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Hydrophobic perfluoro-silane functionalization of porous silicon photoluminescent films and particles
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Applied Surface Science xxx (2016) xxx–xxx
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Hydrophobic perfluoro-silane functionalization of porous silicon photoluminescent films and particles C. Rodriguez a , P. Laplace a , D. Gallach-Pérez a , P. Pellacani a , R.J. Martín-Palma a , V. Torres-Costa a,b , G. Ceccone c , M. Manso Silván a,∗ a
Departamento de Física Aplicada e Instituto de Ciencia de Materiales Nicolás Cabrera, Universidad Autónoma de Madrid, 28049, Madrid, Spain Centro de Microanálisis de Materiales, Universidad Autónoma de Madrid, 28049, Madrid, Spain c European Commission, Joint Research Centre, Institute for Health and Consumer Protection, 21020, Ispra (Va), Italy b
a r t i c l e
i n f o
Article history: Received 15 October 2015 Received in revised form 13 January 2016 Accepted 14 January 2016 Available online xxx Keywords: Porous Silicon Quantum dots Perfluoro-silane Self assembly Luminescence Hydrophobic
a b s t r a c t Luminescent structures based on semiconductor quantum dots (QDs) are increasingly used in biomolecular assays, cell tracking systems, and in-vivo diagnostics devices. In this work we have carried out the functionalization of porous silicon (PSi) luminescent structures by a perfluorosilane (Perfluorooctyltriethoxysilane, PFOS) self assembly. The PFOS surface binding (traced by X-ray photoelectron spectroscopy) and photoluminescence efficiency were analyzed on flat model PSi. Maximal photoluminescence intensity was obtained from PSi layers anodized at 110 mA/cm2 . Resistance to hydroxylation was assayed in H2 O2 :ethanol solutions and evidenced by water contact angle (WCA) measurements. PFOS-functionalized PSi presented systematically higher WCA than untreated PSi. The PFOS functionalization was found to slightly improve the aging of the PSi particles in water giving rise to particles with longer luminescent life. Confirmation of PFOS binding to PSi particles was derived from FTIR spectra and the preservation of luminescence was observed by fluorescence microscopy. Such functionalization opens the possibility of promoting hydrophobic-hydrophobic interactions between biomolecules and fluorescent QD structures, which may enlarge their biomedical applications catalogue. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor quantum dots (QDs) are increasingly used in biomedical applications in view mainly of their luminescent properties. They have been tested both in-vitro as cell labelling [1] and diagnostic systems [2], as well as in-vivo in animal models for tissue imaging purposes [3]. Given the temperature induced emission shift they have further been explored as thermometers [4]. Porous silicon (PSi) in the form of particles (PSps) is a well known source for silicon QDs specially adapted to biomedical applications [5–7] in view of the reliability of the biofunctionalization processes through silanization and the intrinsic biocompatibility of Si degradation products [8,9]. Furthermore, PSps can be used to allocate nanomaterials to provide additional functionalities such as magnetic properties [10] or drug delivery potential [11,12]. The drawbacks stem from the chemical instability of PSi, which leads to a decay of intensity and red-shift of luminescence that should be prevented through surface passivation [8].
∗ Corresponding author. Tel.: +34 914974918; fax: +34 914973969. E-mail address: [email protected] (M. Manso Silván).
Biofunctionalization has been traditionally focused on providing surface groups on semiconductor nanostructures allowing a covalent binding with target biomolecules. Amino-silanes stand as the most prominent molecules for surface biofunctionalization of PSps [13]. In the opposite approach, hydrophilic antifouling groups from polyethylene-glycol have been promoted on PSps surface to inhibit the interactions with biomolecules [8]. However, covalent binding does not allow exploiting the multiple interaction mechanisms among PSps and biomolecules. In particular, reversible interactions with drugs are preferred for delivery applications. Furthermore, proteins carry a diversity of hydrophobic residues that shall be exploited for promoting such reversible interaction with PSps. For instance, serum proteins such as serum albumin exhibit pH dependent conformational changes induced by hydrophobic interactions [14]. The relevance of these processes on interactions with surfaces has been outlined previously [15]. Hydrocarbon surfaces are reputed hydrophobic systems and cover typically silicones [16], thermally passivated porous silicon [17], and carbonaceous materials [18], unless oxidative processes are used to induce surface polar (hydrophilic) groups. To maximize the hydrophobicity of the surfaces, fluorocarbon groups [19] are preferred to natural hydrocarbon moieties. When dealing
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Please cite this article in press as: C. Rodriguez, et al., Hydrophobic perfluoro-silane functionalization of porous silicon photoluminescent films and particles, Appl. Surf. Sci. (2016), http://dx.doi.org/10.1016/j.apsusc.2016.01.119
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Fig. 1. (a) Optimization of photoluminescence of the PSi layers by varying current density and (b) typical Si QDs within PS as observed by TEM.
