Self-Assembled Multilayers of CdSe Nanocrystals and Hydrazine or

ARTICLE pubs.acs.org/JPCC

Self-Assembled Multilayers of CdSe Nanocrystals and Hydrazine or Linear Diamines B. Vercelli and G. Zotti* Institute for Energetics and Interphases-IENI CNR, C.o Stati Uniti 4, 35127 Padova, Italy

A. Berlin Institute for Molecular Science and Technologies-ISTM CNR, Via C. Golgi 19, 20133 Milano, Italy ABSTRACT: Hydrazine and two linear diamines, H2N-(CH2)n-NH2 (n = 2 and 6), were reacted with (hexadecylamine/stearate)-capped CdSe nanoparticles (7.5 nm diameter) to form multilayers on ITO glass via layer-by-layer alternation. The new materials were investigated by cyclic voltammetry (CV), UV-vis and FTIR spectroscopies, and photoconductivity. FTIR analysis of multilayer growth showed the complete removal of both L (hexadecylamine) and X (stearate) caps; the latter occurred via complexation of the external cadmium layer. Bulk CdSe/hydrazine polymer, obtained from treatment of a CdSe nanocrystal (CdSe-NC) dispersion with excess hydrazine, included massive amounts of hydrazine in a N2H4/CdSe molar ratio of approximately 24. On the contrary, CV analysis showed that hydrazine covers the NC surface of multilayers with approximately 2 10-9 mol cm-2; specifically, the N2H4/CdSe molar ratio was much lower (ca. 0.5). FTIR analysis of bulk CdSe/diamine polymers showed the same NC surface coverage, (1-2) 10-9 mol cm-2, of the CdSe/hydrazine multilayers. The shortest hydrazine linker results in the highest photocurrent values. Diamines display lower photoconductivity values, decreasing as the N-N distance is increased.

1. INTRODUCTION The insulating nature of organic ligands commonly used in nanocrystal synthesis results in poor interparticle coupling. Highly photoconductive films of CdSe nanocrystals have thus been prepared by exchanging the original bulky long-chain ligands with short diamines, such as ethylenediamine (EDA) in layer-by-layer (LBL) methods.1 The introduction of EDA in simple drop-cast films results in increased photoconductivity from interparticle transport of charge carriers. In addition, LBL films exhibit an even higher photoconductivity due to the stronger electronic coupling, higher order, and full photoluminescence quenching.1 Hydrazine, which can be viewed as the shortest diamine, can serve as a more convenient ligand for colloidal nanocrystals by replacing the original long-chain organic ligands. Chemical treatments with hydrazine enabled considerable progress in the preparation of conductive PbSe-NC solids.2 Treatment of pristine PbSe-NC films in an acetonitrile solution of hydrazine enhances the charge transport.3,4 Recently, CdSe as an aligned nanorod structure has been investigated, and again, photocurrent was enhanced by hydrazine treatment.5 Several points, however, require clarification, such as the amount and disposition of the hydrazine or diamine linker in the NC polymer solid and the dependence of photoconduction on the linker length. The aim of this work was to provide insight into the extent to which the common bulky surface ligands, capping spherical CdSe-NCs, are replaced by hydrazine and how much the r 2011 American Chemical Society

photoconductance of the relevant LBL films is enhanced. The obtained results were compared with those from layers produced with longer linkers including, ethylenediamine (EDA) and hexamethylenediamine (HMDA) (Scheme 1).

2. EXPERIMENTAL SECTION 2.1. Chemicals and Reagents. Acetonitrile was reagent grade (Uvasol, Merck) with a water content < 0.01%. The supporting electrolyte tetrabutylammonium perchlorate (Bu4NClO4), pure hydrazine hydrate (N2H4 3 H2O), ethylenediamine (EDA), hexamethylenediamine (HMDA), 3-mercaptopropionic acid (MPA), poly(acrylic) acid (PAAH), and all other chemicals were reagent grade and were used as received. Poly(vinylbenzoic) acid (PVBH) was produced as reported previously.6 Soluble CdSe-NCs with the surface capped by hexadecylamine and stearic acid were produced as previously reported.6 The NCs display the first exciton peak at 645 nm, corresponding to an average size of 7.5 nm.7 CdSe-NCs were used ca. 10-2 M (as CdSe units) in CHCl3. 2.2. Substrates and Multilayer Film Formation. Transparent conducting surfaces were prepared from indium tin oxide (ITO)/glass electrodes (20 Ω sq-1 from Kintec, Hong Kong). Received: October 26, 2010 Revised: February 9, 2011 Published: March 02, 2011 4476

