Atomic Ligand Passivation of Colloidal Nanocrystal Films via their

Jan 29, 2013 - Marco Zanella,* Lorenzo Maserati, Manuel Pernia Leal, Mirko Prato, Romain Lavieville, Mauro Povia,. Roman Krahne, and Liberato Manna*...
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Atomic Ligand Passivation of Colloidal Nanocrystal Films via their Reaction with Propyltrichlorosilane Marco Zanella,* Lorenzo Maserati, Manuel Pernia Leal, Mirko Prato, Romain Lavieville, Mauro Povia, Roman Krahne, and Liberato Manna* Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy S Supporting Information *

ABSTRACT: Colloidal nanocrystal films of different materials (semiconductors, metals) and shapes (spheres and rods) were dipped in solutions of propyltrichlorosilane (PTCS) in acetonitrile. This process removed most of the surfactants covering the surface of the tested nanocrystals, leaving their surface either unpassivated or passivated with chlorine atoms, depending on their composition. PTCS was reactive toward most of the surfactants used in nanocrystal synthesis and therefore such a procedure could be applied to a large variety of materials. All samples were characterized with FTIR, XRD, and XPS measurements. In nanocrystal films, the reduction of the separation between the nanocrystals resulting from the removal of surfactants led to an enhancement in both dark and photocurrent. The surface of Au nanocrystals is left unpassivated by the reaction with PTCS, which makes the process potentially useful for applications in catalysis and plasmonics. KEYWORDS: nanocrystals, ligand exchange, chlorosilane, photoconductivity, catalysis metal chalcogenides,18−22 with metal free inorganic ligands,23 and with Meerwein’s salts.24 An interesting aspect of some of these latter procedures resides in the fact that the replacement is done on the NCs in solution and yields surfactant-free NCs that are stable in solution. These new approaches should pave the way to the preparation of easily processable NC inks free from organic ligands. Furthermore, films of NCs coated with inorganic ligands exhibit large values of carrier mobilities, with band-like behavior,20,25 and films of NCs with atomic ligand passivation have shown much improved efficiency in photovoltaic devices.26,27 Some of the ligand replacement methods could decrease interparticle separation down to one or a few atoms,26 as the native ligands on the NC surface were exchanged (for example, by addition of ammonium halides and thiocyanates) with ions such as Br−, Cl−, I−, and SCN−. The film deposition was usually performed following a layer-bylayer spin coating method. Moreover, previous works28,29 have reported ligand exchange procedures involving trimethylsilylating agents as chemicals able to displace oxygen-bearing surfactants and yield solutions of colloidal NCs with unique functionalities: for example, the ligand-exchanged NCs could retain their optical properties even

1. INTRODUCTION Colloidal inorganic nanocrystals (NCs) are promising materials for the fabrication of cost-effective films that are potentially employable in a broad range of applications. Coordinating ligands, such as the surfactant used for their synthesis, play an important role during the NCs synthesis by regulating their size, chemical composition, and morphology. They are also important in driving postsynthesis assembly of NCs and in guaranteeing their long-term stability.1−4 These ligands are however detrimental for the NCs-based applications that require the NCs to be electrically accessible, for example, in thin film transistors, electroluminescent devices, solar cells, and catalysis.5−7 This is because the surfactants commonly employed in synthesis have long aliphatic chains and therefore they are generally insulators and act as large potential barriers for charge carriers to be transferred in and out of the NCs. In the last years, several strategies have been developed for the replacement of the surfactants used during the synthesis with other molecules. These include the use of thermally degradable ligands that can be removed from the surface of the NCs by thermal annealing8−10 or by photodegradation;11 ligand exchange performed directly on NC films with short bifunctional ligands, such as hydrazine, ethylenediamine, ethanedithiol, and others;12−15 or preparation of NC films via layer by layer (lbl) deposition in which the ligand exchange process is performed at each step of the film deposition.12,16,17 These ligands ensure increased electrical accessibility of the NCs by reducing the potential barrier for charge transfer. Recent developments have included ligand replacement with molecular © 2013 American Chemical Society

