ARTICLE pubs.acs.org/Langmuir
Electroless Silver Plating of the Surface of Organic Semiconductors Marcello Campione,*,‡ Matteo Parravicini,† Massimo Moret,† Antonio Papagni,† Bernd Schr€oter,§ and Torsten Fritz§ †
Department of Materials Science, Universita degli Studi di Milano Bicocca, Via Cozzi 53, I-20125 Milan, Italy Department of Geological Sciences and Geotechnologies, Universita degli Studi di Milano Bicocca, Piazza della Scienza 4, I-20126 Milano, Italy § Institute of Solid State Physics, Friedrich-Schiller-Universit€at, Helmholtzweg 5, D-07743 Jena, Germany ‡
bS Supporting Information ABSTRACT: The integration of nanoscale processes and devices demands fabrication routes involving rapid, cost-effective steps, preferably carried out under ambient conditions. The realization of the metal/organic semiconductor interface is one of the most demanding steps of device fabrication, since it requires mechanical and/or thermal treatments which increment costs and are often harmful in respect to the active layer. Here, we provide a microscopic analysis of a room temperature, electroless process aimed at the deposition of a nanostructured metallic silver layer with controlled coverage atop the surface of single crystals and thin films of organic semiconductors. This process relies on the reaction of aqueous AgF solutions with the nonwettable crystalline surface of donor-type organic semiconductors. It is observed that the formation of a uniform layer of silver nanoparticles can be accomplished within 20 min contact time. The electrical characterization of two-terminal devices performed before and after the aforementioned treatment shows that the metal deposition process is associated with a redox reaction causing the p-doping of the semiconductor.
1. INTRODUCTION Metalsemiconductor interfaces represent fundamental elements in the fabrication of optoelectronic devices and integrated circuits. Their relevance is recognized in the field of both inorganic and organic semiconductors.1,2 Control on the nanometer scale of the growth of metalsemiconductor interfaces has motivated the development of metallization methods other than thermal evaporation and electrodeposition. With the aim to ensure also a substantial impact on costs and versatility, electroless deposition methods of metallic nanostructures have recently gained much attention.310 Electroless methods rely on the reduction of the metal cations in aqueous solution by direct contact with the target surface, inducing the spontaneous deposition of a metal layer (plating).11 In the case of the presence of a reducing agent in the solution, the target surface and/or metal deposit act as catalyzers of the electroless deposition reaction; otherwise, the target surface itself can serve as the reducing agent, giving rise to the mechanism of the so-called galvanic displacement. This process advances by the progressive transfer (displacement) of oxidized target atoms (ions) into the solution through their permeation in the metal film. The deposition stops when ions are no longer able to diffuse through the metal layer or when a nonconductive layer of oxide forms. Electroless methods for the deposition of metals are well documented for inorganic semiconductors, which induce the deposition process through donation of their valence band electrons. As far as we are aware, no such process was described in the case of the surface of organic r 2011 American Chemical Society
semiconductors. This class of materials necessitates all the more a nondestructive metallization method due to their inherent chemical and structural vulnerability. So far, some soft-metallization techniques successfully applied to organic semiconductors have been described. They rely basically on three approaches: lamination,12,13 ion beam assisted deposition,14 and inkjet printing.1517 The common prerogative of the three approaches is to reduce the process temperature in order to not alter the semiconductor surface. Electroless plating methods are already reliable metallization methods of organic materials. To give an example, their use is well documented in the manufacturing of integrated circuit components based on dielectric polyimide films.18,19 However, organic semiconductors were never subjected to electroless processes with the scope to control the nanoscale assembly of metal layers on their surface. Here, we describe a room temperature electroless deposition of silver nanostructures on the surface of organic semiconductors in single crystal and thin film phase using an aqueous solution of AgF as metal precursor. The influence of the deposition conditions on the process is inferred by X-ray diffraction and atomic force microscopy analysis. We identify a correlation between the maximum metal mass deposited on the semiconductor surface and the semiconductor volume, indicating that the metallization Received: July 8, 2011 Revised: August 9, 2011 Published: August 29, 2011 12008
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involves an oxidation process which is not limited to the semiconductor surface but, if not deliberately halted, propagates into its volume. Interestingly, the semiconductor structure and morphology are observed to be preserved after the plating process, whereas the electrical response of the material changes, indicating the occurrence of doping. These observations combined with the analysis of other physicochemical parameters monitored during the electroless deposition allow us to propose a deposition mechanism having unique characteristics, which substantially differ from traditional galvanic displacement.
