloid Interface Sci. 250 (2002) 303. [34] F. Dubois, B. Mahler, B. Dubertret, E. Doris, C. Mioskowski, J. Am. Chem. Soc. 129 (2007) 482. [35] L. Dreesen, C. Humbert, M. Celebi, J.-J. Lemaire, A.A. Mani, P.A. Thiry, A. Peremans, Appl. Phys. B 74 (2002) 621. [36] C. Humbert, O. Pluchery, E. Lacaze, A. Tadjeddine, B. Busson, Gold Bull. 46 (2013) 299. [37] R.A. Sperling, W.J. Parak, Phil. Trans. R. Soc. A 368 (2010) 1333. [38] L. Dreesen, C. Humbert, Y. Sartenaer, Y. Caudano, C. Volcke, A.A. Mani, A. Peremans, P.A. Thiry, S. Hanique, J.M. Frere, Langmuir 20 (2004) 7201. [39] K.C. Chou, S. Westerberg, Y.R. Shen, P.N. Ross, G.A. Somorjai, Phys. Rev. B 69 (2004) 153413.
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Linear and nonlinear optical properties of functionalized CdSe quantum dots prepared by plasma sputtering and wet chemistry Christophe Humbert a, Abdellatif Dahi b, Laetitia Dalstein a, Bertrand Busson a, Marjorie Lismont b, Pierre Colson c, Laurent Dreesen b,⇑ a b c
Univ Paris-Sud, Laboratoire de Chimie-Physique, CNRS, Bâtiment 201P2, 91405 Orsay, France GRASP – Biophotonics, University of Liege, Institute of Physics, Allée du 6 août 17, 4000 Liège, Belgium GREEnMat – LCIS, University of Liege, Institute of Chemistry, Allée de la Chimie 3, 4000 Liège, Belgium
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 17 October 2014 Accepted 22 December 2014 Available online 31 December 2014 Keywords: Quantum dots Magnetron sputtering Optical properties Sum-frequency generation
a b s t r a c t We develop an innovative manufacturing process, based on radio-frequency magnetron sputtering (RFMS), to prepare neat CdSe quantum dots (QDs) on glass and silicon substrates and further chemically functionalize them. In order to validate the fabrication protocol, their optical properties are compared with those of QDs obtained from commercial solutions and deposited by wet chemistry on the substrates. Firstly, AFM measurements attest that nano-objects with a mean diameter around 13 nm are located on the substrate after RFMS treatment. Secondly, the UV–Vis absorption study of this deposited layer shows a specific optical absorption band, located at 550 nm, which is related to a discrete energy level of QDs. Thirdly, by using two-color sum-frequency generation (2C-SFG) nonlinear optical spectroscopy, we show experimentally the functionalization efficiency of the RFMS CdSe QDs layer with thiol derived molecules, which is not possible on the QDs layer prepared by wet chemistry due to the surfactant molecules from the native solution. Finally, 2C-SFG spectroscopy, performed at different visible wavelengths, highlights modifications of the vibration mode shape whatever the QDs deposition method, which is correlated to the discrete energy level of the QDs. Ó 2015 Elsevier Inc. All rights reserved.
1. Introduction
⇑ Corresponding author. E-mail address: [email protected] (L. Dreesen). http://dx.doi.org/10.1016/j.jcis.2014.12.075 0021-9797/Ó 2015 Elsevier Inc. All rights reserved.