Special Issue: Synthetic and Mechanistic Advances in Nanocrystal Growth Received: September 19, 2012 Revised: December 19, 2012 Published: January 29, 2013 1423

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NC films were immersed in a 0.1 M solution of hydrazine in ethanol for 20 min and rinsed in ethanol to quench the reaction. All ligand exchange procedures were performed in a glovebox under nitrogen atmosphere. Fourier Transform Infrared (FTIR) Spectroscopy. For FTIR characterization the film was deposited on a glass slide. FTIR spectra before and after PTCS treatment were taken on the same sample and were recorded with a Bruker vertex 70v spectrometer. X-ray Diffraction (XRD) Characterization. This was carried out on a Smartlab 9 kW Rigaku diffractometer equipped with a copper rotating anode. The X-ray source was operated at 40 kV and 150 mA. X-ray Photoelectron Spectroscopy (XPS) Characterization. This was performed before and after the PTCS treatment on a Kratos Axis Ultra DLD spectrometer, using a monochromatic Al Kα source (15 kV, 20 mA). Wide scans were acquired at an analyzer pass energy of 160 eV. High resolution narrow scans were performed at a constant pass energy of 40 eV and steps of 0.1 eV. The photoelectrons were detected at a takeoff angle Φ = 0° with respect to the surface normal. The pressure in the analysis chamber was maintained below 5 × 10−9 Torr for data acquisition. The data were converted to VAMAS format and processed using CasaXPS software, version 2.3.15. The binding energy (BE) scale was internally referenced to the C 1s peak (BE for C−C = 284.6 eV). Electrical Transport Measurements. Interdigitated electrode devices were fabricated by photolithography and metal evaporation (5 nm Ti, 40 nm Au) on Si/SiO2 substrates with electrode spacing of 1 μm. For the electrical characterization the NCs films were deposited onto the devices by drop casting. All the samples underwent a thermal annealing at 180 °C for 20 min under nitrogen prior to the measurements to remove eventual excess organics left in the films. The conductivity of the NC films was probed in a micromanipulated closed cycle cryostat from Janis Research under vacuum (10−5 Torr), and a Keithley 2612 sourcemeter was used to record the current−voltage curves. All measurements were performed at a temperature of 15 K in order to freeze out contributions from trap states which resulted in current instabilities in NC films at room temperature.30 For the photocurrent measurements the samples were illuminated by a Xenon lamp coupled to a glass fiber with a power density of 20 mW/mm2. Nuclear Magnetic Resonance (NMR) Spectroscopy. 1H and 13 C NMR spectra were recorded on a Bruker DRX 400 spectrometer at 298 K. Acetonitrile-d3 and toluene-d8 were used as deuterated solvents.

in aqueous solvents. The surface of these ligand exchanged NCs is not passivated with single atoms; however, it was clearly shown that the use of silylating agents can remove the native ligands and leave halogen or chalcogen atoms on the surface of the NCs. Finding a simple and fast ligand exchange/removal procedure to prepare NC films in which the interparticle separation is reduced to the atomic level can be very useful for large scale applications. Here we exploit both the high efficiency of silylating agents in removing the oxoanionic ligands from the NC surface and their high reactivity toward other functional groups of various surfactants commonly used in NC synthesis (such as −NH2 and −SH) to devise a general ligand exchange approach (see Scheme 1 and Figure S1 of the Supporting Information). The Scheme 1. (top) General Scheme of the Reaction of NCs with PTCS in Acetonitrilea

a

In most cases, Cl atoms are left off the surface of the NCs; in the case of Au NCs, however (bottom), their surface is left unpassivated.

approach is based on simply reacting a film of NCs with a solution of propyl trichlorosilane (PTCS) in acetonitrile. The latter is a good solvent for PTCS and, besides being unable to oxidize this silylating agent, it does not dissolve the NC films. The approach removed the original ligands bound to the surface of NCs and reduced the interparticle separation. Also, depending on chemical composition of the NCs, it left the surface either passivated with chlorine atoms (in most of the NCs tested) or unpassivated (in the case of Au NCs). We also tested the effect of PTCS treatment on the electrical transport properties of several types of NCs. In all the tested cases, the reduction of the separation between the NCs resulting from the removal of surfactants led to a strong enhancement in both dark and photocurrent. Also the possibility to completely remove the passivation in metal NC surfaces, as in Au, is of paramount importance for catalytic applications.

