Directionally Selective Sensitization of ZnO Nanorods by TiOPc: A

Article pubs.acs.org/JPCC

Directionally Selective Sensitization of ZnO Nanorods by TiOPc: A Novel Approach to Functionalized Nanosystems N. Coppedè,*,† D. Calestani,† M. Villani,† M. Nardi,‡,§ L. Lazzarini,† A. Zappettini,† and S. Iannotta†,‡ †

IMEM-CNR Parma, Parco Area delle Scienze 37/A, 43124 Parma, Italy IMEM-CNR Trento, Via alla Cascata 56c, 38123 Trento, Italy

‡

ABSTRACT: We demonstrate for the first time the ability to selectively sensitize a specific area of nanorods in a forest, by kinetically activated processes, induced by a supersonic beam source. The experiments have been carried out on forests of ZnO grown by vapor phase deposition on aluminum doped ZnO. The highly supersonic beam, produced by seeding TiOPc in He, achieves a molecular kinetic energy larger than 15 eV. By exposing the ZnO forest at different angles with respect to the beam direction, we sensitized the nanorods on different specific sites, leaving the other parts completely clean, as confirmed by SEM and HR-TEM studies. The effective sensitization is demonstrated by photoexcitation spectroscopies, where the PL spectra indicate efficient energy transfer from the TiOPc molecule to the ZnO nanorods. Since this kinetical functionalization approach does not need molecules with specific linkers, the present results pave the way to novel multifunctional sensitization processes that we envisage to be uniquely suitable for hybrid photovoltaics, sensing, and biomedical applications.

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INTRODUCTION Controlled multisensitization of metal and oxides nanostructures presents a wide spectra of key applications in different fields of scientific research. The ability to control and select the surface area where each molecular species could sensitize the nanostructure is of paramount importance to produce a novel class of multifunctional interfaces capable to optimize innovative device performances. The ability to design and realize multifunctionalized nanostructures implies high control in the wanted sensitization processes, with specific approaches to selectively control specific areas to sensitize. Moreover, the effective formation of stable bonds at the interface is needed, in most application, in order to optimize specific effects such as light-harvesting, charge and excitation transfer, and stable biofunctions. Among the many fields of application of multiple functionalization, great efforts have been devoted to biomedical diagnostics,1 imaging,2 drug delivery and dosage,3 energy production,4 and sensing devices.5 In this framework, there have been major developments in the synthesis of multicomponent nanorods and their subsequent surface functionalization. Nanorods, among other nanosystems, deserve specific attention,6 because they could be tailored to show different specific properties on the different faces while they can be easily synthesized via both top-down or bottom-up approaches by using lithographic methods7 or synthetic chemical methods,8 respectively. A typical approach to biomedical systems based on nanorods is to exploit the different free energies at the different surfaces and promote molecular recognitions and selfassemblies with specific organic counterparts9 or by the introduction of different metals for the selective functionalization of portions of the nanorods.10 In biomedical applications, © 2012 American Chemical Society

examples of multiple functionalization are related to multiplexing in determining DNA sequence11 or the detection of a variety of protein molecules12 as multiwavelength fluorescent tags in imaging.13 Another potentially very promising application is in DSSC solar cells,14 where the optimization of the efficiency in energy conversion faces the need to fully cover the solar spectrum, a question that is usually approached by chemically tailoring a single molecule and/or multiple layer functionalization. This approach is unfortunately limited by the short exciton diffusion length in organic materials and the need of a direct contact between the organic molecule and the oxide nanostructure. Another key application is in gas and liquid phases sensing,15 where a key advantage of a multifunctionalization approach of nanosystems with different selective molecular species will pave the way to the ability to optimize differential detection with increased specific selectivity and sensitivity of the devices. In this work we demonstrate directionally specific functionalization to ZnO nanorods as a viable basic process for future multiple functionalization. The choice of ZnO nanocrystals has been made because they have already been exploited for several of the mentioned applications. In particular, photovoltaic cell as well as gas sensor devices have been explored using ZnO in different controlled shapes: nanocrystals,16 nanotetrapods,17 and nanorods.18 Nanorods (NR) show interesting advantages. First of all, the NRs have a continuous crystalline structure and they can be grown by vapor phase at low temperature, directly Received: January 16, 2012 Revised: March 9, 2012 Published: March 15, 2012 8223