Recent years have seen an increase in research works on quantum dots (QDs), i.e. semiconducting nanoparticles (NPs) characterized by a diameter around 10 nm and, consequently, exhibiting particular electronic and optical properties arising from quantum
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confinement [1]. Their specific electronic and optical properties can lead to new and interesting applications in various fields, such as catalysis, semiconductors industry, health care, pharmacology, medicine and biotechnologies [2–6]. For instance, QDs are used for fluorescence labeling instead of classical fluorophores due to their higher emission yield and stability. More particularly, this advantage is of great interest for FRET (Fluorescence Resonant Energy Transfer) technique and biosensor applications [7,8]. In the former case, QDs allow the study of molecular interaction with a better sensitivity while, in the latter case, they allow the detection of lower amounts of molecular species. Among the various II–VI semi-conductors, bulk CdSe is one of the most promising for these applications. The reason stems in its band gap located around 1.74 eV (710 nm) and therefore tunable in the visible spectral range if NPs are used instead of bulk semi-conductor [9]. Indeed, the nanometric size gives rise to localized energy levels whose energies depend on the QDs diameter [9]. Commercial CdSe QDs solutions are generally synthesized by techniques such as sol–gel and colloidal chemistries which require the use of toxic and volatile precursors and solvents [9–12]. Moreover, to prevent NPs aggregation and, therefore, the loss of their interesting optical properties, stabilizing agents or capping molecules are required. These generally complicate the functionalization process, especially when NPs need to be deposited on a substrate. As a consequence, the manufacturing of biosensors or molecular labels is compromised if this difficulty cannot be overcome. Physical vapor deposition (PVD) techniques such as radio-frequency magnetron sputtering (RFMS) do not suffer from the aforementioned limitations. Indeed, due to the physical nature of the process, the use of precursors and solvents is not required. Moreover, the method is performed under vacuum and with high purity targets resulting in low contamination of the deposited sample. Controlling the sputtering parameters, such as the pressure in the vacuum chamber, also allows the tuning of the NPs size and therefore of their optical properties [13]. RFMS was previously successfully used for the deposition of TiO2, Ge, Si, ZnO, GaAs, CuInSe2 and CdS QDs [13–22]. RFMS was also applied to the deposition of CdSe thin film and CdSe quantum dots embedded in SiO2 or in an organic matrix [23,24]. However, to the best of our knowledge, there is no work devoted to the preparation of layers of clean CdSe QDs on a solid surface using RFMS. As a consequence, the functionalization of such QDs layer by chemical species has also not been reported. To reach that goal, a characterization tool sensitive at the molecular layer level is needed. We therefore used two-color sum-frequency generation (2C-SFG) spectroscopy, a surface-sensitive nonlinear optical vibrational spectroscopy [25], to probe at the nanoscale and molecular scale the electronic and chemical properties of functionalized CdSe QDs as well as their potential coupling. This technique is, at present, successfully used to probe the surface vibrational properties of functionalized metal nanoparticles as shown in works mostly related to gold spherical (17 nm diameter) nanoparticles (AuNPs) [26]. Furthermore, it has been demonstrated that sum-frequency generation (SFG) can assign the geometrical orientation of organic ligands adsorbed on CdSe QDs synthesized by wet chemistry [27]. In the present paper, we firstly show with AFM and UV–Vis measurements that the RFMS deposition process of CdSe QDs is efficient on silicon and glass. Then, 2C-SFG spectroscopy is used to prove that thiol adsorption chemistry is directly and easily feasible on CdSe QDs prepared by RFMS. Finally, we use 2C-SFG spectroscopy to highlight a possible doubly resonant sum-frequency generation process (DRSFG), i.e. a coupling of the vibrational properties of an adsorbed molecular layer and the QDs electronic properties. The results obtained on RFMS samples are compared with the ones obtained by wet chemistry using commercial QDs solutions.