3. RESULTS AND DISCUSSION Ligand Exchange on CdSe/CdS NCs. We initially tested CdSe/CdS core/shell NCs, which have been widely studied in the past for their optical and transport properties as well as their surface characteristics. Figure 1 displays TEM images of NCs before and after their native surfactants, namely, octadecylphosphonic acid (ODPA), trioctylphosphine oxide (TOPO), and trioctylphosphine (TOP), had been exchanged. The reaction with PTCS did not appear to modify the average size of the NCs, although these were slightly reshaped, most likely due to a moderate etching of their surface (this can be seen by comparing Figure 1c with Figure 1d). A reduction in interparticle separation, consistent with removal of surfactants, is visible in images of NC films (Figure 1a,b), although it is more evident in more diluted samples (Figure 1d). This was also supported by high-resolution TEM (HRTEM, see Figure S2a-b of the Supporting Information). FTIR analysis (Figure 2a) indicated that after the treatment with PTCS the peaks around 2900 cm−1, which are ascribable to the aliphatic chains of the surfactants, were strongly reduced in intensity, suggesting that most of the surfactants had been removed from the NC surface. A strong reduction in intensity of these peaks was already observed by Rosen et al.24 after performing the ligand exchange with Meerwein’s salts on films of PbSe NCs.

2. EXPERIMENTAL SECTION Chemicals. Propyl trichlorosilane (PTCS) (97%) and hydrazine monohydrate (98%) were purchased from Alpha Aesar. Water-free acetonitrile and ethanol were purchased from Sigma. All chemicals were used as shipped. Synthesis of NCs. In order to generalize the process, several types of NCs with different compositions (CdSe/CdS, PbSe, CdTe, Cu2Se, CuInS2, Au, Pt), shapes (dots and rods), and surfactants (phosphonic acids, carboxylic acids, amines, and thiols) were synthesized. Synthesis details for these materials are found in the Supporting Information. Ligand Exchange Procedures. PTCS ligand exchange was performed by dipping the film of NCs (drop cast on substrate and having a thickness between 50 and 200 nm) in a solution (0.5 M) of PTCS in acetonitrile for 10−30 s. After that, the sample was rinsed with toluene and left to dry. For ligand exchange with hydrazine the 1424

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Figure 1. (a, b) TEM images of a film of CdSe/CdS NCs as deposited on the TEM grid (a) and after treatment with PTCS (0.5 M for 15 s) (b); (c, d) TEM images of a more diluted sample of the same NCs, as prepared (c) and after treatment (d). The initial NCs are slightly elongated, due to the hexagonal structure of both the CdSe core and the CdS shell, which facilitates preferential growth along their 001 crystallographic direction. After treatment, both a slight reshaping and a considerable decrease in interparticle separation can be seen (d). Statistical analysis of the TEM images (see Table S1 of the Supporting Information) indicates that the reaction with PTCS does not lead to an appreciable reduction of the NCs size.