2. EXPERIEMENTAL SECTION We focused our study on the paradigmatic semiconductor family of oligothiophenes in crystalline phase.20 With the plan to investigate the role of surface chemistry and conjugation length on the metal deposition process, we selected initially the two couples terthiophene (3T) and α,ω-dibromoterthiophene (3T-Br2, commercially available), and quaterthiophene (4T) and α,ω-dibromoquaterthiophene (4T-Br2, appositely synthesized). These four materials form crystalline phases with a layered structure of close-packed molecules arranged in a herringbone motif,21 giving rise to a low energy molecularly flat basal surface which is an ideal substrate for investigating morphological changes induced by external agents. The basal surface of 3T and 4T crystals is lined up with H-atoms, whereas the basal surface of 3T-Br2 and 4T-Br2 crystals is lined up with Br-atoms. Unfortunately, the surface of 3T crystals sublimates under ambient conditions, and 4T-Br2 could not be processed for obtaining suitable single crystals due to its low solubility in most solvents and its low tendency to sublimate. Therefore, we limited our study to 3T-Br2 and 4T single crystals, while using 4T-Br2 and 4T vapor phase deposited thin films. Additionally, we tested our plating method on other common organic semiconductors. Especially, we selected tetracene (TEN) single crystals for studying the effect of Ag nanoparticle (NP) deposition in the electrical response of two-terminal devices.
2.1. Chemical Synthesis of 2,20 :5,20 :500 ,2000 -Quaterthiophene (4T) and 5,5000 -Dibromo-2,20 :5,20 :500 ,2000 -quaterthiophene (4TBr2). 4T was synthesized following the procedure reported in ref 22. For
4T-Br2, we followed the procedure described by B€auerle et al.23 Briefly, N-bromosuccinimide (0.36 g, 2 mmol) dissolved in dimethylformamide (DMF; 10 mL) was rapidly dropped to a stirred solution of quaterthiophene (0.34 g, 1 mmol) in DMF (80 mL) at 80 °C. After 3 h, the solution was cooled to room temperature, poured onto ice, and extracted several times with CH2Cl2. The organic phases were combined, washed with water, and dried with Na2SO4. Evaporation of solvent and recrystallization from hexane afforded pure 4T-Br2 as a orange solid; yield: 0.40 g (82%); mp 264 °C. 1H NMR (300 MHz) CDCl3: δ = 6.93 (d, 2H), 7.01 (d, 2H), 7.03 (d, 2H), 7.09 (d, 2H).