with Si-based materials (silicas, glass, silicates, silicon) and other affine materials, the hydrophobic functionalization can be however performed through perfluoro-silanes [20] as an alternative to more generic plasma [21], or photoactivated [22] hydrosilylation processes. Such functionalization provides the surfaces not only with hydrophobic and oleophobic properties, but also increases the range of functionality to frosting conditions [23]. Thus, perfluoro-silane modified materials present improved dielectric [24], anticorrosion and tribological properties [25], and increase the stability of organic solar cells [26] or transistors [27]. In the present work we aim at the surface functionalization of PSi structures with Perfluorooctyl-triethoxysilane (PFOS) in order to provide a compact synthetic surface on PSi, analogue to what promoted naturally using hydrophobin proteins for PSi based drug delivery systems [28]. Silane self-assembly is generally driven by surface OH groups, but such intermediate modification is detrimental for photoluminescence properties. We explore the influence of exciton generation using white light to carry out the surface modification with PFOS as an alternative to other methods that preclude deep PSi oxidation [29] and use intermediate Li compounds [30]. We aim at the description of the assembly efficiency by using Xray photoelectron spectroscopy (XPS), water contact angle (WCA), Fourier transformed infrared spectroscopy (FTIR) and study the stability in critical conditions by the photoluminescence aging in particularly oxidizing conditions. 2. Experimental 2.1. Porous Silicon preparation and functionalization PSi layers were formed by the electrochemical etch of p-type low resistivity Si wafers (boron doped, single side polished, orientation ((1 0 0) and 0.01–0.02 cm resistivity) in an HF:ethanol (1:2) solution. The non-polished side of the Si wafers was previously coated with an Al layer and annealed to provide low resistance ohmic contacts. Samples were galvanostatically etched without illumination. The etching current density was varied from 90 to 110 mA cm−2 and the etching time was 30 s. To produce PSps, an additional final etch step of 5 s at 180 mA cm-2 was applied to sacrifice the PSi layer. The induced instability of the PSi/Si interface induced a fragmentation of the layer in the form of PSi flakes, which were easily scrapped from the surface and were further mortar grinded to reduce their size. The whole extraction process was carried out in ethanol to prevent atmospheric exposure thus
minimizing oxidation and photoluminescence extinction. Surface functionalization with solutions of PFOS 0.2% (pur. > 98%, Sigma Aldrich) in ethanol (Panreac) were carried out under a 100 W halogen illumination for 30 min, which is known to slightly oxidize the PSi surface [31]. After modification, the PSi structures (whether films or particles) were changed of ethanol medium twice and dried at room temperature for analysis. When aiming at studying the stability of the particles, two aging conditions were considered. For wettability aging purposes H2 O2 :ethanol (1:2) solutions were used in view of their oxidation potential. For PL studies, milder oxidation conditions in milliQ water were used to avoid a drastic decay of emission intensity. 2.2. Characterization techniques Photoluminescence (PL) was measured using a fluorimeter (Aminco Bowman series 2) operating with a 150 W continuous wave Xe lamp. Samples were excited with wavelengths between 350 and 400 nm (2 nm bandwidth) in order to excite the different energy levels. Signal was recorded using a photomultiplier tube detector polarized at 500 V. The emission scan was performed between 475 and 675 nm (4 nm bandwidth), avoiding the second harmonic diffraction peak from the excitation wavelength. PL response from PSps has been also observed directly by fluorescence microscopy using a IX81 Olympus inverted microscope linked to a DP72 digital camera controlled by cellD software. The excitation source of this device is a 100 W Hg lamp operating at stabilized direct current, suitable for UV fluorescence. The internal structure of PSi was observed by transmission electron microscopy (TEM) in a JEOL 2100F operated at 200 kV. Plane sections were prepared by subsequent steps of mechanical (dimpler) and ion beam milling (dual Ar ion gun). XPS measurements were performed in an AXIS ULTRA Spectrometer (KRATOS Analytical, UK). The samples were irradiated with monochromatic Al-K␣ X-rays (h = 1486.6 eV) using X-ray spot size of 400 × 700 m2 and different take-off angles (TOA) with respect to the sample surface. Surface charging was compensated by means of a filament inserted in a magnetic lens system and all spectra were corrected by setting the C1s hydrocarbon component to 284.50 eV. For the core level spectra, a pass energy of 40 eV was selected, determining a resolution of 0.63 eV as measured both on the Ag 3d5/2 peak and on the Ag Fermi edge.The data were processed using the Vision2 software (Kratos, UK) and CasaXPS v16R1 (Casa Software, UK). Curve fitting of core level peaks was carried out using the same
Please cite this article in press as: C. Rodriguez, et al., Hydrophobic perfluoro-silane functionalization of porous silicon photoluminescent films and particles, Appl. Surf. Sci. (2016), http://dx.doi.org/10.1016/j.apsusc.2016.01.119
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3
Name Si 2p3 Si 2p1 Si 2pOx
Pos. FWHM %Are a 99.39 0.655 51.850 99.99 0.623 25.919 103.44 1.696 22.231
a
105
100
Name C0 C2 C3 C4 C5
b
Pos. FWHM %Are a 284.89 1.600 41.166 286.13 1.600 19.495 288.03 1.600 14.162 290.52 1.600 15.220 293.00 1.486 9.957
95
20
90
CPS
CPS
15
85 80
10
75 70 5
65 60
0 105 2 x 10
102 Si 2p 99 Binding Energy (eV)
296
96
292 288 Binding Energy (eV)
3 x 10
284
55 50
Name Si 2p3 Si 2p1 Si 2pOx
Pos. FWHM %Are a 97.09 1.363 35.235 97.69 1.700 17.613 100.33 2.360 47.152
22
c
20
45
Name C0 C1 C3 C4
Pos. FWHM %Are a 282.29 1.340 75.238 283.52 1.600 16.789 289.13 1.557 7.057 291.61 1.000 0.916
d
18
40 16
35 CPS
CPS
14
30
12
25 Si 2p1
10
20 8
15
6
10
4
5 105
102 99 Binding Energy (eV)
96
292
288 284 Binding Energy (eV)
280
Fig. 2. Si2p and C1s core level spectra from PFOS condensate reference (a) and b), respectively) and PFOS functionalized PSi (c) and d), respectively).