Annealing of these samples for 20 min at 180 °C did not have much influence on their FTIR spectra (see Supporting Information).31 XRD patterns recorded on films before and after ligand exchange did not show any significant variation in the crystal structure of the sample (Figure 2b), indicating that it was not altered by the process and by the annealing (Supporting Information Figure S4). In the XPS spectra (Figure 2c) of a film of CdSe/CdS NCs treated with PTCS, Cl related peaks at around 200 eV (Cl 2p) could be seen. The position of the Cl 2p3/2 peak (198.6 eV) is consistent with the value reported by Owen et al.29 for CdSe NCs decorated with Cl atoms. According to the NIST database,32 transition metal chlorides have the Cl 2p3/2 peak at 199 ± 0.7 eV, which is comparable with our data. Figure 2d reports XPS spectra, before and after treatment, in a binding energy range between 220 and 234 eV, where the S 2s and Se 3s peaks are located. After treatment with PTCS, the signal from Se was slightly more defined: this could be explained by a moderate etching of the CdS shell. No appreciable change in the optical absorbance spectra (in the UV−visible range) of NC films before and after treatment could be seen (Figure 2e). In both cases, the absorbance was dominated by the CdS shell (peak around 550 nm), while the absorbance from the CdSe cores was centered at 610 nm (Figure 2f). Along with XRD, FTIR, and XPS, these data suggest that the reaction with PTCS leads to a removal of the native ligand and to a passivation of the NCs surface with chlorine atoms without major disruption of the core/shell structure. Ligand exchange did quench the photoluminescence of the NCs, by almost 90% (see Figure 2f). This can be ascribed to possible concomitant causes: (i) a moderate etching of the CdS shell by PTCS, which should induce the formation of trap states on the NCs surface, and (ii) the presence of Cl atoms on the NCs surface. Photoluminescence quenching was indeed already observed by Owen et al.29 when reacting the CdSe/ZnS core/shell NCs with trimetylchlorosilane. The exact mechanism of such quenching (whether, for example, Cl passivation induces important variations in the electronic structure) will require additional investigations.

Figure 2. (a) FTIR spectra of a film of CdSe/CdS NCs drop-cast on glass: transmittance of bare glass slide (blue) and CdSe/CdS film before (black) and after (red) ligand exchange. The two main peaks below 3000 cm−1 are due to the symmetric and antisymmetric stretching of the CH bond; (b) XRD patterns of CdSe/CdS NCs before (black) and after (red) ligand exchange. Spectra are compared with the chart 01-075-5679 for hexagonal CdSe (green) and 00-0010780 for CdS (blue); (c) XPS spectra of a film of CdSe/CdS NCs drop-cast on a silicon wafer in the region between 194 and 204 eV. As deposited (black spectrum) and after the treatment with PTCS (red spectrum); (d) XPS spectra in the region between 220 and 234 eV. As deposited (black spectrum) and after the treatment with PTCS (red spectrum); (e) comparisons of the absorbance spectra of a film of CdSe/CdS NCs before (red) and after (black) ligand exchange. Inset: absorbance of the CdSe cores. (f) Comparison of the PL intensity before and after ligand exchange for the same film reported in (e). Arrows show a tiny blue shift of the spectra maxima that is not detectable in the absorbance spectrum due to the broadness of the peaks. This shift is likely due to a moderate etching of the CdS shell upon reaction with PTCS.

Figure 3 shows data on low-temperature photocurrent, dark current, and photo-to-dark current ratios at various voltages for films of CdSe/CdS NCs that had undergone different treatments. These were (i) as-deposited films (Figure 3a); (ii) films treated with hydrazine, which is also known to replace the native ligands and reduce the inter-NCs separation (Figure 3b); and films treated with PTCS (Figure 3c). The asdeposited, annealed NC films exhibited almost no conduction up to a threshold voltage of around 80 V (corresponding to an electric field of 8 × 105 V/cm), after which current sets in, due to breakdown of the device, and we observed no significant difference between dark and photocurrent (Figure 3a). Hydrazine treated films showed an appreciable photoconductivity with nonlinear characteristics (Figure 3b) that can be explained by exciton dissociation within the individual NCs in the film33 due to the applied electric field, in agreement with previous studies.34−36 The PTCS treatment of the NC films resulted in a significant increase in photoconductivity and in an almost linear voltage 1425