2.2. Crystal Growth and Device Assembly and Characterization. Single crystals of commercial 3T-Br2 and TEN (Sigma-
Aldrich) were grown by the physical vapor transport method.24,25 3TBr2 was heated with a three-zone furnace at 140 °C at the extremity of a 75 cm long glass tube with 25 mm inner diameter, through which 50 mL/min of nitrogen was fluxed. The middle and end temperatures were 135 and 120 °C, respectively. After 36 h, thin flakes of crystalline 3T-Br2 were found at the central zone of the tube and placed on 1 1 0.1 cm3 quartz plates. TEN was heated at 180 °C in a 20 cm long glass tube with 20 mm diameter using an aluminum cylinder wound by a NiCr filament. The nitrogen gas was fluxed at 20 mL/min through an inner tube of 5 mm diameter. Crystals formed as thin flakes after 12 h of sublimation on the external surface of the inner tube. Single crystals of TEN of the order of 20 mm2 were then placed on ITO-covered glass plates (3060 Ω/sq) serving as anode in diode devices. The cathode was fabricated using Ag tape.26,27 The so-fabricated diodes were
Figure 1. (a) AFM topography of the growth surface of a 3T-Br2 single crystal. A space-filled model of the molecular structure of 3T-Br2 is reported on top. A cross-sectional profile of the monomolecular deep hollow in the center of the image is shown in (b). An XRD specular scan pattern collected on the single crystal (log scale) is reported in (c), showing reflections corresponding to an interlayer spacing of 14.91 Å and a unit cell extending over one molecular layer in depth. In (d), a molecular-scale AFM raw image collected on the crystal surface is reported (the vertical bright band on the left is a scanner-induced artifact associated to the collection of trace scan data). The zoomed-in Fourier-filtered image is reported in (e), showing a rectangular unit cell (white lines) and surface corrugations virtually identical to those of the (010) surface of 2T-Br2 (f) (brown, Br-atoms; light gray, H-atoms; dark gray, C-atoms). characterized by collecting current densityvoltage (JV) characteristics with a Keithley 2400 1A sourcemeter in air (grounded cathode). 2.3. Thin Film Growth. The deposition of thin films was carried out via sublimation at room pressure under nitrogen atmosphere using a close-space sublimation apparatus similar to that used in ref 28. A heating stage (Linkam THMS600) was used for inducing the sublimation at a controlled temperature of the semiconductor powder spread over a glass plate. The substrate was placed at a distance of 3 mm from the sublimating powder, and the film growth was monitored with an optical microscope focusing the substrate plane through the heating stage sealed glass window while a direct thickness measurement was not carried out. The substrate temperature was measured with a thermocouple. 4T-Br2 was deposited at 225 °C, with the substrate held at 100 °C. 4T was deposited at 160°, with the substrate at 70 °C. The deposition time was set to 4 min for both materials. 2.4. X-ray Diffrraction (XRD) Characterization. X-ray diffraction specular scans on single crystals and films were performed with a PANalytical X’Pert Pro powder diffractometer with Cu Kα radiation. 2.5. Atomic Force Microscopy (AFM) Characterization. Surface topography over microscales was analyzed with a Nanoscope V MultiMode atomic force microscope (Veeco) using single-beam silicon cantilevers (force constant of 40 N/m) operating in intermittent contact (tapping) mode. Nanoscale images were collected with single-beam silicon nitride cantilevers (force constant 0.6 N/m) operating in contact mode. Image analysis was performed with the program WSXM.29 12009
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Figure 2. AFM topography (2 2 μm2 areas) of the growth surface of 3T-Br2 single crystals after immersion in dark in a 0.2 mM solution of AgF. In panels (a), (c), (e), and (g), the surface of different crystals immersed for a period (min) reported on the top-right is displayed; in panels (b), (d), (f), and (h), the surface of the same crystal immersed in the AgF solution for subsequent periods (min) reported on the top-right is displayed. The color scale at the bottom of the panels applies to all images.
2.6. X-ray Photoelectron Spectroscoy (XPS) Characterization. The chemical composition of the film surface was analyzed by X-ray excited photoelectron spectroscopy using a nonmonochromatized Mg Kα X-ray source (1253.6 eV) and an EA200 (SPECS GmbH) hemispherical electron analyzer. A metallic Ag film and a AgF deposit on ITO were measured as reference materials for chemical shift analysis.