initial parameters and inter-peak constrains to reduce scattering. The core level envelopes were fitted with Gaussian-Lorentzian function (G/L = 30) and variable full width half maximum. The characterisation of the PSps was performed by FTIR, using a Bruker Vector 22 (resolution 8 cm−1 , 4000–400 cm−1 , 32 scans at 10 kHz) in transmission configuration after preparation of KBr disks. Static Water Contact Angle (WCA) measurements were carried out in a KSW 100 with droplet volumes of 3 l. Results are shown as the mean value of five droplets deposited on the different preparations of PSi films.
3. Results and discussion 3.1. Optimization of PFOS functionalized photoluminescent PSi PSi layers were studied mainly as model material for the optimization of the processing of PSps. First steps were devoted to the
optimization of the photoluminescence of the structures. P-type Si wafers were anodized at three different current densities (90, 100 and 110 mA/cm2 ) and the PL was acquired. The PL of the three different samples is plotted in Fig. 1a. The results show that an increase of emitted intensity is induced for increasing anodization current densities (within a range of previously described conditions [32]) and with a slight blue shift. This latter feature agrees with the formation of Si QDs within the PSi matrix with increasing band gap for increasing current density (smaller QD size). The application of theories for indirect band gap determination in nanoscaled materials allows extracting an estimation of QD size from the PL curves [33]. The results show that QDs sizes range from 2.4 ± 0.4 to 3.2 ± 0.2 nm. The error value is linked to the width of the PL curve, and it shows that for an optimization of intensity one has to slightly sacrifice the homogeneity of the QD size distribution. The direct observation of the samples by TEM allowed identifying irregular Si QD structures (Fig. 1b) with mean sizes that agree with those estimated by PL measurements.
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The PSi functionalization efficiency by PFOS was studied by XPS using again PSi layers as model surface. Table 1 shows the results of the elemental composition as extracted from survey spectra of functionalized layers and PFOS condensates prepared on Si as composition reference. It can be seen that a considerable amount of F is detected on the PSi surface after functionalization, although not to the level of the reference PFOS condensate. It is worth noting that, in spite of the mainly organic origin of the PFOS condensate, the amount of C detected on the functionalized PSi surface was higher than on the reference PFOS condensate. This was compensated by an inverse tendency in the Si composition among the samples. This led to suggest that the PFOS reference condensate is most probably not conformably coating the Si substrate and invited to a detailed study of the core level spectra of these two elements. The analysis of the Si2p signal from the PFOS condensate reference (Fig. 2.a) clearly demonstrated the non conformal covering of the layer. The Si2p signal could be deconvoluted with two contributions related to the 2p1/2 and 2p3/2 electrons arising from the Si substrate and a SiOx contribution consistent with the chemical structure of the Si atom in the PFOS molecule. The C1s spectrum from this reference surface (Fig. 2.b) showed 5 main components related (from lower to higher binding energy) to C H, C O, C O, CF2 and CF3 . It looks thus on the one hand that the PFOS functionalization is accompanied by a considerable adsorption of oxidized C species integrated most probably during light irradiation. However, the surface affinity of fresh PSi for a wide range of C species has been already evidenced and their undirected coverage with C C, C O and C O compounds has been already evidenced [8]. On the other hand, we can observe that the two highest BE peaks represent a good label of the perfluorooctyl group, which clearly indicates that neither the white light irradiation, nor the X-ray excitation for analysis has notably degraded the formed PFOS layer. The core level spectra corresponding to the PFOS functionalized PSi showed a considerable energy shift (circa 2 eV) related to surface charging. The Si2p spectrum (Fig. 2.c) presented a notable increase of the SiOx contribution. This effect can be directly related to a slight oxidation of the PSi surface prior and upon functionalization. However, it shows that the oxidation level is below 50%, which can be taken as an indication of the preservation of the intrinsic semiconductor state of the Si QDs. With respect to the C1s spectrum (Fig. 2.d), it shows that the surface presents only four contributions. By comparing with the reference sample, it appears that the amount of C O can be considered as negligible on the PSi surface. Relevantly, the two higher BE contributions denote the presence of the perfluorooctyl group on the surface. However, the study of the relative intensity of the CF2 to CF3 contribution denotes a decrease of the terminal group. Such discrepancy arises most probably from the presence of F− ions within the PSi network, which contribute to the composition in spite of intense rinsing (and up to 4.5 at.%) as previously detected [8]. No hint could be extracted from the strict monocomponent observed in all F1s spectra. These assumptions led to the conclusion that the CF3 contribution to the C1s peak shall be used for determining the PFOS assembly efficiency on PSi. From data in the table and in the spectrum it can be concluded that the amount of CF3 on the surface is 0.16% of the total surface composition. In order to determine how this modification of the surface influences the surface properties, WCA measurements were
Fig. 3. WCA measurements on PSi and PFOS-PSi before (top) and after (bottom) aging in 24 h H2 O2 :EtOH solution.