dx.doi.org/10.1021/jp110223f | J. Phys. Chem. C 2011, 115, 4476–4482

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atmosphere (50 cm3 min-1). The samples were heated in an open Al2O3 crucible at a rate of 5 °C min-1 from 30 to 350 °C. 2.3.3. Photoconductivity. Photoconductivity measurements of the multilayers were performed with a special Hg electrode, contacting the multilayer-covered ITO as described previously.6 Bias was applied to the ITO versus Hg electrode. Illumination was performed on the back glass side of the ITO/multilayer with a water-filtered 100 W halogen lamp spotted over an area of ca. 10 cm2. The resulting light power, calibrated with a silicon photodiode, was ca. 100 mW cm-2.

3. RESULTS AND DISCUSSION

The buildup of multilayers was performed according to the LBL methodology. Specifically, the electrodes were dipped alternatively into solutions of the two components. After each immersion, the substrate was carefully washed and dried in air. Multilayers were built on PVBH- or PAAH-primed ITO (from 10-3 M polymer in EtOH)6 via alternation of 10-2 M CdSe-NCs in CHCl3 and 10-3 M linker in EtOH. The exposure time was 5 min in all cases. Adsorption occurs equally on both sides of the ITO/glass electrodes. The gold surface for infrared reflection absorption spectroscopy (IRRAS) measurements was a 2 5 cm2 slab primed with 10-3 M MPA in EtOH for 3 days.8 The resulting thiolate bond produces a carboxylic-acid-modified surface for efficient coordination to CdSe-NCs. 2.3. Apparatus and Procedure. 2.3.1. Electrochemistry, Spectroscopy, and Profilometry. Electrochemical experiments were performed in acetonitrile þ 0.1 M Bu4NClO4 at room temperature and under nitrogen using three electrode cells. The working electrodes for cyclic voltammetry (CV) of layers were 1 4 cm2 ITO/glass sheets. The counter electrode was platinum, and the reference electrode was a silver/0.1 M silver perchlorate in acetonitrile (0.34 V vs SCE). The voltammetric apparatus (AMEL, Italy) included a 551 potentiostat modulated by a 568 programmable function generator. UV-vis analysis was performed on a PerkinElmer Lambda 15 spectrometer. FTIR spectra were obtained using a PerkinElmer 2000 FTIR spectrometer. IRRAS of the layers was performed with a grazing incidence reflection unit (Specac). All spectra were recorded with 2 cm-1 resolution at an angle of incidence of 80° relative to the surface normal. Ten cycles were run for each spectrum, and weighted subtraction of the background at the end of the series of measurements was applied. No gas purging of the chamber was necessary. The multilayer thickness was determined with an Alphas-step IQ profilometer from KLA Tencor. 2.3.2. Thermogravimetry-Differential Thermal Analysis (TG-DTA). Thermal decomposition was carried out in a Netzsch STA 449C thermoanalytical instrument in a flowing nitrogen