2. Materials and methods 2.1. CdSe quantum dots deposition and functionalization Glass (borosilicate) and Si (p-type, orientation: 0 0 1, resistivity: 4.6 X cm) substrates were utilized. Both were carefully rinsed in ultrasonic baths of acetone, ethanol and water before their use. Two methods of QDs deposition were used: radio-frequency magnetron sputtering (RFMS) and wet chemistry. In the former case, the distance between the target, CdSe (Ampere Industrie; diameter 33 mm; thickness 3 mm; purity 99.99%), and the substrate (Si or glass) was set at 15 cm. The base pressure of the plasma chamber was 10 4 Pa. The CdSe target was sputtered with RF plasma (15 W) of argon (99.99%) at 20 Pa. The deposition was performed at room temperature during 7.5 min for the samples analyzed here. When samples were prepared by wet chemistry, the substrates (Si or glass) were immersed in a 10 3 M CdSe quantum dots commercial solution in toluene (Ref. 662607, Lumidot™ CdSe 590, Sigma–Aldrich) during 72 h, then thoroughly rinsed with high purity water and dried. The Lumidot™ CdSe 590 QDs, named L590 throughout the paper, exhibit absorption and fluorescence bands at 570 and 590 nm, respectively [28]. For comparison, the QDs expected diameter size including the capping shell is around 8 nm as mentioned by the manufacturer [29]. For their functionalization, RFMS QDs substrates were immersed in a Dodecanethiol (DDT 98%, Sigma–Aldrich, 10 3 M) or in Thiophenol (TP 99%, Sigma–Aldrich, 10 3 M) solutions in ethanol during 18 h. Samples prepared by wet chemistry were immersed in Thiophenol solution (10 3 M) in ethanol only or used directly after the deposition of the commercial QDs. All samples were thoroughly rinsed, first in ethanol, then in high purity water (18 MX). Finally, they were dried under nitrogen flow before their characterization. 2.2. AFM AFM imaging was performed with a commercial STM/AFM (Multimode AFM equipped with a Nanoscope IIIa electronics, Digital Instruments, Santa-Barbara, CA) operating in the Tapping-Mode™ (TM-AFM). The AFM images were performed using a cantilever from BudgetSensors with a 75 kHz resonance frequency, a spring constant of 42 N/m and an apex radius of 7 nm as nominal values [30]. The background slope was resolved using first or second order polynomial functions. No further filtering was performed. 2.3. UV–Vis spectroscopy The UV–Vis transmission spectra on the glass substrates were recorded using a QE6500 Spectrometer from Ocean Optics and a deuterium tungsten-halogen light source. For the characterization of the CdSe QDs solutions, we used a cell with a 1 cm optical path. On solid substrates, the cell was removed and replaced by the glass samples. The UV–Vis linear absorbance spectra were recorded using a CARY 5000 spectrophotometer (Agilent) equipped with a specular reflectance accessory (UV–Vis VeeMax, Pike) set at 55° incidence angle for the light beam. The goal was to be in similar conditions as for the incident visible beam used in nonlinear SFG spectroscopy as detailed further. Each absorbance curve was obtained by differential measurements between the corresponding bare silicon wafer used as a reference and the two samples covered with QDs. 2.4. Sum-frequency generation vibrational spectroscopy The experimental setup used to perform nonlinear optical vibrational spectroscopy is detailed elsewhere [31]. Briefly, it is
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currently based on a Nd:YVO4 laser source at 1064 nm wavelength. After the shaping of its temporal structure and power amplification, it is sent through a rotating LiNBO3 crystal for one part to obtain an IR beam tunable in the 2.6–4.2 lm range. The other part, after conversion to a UV beam (355 nm) through b-Barium Borate (BBO) and Lithium Triborate (LBO) crystals, is sent through a rotating BBO crystal to obtain a visible tunable beam in the 440–710 nm range (i.e. from blue to red). A ppp-polarization scheme is established for the three beams (SFG, visible, IR). The IR and visible beams are mixed at the same point of the probed sample with incidence angles of 65° and 55° with respect to the z-normal direction to the sample surface, respectively (see Fig. S1 in supplementary information). As mentioned earlier, the visible beam has the same incidence angle as for the UV–Visible specular reflectance measurements in order to probe the QDs electronic properties in similar conditions for linear and nonlinear optical spectroscopy. Furthermore, as SFG is a spatial and temporal coherent process, this spectroscopy follows the energy (⁄xSFG = ⁄xvis + ⁄xir) and momentum (⁄kSFG = ⁄kvis + ⁄kir) conservation rules [25]. Two-color SFG measurements are performed on both samples in the same conditions by selecting 13 visible wavelengths (442, 488, 500, 514, 532, 550, 575, 590, 610, 650, 670, 690 and 710 nm) and by tuning the IR beam in the 3.3–3.6 lm spectral range. Therefore, SFG photons are collected by photomultipliers after spatial and spectral filtering through a monochromator. SFG sample spectra are normalized by the power of the IR and visible beams in order to compensate for laser fluctuations. SFG photons are generated by the enhancement of the nonlinear second order susceptibility of the probed interface as explained in literature [25]. In these conditions, it is possible to probe the QDs surface chemistry and to highlight the eventual coupling between the vibrational properties of the adsorbed molecules and the QDs electronic properties.