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annealing did not differ much from each other (Figure S4d). Transport measurements on films of CdSe/CdS rods are reported in the Supporting Information (Figure S5): they show a photocurrent enhancement, upon ligand exchange, that is comparable to that of the spherical core−shell particles discussed above.31 Recently, in our group we reported the fabrication of films consisting of multilayers of vertically aligned nanorods on a substrate.37 We were able to perform the ligand exchange with PTCS also on such films of close-packed, vertically aligned CdSe/CdS nanorods, and Supporting Information Figure S3a-b reports top views of a film of CdSe/CdS rods before and after the ligand exchange with PTCS. After the ligand exchange, the rods are still vertically aligned, and the interparticle distance is strongly reduced due to the ligand removal (Figure S3b). Similarly to the case of the slightly elongated CdSe/CdS core shell NCs, CdSe/CdS rods are structurally stable to the reaction with PTCS and native ligands are stripped from their surface and replaced with Cl atoms (Figure S3). Ligand Exchange on Other Semiconductor NCs. In order to prove the generality of the PTCS treatment, we extended it to NCs with different shapes, compositions, and types of surfactants coating their surface. We discuss in the following the results on semiconductor CdTe, PbSe, Cu2Se, and CuInS2 NCs. CdTe NCs (dots and rods) could withstand the treatment with PTCS without being degraded. FTIR (Supporting Information Figure S6e) shows that most of the ligands (phosphonic acids) are removed from the NCs surface and the shape, size, and crystal structure are preserved (Figure S6a-d,f for dots and Figure S7 for rods). XPS data (Figure S6g) indicate that the former ligands are replaced with chlorine atoms.31 Transport measurements on films of CdTe quantum dots show clear increase of photocurrent and photo- to dark current ratio after PTCS treatment (Figure 4), confirming the success of this treatment as in the CdSe/CdS NC case discussed above.

Figure 3. Photo (red) and dark (black) current and their ratios (empty circles) are shown for films of CdSe/CdS NCs that had followed various treatments, recorded at 15 K. (a) As-deposited films; (b) films treated with hydrazine; (c) films treated with PTCS. In all cases the films underwent a thermal annealing at 180 °C for 20 min prior to the measurements, to remove volatile residual organics.

dependence, as demonstrated in Figure 3c. Compared to the hydrazine treated films, in the high bias range the photoconductivity increased by 5 orders of magnitude. This behavior can be attributed to the reduction of the interparticle distance within the film. Such small interparticle separation decreases the tunnel barrier in between the NCs significantly, leading to strong electrical coupling. These effects result also in an appreciable conductivity in the dark, because due to the reduced film resistance a large fraction of the applied bias drops on the interfaces of the NCs film with the metal electrodes, which facilitates charge injection. Figure 3c shows that PTCS treatment led to large photocurrent to dark current ratios, exceeding 103 over the full measured bias range, which makes this process interesting for photovoltaic and photosensing applications. Figure S1 of the Supporting Information reports SEM images of the films of CdSe/CdS NCs drop-cast on top of the gold electrodes. Figure S1a-b refers to the film before the treatment with PTCS, while Figure S1c-d refers to the film after the treatment. In the latter case, the film presents cracks due to the reduction of the interparticle separation following surfactant removal. The formation of cracks in the NCs film was found even when the ligand exchange was performed with other molecules such as hydrazine and amines.10 Tests were carried out also on films of CdSe/CdS nanorods. Figures S3 and S4 in the Supporting Information demonstrate that PTCS treatment of CdSe/CdS rods, followed by annealing at 180 °C for 20 min, leaves the particles structurally stable. Annealing was found to be responsible for a certain loss of Cl atoms from the NC surface (Figure S4b). Analysis of XPS spectra show that the ratio Cd:Cl decreases from 83:17, before annealing, to 89:11 after it. This could be due to the thermal desorption of Cl atoms from the nanoparticles surface since the FTIR spectra taken on the sample before and after the

Figure 4. I−V curves on a CdTe quantum dots film at 15 K. Photo (red) and dark (black) current and their ratio (empty circles) are shown for (a) as-deposited film; (b) film treated with PTCS. Both samples were annealed at 180 °C for 20 min. 1426