performed on the model PFOS functionalized PSi layers and compared with freshly synthesized PSi layers. The results in Fig. 3 show that the surfaces of pristine PSi present a strong hydrophobic behavior with WCA values of 115 ± 3◦ as expected from the dominant S H bonding present on fresh surfaces. The analysis of the PFOS functionalized samples denoted a slight increase of the hydrophobic behavior with WCA values of 123 ± 4◦ . As such, the gain in hydrophobicity may not justify the functionalization step. However, we decided to study the stability of the hydrophobic properties by accelerating the aging of the PSi surfaces in extremely oxidation conditions. After aging for 24 h in H2 O2 :EtOH solutions the WCA on PSi was not even measurable in view of the extreme hydrophilicity of the surface. The same aging process on PFOS functionalized PSi also induced a clear hydrophilic transformation although the WCA was limited to 33 ± 2◦ , which refers to a clear protective effect of the PFOS assembled layer. The aging effect was also evaluated on the PL emission. In view of the extremely fast oxidation observed in H2 O2 based solutions, the aging was studied in this case in milliQ water. The PL intensity was recorded each day along 8 days aging time as plotted in Fig. 4. The results illustrate again that the PFOS functionalization has a protective effect, although this protection appears to be inhibited at the longer aging times studied, when the emission intensity almost converges with that of the aged pristine PSi. 3.2. Performance of PFOS functionalized PSps In view of the previously obtained results for PSi layers, PSps were processed by using current densities of 110 mA/cm2 and functionalized in 0.2 v.% PFOS solutions. In order to check the functionalization process, FTIR spectroscopy was used to identify the presence of organic groups on the PSi surfaces. Fig. 5 shows the compared spectra obtained from pristine PSps and PFOS
Table 1 Surface stoichiometry of PFOS functionalized PSi compared to PFOS condensate references on Si as derived from XPS analysis (at.% ± 0.5%).
PFOS-PSi PFOS ref on Si
F 1s
O 1s
C 1s
Si 2p
14.3 22.3
24.4 23.4
38.7 16.1
22.6 38.2
Fig. 4. Decrease of PL intensity upon aging of PSi films (pristine and PFOS functionalized) in water during 8 days.
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aging in extreme (H2 O2 :EtOH solutions) oxidation conditions. The protective effect is however mild on the PL properties, which were measured after etching in milder water conditions. An initial protective effect preserving PL intensity can be identified, though the effect was almost mitigated after 8 days aging. These information was used to produce PFOS functionalized PSps. The success of the functionalization was monitored in this case by FTIR. It was shown that the functionalization is compatible with the preservation of the PL. Studies are in progress to functionally exploit these particles in protein assays.
Acknowledgments
Fig. 5. FTIR spectra of PSps and PFOS functionalized PSps and corresponding fluorescence microscopy images, top and bottom, respectively.
functionalized PSps. The attribution to the main observed bands is labelled also in the figure. It could be observed that the pristine PSps age considerably prior to measurement and exhibit relatively intense Si O, Si OH and adsorbed water bands and very poor Si H bands. Additionally, the sample presents relatively lower bands associated to carbon species with respect to the Si O ones. For the PFOS functionalized PSps, the spectrum reflects also a consumption of the Si H species normally observed on fresh PSi. However, in this case the oxidation and specially hydroxylation and adsorbed water bands are clearly inhibited. The presence of organic groups is in fact extremely relevant. However, the presence of alkyl is more relevant than fluoroalkyl groups, in agreement with what analyzed by XPS. In fact, again the CF3 group (sharp band at the low wavenumbers edge) appears to be the ideal group for tracing the functionalization in view of the partial coincidence of CF2 bands with the Si O region [20]. However, the contribution of CF2 to PFOS-PSi considerably widens the spectrum at the 1000–1200 cm−1 range with respect to what observed in the control PSi sample. The PL of the PSps was studied by direct observation in the fluorescence microscope of fractions standing on a Si substrate. The images show a relatively similar behavior with the typical red-orange colour. The emission is slightly diffusive in the PFOS functionalized PSps, though this may be simply attributed to the bigger size of the sampled PSps. These results further prove that, in spite of the increase of the surface area of PSps with respect to PSi layers, the functionalization process with PFOS is compatible with an efficient PL emission and the oxidation of the structures does not consume the Si QDs within PSps. Further studies are required to analyze the effects of protein adsorption on the PL emission. 4. Conclusions Photoluminescent PSi layers and particles with hydrophobic functionalization have been prepared by assembly of perfluorooctyl-silane. The PSi layers have been used as models to investigate the surface modifications taking place during functionalization and the effects on the wettability. PSi formation conditions have been selected at 110 mA/cm2 in order to maximize the PL emission, which can be used to estimate the presence of a distribution of Si QDs within the 2.4 ± 0.4 nm range. It has been determined by XPS that the perfluorooctyl group is present on the surface and its terminal CF3 carbon represents 0.16% of the total surface composition. The effect of such functionalization is specially remarkable when the PSi structures are aged; the surfaces of pristine PSi are hydrophobic irrespective of the PFOS functionalization, but PFOS protects PSi against oxidation, which reduces the WCA decay after
The authors thank Dr Laura Pascual for operation of HRTEM. Funding through grants MAT2013-46572-C2-1-R and MAT201454826-C2-1-R from Ministerio de Economía y Competitividad is greatly appreciated. C. Rodríguez and P. Pellacani specially thank the EU Commission for the funding received through Marie Sklodowska Curie ITN project Thinface (GA 607232).