3. Results and discussion 3.1. AFM measurements AFM was used to analyze the topography and roughness of a bare silicon substrate (Fig. 1a), a silicon substrate covered by QDs deposited by wet chemistry (Fig. 1c and d) and a QDs-covered silicon substrate using RFMS (Fig. 1e and f). The comparison of Fig. 1a with 1c and e clearly attests the deposition of layers of CdSe QDs whatever the deposition method. For the sample prepared by wet chemistry, Fig. 1c and d shows that the CdSe QDs agglomerate forming clusters on the Si substrate. For the QDs prepared by RFMS, Fig. 1e and f highlight a relatively homogenous coverage of the surface by the QDs. Using a standard image analysis program (Image J), we evaluate the surface coverage by the nanoparticles at 25–30% of the overall surface assuming circular shape [32]. The surface roughness was calculated over 3 images (2.5 2.5 lm2). The measured root-mean-square roughnesses, defined as the average height deviations taken from the mean plane, are equal to 0.7, 6.25 and 3.7 nm for bare, wet chemistry and RFMS covered Si, respectively, confirming the deposition of a QDs layer. The higher value of the roughness associated to the sample prepared by wet chemistry is explained by the formation of clusters on the surface. Considering Fig. 1f, we measured the apparent NPs diameter mean value over 100 NPs. The histogram of the diameter distribution is reported in Fig. 1b and shows the diameter mean value is equal to 39 nm (assuming a Gaussian distribution) which is an overestimation due to the tip-particle interaction [33]. In order to correct this effect, we applied the tip size deconvolution process as explained in Ref. [33] and obtained a mean diameter value around 13 nm. Although slightly higher than the diameter (8 ± 0.8 nm with
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the surface ligand capping shell) of commercial CdSe QDs (Lumidot™ CdSe 590, Sigma–Aldrich) exhibiting similar optical absorption and used hereafter, this value attest the deposition of nano sized regular objects on the substrate. The growth mechanisms will be discussed in detail as a function of the RFMS parameters in a forthcoming paper where it will be shown that the QDs adopt dome-shaped structures similar to the ones reported by Samavati et al. for germanium quantum dots [18] as depicted in Fig. S1. In the present paper, we focus our attention on the optical properties of the sample in the linear and nonlinear regimes as on the possibility to functionalise them or not. 3.2. UV–Vis measurements on glass substrates The UV–Vis absorption spectra are reported in Fig. 2a–c for QDs deposited on glass substrates by RFMS, wet chemistry, and QDs in solution (solvent: toluene), respectively. Each spectrum exhibits an absorption band at 570 nm. The differences in the amplitudes of the absorption bands reflect the number of QDs probed by the UV–Vis beam, higher in solution and depending on the surface coverage for the glass samples. The origin of the bands lies in the localized energy states resulting from the nanometer size of the deposited objects, their semi-conducting nature and their separation [1]. The similarities in the spectral features between the measurements in solution and after deposition (from solution), and the samples manufactured by RFMS technique confirm the success of QDs layer deposition on the glass substrates. 3.3. UV–Vis measurements on silicon substrates The UV–Vis reflection spectra are shown in Fig. 3a and b for the QDs prepared on Si by RFMS and wet chemistry, respectively. Fig. 3a is characterized by two bands centered at 550 and 640 nm while Fig. 3b is dominated by features located around 560 and 630 nm. They are pointed out by arrows in Fig. 3. The lower wavelengths bands are similar to the ones observed on the QDs layer fabricated on glass and are therefore assigned to the QDs localized absorption energy level. They attest the deposition of a QDs layer on the Si substrate. The origin of the higher wavelength features remains unclear for the moment but is probably related to the substrate. Indeed, they are only observed for the deposition on Si substrates but not on glass ones. The full widths at half maximum of the lower wavelength (FWHM) features are typically 20 nm and 50 nm