3. RESULTS AND DISCUSSION 3.1. Semiconductor Single Crystals. To investigate precisely morphological changes consequent to surface treatments, we selected single crystals having a molecularly flat surface. In Figure 1, an AFM image collected on the widest growth surface of a 3T-Br2 single crystal is reported. The crystal surface exhibits smooth terraces of the order of tens of micrometers. Occasionally, steps could be imaged, having a height of 1.5 ( 0.3 nm, as those shown in the cross-sectional profile in Figure 1b. XRD specular scans performed on a single crystal placed on a glass substrate show reflections corresponding to an interlayer spacing of 14.91 Å, in close agreement with the morphological features observed by AFM. These results are consistent with a layered structure of close-packed molecules (length 17.4 Å) tilted by ca. 25° toward the layer plane, similarly to the known structure of the 2T-Br2 crystal.30 This molecular arrangement gives rise to a basal surface being lined up with Br-atoms (Figure 1f). The investigation
with AFM of the nanoscale surface corrugation of the 3T-Br2 crystal (Figure 1d and e) fully validates this conclusion. Indeed, a pseudocentered rectangular unit cell can be identified, having close similarity with that of 2T-Br2(010). When 3T-Br2 crystals are immersed in AgF solution (0.02 mg mL1 = 0.2 mM), deposition of NPs occurs immediately. Two kinds of treatments in solution were carried out: under dark (Figure 2) and under illumination with UV light (254 nm, 4W fluorescent tube) (Figure 3b, d, f, and h). The crystal surface after 2 min of immersion in solution in dark, rinsing with distilled water, and drying with nitrogen, is displayed in Figure 2a. NPs, appearing in bright contrast, are observed to decorate uniformly the surface. By increasing the immersion time, the number density of NPs increases, while keeping their average size substantially unvaried (Figure 2c and e), until after 20 min when an almost complete coverage is reached (Figure 2g). As shown in Figure 3g, NPs possess a lateral size of a few tens of nanometers, whereas their height falls in the interval 15 nm. The AFM morphology of the obtained NPs corresponds to an oblate hemispheroid, in very close agreement to the morphology of initial Au grains deposited by electroless plating on Ge surfaces.5 By scanning the crystal surface over larger areas, one can note the sporadic presence of some clusters of NPs, with a ringlike morphology and surrounded by a depletion region (Figure 3a and c). These clusters, probably deriving from NP coalescence, 12010
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Langmuir are more common on the surface of crystals treated with UV light during immersion in AgF solution. Figure 3b, d, and f shows the surface of a 10 min treated crystal. Besides the presence of single NPs, the deposited overlayer is observed to form a continuous branched lattice (percolation lattice). The average height of the deposit is similar to that obtained under dark conditions, whereas continuous clusters are observed to extend up to 1 μm. NP coalescence is observed to be induced also by subsequent immersion treatments. Figure 2b shows the surface morphology of a crystal after a first immersion in dark for 2 min. A second immersion of 3 min gives rise to the morphology reported in Figure 2d. Some 1D clusters of NPs can be seen on the surface. Subsequent immersions give rise to the formation of 2D clusters which, different from the case in Figure 3, have a more uniform
Figure 3. AFM topography (progressive zoom-in from top to bottom) of the growth surface of 3T-Br2 single crystals after treatment for 10 min with a 0.2 mM solution of AgF in the dark ((a) 10 10 μm2, (c) 5 5 μm2, (e) 1 1 μm2) and under UV illumination ((b) 10 10 μm2, (d) 5 5 μm2, (f) 1 1 μm2). Cross-sectional profiles taken along the blue lines of images (e) and (f) are reported in panels (g) and (h), respectively.