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remained unpassivated. This could be explained considering that the bond between the chlorine and the gold atoms is weaker than for other materials: Raman measurements performed on bulk electrodes have shown that the bond between Cl and Au has the lowest strength constant among the bonds that Cl can form with other metals such as Pt, Ag, and Cu.40 Mrozek and Weaver40 showed that the strength constant with chlorine atoms increases sharply toward the left of the 4d (Rh > Pd > Ag) and 5d (Ir > Pt > Au) series tested, suggesting a metal dependent covalency of the metal-chlorine bond. Similar results were obtained when starting from Au NCs passivated with dodecanethiol,31 although a slight signal from Cl could be detected from XPS in the NC film before the reaction with PTCS. Our conclusion is that the final outcome of the ligand exchange procedure, at least for Au NCs, is not much dependent on the type of ligands that are originally present on the surface of the NCs. The absence of chlorine peaks in the samples of Au NCs, after the ligand exchange, supports the hypothesis that in all the other tested materials the Cl signal in the XPS measurements was coming from atoms that are passivating the NC surface and not from contaminations of PTCS left in the film after the rinsing with toluene. Having gold NCs with a surface that is ligand free is important for catalytic purposes, and the absence of chlorine is preferable since it behaves as a poisonous agent in several catalytic reactions.41 The removal of the surfactants passivating the surface of Au NCs reduced the interparticle separation between the NCs (Figure 5b and Figure S10b, Supporting Information), and this influences both film conductivity, as it increased by more than 7 orders of magnitude (Figure 5d), and the plasmonic properties of the NC film. Figure 5c clearly demonstrates how the absorbance of a film of Au NCs changes after the reaction with PTCS. The reduction of the interparticle separation between Au NCs enhances the plasmon coupling between the NCs, and it is responsible for the formations of the plasmon peaks observed at around 600 and 650 nm, whose contribution to the film absorbance becomes dominant after the reaction with PTCS. This effect was previously observed for example on films of gold nanoparticles either subject to pressure high enough to reduce the interparticle distance or passivated with diamine ligands having different lengths.42 Generally, the reactivity of the PTCS toward the surfactants at the surface of the NCs was found to be enhanced by the presence of acetonitrile. An example is given by gold NCs, for which the reaction with pure PTCS took several hours (Supporting Information Figure S11a(top),b,c) while after 10 s of treatment with PTCS in acetonitrile the NC film was no longer soluble in toluene (Figure S11a(bottom)). The enhancement of the reactivity of trichlorosilane in acetonitrile was already reported by other groups: for example, a mixture of trichlorosilane with diphenylsulfoxide in acetonitrile was found to detonate at 10 °C while the mixture was found to be stable with several other solvents.43 One possible explanation for the detonation was the formation of HCl due to the hydrolysis of chlorosilane in acetonitrile. This highlights the importance of the reaction environment which was found to be fundamental also in the case of hydrazine, which requires ethanol to remove efficiently the ligands from the surface of the NCs.10 In this respect, one could hypothesize the existence of a mechanism involving acetonitrile in the reaction of PTCS and the surfactants covering the surface of NCs. In this respect, we compared NMR spectra at 298 K of

PbSe NCs are a promising material for photovoltaic applications due to the large Bohr radius and small band gap of PbSe,38 and the removal of the native ligands was found to improve significantly the performances of devices based on these NCs.24,26,39 We found that the PbSe NCs were stable during the ligand exchange with PTCS and they did not undergo dissolution, nor appreciable variation in size, as can be seen from Figure S8a-b of the Supporting Information (see also Table S1).31 Moreover, in Figure S8b the reduction of the interparticle separation is clearly visible and is consistent with the removal of the original surfactants. FTIR measurements (Figure S8c) show that the oleic acid molecules present on their surface are removed. In the XPS spectra of the sample after the reaction with PTCS, the presence of the 2p peaks of chlorine around 200 eV of binding energy suggests again that the native ligands have been replaced with chlorine atoms (Figure S8e). On the basis of XRD analysis, the crystalline structure of the particles remained unchanged throughout the ligand exchange process (Figure S8d). Even copper-based NCs were found to lose their native ligands in the reaction with PTCS. The ligand exchange with PTCS was performed on samples of Cu2Se and CuInS2 NCs passivated with various common ligands (such as oleic acid, oleylamine, or phosphonic acids). Various analyses confirmed exchange of ligand also for these NCs; in particular, in Supporting Information Figure S9 it is shown how the FTIR spectra of these particles change after the reaction with PTCS. Ligand Exchange on Metal NCs (Au, Pt). Au NCs could withstand the reaction with PTCS (Figure 5a-b). By combining