for the samples prepared by wet chemistry and RFMS, respectively. They reflect the nanoparticles size dispersion, higher for the RFMS sample as mentioned in the AFM section. The observation of bands related to the localized absorption energy level is a strong indication that the QDs are isolated from each other due to the quantum confinement [1]. In other words, for the sample prepared by wet chemistry, it means the capping molecules (Hexadecylamine, as enlightened by SFG spectroscopy in Section 3.4) prevent a direct contact in the agglomerates shown in Fig. 1c and d. For the layer deposited by RFMS, it means no CdSe film is present on the surface otherwise a continuous decrease of the UV–Vis absorbance with wavelength would have been observed instead of well resolved bands. 3.4. SFG measurements at fixed visible wavelength We use thiophenol (TP, C6H5SH) molecules to study the efficiency of the functionalization process of the QDs prepared by RFMS with respect to wet chemistry. Indeed, thiol molecules easily adsorb on various surfaces, including CdSe, and their spectral characteristics can be probed using sum-frequency generation spectroscopy [34–36]. Moreover, TP is characterized by a vibrational SFG resonance around 3060 cm 1 which is out of the CH2 and
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Fig. 1. AFM images of a bare (a) silicon substate, a CdSe layer prepared by wet chemistry on silicon (c, d) and a CdSe layer prepared by RFMS on silicon (e, f). (b) Is the histogram of the apparent diameters distribution of sample prepared by RFMS. a, c and e stand for 2.5 lm 2.5 lm images while d and f stand for 500 nm 500 nm images.
CH3 spectral range [36]. The SFG measurements on the TP/wet chemistry QDs/Si and TP/RFMS QDs/Si systems are reported on Fig. 4. The spectra are both acquired with the incident visible laser wavelength set at 575 nm. For IR wavelengths between 3000 and 3150 cm 1, no SFG vibrational feature is observed in the former spectrum while a band, located at 3063 cm 1, appears on the RFMS related curve. It is assigned to the specific CH stretching vibration mode of the TP molecule and therefore attests its grafting on the QDs prepared by RFMS [36]. This observation confirms that there are no polluting species on the substrate during the RFMS process. In the 2800–3000 cm 1 range, no vibration mode is observed for the sample prepared by RFMS while 5 vibration modes characterize the wet chemistry one. This spectral region can therefore be fitted, as explained in the supporting information (Formula S1 and S2, Table S1), using 5 vibration modes located at 2862, 2887, 2928, 2952 and 2975 cm 1. These ones are typical of symmetric
and asymmetric CH stretching vibration modes of the methyl and methylene groups of alkyl chains [35]. While a toluene (C7H8) contribution to the CH3 vibration cannot be excluded, they origin lies more probably in the capping hexadecylamine (HDA, CH3(CH2)14CH2NH2) molecules, used to prevent QDs aggregation in solution [29]. These chemical entities seems therefore not replaced by thiophenol during the functionalization process. As a matter of fact, we highlight a key problem when commercial QDs are used for deposition on a substrate: the replacement of the adsorbed stabilizing species by the molecules of interest. This problem is well-know and is solved by different procedures: multiple washing of the QDs, ligands exchange, use of conjugation chemistry [37]. The efficiencies of the different techniques are variable and these ones obviously complicate the preparation process. On the contrary, RFMS QDs samples are neat (at the exit of the vacuum PVD chamber) and therefore more easily chemically modified
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Fig. 4. SFG spectra of the QDs deposited by RFMS (circles) and wet chemistry (triangles) on Si substrates and functionalized by thiophenol molecules. Continuous lines are fits to the experimental data.
3.5. SFG measurements at various visible wavelengths
Fig. 2. UV–Vis transmission spectra of CdSe quantum dots deposited on glass substrates by RFMS (a), wet chemistry (b) and in solution (c). The numbers in parenthesis are the spectrum intensity multiplying factors. The zero of each curve is indicated by dashes on the right axis.