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height and form something more similar to a mosaic tessellation (Voronoi pattern) than to a percolation lattice. 3.2. Semiconductor Thin Films. The low tendency to sublime and the low solubility of 4T-Br2 did not allow us to process this material for obtaining suitable single crystals. Rather, this material forms crystalline film phases reaching high coverages. Figure 4a and c show the surface topography of a thin film obtained using a quartz plate as substrate (rms roughness over a 1 μm2 area: 0.2 nm) kept at 100 °C. The profiles reported in Figure 4b and d reveal a uniform height of the crystalline domains (bright branched islands) nucleated on the substrate surface (dark background), having a layered structure. The results of an XRD analysis confirm the layered structure of the islands, being consistent with an interlayer spacing of 18.01 Å and a unit cell containing two molecular layers (Figure 4e). This spacing is identical to that of the orthorhombic phase of α,ω-dimethylquaterthiophene,31 a molecule having the same length as 4T-Br2. Hence, a similar structure of the film phase of 4T-Br2 can be inferred (Figure 4). On the basis of the structural analysis reported in Figure 4, the surface exposed by the crystalline film of 4T-Br2 is virtually identical to that shown in Figure 1f. The behavior of 4T-Br2 films with respect to the treatment with AgF solution is rather different in comparison to single crystals. We observed that the contact with the metal precursor solution gives rise to the deposition of NPs. However, the number density of NPs and their coverage was observed to be independent of immersion time (for times longer than 2 min).
Figure 5. (a) AFM topography of the surface of a 4T-Br2 thin film grown on silica substrate and immersed for 10 min in AgF 0.2 mM and (b) zoom-in of image (a) as taken over the highlighted rectangular area. (c) Cross-sectional profile taken along the blue line of image (b) (the same scale as that of Figure 3 is used for helping comparison).
Figure 4. (a) AFM topography of the surface of a 4T-Br2 thin film grown on a quartz substrate and (b) cross-sectional profile taken along the blue line of the image. (c) Zoom-in of image (a), and (d) cross-sectional profile taken along the blue line. (e) XRD specular scan pattern collected on the thin film, showing reflections corresponding to an interlayer spacing of 18.01 Å and a unit cell containing two molecular layers. On the right side of panels (d) and (e), a structural model of the film is reported. 12011
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Figure 6. 100 75 μm2 optical micrographs (differential interference contrast) collected on (a) 4T-Br2 and (b) 4T thin films grown on ITO substrate for XPS analysis and corresponding 10 10 μm2 AFM images ((c) and (d), respectively).
Figure 7. (a) XPS spectrum of a Ag-plated 4T-Br2 film deposited on ITO. The black rectangle indicates the spectral position of Ag 3d signal. (b) Long integrated Ag 3d photoemission signals of films deposited on ITO of (1) 4T-Br2, (2) 4T-Br2 after AgF-treatment, (3) 4T, and (4) 4T after AgF-treatment.
The surface topography of crystalline domains after an immersion time of 10 min under dark conditions is reported in Figure 5. By comparing the cross-sectional profile in Figure 5c with that in
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Figure 3g corresponding to samples undergone the same treatment, one can note that the number density of NPs on the crystal and film surface is similar. However, NPs are only 1 nm high on the film surface, and their lateral size is close to the size of the AFM tip apex (ca. 20 nm). To characterize the chemical nature of NPs deposited on the semiconductor surface via X-ray photoemission spectroscopy (XPS) analysis, we deposited 4T-Br2 films on ITO substrates (3060 Ω/sq, surface roughness 3.5 nm (rms) over a 1 μm2 area) (Figure 6a). While the crystal structure and orientation of the film remain unchanged (Figure S1, Supporting Information), the film morphology changes with respect to that of films deposited on quartz (Figure 4): crystalline islands are elongated, and under the same deposition conditions the film substrate coverage is lower. To evaluate the influence of terminal Br-atoms on the propensity of the surface to NP adsorption, we analyzed also isostructural α,ω-quaterthiophene (4T) films deposited on ITO (Figure S1, Supporting Information), grown under conditions