3.1. CdSe/Linker Bulk Polymers. Addition of excess hydrazine, EDA, or HMDA to the CdSe-NC dispersion in CHCl3 causes the precipitation of aggregated NCs. After stirring overnight, the insoluble material was filtered off, washed with CHCl3 and EtOH, dried under vacuum, and FTIR analyzed as a KBr pellet. 3.1.1. CdSe/N2H4. FTIR Analysis. The FTIR spectrum of the original CdSe-NCs (Figure 1a1) shows two bands at 3310 and 1635 cm-1, corresponding to the N-H stretching and bending modes of the surface-capping hexadecylamine ligand. The two bands at 1540 and 1415 cm-1 are due to the COO asymmetrical and symmetrical stretching modes of the stearate ligand. Strong bands at 2930 and 2850 cm-1 (stretching), medium at 1470 cm-1 (scissoring), and weak at 720 (rocking) cm-1 are due to the methylene moieties in the alkyl chain of the capping ligands. Upon treatment with N2H4 (Figure 1a2), all the previous bands disappeared, revealing the complete substitution of both hexadecylamine and stearate ligands. New bands were present, namely, a dominant and broad band at 3040 cm-1 with a quartet structure (3300, 3160, 3040, and 2940 cm-1) and two medium bands at 1080 and 955 cm-1. The former are attributable to the N-H stretching modes of hydrazine.9 The bands at 1080 and 955 cm-1 are typical for the N-N stretching and asymmetric NH2 wagging modes of hydrazine, respectively.9 The weaker bands at approximately 1600 cm-1 are due to the hydrazine NH2 scissoring mode.9 Elemental and TG-DTA Analysis. Elemental analysis of the CdSe/N2H4 polymer gave Cd = 6.33% (weight) and N = 37.16% (weight), corresponding to a N2H4/CdSe molar ratio of 24. Because the ratio of surface and bulk molecules is normally around 0.1 in our CdSe-NCs,10 the result indicates a large amount of hydrazine, possibly caged in large cavities of a CdSe-chain gel structure. This was confirmed by thermogravimetry. The thermal behavior of the CdSe/N2H4 polymer under nitrogen atmosphere and dynamic heating conditions (5 °C min-1 heating rate) is shown in Figure 2. The TG (black) shows a region of high stability, up to 210 °C. A massive weight loss is shown between 210 and 320 °C, until a constant weight of 7% is attained. The loss is attributable to hydrazine, which is in substantial agreement with the elemental analysis data. The corresponding DTA curve (red) shows two strong endothermic peaks around 98 and 311 °C. The latter is related to the end of elimination of intercalated hydrazine and water from the gel structure with aggregation of the CdSe-NCs. The former, however, is more difficult to interpret. We suggest that such a process, which occurs without any appreciable weight loss, is attributable to an internal rearrangement of the hydrazinewater caging CdSe-NC network. 4477

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Figure 1. FTIR spectra (KBr pellet) of CdSe-NCs (a1) before and (a2) after hydrazine treatment, (b) of the CdSe/EDA bulk polymer, and (c) of the CdSe/HMDA bulk polymer.

On the basis of previous results, it appears that the bulk CdSe/ N2H4 polymer is disordered and mainly composed of the linker itself. The bulk polymer from the diamines and the LBL multilayers described below are, in this respect, completely different. 3.1.2. CdSe/EDA and CdSe/HMDA. The FTIR spectra (Figure 1b,c) show the disappearance of all bands corresponding to the original capping ligands, only showing bands from the diamine. The strong NH2 wagging band at 900 cm-1 of EDA is shifted to 998 cm-1 (Figure 1b), suggesting coordination to the CdSe-NC surface. In the case of CdSe/HMDA (Figure 1c), the band is shown at 1050 cm-1. Infrared spectra of complexes of ethylenediamine with bivalent ions, such as cadmium(II), have shown a similar shift.11 The 100-150 cm-1 shift may be due to complexation electronic factors. The intensity of the NH2 wagging band in the KBr pellet has allowed the evaluation of the diamine content of the particles. To this end, the extinction coefficient, ε, of EDA was evaluated from the intensity of the band in a toluene solution. In this medium, EDA shows a band at 878 cm-1, with ε = 55 M-1 cm-1. Using this value, the EDA content of the nanoparticles was evaluated to be approximately 13% in weight, corresponding to an EDA/ CdSe molar ratio of 0.5. In the case of HMDA, the diamine content of the nanoparticles was evaluated to be approximately 16% in weight, corresponding to an HMDA/CdSe molar ratio of 0.3. These values are quite low, compared with hydrazine, and provide clear evidence that no gel is formed by the organic scarcely hydrophilic linker. The lower content (CdSe-NC surface

Figure 2. TG-DTA analysis of the CdSe-NCs/hydrazine polymer.