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Hydrophobic perfluoro-silane functionalization of porous silicon photoluminescent films and particles C. Rodriguez a , P. Laplace a , D. Gallach-Pérez a , P. Pellacani a , R.J. Martín-Palma a , V. Torres-Costa a,b , G. Ceccone c , M. Manso Silván a,∗ a
Departamento de Física Aplicada e Instituto de Ciencia de Materiales Nicolás Cabrera, Universidad Autónoma de Madrid, 28049, Madrid, Spain Centro de Microanálisis de Materiales, Universidad Autónoma de Madrid, 28049, Madrid, Spain c European Commission, Joint Research Centre, Institute for Health and Consumer Protection, 21020, Ispra (Va), Italy b
a r t i c l e
i n f o
Article history: Received 15 October 2015 Received in revised form 13 January 2016 Accepted 14 January 2016 Available online xxx Keywords: Porous Silicon Quantum dots Perfluoro-silane Self assembly Luminescence Hydrophobic
a b s t r a c t Luminescent structures based on semiconductor quantum dots (QDs) are increasingly used in biomolecular assays, cell tracking systems, and in-vivo diagnostics devices. In this work we have carried out the functionalization of porous silicon (PSi) luminescent structures by a perfluorosilane (Perfluorooctyltriethoxysilane, PFOS) self assembly. The PFOS surface binding (traced by X-ray photoelectron spectroscopy) and photoluminescence efficiency were analyzed on flat model PSi. Maximal photoluminescence intensity was obtained from PSi layers anodized at 110 mA/cm2 . Resistance to hydroxylation was assayed in H2 O2 :ethanol solutions and evidenced by water contact angle (WCA) measurements. PFOS-functionalized PSi presented systematically higher WCA than untreated PSi. The PFOS functionalization was found to slightly improve the aging of the PSi particles in water giving rise to particles with longer luminescent life. Confirmation of PFOS binding to PSi particles was derived from FTIR spectra and the preservation of luminescence was observed by fluorescence microscopy. Such functionalization opens the possibility of promoting hydrophobic-hydrophobic interactions between biomolecules and fluorescent QD structures, which may enlarge their biomedical applications catalogue. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor quantum dots (QDs) are increasingly used in biomedical applications in view mainly of their luminescent properties. They have been tested both in-vitro as cell labelling [1] and diagnostic systems [2], as well as in-vivo in animal models for tissue imaging purposes [3]. Given the temperature induced emission shift they have further been explored as thermometers [4]. Porous silicon (PSi) in the form of particles (PSps) is a well known source for silicon QDs specially adapted to biomedical applications [5–7] in view of the reliability of the biofunctionalization processes through silanization and the intrinsic biocompatibility of Si degradation products [8,9]. Furthermore, PSps can be used to allocate nanomaterials to provide additional functionalities such as magnetic properties [10] or drug delivery potential [11,12]. The drawbacks stem from the chemical instability of PSi, which leads to a decay of intensity and red-shift of luminescence that should be prevented through surface passivation [8].
∗ Corresponding author. Tel.: +34 914974918; fax: +34 914973969. E-mail address: [email protected] (M. Manso Silván).
Biofunctionalization has been traditionally focused on providing surface groups on semiconductor nanostructures allowing a covalent binding with target biomolecules. Amino-silanes stand as the most prominent molecules for surface biofunctionalization of PSps [13]. In the opposite approach, hydrophilic antifouling groups from polyethylene-glycol have been promoted on PSps surface to inhibit the interactions with biomolecules [8]. However, covalent binding does not allow exploiting the multiple interaction mechanisms among PSps and biomolecules. In particular, reversible interactions with drugs are preferred for delivery applications. Furthermore, proteins carry a diversity of hydrophobic residues that shall be exploited for promoting such reversible interaction with PSps. For instance, serum proteins such as serum albumin exhibit pH dependent conformational changes induced by hydrophobic interactions [14]. The relevance of these processes on interactions with surfaces has been outlined previously [15]. Hydrocarbon surfaces are reputed hydrophobic systems and cover typically silicones [16], thermally passivated porous silicon [17], and carbonaceous materials [18], unless oxidative processes are used to induce surface polar (hydrophilic) groups. To maximize the hydrophobicity of the surfaces, fluorocarbon groups [19] are preferred to natural hydrocarbon moieties. When dealing
http://dx.doi.org/10.1016/j.apsusc.2016.01.119 0169-4332/© 2016 Elsevier B.V. All rights reserved.