dx.doi.org/10.1021/jp3005184 | J. Phys. Chem. C 2012, 116, 8223−8229

The Journal of Physical Chemistry C

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films24,25 compared to the Langmuir−Blodgett (LB)33 and organic molecular beam deposition (OMBD) techniques.34 Here we demonstrate the ability to sensitize selectively a specific area of nanorods in a forest, by kinetic activated processes, induced by a supersonic beam source. The experiments have been carried out on forests of ZnO grown by vapor phase deposition on aluminum-doped ZnO.

on a conductive transparent metal oxide (TCO) made of ZnO doped with aluminum (AZO) deposited on commercial glass.19 In such a way, the structural homogeneity and crystalline properties are maintained all over the NR length, allowing a continuous path for the electrons in the material. In fact, with these structural properties, NRs minimize the potential barriers such as grain boundaries and heterojunction barriers, improving electron conduction and reducing recombination. Moreover, NRs present a high surface and volume ratio, keeping their lateral surface much more accessible to the sensitization with organic dyes20 than in the case of packed nanocrystals. Finally, the ZnO NRs make the anode surface “rough” enough to induce scattering phenomena on incoming light and enable multiple trapping processes at the interface. Aligned and regular structures, like NRs forests, represent hence the shape of choice to demonstrate selective sensitization of the metal oxide. The other key feature of our approach is the kinetic advanced functionalization process induced by supersonic molecular beam deposition (SuMBD).21,22 It is worth recalling that SuMBD has been already successfully used as a high controlled growth technique for organic molecules,23,24 showing a unique ability to control structure and morphology in the growth of thin films25 and consequently in achieving improved performances in device applications.26 Kinetic activated surface processes have also been explored to produce functional layers.27 Supersonic molecular beam deposition is based on the controlled expansion in high vacuum of a carrier gas (He, Ar, Kr), seeded by the organic molecules. The molecular beam properties, when properly selected and skimmed, can be tuned in terms of kinetic energy, alignment, and directionality. Such parameters play a critical role in the growth of the organic films. In the Knudsen or traditional regime (molecular flow), molecules will not experience collisions, so the velocity and internal energy distributions of the molecules follow a Maxwell−Boltzmann distribution function, whereas the angular distribution is typically cosine-like.21 In SuMBD the presence of channeled openings could improve the directionality of the beam, giving rise to a cosn θ (n ≥ 2) forward intensity, with a large number of collisions occurring in the free-jet expansion, determining a supersonic flow, where the final speed of the seeded organic molecules will reach that of the carrier gas, so that their final kinetic energy could be tuned up to tens of electronvolts. In particular, considering the characteristic dimensions of the deposition apparatus, the supersonic beam presents an angular spread of about 5° on the sample substrate. The much sharper forward angular distribution of supersonic free jets gives rise to an intensity at the beam target that could be a factor up to 100 times larger than that for Knudsen cells. The result is that SuMBD technique allows the formation of a highly directional beam, with narrow velocity and angular distributions, with high controlled kinetic energy (far outside the range of thermal evaporation) and with a fast cooling of the internal degrees of freedom.28 Finally, titanylphthalocyanine (TiOPc) was chosen as the organic sensitizing molecule because it exhibits a wide absorption spectrum in the visible range, it has been used as an active material in organic solar cells,29,30 it has applications in organic light-emitting diodes, it has been investigated as a conducting layer in organic field effect transistors,31 and it has been used as an active material in the infrared region and as a photoconductive materials for printers and copying machines.32 Two of the authors have previously shown the ability of the SuMBD approach to produce better phase controlled TiOPc