coverage) of HMDA compared with EDA suggests weaker bonding and may be responsible for the lower multilayer growth rate (see below). 3.2. CdSe/Linker Multilayers. Multilayers are built on the ITO electrodes with alternate exposure to the linker and CdSe dispersions. UV-visible spectroscopy was used to monitor the assembling process of the LBL films. The obtained optical and thickness parameters are summarized in Table 1. All the structures are uniform under SEM examination and robust enough to withstand a standard sticky-tape test with no appreciable loss. 4478



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3.2.1. CdSe/N2H4. The CdSe-NC monolayer, which is regularly formed on the ITO/PVBH support, was not dissolved in pure hydrazine hydrate. Additionally, the absorbance of the exciton band was changed, but only a modest blue shift of 5-7 nm was observed (Figure 3a). This shift, which was not registered in the treatment with a diluted (10-3 M) solution of hydrazine in EtOH, may be attributed to dissolution of the external shell of the capped CdSe-NC layer (see below). Subsequent treatment with the CdSe-NC suspension formed a second CdSe layer with the same absorbance and at the same maximum wavelength, suggesting that hydrazine formed a layer that is able to further coordinate to CdSe-NCs. It is noteworthy that this happens at the same level with both pure and diluted hydrazine. The absorption spectra of multilayer films prepared with different numbers of layers from 10-3 M N2H4 in EtOH is shown in Figure 3b1. The observed linear increase of CdSe-NC absorbance versus number of layers indicated a stepwise uniform assembly process (Figure 3b2). The two-side absorbance differential increase (ΔA) at 645 nm due to CdSe-NC is 410-3 AU layer-1. This band was recorded at the same energy of the first exciton band of CdSe colloidal particles in solution. This result indicates that the CdSe-NCs are successfully assembled without aggregation and with directly assembly process (Figure 3b2). Table 1. Two-Side CdSe Exciton-Band Differential Absorbance (ΔA), Total Thickness (d) of (CdSe/linker)10 Multilayers on PVBH-Primed ITO, and Solid-State Photocurrents iph under 100 mW cm-2 Illumination (at 1 V Applied Potential on 0.023 cm2 ITO-Hg Contacts) Using Different Linkers linker

ΔA/10-3 AU layer-1

d/nm

iph/mA cm-2

N2H4

4.0

42

40

EDA

5.0

45

10

HMDA

2.0

16

PAAHa12 a

PAAH = poly(acrylic) acid.

2 10

The thickness of a 10-multilayer, (CdSe/N2H4)10, is 40 nm. Compared with the optical growth, this thickness indicates a compact multilayer structure.12 3.2.2. CdSe/EDA and CdSe/HMDA. EDA also caused the 5 nm blue shift of the exciton band of a CdSe-NC monolayer; however, even a 10-3 M solution in EtOH can cause this change. This shift indicates an action on the NC surface stronger than that operated by N2H4, which may be attributed to the higher electron donor properties of the diamine, compared with hydrazine, and to its high chelating ability, leading to very stable five-atom rings. A shift difference was not found after HMDA treatment under the same conditions. It was recently reported that excess propylamine blue shifts analogously to the absorption spectrum of carboxylate (stearate or oleate)-capped CdSe-NCs while removing the caps.13 Therefore, it appears that primary amines dissolve surface atoms and cooperatively passivate the surface dangling bonds on CdSe-NCs. For both R,ω-diaminoalkane linkers, EDA and HMDA, the multilayer growth performed with 10-3 M linker in EtOH was regular (see Table 1). The HMDA efficiency was lower compared with others, which may be attributed to a weaker action on the NCs by the long-chain linker. 3.3. Decapping of CdSe-NC Layers by Hydrazine and Diamines. In a recent report on the surface composition and stability of capped CdSe-NCs,10 the Cd/Se ratio was higher than unity, decreasing with increasing diameter so that approximately 85% of the surface of the NCs was covered by cadmium ions. The results were obtained using a combination of XPS, ICP-AES, and NMR analysis. This cadmium enrichment on the surface is mainly attributed to negatively charged (X-type, stearate in our case) ligands available to balance the positive charge of the cadmium-rich surface. We expected the hydrazine or diamine ligands to substitute the L-type hexadecylamine ligands because the X-type stearate ligands bind more strongly to the cadmium-rich surface. The primary origin of the high affinity of the X-type ligands for the NCs is their capacity for multidentate coordination to the NC surface and their negative charge, which balances the net positive charge of the cadmium-enriched surfaces of the NCs.