leading to similar substrate coverage (50% for 4T-Br2 and 20% for 4T) and average island height (20 nm) (Figure 6b). The elemental composition of the film surface determined by XPS measurements (Figure 7) indicates the presence of metallic Ag on both 4T and 4T-Br2 films, therefore allowing us to refer to the NPs observed in Figures 2, 3, and 5 as Ag NPs. The chemical shift and the line shape of Ag photoemission signal correspond to that of metallic Ag, with a surface coverage of ca. 0.2% (Figure S2, Supporting Information). The amount of F-atoms on the film surface is below the detection limit of the instrument and nonetheless lower than 0.3 times the amount of Ag-atoms. The intensity of the Ag signal appears higher in the case of 4T-Br2 films (Figure 7b); however, it is proportionated to the estimated film coverage, being higher in 4T-Br2 films, thus indicating a substantially equal attitude of different semiconductors to adsorb Ag on their surface. The formation of Ag NPs is independent of the surface composition. Rather, it depends on the bulk properties of the semiconductor. In particular, the above results show that the quantity of NPs deposited is dependent on the semiconductor volume. By assuming that the semiconductor acts as electron donor promoting the reduction of Ag+(aq) to Ag(s) on its surface and that it can provide for one electron per molecule to reduce Ag ions in solution, one molecular layer of 4T, 4T-Br2, or 3T-Br2 (having a molecular density of ca. 4 nm2) is sufficient for reducing a quantity of metallic Ag corresponding to a uniform layer with an equivalent thickness of 0.1 nm. The crystalline islands of 4T-Br2 of Figure 5, comprising ca. 10 molecular layers, are then expected to be plated with a Ag layer of ca. 1 nm, while single crystals of 3T-Br2, comprising ca. 100 molecular layers (thickness ca. 200 nm), are expected to be plated with a Ag layer of ca. 10 nm. These estimates are fully comparable with those deducible from Figures 5c and 3g. The oxidation of the semiconductor molecules would result in the formation of radical cations, possibly coordinated by an anion (F or OH, diffused though the crystal interstitials during the plating reactions). The migration of replenishing negative charges provided by OH toward the semiconductor cations was suggested by Kalkan and Fonash for the electroless synthesis of Ag NPs on Si.9 Silanol groups function as precursor for the further oxidation to SiO2, and the simultaneous production of H+ replacing Ag+. The monitoring of the pH at 25 °C of a suspension of 3T-Br2 in an equimolar solution of Ag salt shows a linear increase from the acidic pH of the metal aquocation solution 12012
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Figure 8. (a) JV characteristic (inset: semilog plot) of a TEN single crystal between ITO and Ag electrodes before (green) and after (blue) Ag electroless plating. (b) AFM surface topography of a TEN crystal after immersion in AgF 0.01 g mL1 for 20 min (height scale: 10 nm).
(4.9 at 0.1 M) to the pH of deionized water under ambient conditions (6.15), with a rate of 0.07 min1 (Figure S3, Supporting Information). This observation indicates an exponential kinetics of the deposition process and corroborates the assumption of equimolarity between deposited Ag atoms and molecular radical cations. Concurrently, it suggests that the electroneutrality of the semiconductor is ensured by the permeation of a spectator anion (F), without releasing of protons. Radical cations may give rise to a solid-state polymerization. This would cause changes of the interlayer spacing of the structure of the order of 20%. Contrary to this hypothesis, X-ray diffraction analysis performed after the solution treatment on both single crystals and thin films reveals no change in intensity, width, and position of the peaks. Rather, radical cations act as additional charge carriers, similarly to a p-doping in traditional semiconductors. The doping effect is apparent in the modification of JV characteristics collected in diode devices having a semiconductor single crystal as active layer. The green curve in Figure 8a represents the JV characteristic collected on an as-grown (001)-oriented TEN single crystal (surface area: 20 mm2) sandwiched between an anode of ITO-coated glass-plate (1 cm2) and a cathode of Ag tape (0.17 mm2) placed at the center of the crystal. A clear rectifying behavior can be observed, with the current onset at 0.25 V forward bias (for bias