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EXPERIMENTAL SECTION AZO films have been deposited directly on glass substrates by the pulsed electron deposition technique. Part of the glass substrate was masked so that the deposition covers about half of the sample. The films show preferential grain orientation with the c-axis of the hexagonal wurtzitic crystal cell perpendicular to the substrate. The ZnO NRs are grown on AZO films, basically using a chemical vapor deposition at temperatures compatible with glass substrates (typically 480 °C). Details of the growth process have been given elsewhere.19 The growth of NRs has been limited only to the substrate region covered with the AZO films. The growth of organic thin films has been performed in a tailor-made supersonic beam deposition apparatus, previously described in more detail.28 It basically consists of a differentially pumped supersonic beam, a TOF mass spectrometer, and a deposition chamber. The supersonic beam source, placed in a high vacuum chamber, is made of a quartz tube with a micrometric nozzle at the front end (typically 50−130 μm in diameter). An inert carrier gas (helium in this experiment) is injected in the quartz tube at a controlled pressure (2−3 bar). Inside the tube, a vessel with the organic material powder is used to sublimate by Joule heating the molecules, dispersing them at very low concentrations into the gas, and expanding both through the source nozzle into the deposition chamber. A conical skimmer selects the central part of the beam, which proceed to the sample in a ultrahigh vacuum chamber. By changing the working parameters (nature and pressure of the carrier gas, sublimation temperature, nozzle diameter, and temperature), it is possible to finely control key properties of molecules in the supersonic beam, such as kinetic energy, momentum, and cooling of the internal degrees of freedom typically induced by expansion. The source is typically operated using a He carrier gas pressure in the range of 100−200 kPa. The central part of the beam is selected by skimming the free jet expansion via a sharp edged conical collimator, which separates the source from the deposition chambers (base pressure 10−8 mbar). Here, the molecular beam is intercepted by the substrate, the temperature of which can be varied from −100 up to 350 °C, with a stability of about 1 °C. TiOPc films have been grown at room temperature while a quartz microbalance was measuring the deposition rates, chosen at 0.5 nm/min. The source operating conditions have been tuned to keep the same high kinetic energy of 15 eV for all the deposited films. The in-line time-of-flight mass spectrometry (TOF-MS) coupled to laser (fourth harmonic of a Nd:YAG laser at 266 nm) multiphoton ionization has been used to monitor the intensity, purity, and stability of the beam. The duration of the deposition has been set to produce films of the same thickness (20 nm). We used TiOPc row material coming from the same batch (Syntec-Sensient GmbH) for all experiments. The starting material has been purified by repeated vacuum gradient sublimation cycles and then measured by the TOF mass spectra, and no significant residual contamination was visible. 8224



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Figure 1. Supersonic molecular beam deposition (SuMBD) source scheme on two different samples of ZnO nanorods (grown on Al-doped ZnO thin film prepared on a glass substrate). Sample a presents an angle of 45° with respect to the beam direction, and sample b is deposited perpendicular to the beam direction.

In Figure 2 is shown the detailed scheme of the TiOPc growth in the 45° configuration, with the nanorods on AZO

We deposit TiOPc, as explained in Figure 2, with the same beam characteristics and thickness, both on ZnO NRs on AZO and on bare glass substrate, contemporary in the same deposition. Conventional analytical and high-resolution TEM studies have been carried out on a JEOL 2200FS field emission microscope operated at 200 kV. Scanning TEM (STEM) with a high-angle annular dark-field (HAADF) detector was carried out to obtain composition sensitive (Z contrast) images. The compositional evaluation of the samples has been performed by X-ray microanalysis. The NRs have been removed from the substrate and dispersed over a holey carbon grid for the TEM analysis. To investigate the optical properties of the sensitized NR, room temperature photoluminescence (PL) measurements were performed using the 325 nm line of a He−Cd laser as excitation source modulated at 30 Hz by a mechanical chopper. Luminescence light was collected through a condenser lens and entered the slits of the dispersive monochromator. A photomultiplier tube was used as a photon to electron conversion element and the signal was then sent to a lock-in amplifier.