Figure 3. (a) UV-vis spectra of the ITO/PVBH/CdSe monolayer (red) before and (black) after tratment with hydrazine for 5 min. (b1) UV-vis spectra of ITO/(PVBH/CdSe)/(N2H4/CdSe)n multilayers (n = 0-9). (b2) Relevant plots of CdSe absorbance vs number of layers. Spectra are background-corrected. 4479



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the LBL layers is expected to be much lower than that in the bulk polymer described above. The CdSe-NC monolayer deposited on top of the priming MPA layer kept the -CH2- stretching band at 2930 and 2850 cm-1, but the following N2H4 treatment caused the complete disappearance of the alkylated caps in the surfacebound CdSe-NCs (Figure 4). With subsequent layering, the signal reappeared with the same intensity at NC treatment and disappeared with N2H4 treatment. Thus, it is inferred that the layers are composed of CdSe-NCs and hydrazine, with the full loss of the originally bound caps. Failure in detecting the hydrazine sublayers by FTIR pushed us to an alternative method, which took advantage of the relatively easy oxidation of this linker, namely, CV oxidative analysis (see the next section). 3.5. CV Analysis of CdSe/N2H4 Multilayer Growth. CV investigation was performed on layers of PAAH-primed ITO with ΔA at 645 nm of 3.5 10-3 AU. The CdSe content was close to that of PVBH-primed ITO but with the electrochemical advantages of a single oxidation peak for the CdSe units.6 The CV of the ITO/PAAH/CdSe/N2H4 monolayer in acetonitrile þ Bu4NClO4 (Figure 5a) shows a sharp irreversible oxidative process due to the CdSe units6 at Ep = ca. 1.4 V, preceded by a lower process at approximately 0.8 V. We attribute the latter process to the oxidation of the N2H4 linker molecules in the nanocrystals. In fact, N2H4 in acetonitrile displayed on the ITO electrode an irreversible oxidation peak at 0.5 V (Figure 5a). The 0.3 V positive potential shift on the CdSe-NC surface may be attributed to coordination to the surface cadmium sites. The oxidation process, performed in the presence of an efficient proton scavenger, requires 2 F mol-1.15 Here, it appears that the CdSe-NCs may act as proton scavengers (eq 1): N2 H4 þ CdSe - 2e f HNdNH þ H2 Se þ Cd2þ

Figure 4. IRRAS spectra of progressive sublayers (1-10) from alternation of CdSe (even) and N2H4 (odd) on MPA-primed gold (1).

In our case, the mild hydrazine or HMDA treatment does not corrode the NCs but removes all of the L-type ligands (hexadecylamine) and unexpectedly removes all of the majority surface capping X-type (stearate) ligands (see FTIR analysis below), with no perturbation of the electronic spectrum of the NCs. The more aggressive treatment with pure hydrazine or with EDA removed the cadmium sites of the semiconductor just below the outer monolayer for a depth that the exciton band shift (approximately 6 nm lower than the starting 645 nm value) allows to evaluate7 as approximately 0.3 nm, that is, comparable with the Cd-Se bond length (0.36 nm). Hydrazine or diamine complexation of surface cadmiumstearate is feasible. As a typical example, nickel hydrazine complexes, such as [Ni(N2H4)2]Cl2 and [Ni(N2H4)3]Cl2, are known.14 3.4. FTIR Analysis of CdSe/N2H4 Multilayer Growth. IRRAS analysis of multilayer formation on a MPA-primed gold surface has resulted in the progression of ligand substitution in the NCs. The fate of CdSe-NC capping ligands was followed by the strong C-H stretching bands at 2930 and 2850 cm-1 due to stretching of the methylene moieties of the alkyl-chain caps. The NH2stretching band at 3400 cm-1 or the NH2-asymmetric-wagging band at approximately 950 cm-1, which would have marked the contribution of hydrazine, were not detected in the layers due to weakness, broadening, and possibly exclusion by surface selection rules in ordered structures. In fact, the hydrazine content of