Figure 5. TEM images of Au NCs before (a) and after treatment with PTCS (b). (c) Absorbance spectra of Au NCs in solution (blue), as film on glass (black), and as film on glass after the treatment with PTCS (red). (d) I−V semilog plot referred to as deposited (red, upper x-axis) and PTCS treated sample (black, bottom x-axis).

data from TEM and XRD we concluded that there was no change in size, morphology, (Figure 5b and Figures S2c, S10a,b,d of the Supporting Information) and crystal structure of the particle upon ligand removal (oleylamine), the latter supported by FTIR (Figure S10c). In this case, however, no signal from Cl species was detected in XPS (Figure S10e). It appears therefore that the surface of the Au NCs should have 1427

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solutions of PTCS in acetonitrile-d3 and toluene-d8 with acetonitrile. The comparison appeared to rule out reaction of acetonitrile with PTCS.31 The reaction with PTCS on Pt NCs passivated with oleic acid and oleylamine removes the native surfactants, preserves the NCs morphology and structure, and leaves the particle surface covered with chlorine atoms (Supporting Information Figure S13). Furthermore, Cl atoms could be removed from the Pt NCs surface by reacting the NCs film with ammonium hydroxide solution (here again, ligand free metal NCs are especially suited for catalysis). NH4OH was previously used to prepare plating solutions from platinum chloride powder for the deposition of platinum catalyst to use in fuel cells.44 After the treatment with ammonium hydroxide, while the average size of the nanoparticles remained unchanged (Supporting Information Figure S13c and Table S1), XPS clearly showed the removal of Cl atoms from the NC surface (Figure S13g). In addition, the XPS spectrum of the NCs film presented a small peak centered around 398 eV, which was identified as the 1s peak of nitrogen (Figure S13h). It is likely that some ammonium hydroxide molecules remained bound to the surface of the Pt NCs after the treatment.31

ASSOCIATED CONTENT

S Supporting Information *

Additional details on the characterization of the films of nanocrystals of CdSe/CdS, PbSe, CdTe, Cu2Se, CuInS2, Au, and Pt NCs before and after the ligand exchange. This material is available free of charge via the Internet at http://pubs.acs.org.

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4. CONCLUSIONS In conclusion, in this work we have reported a simple approach to perform an atomic passivation on films of a wide variety of NCs by using propyltrichlorosilane. The process was found applicable to NCs passivated with the most common ligands used in colloidal synthesis. In test studies, the reduction of the interparticle distance strongly improves the film conductivity. In some cases, for example, metal NCs such as Au NCs, the removal of the ligands from the NC surface could be helpful for their exploitation in catalysis. The process presented here should be easily extended to other types of silylating agents, such as bis(trimethylsilyl) sulfide or selenide, which should be able to replace the surface ligands with other atomic species, such as chalcogen atoms.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.Z.), [email protected] (L.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge partial financial support from the European Union through the FP7 starting ERC Grant NANOARCH (Contract No. 240111). M.P.L. thanks the Junta de Andaluciá for a postdoctoral fellowship. The authors would like to thank Iwan Moreels and Tania Montanari for helpful discussions, Francesco De Donato for help with the synthesis of CdSe/CdS NCs, and Alessandro Genovese for assistance in HRTEM imaging. 1428

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