Figure 2. Image and schematic of the sample sensitized with SuMBD deposited TiOPc with beam at 45° with respect to the substrate. The schematic pictures the layout of the ZnO NR grown on a glass substrate. In this configuration it is possible to measure PL spectra of bare ZnO NR, TiOPc sensitized NR, and TiOPc alone.

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RESULTS AND DISCUSSION To achieve area specific functionalization we exploited the supersonic beam directionality to induce kinetic activated reactivity of the molecules at the impact with the surface. To this end the angle of impact becomes the controlling parameter, and hence, we performed a comparison between two different substrate orientation with respect to the supersonic beam. Figure 1 shows a scheme of the geometries used in the experiments. In particular, when the angle between the beam and the axis of nanorods is 45° (configuration a), only one side of the nanorods will be exposed to the beam and we expect that, if kinetic activated processes will occur, diffusion of molecules toward the other sides of the nanorods will be avoided. When the beam is parallel to the axis of the nanorods, the whole forest will be exposed to the arriving molecules. The experiment will then consists of comparing the growth of the TiOPc on the nanorods with identical beam deposition parameters, for two different directions of impact. This will therefore make it possible to determine the sensitization process induced by directionality of the beam on the TiOPc growth.

substrate represented in gray and the TiOPc molecule in blue, with the expected growth reported on the nanorods sides. It is important to underline that, as described later on, the same film has been optically measured in two different areas, on the glass substrate and on the nanorods forest, allowing a quantitative comparison of its optical properties. Figure 3 shows typical scanning electron microscopy (SEM) micrographs of the untreated ZnO forest and of the same sample after exposure to the TiOPc supersonic beam impinging at an angle of 90°. In more detail, Figure 3a shows an as grown ZnO NR at 40 000× magnification; a regular organized forest is observed with a uniform distribution of NR evenly aligned over a wide area of the sample. A more detailed image is shown in Figure 3b (80 000×): NRs length is typically 1 μm, while the average NR section is about 50 nm in diameter. Figure 3c shows the same sample sensitized with 30 nm of TiOPc (80 000×). The NR is fully covered in a symmetric and homogeneous way and each nanorod has a larger section that becomes about 80 nm in diameter, indicating a TiOPc coverage 8225



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Figure 3. SEM image of as grown ZnO nanorods magnified 40 000× (left image A) and 80 000× (center image B). SEM image of ZnO nanorods exposed to the TiOPc supersonic beam in the 90° configuration magnified 80 000× (right image C). The thickness of nanorods covered with TiOPc is uniformly larger than the as grown nanorod, and no evidence of preferential growth is present.

Figure 5. (a) Zero-loss TEM image of ZnO nanorods (dark gray) clearly showing the directional covering of titanylphthalocyanine (light gray) only on the right side. (b) High resolution TEM image of the wire portion squared in part a: no phthalocyanine layer is present on the left side of the nanorod; the typical wurtzite lattice in the (2−1− 10) projection is shown, as proved by the diffractogram reported as inset.