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Fig. 1. (a) Optimization of photoluminescence of the PSi layers by varying current density and (b) typical Si QDs within PS as observed by TEM.
with Si-based materials (silicas, glass, silicates, silicon) and other affine materials, the hydrophobic functionalization can be however performed through perfluoro-silanes [20] as an alternative to more generic plasma [21], or photoactivated [22] hydrosilylation processes. Such functionalization provides the surfaces not only with hydrophobic and oleophobic properties, but also increases the range of functionality to frosting conditions [23]. Thus, perfluoro-silane modified materials present improved dielectric [24], anticorrosion and tribological properties [25], and increase the stability of organic solar cells [26] or transistors [27]. In the present work we aim at the surface functionalization of PSi structures with Perfluorooctyl-triethoxysilane (PFOS) in order to provide a compact synthetic surface on PSi, analogue to what promoted naturally using hydrophobin proteins for PSi based drug delivery systems [28]. Silane self-assembly is generally driven by surface OH groups, but such intermediate modification is detrimental for photoluminescence properties. We explore the influence of exciton generation using white light to carry out the surface modification with PFOS as an alternative to other methods that preclude deep PSi oxidation [29] and use intermediate Li compounds [30]. We aim at the description of the assembly efficiency by using Xray photoelectron spectroscopy (XPS), water contact angle (WCA), Fourier transformed infrared spectroscopy (FTIR) and study the stability in critical conditions by the photoluminescence aging in particularly oxidizing conditions. 2. Experimental 2.1. Porous Silicon preparation and functionalization PSi layers were formed by the electrochemical etch of p-type low resistivity Si wafers (boron doped, single side polished, orientation ((1 0 0) and 0.01–0.02 cm resistivity) in an HF:ethanol (1:2) solution. The non-polished side of the Si wafers was previously coated with an Al layer and annealed to provide low resistance ohmic contacts. Samples were galvanostatically etched without illumination. The etching current density was varied from 90 to 110 mA cm−2 and the etching time was 30 s. To produce PSps, an additional final etch step of 5 s at 180 mA cm-2 was applied to sacrifice the PSi layer. The induced instability of the PSi/Si interface induced a fragmentation of the layer in the form of PSi flakes, which were easily scrapped from the surface and were further mortar grinded to reduce their size. The whole extraction process was carried out in ethanol to prevent atmospheric exposure thus
minimizing oxidation and photoluminescence extinction. Surface functionalization with solutions of PFOS 0.2% (pur. > 98%, Sigma Aldrich) in ethanol (Panreac) were carried out under a 100 W halogen illumination for 30 min, which is known to slightly oxidize the PSi surface [31]. After modification, the PSi structures (whether films or particles) were changed of ethanol medium twice and dried at room temperature for analysis. When aiming at studying the stability of the particles, two aging conditions were considered. For wettability aging purposes H2 O2 :ethanol (1:2) solutions were used in view of their oxidation potential. For PL studies, milder oxidation conditions in milliQ water were used to avoid a drastic decay of emission intensity. 2.2. Characterization techniques Photoluminescence (PL) was measured using a fluorimeter (Aminco Bowman series 2) operating with a 150 W continuous wave Xe lamp. Samples were excited with wavelengths between 350 and 400 nm (2 nm bandwidth) in order to excite the different energy levels. Signal was recorded using a photomultiplier tube detector polarized at 500 V. The emission scan was performed between 475 and 675 nm (4 nm bandwidth), avoiding the second harmonic diffraction peak from the excitation wavelength. PL response from PSps has been also observed directly by fluorescence microscopy using a IX81 Olympus inverted microscope linked to a DP72 digital camera controlled by cellD software. The excitation source of this device is a 100 W Hg lamp operating at stabilized direct current, suitable for UV fluorescence. The internal structure of PSi was observed by transmission electron microscopy (TEM) in a JEOL 2100F operated at 200 kV. Plane sections were prepared by subsequent steps of mechanical (dimpler) and ion beam milling (dual Ar ion gun). XPS measurements were performed in an AXIS ULTRA Spectrometer (KRATOS Analytical, UK). The samples were irradiated with monochromatic Al-K␣ X-rays (h = 1486.6 eV) using X-ray spot size of 400 × 700 m2 and different take-off angles (TOA) with respect to the sample surface. Surface charging was compensated by means of a filament inserted in a magnetic lens system and all spectra were corrected by setting the C1s hydrocarbon component to 284.50 eV. For the core level spectra, a pass energy of 40 eV was selected, determining a resolution of 0.63 eV as measured both on the Ag 3d5/2 peak and on the Ag Fermi edge.The data were processed using the Vision2 software (Kratos, UK) and CasaXPS v16R1 (Casa Software, UK). Curve fitting of core level peaks was carried out using the same
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Name Si 2p3 Si 2p1 Si 2pOx
Pos. FWHM %Are a 99.39 0.655 51.850 99.99 0.623 25.919 103.44 1.696 22.231
a
105
100
Name C0 C2 C3 C4 C5
b
Pos. FWHM %Are a 284.89 1.600 41.166 286.13 1.600 19.495 288.03 1.600 14.162 290.52 1.600 15.220 293.00 1.486 9.957
95
20
90
CPS
CPS
15
85 80
10
75 70 5
65 60
0 105 2 x 10
102 Si 2p 99 Binding Energy (eV)
296
96
292 288 Binding Energy (eV)
3 x 10
284
55 50
Name Si 2p3 Si 2p1 Si 2pOx
Pos. FWHM %Are a 97.09 1.363 35.235 97.69 1.700 17.613 100.33 2.360 47.152
22
c
20
45
Name C0 C1 C3 C4
Pos. FWHM %Are a 282.29 1.340 75.238 283.52 1.600 16.789 289.13 1.557 7.057 291.61 1.000 0.916
d
18
40 16
35 CPS
CPS
14
30
12
25 Si 2p1
10
20 8
15
6
10
4
5 105
102 99 Binding Energy (eV)
96
292
288 284 Binding Energy (eV)
280
Fig. 2. Si2p and C1s core level spectra from PFOS condensate reference (a) and b), respectively) and PFOS functionalized PSi (c) and d), respectively).
initial parameters and inter-peak constrains to reduce scattering. The core level envelopes were fitted with Gaussian-Lorentzian function (G/L = 30) and variable full width half maximum. The characterisation of the PSps was performed by FTIR, using a Bruker Vector 22 (resolution 8 cm−1 , 4000–400 cm−1 , 32 scans at 10 kHz) in transmission configuration after preparation of KBr disks. Static Water Contact Angle (WCA) measurements were carried out in a KSW 100 with droplet volumes of 3 l. Results are shown as the mean value of five droplets deposited on the different preparations of PSi films.