Fig. 3. UV–Vis reflection spectra of CdSe quantum dots deposited on Si substrates by RFMS (a) and by wet chemistry (b). The zero of each curve is indicated by dashes on the right axis.
by molecules possessing a strong affinity for CdSe such as thiol ones. This is really interesting for the development of biological sensors where the QDs surface needs to be easily and softly modified by probe molecules designed to specifically bind target species in solution.
As shown in Paragraphs 3.2 and 3.3, RFMS and wet chemistry deposited QDs are both characterized by a visible absorption band which is related to a discrete energy level. It has also been shown that the SFG signal of adsorbed species may be enhanced by selecting a visible laser wavelength matching a particular visible feature such as a molecular visible energy level [38,39] or a plasmon resonance [26,36]. In both cases, the signal increase and shape modifications are due to a coupling between the electronic properties of the molecule or the substrate and the vibrational properties of the absorbed species [26,36,38,39]. Now, we would like to assess the possibility of a similar coupling between QDs and adsorbed species which has not been reported till now. We therefore immersed RFMS QDs (on Si substrates) in Dodecanethiol (DDT, CH3(CH2)11SH) solutions and acquired SFG spectra at various visible laser wavelengths ranging between 442 and 710 nm. We used DDT instead of TP to functionalize the RFMS QDs sample because DDT gives a stable adsorbed layer, its alkane chain allows the formation of more ordered monolayer and the molecule gives well characterized SFG resonances [35]. The SFG spectra are shown in Fig. 5 for the RFMS (left panel) and the wet chemistry (right panel) samples, respectively. All the spectra are fitted, as explained in the supporting information (Formula S1 and S2), using 5 vibration modes located at 2862, 2887, 2928, 2952 and 2975 cm 1 (see Tables S2 and S3 of the supporting information for the values of the parameters giving the best fits) which are particularly distinguishable on the 575 nm curves. As explained in the previous paragraph, they are due to the HDA vibration modes for the sample prepared by wet chemistry. For the RFMS sample, these molecules being absent in the manufacturing process, the vibrational features are attributed to DDT. It is worth noting that the similar chemical nature of HDA and DDT in the alkane skeleton implies the presence of similar vibrational features in the 2800–3000 cm 1 spectral range. Anyway, our measurements of Fig. 5(left) attest the formation of DDT layer on the QDs surfaces and confirm the interest of thiol function as anchoring group. What is surprising in Fig. 5 for both samples is the evolution of the vibrational shapes with the visible wavelength. A theoretical model of the SFG signal evolution with the visible wavelength is out of the paper scope because it requires the knowledge of the CdSe QDs nonlinear susceptibility which is unknown at present. Nevertheless, we fit the SFG spectral features using the procedure described in the supporting information. We only provide here a qualitative discussion of the observed features (see Tables S2 and
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Fig. 5. 2C-SFG spectra of the QDs deposited by RFMS (left) and wet chemistry (right) on Si substrates. RFMS QDs are functionalized by DDT molecules. Continuous lines are fits to the experimental data. The zero of each curve is indicated by dashes on the left and right axes.
S3 of the supporting information for the values of the parameters giving the best fits). For the sample fabricated by RFMS, the resonances evolve from a derivative (Fano) shape at 442 nm toward a peak shape at 575 nm for finally recovering a Fano shape when the visible laser wavelength reaches 710 nm. The resonances amplitudes do not evolve significantly with the visible laser wavelength (see Table S3 of the supporting information for more details). For the sample prepared by wet chemistry, the spectral features are more drastically modified with the visible laser wavelength. For wavelengths between 442 and 575 nm, the resonances are also characterized by a Fano shape. They are well peak-shaped for wavelengths between 575 and 610 nm and a significant amplitude increase is also noticed. The resonances recover a Fano shape after 610 nm. It is worth noting that for the higher visible wavelengths the amplitude of the 2975 cm 1 vibration mode strongly increases. We correlate the above behavior to the electronic properties of the QDs layers and more precisely to the wavelength position and width of the main absorption band. Indeed, the shape is modified when the visible wavelength lies in the main absorption band centered around 550 and 560 nm for the RFMS and wet chemistry layers, respectively. The absorbance feature being narrower for the QDs prepared by wet chemistry with respect to RFMS (20 nm vs. 50 nm, see Fig. 3b), the SFG signal undergoes spectral changes in a more limited wavelength range for this sample. The particular behavior of the highest wavenumber vibrational mode at 690 and 710 nm observed in Fig. 5, right panel, is indicative of a DRSFG process [38]. Indeed, the SFG wavelength is around 570 nm in both cases which is close to the maximum of the associated QDs main absorption band on Si. Systematic SFG measurements at higher incident visible wavelengths (>710 nm) should be performed to validate quantitatively this effect but, at this stage, experimental limitations in our SFG setup have to be overcome to reach that goal in future works.