ð1Þ

As the multilayer was progressively built, we observed that the CdSe content, which is measured by the optical intensity and CV total charge,6 increased linearly with the number of layers. The charge assigned to the N2H4 oxidation (at the first CV process) increased sublinearly (Figure 5b), with an initial charge of 0.35 mC cm-2. Given a two-electron stoichiometry for N2H4 oxidation, the surface content at the first layer is 2 10-9 mol cm-2 layer-1. Therefore, a dense coverage was formed, about twice that of alkanethiols on gold.16 Because the theoretical ratio of bulk CdSe (Nb) and surface ferrocene (Ns) molecules, Nb/Ns, is approximately 10 for a 7.5 nm CdSe-NC,6,12 the N2H4/CdSe molar ratio in multilayers was approximately 0.5, much lower than that in the bulk polymer (approximately 24; see above) but essentially the same as that of the bulk CdSe/diamine polymers (see above). In the (N2H4/CdSe)10 multilayer, the N2H4 oxidation charge was only 1.5 mC cm-2, approximately 50% of that expected for a full oxidation. It has been suggested that the progressive decrease of the N2H4 oxidation charge is due to the stabilization of the linkers as bridges. In other words, the most easily oxidized molecules are the one-end coordinated molecules. The bridging molecules are progressively buried into the structure as the multilayer grows, becoming harder to be electrochemically oxidized. 3.6. Thermal Stability of Multilayers. Thermal stability was determined for hydrazine-based multilayers only, due to high 4480



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Figure 5. (a) Single-sweep oxidative voltammogram of the (black) ITO/PAAH/CdSe/N2H4 layer and (red) ITO/N2H4 (2 10-4 M). (b) First process and (inset) total oxidation charge vs the number of layers in single-sweep voltammograms of ITO/PAAH/(CdSe/N2H4)n multilayers. Voltammograms were run in acetonitrile þ 0.1 M Bu4NClO4 at a scan rate of 0.02 V s-1.

Figure 6. (a) UV-vis spectra (background-corrected) of ITO/(PVBH/CdSe)/(N2H4/CdSe)9 multilayers (red) before and (black) after annealing at 220 °C for 2 h under vacuum. (b) Solid-state photocurrent transient of the ITO/(PVBH/CdSe)/(N2H4/CdSe)9/Hg multilayer at 1 V applied voltage and 100 mW cm-2 illumination.

linker volatility and for a comparison with the bulk polymer (see above). Heating (N2H4/CdSe)10 multilayers at 120 °C for 15 min did not change the optical response, indicating remarkable stability of the hydrazine linkers. Changes were registered after curing at 220 °C and under vacuum. Figure 6a shows that the first excitonic absorption band broadens and red shifts in response to thermal treatment. We mainly attribute this result to a moderate growth of the NCs.4 It should be stated that a similar aggregation process is shown also by the bulk polymer, which starts to decompose at a comparable temperature (see above). 3.7. Photoconductivity of Multilayers. The high PL of capped CdSe-NCs dispersed in CHCl3 was completely quenched when the nanoparticles were assembled to multilayers. This is a favorable condition for photoconductivity, which reaches high values. The (CdSe/linker)10 layers at 1 V bias and 100 mW cm-2 illumination display high photocurrents with response times of seconds (Figure 6b). Under the same conditions, thin cast films of the original CdSe-NCs do not show any response. Table 1 summarizes solid-state photocurrent values for multilayers of the different linkers. It is interesting to note that substantial photocurrent values were obtained for all the investigated linkers, with levels in the following order: N2H4 > EDA > HMDA. The N2H4 sample gave reproducible results of high conductivity (Table 1), providing the

record value in our investigated series, including the highly photoconductive polyacrylate PAAH structures.6,12 The N-N bond distance is short, 1.46 A in solid hydrazine.17 The longer linkers, EDA and HMDA, gave progressively lower photoconductivity due to the increased interdot distance.