3. Results and discussion 3.1. Optimization of PFOS functionalized photoluminescent PSi PSi layers were studied mainly as model material for the optimization of the processing of PSps. First steps were devoted to the
optimization of the photoluminescence of the structures. P-type Si wafers were anodized at three different current densities (90, 100 and 110 mA/cm2 ) and the PL was acquired. The PL of the three different samples is plotted in Fig. 1a. The results show that an increase of emitted intensity is induced for increasing anodization current densities (within a range of previously described conditions [32]) and with a slight blue shift. This latter feature agrees with the formation of Si QDs within the PSi matrix with increasing band gap for increasing current density (smaller QD size). The application of theories for indirect band gap determination in nanoscaled materials allows extracting an estimation of QD size from the PL curves [33]. The results show that QDs sizes range from 2.4 ± 0.4 to 3.2 ± 0.2 nm. The error value is linked to the width of the PL curve, and it shows that for an optimization of intensity one has to slightly sacrifice the homogeneity of the QD size distribution. The direct observation of the samples by TEM allowed identifying irregular Si QD structures (Fig. 1b) with mean sizes that agree with those estimated by PL measurements.
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The PSi functionalization efficiency by PFOS was studied by XPS using again PSi layers as model surface. Table 1 shows the results of the elemental composition as extracted from survey spectra of functionalized layers and PFOS condensates prepared on Si as composition reference. It can be seen that a considerable amount of F is detected on the PSi surface after functionalization, although not to the level of the reference PFOS condensate. It is worth noting that, in spite of the mainly organic origin of the PFOS condensate, the amount of C detected on the functionalized PSi surface was higher than on the reference PFOS condensate. This was compensated by an inverse tendency in the Si composition among the samples. This led to suggest that the PFOS reference condensate is most probably not conformably coating the Si substrate and invited to a detailed study of the core level spectra of these two elements. The analysis of the Si2p signal from the PFOS condensate reference (Fig. 2.a) clearly demonstrated the non conformal covering of the layer. The Si2p signal could be deconvoluted with two contributions related to the 2p1/2 and 2p3/2 electrons arising from the Si substrate and a SiOx contribution consistent with the chemical structure of the Si atom in the PFOS molecule. The C1s spectrum from this reference surface (Fig. 2.b) showed 5 main components related (from lower to higher binding energy) to C H, C O, C O, CF2 and CF3 . It looks thus on the one hand that the PFOS functionalization is accompanied by a considerable adsorption of oxidized C species integrated most probably during light irradiation. However, the surface affinity of fresh PSi for a wide range of C species has been already evidenced and their undirected coverage with C C, C O and C O compounds has been already evidenced [8]. On the other hand, we can observe that the two highest BE peaks represent a good label of the perfluorooctyl group, which clearly indicates that neither the white light irradiation, nor the X-ray excitation for analysis has notably degraded the formed PFOS layer. The core level spectra corresponding to the PFOS functionalized PSi showed a considerable energy shift (circa 2 eV) related to surface charging. The Si2p spectrum (Fig. 2.c) presented a notable increase of the SiOx contribution. This effect can be directly related to a slight oxidation of the PSi surface prior and upon functionalization. However, it shows that the oxidation level is below 50%, which can be taken as an indication of the preservation of the intrinsic semiconductor state of the Si QDs. With respect to the C1s spectrum (Fig. 2.d), it shows that the surface presents only four contributions. By comparing with the reference sample, it appears that the amount of C O can be considered as negligible on the PSi surface. Relevantly, the two higher BE contributions denote the presence of the perfluorooctyl group on the surface. However, the study of the relative intensity of the CF2 to CF3 contribution denotes a decrease of the terminal group. Such discrepancy arises most probably from the presence of F− ions within the PSi network, which contribute to the composition in spite of intense rinsing (and up to 4.5 at.%) as previously detected [8]. No hint could be extracted from the strict monocomponent observed in all F1s spectra. These assumptions led to the conclusion that the CF3 contribution to the C1s peak shall be used for determining the PFOS assembly efficiency on PSi. From data in the table and in the spectrum it can be concluded that the amount of CF3 on the surface is 0.16% of the total surface composition. In order to determine how this modification of the surface influences the surface properties, WCA measurements were
Fig. 3. WCA measurements on PSi and PFOS-PSi before (top) and after (bottom) aging in 24 h H2 O2 :EtOH solution.