4. Conclusion We showed, using AFM and UV–Vis spectroscopies, the possibility to deposit CdSe quantum dots on glass and silicon substrates using radio-frequency magnetron sputtering technology. The electronic properties are similar to the ones obtained on QDs prepared by wet chemistry using commercial quantum dots solution. The efficiency of the functionalization process by thiol molecules was evaluated using sum-frequency generation spectroscopy. While surfactant/capping molecules prevented adsorption on the QDs surface prepared by wet chemistry, the nonlinear measurements showed the presence of the thiol molecules on the QDs fabricated by RFMS. This result is really important for the development of devices, such as biosensors, where a precise and versatile control of the surface properties is required. Finally, with 2C-SFG measurements, we highlighted drastic modification of the SFG optical response when it is energy-correlated to the QDs discrete energy absorption level. Acknowledgments Research leading to these results has received funding from the PHC Tournesol Franco-Belge (Project N°22541VD) and was supported by the Région Île-de-France in the framework of the funding program C’Nano IdF under Grant agreement CREMOSOFT. This work was also supported by the ‘‘Fonds de la Recherche Scientifique – FRS’’, the University of Liège (Projects N°: D-11/10 and FSRC-14/03) and by ‘‘Wallonie-Bruxelles International’’ (PHC Tournesol 2010 and 2011). We thank Dr. C. Volcke for his help in the AFM interpretation and Prof. Rudi Cloots for the access to AFM facilities (University of Liege – Microscopy Research and Teaching Support Unit (CAREl)).
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Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2014.12.075.
References [1] L. Jacak, P. Hawrylak, A. Wojs, Quantum Dots (Nanoscience and Nanotechnology), Springer, Heidelberg, Germany, 1998. [2] X. Michalet, F.F. Pinaud, L.A. Bentolila, J.M. Tsay, S. Doose, J.J. Li, G. Sundaresan, A.M. Wu, S.S. Gambhir, S. Weiss, Science 307 (2005) 538. [3] P. Juzenas, W. Chen, Y.-P. Sun, M.A.N. Coelho, R. Generalov, N. Generalova, I.L. Christensen, Adv. Drug Delivery Rev. 60 (2008) 1600. [4] Y. Lu, J. Liu, Wiley Interdiscip. Rev. Nanomed. Nanobiotechnol. 1 (2009) 35. [5] P. Zrazhevskiv, M. Sena, X. Gao, Chem. Soc. Rev. 39 (2010) 4326. [6] Q. Dai, C.E. Duty, M.Z. Hu, Small 6 (2010) 1577. [7] W.R. Algar, U.J. Krull, Anal. Bioanal. Chem. 391 (2008) 1609. [8] U. Resch-Genger, M. Grabolle, S. Cavaliere-Jaricot, R. Nitschke, T. Nann, Nat. Methods 5 (2008) 763. [9] V. Biju, T. Itoh, A. Anas, A. Sujith, M. Ishikawa, Anal. Bioanal. Chem. 391 (2008) 24690. [10] M.D. Regulacio, M.-Y. Han, Acc. Chem. Res. 43 (2010) 621. [11] O. Obonyo, E. Fisher, M. Edwards, D. Douroumis, Crit. Rev. Biotechnol. 30 (2010) 283. [12] K. Grieve, P. Mulvaney, F. Grieser, Curr. Opin. Colloid Interface Sci. 5 (2000) 168. [13] L. Dreesen, F. Cecchet, S. Lucas, Plasma Process. Polym. 6 (2009) S849. [14] M. Shiratani, G. Uchida, H.W. Seo, D. Ichida, K. Koga, N. Itagaki, K. Kamataki, Adv. Mater. Res. 783–786 (2014) 2022. [15] A Samavati, Z. Othaman, S.K. Ghoshal, M.R. Dousti, J. Luminesc. 154 (2014) 51. [16] F. Aldaw Idrees, S. Sakrani, Z. Othaman, Adv. Mater. Res. 795 (2013) 721. [17] J.H. Sun, H.C. Kang, TSF 518 (2010) 6522. [18] A. Samavati, Z. Othaman, S.K. Ghoshal, M.R. Dousti, R.J. Amjad, Int. J. Mol. Sci. Chin. Phys. Lett. 29 (2012) 118101.