4. CONCLUSIONS The present investigation shows that hydrazine and linear diamines form regular polymeric multilayers with (hexadecylamine/ stearate)-capped CdSe-NCs on ITO glass via layer-by-layer alternation. During this building process, L (hexadecylamine) and X (stearate) caps are completely removed by hydrazine and diamines via complexation of the external cadmium layer. Whereas bulk CdSe/hydrazine polymer is a (hydrazine hydrate) gel, hydrazine covers the NC surfaces with a monolayer, providing a hydrazine content approximately 50 times lower than that of the bulk. Excess hydrazine is formed by soaking capped semiconductor nanoparticles in concentrated hydrazine solutions, which may be detrimental to the photoconductivity of the structures. Hydrazine, a short bidentate linker, raises the transport rate of the photogenerated charge between dots to the highest level, giving the record photoconductivity in the multilayer structures. Linear diamines give lower photoconductivity values, decreasing as the N-N distance is increased. It should be stressed that 4481



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ethylenediamine, which has efficient phototransport properties, may be preferable to hydrazine in the absence of any possible gel formation and toxicity. The optical and electronic properties of multilayers constituted by CdSe nanocrystals connected by bidentate linkers are useful in the development of efficient photovoltaic devices. Such multilayers are under investigation by our group to elucidate the role of the linker head in controlling the photoconductive properties of these semiconductor structures.

’ AUTHOR INFORMATION Corresponding Author

*Tel: (þ39)049-829-5868. Fax: (þ39)049-829-5853. E-mail: [email protected].

’ REFERENCES (1) Talgorn, E.; Moysidou, E.; Abellon, R. D.; Savenije, T. J.; Goossens, A.; Houtepen, A. J.; Siebbeles, L. D. A. J. Phys. Chem. C 2010, 114, 3441. (2) Talapin, D. V.; Murray, C. B. Science 2005, 310, 86. (3) Luther, J. M.; Beard, M. C.; Song, Q.; Law, M.; Ellingson, R. J.; Nozik, A. J. Nano Lett. 2007, 7, 1779. (4) Law, M.; Luther, J. M.; Song, O.; Hughes, B. K.; Perkins, C. L.; Nozik, A. J. J. Am. Chem. Soc. 2008, 130, 5974. (5) Steiner, D.; Azulay, D.; Aharoni, A.; Salant, A.; Banin, U.; Millo, O. Phys. Rev. B 2009, 80, 195308. (6) Zotti, G.; Vercelli, B.; Berlin, A.; Chin, P. T. K.; Giovanella, U. Chem. Mater. 2009, 21, 2258. (7) Yu, W. W.; Qu, L.; Guo, W.; Peng, X. Chem. Mater. 2003, 15, 2854. (8) Carvalhal, R. F.; Sanches Freire, R.; Kubota, L. T. Electroanalysis 2005, 17, 1251. (9) Tipton, T.; Stone, D. A.; Kubulat, K.; Person, W. B. J. Phys. Chem. 1989, 93, 2917. (10) Morris-Cohen, A. J.; Donakowski, M. D.; Knowles, K. E.; Weiss, E. A. J. Phys. Chem. C 2010, 114, 897. (11) Krishnan, K.; Plane, R. A. Inorg. Chem. 1966, 5, 852. (12) Zotti, G.; Vercelli, B.; Berlin, A.; Pasini, M.; Nelson, T. L.; McCullough, R. D.; Virgili, T. Chem. Mater. 2010, 22, 1521. (13) Kim, W.; Lim, S. J.; Jung, S.; Shin, S. K. J. Phys. Chem. C 2010, 114, 1539. (14) Huang, G.; Xu, S.; Xu, G.; Li, L.; Zhang, L. Trans. Nonferrous Met. Soc. China 2009, 19, 389. (15) Cao, X.; Wang, B.; Su, Q. J. Electroanal. Chem. 1993, 361, 211. (16) Kakiuchi, T.; Usui, H.; Hobara, D.; Yamamoto, M. Langmuir 2002, 18, 5231. (17) Collin, R. L.; Lipscomb, W. N. Acta Crystallogr. 1951, 4, 10.

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Self-Assembled Multilayers of CdSe Nanocrystals and Hydrazine or

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