performed on the model PFOS functionalized PSi layers and compared with freshly synthesized PSi layers. The results in Fig. 3 show that the surfaces of pristine PSi present a strong hydrophobic behavior with WCA values of 115 ± 3◦ as expected from the dominant S H bonding present on fresh surfaces. The analysis of the PFOS functionalized samples denoted a slight increase of the hydrophobic behavior with WCA values of 123 ± 4◦ . As such, the gain in hydrophobicity may not justify the functionalization step. However, we decided to study the stability of the hydrophobic properties by accelerating the aging of the PSi surfaces in extremely oxidation conditions. After aging for 24 h in H2 O2 :EtOH solutions the WCA on PSi was not even measurable in view of the extreme hydrophilicity of the surface. The same aging process on PFOS functionalized PSi also induced a clear hydrophilic transformation although the WCA was limited to 33 ± 2◦ , which refers to a clear protective effect of the PFOS assembled layer. The aging effect was also evaluated on the PL emission. In view of the extremely fast oxidation observed in H2 O2 based solutions, the aging was studied in this case in milliQ water. The PL intensity was recorded each day along 8 days aging time as plotted in Fig. 4. The results illustrate again that the PFOS functionalization has a protective effect, although this protection appears to be inhibited at the longer aging times studied, when the emission intensity almost converges with that of the aged pristine PSi. 3.2. Performance of PFOS functionalized PSps In view of the previously obtained results for PSi layers, PSps were processed by using current densities of 110 mA/cm2 and functionalized in 0.2 v.% PFOS solutions. In order to check the functionalization process, FTIR spectroscopy was used to identify the presence of organic groups on the PSi surfaces. Fig. 5 shows the compared spectra obtained from pristine PSps and PFOS
Table 1 Surface stoichiometry of PFOS functionalized PSi compared to PFOS condensate references on Si as derived from XPS analysis (at.% ± 0.5%).
PFOS-PSi PFOS ref on Si
F 1s
O 1s
C 1s
Si 2p
14.3 22.3
24.4 23.4
38.7 16.1
22.6 38.2
Fig. 4. Decrease of PL intensity upon aging of PSi films (pristine and PFOS functionalized) in water during 8 days.
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aging in extreme (H2 O2 :EtOH solutions) oxidation conditions. The protective effect is however mild on the PL properties, which were measured after etching in milder water conditions. An initial protective effect preserving PL intensity can be identified, though the effect was almost mitigated after 8 days aging. These information was used to produce PFOS functionalized PSps. The success of the functionalization was monitored in this case by FTIR. It was shown that the functionalization is compatible with the preservation of the PL. Studies are in progress to functionally exploit these particles in protein assays.
Acknowledgments
Fig. 5. FTIR spectra of PSps and PFOS functionalized PSps and corresponding fluorescence microscopy images, top and bottom, respectively.
functionalized PSps. The attribution to the main observed bands is labelled also in the figure. It could be observed that the pristine PSps age considerably prior to measurement and exhibit relatively intense Si O, Si OH and adsorbed water bands and very poor Si H bands. Additionally, the sample presents relatively lower bands associated to carbon species with respect to the Si O ones. For the PFOS functionalized PSps, the spectrum reflects also a consumption of the Si H species normally observed on fresh PSi. However, in this case the oxidation and specially hydroxylation and adsorbed water bands are clearly inhibited. The presence of organic groups is in fact extremely relevant. However, the presence of alkyl is more relevant than fluoroalkyl groups, in agreement with what analyzed by XPS. In fact, again the CF3 group (sharp band at the low wavenumbers edge) appears to be the ideal group for tracing the functionalization in view of the partial coincidence of CF2 bands with the Si O region [20]. However, the contribution of CF2 to PFOS-PSi considerably widens the spectrum at the 1000–1200 cm−1 range with respect to what observed in the control PSi sample. The PL of the PSps was studied by direct observation in the fluorescence microscope of fractions standing on a Si substrate. The images show a relatively similar behavior with the typical red-orange colour. The emission is slightly diffusive in the PFOS functionalized PSps, though this may be simply attributed to the bigger size of the sampled PSps. These results further prove that, in spite of the increase of the surface area of PSps with respect to PSi layers, the functionalization process with PFOS is compatible with an efficient PL emission and the oxidation of the structures does not consume the Si QDs within PSps. Further studies are required to analyze the effects of protein adsorption on the PL emission. 4. Conclusions Photoluminescent PSi layers and particles with hydrophobic functionalization have been prepared by assembly of perfluorooctyl-silane. The PSi layers have been used as models to investigate the surface modifications taking place during functionalization and the effects on the wettability. PSi formation conditions have been selected at 110 mA/cm2 in order to maximize the PL emission, which can be used to estimate the presence of a distribution of Si QDs within the 2.4 ± 0.4 nm range. It has been determined by XPS that the perfluorooctyl group is present on the surface and its terminal CF3 carbon represents 0.16% of the total surface composition. The effect of such functionalization is specially remarkable when the PSi structures are aged; the surfaces of pristine PSi are hydrophobic irrespective of the PFOS functionalization, but PFOS protects PSi against oxidation, which reduces the WCA decay after
The authors thank Dr Laura Pascual for operation of HRTEM. Funding through grants MAT2013-46572-C2-1-R and MAT201454826-C2-1-R from Ministerio de Economía y Competitividad is greatly appreciated. C. Rodríguez and P. Pellacani specially thank the EU Commission for the funding received through Marie Sklodowska Curie ITN project Thinface (GA 607232).
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