75
[19] S. Lee, D.W. Shin, W.M. Kim, B.-K. Cheong, T.S. Lee, K.S. Lee, S. Cho, TSF 514 (2006) 296. [20] K.-K. Kim, N. Koguchi, Y.-W. Ok, T.-Y. Seong, S.-J. Park, Appl. Phys. Lett. 19 (2004) 3810. [21] S. Huang, Z. Dai, F. Qu, L. Zhang, X. Zhu, Nanotechnology 13 (2002) 691. [22] A.G. Rolo, O. Conde, M.J.M. Gomes, TSF 318 (1998) 108. [23] S. Levichev, A. Chahboun, A.G. Rolo, O. Conde, M.J.M. Gomes, Thin Solid Films 517 (2009) 2538. [24] E. Campos-González, P. Rodríguez-Fragoso, G. Gonzalez de la Cruz, J. SantoyoSalazar, O. Zelaya-Angel, J. Cryst. Growth 338 (2012) 251. [25] C. Humbert, A. Tadjeddine, B. Busson, J. Phys. Chem. Lett. 2 (2011) 2770. [26] C. Humbert, O. Pluchery, E. Lacaze, A. Tadjeddine, B. Busson, PCCP 14 (2012) 280. [27] M.T. Frederick, J.L. Achtyl, K.E. Knowles, E.A. Weiss, F.M. Geiger, J. Am. Chem. Soc. 133 (2011) 7476. [28] Lumidot™ CdSe 590, (accessed June 2014). [29] Lumidot™ CdSe-6 quantum dot nanoparticles kit, (accessed June 2014). [30] AFM probe, (accessed December 2014). [31] C. Humbert, B. Busson, C. Six, A. Gayral, M. Gruselle, F. Villain, A. Tadjeddine, J. Electroanal. Chem. 621 (2008) 314. [32] M. Haidopoulos, F. Mirabella, M. Horgnies, C. Volcke, P.A. Thiry, P. Rouxhet, J.-J. Pireaux, J. Microsc. 228 (2007) 227. [33] M. Rasa, B.W.M. Kuipers, A.P. Philipse, J. Colloid Interface Sci. 250 (2002) 303. [34] F. Dubois, B. Mahler, B. Dubertret, E. Doris, C. Mioskowski, J. Am. Chem. Soc. 129 (2007) 482. [35] L. Dreesen, C. Humbert, M. Celebi, J.-J. Lemaire, A.A. Mani, P.A. Thiry, A. Peremans, Appl. Phys. B 74 (2002) 621. [36] C. Humbert, O. Pluchery, E. Lacaze, A. Tadjeddine, B. Busson, Gold Bull. 46 (2013) 299. [37] R.A. Sperling, W.J. Parak, Phil. Trans. R. Soc. A 368 (2010) 1333. [38] L. Dreesen, C. Humbert, Y. Sartenaer, Y. Caudano, C. Volcke, A.A. Mani, A. Peremans, P.A. Thiry, S. Hanique, J.M. Frere, Langmuir 20 (2004) 7201. [39] K.C. Chou, S. Westerberg, Y.R. Shen, P.N. Ross, G.A. Somorjai, Phys. Rev. B 69 (2004) 153413.