typically of about 15 nm. The deposition of TiOPc under these conditions is hence regular and continuous. Switching to the other set of experiments where the NR forest is being exposed to the same beam but at an impact angle of 45° (see Figure 2), Figure 4 shows a typical STEM HAADF

enlargement of the region delimited by the square defined by the white perimeter in Figure 5a. The lattice is in the (2−1− 10) projection, as proved by the diffractogram reported in the inset. The NRs grow parallel to the c axis. It is important to underline that the side opposite to the growth is clean from the TiOPc deposition, as shown in the magnification in Figure 5b. To determine the composition and the spatial distribution of the chemical species, high-resolution X-ray microanalysis has been performed. Figure 6a reports the X-ray intensity map of the K lines of Zn (in blue) and Ti (in green). While Zn is homogeneously distributed in the whole NR structure, the Ti signal is localized in correspondence to the area exposed to TiOPc. Due to the very low quantity of Ti, this analysis has been somewhat difficult and required very long time to improve the counting statistic. The asymmetric Ti distribution is even clearer in Figure 6b, where the signals are integrated over the region delimited by the white rectangle of Figure 6a and the Ti signal is multiplied by a factor 3 to show the effect better. It appears quite clear that the Ti signal is strongly asymmetric and is higher outside the NR. This behavior has been confirmed over several different NRs and demonstrates the asymmetrical TiOPc coverage due to the 45° beam exposure configuration. To demonstrate the functional sensitization of the NR we carried out photoluminescence studies. Figure 7 compares PL spectra of the samples sensitized in the 45° configuration with the emission from the as grown ZnO NRs forest and a pure TiOPc thin film on glass. The green line (letter C) is the typical room temperature luminescence spectra of the as grown ZnO NR by vapor phase techniques35 characterized by two main bands: the near band edge emission, centered at 380 nm, which is more intense than the other band, which is defect-related and centered at 500 nm. The blue line (letter B) in Figure 7 is the emission band of the TiOPc film grown on clean glass and is centered at 430 nm. Finally the red line (letter A) is the PL of the ZnO NRs forest sensitized with TiOPc by SuMBD in the 45° configuration. It is worth recalling that the amount of TiOPc deposited per unit surface is the same for both sets of data for the blue and the red lines in Figure 7. The comparison of the spectra in Figure 7 shows two main features: (i) the band at 430 nm of TiOPc has completely disappeared in the case of TiOPc-sensitized NRs and (ii) sensitized NRs have an overall lower emission with respect to bare NR due to TiOPc laser absorption.

Figure 4. STEM HAADF image of a ZnO nanorods bunch for which the TiOPc has been deposited with an angle of 45° of the beam with respect to the sample surface. It is possible to distinguish a directional covering of titanylphthalocyanine as a light halo surrounding the rods only on one side.

image of a bunch of ZnO NRs. In these imaging conditions the intensity is a function of the atomic number Z, so the ZnO NRs appear very bright in comparison to the C background of the supporting grid. The NRs diameter at the base is typically of 40−50 nm while their average length is about 1 μm. They show a diameter reduction from the base to the top, with a tapering of approximately 3%. A careful inspection of the image allows one to distinguish a light halo, of intermediate gray, only on one side of the NR, which suggests directional coverage by TiOPc. The rods that do not show the halo are very likely the ones that, due to the random deposition of the NR on the C grid, have been twisted face down with the TiOPc covered area hidden to the electron beam. In Figure 5a the specific sensitized area is shown by means of a bright-field, zero-loss TEM image: the ZnO NRs appear now dark gray while the background carbon grid is bright. The intermediate gray layer surrounding the NRs only by their right side demonstrates the directionality of the coverage. The deposited layer shows a quite uniform thickness and covers the same side of the NR along its full length. The NRs are single crystals with the wurtzite structure, as shown in Figure 5b, which represents a high-resolution TEM 8226



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Figure 6. (a) X-ray map of the same rods as in Figure 5a obtained with the signals of the Zn and Ti K lines, in blue and green respectively. (b) Intensity profile of the Zn and Ti X-ray signals integrated in the squared area as in part a. The Ti asymmetric distribution with respect to Zn is clear.

the side exposed to the beam. The single NR has been sensitized along all its length. Moreover, the coverage is quite homogeneous on the treated sides and its thickness could be easily controlled by simply changing the deposition time. From the compositional analysis in Figure 6a, the asymmetric presence of Ti species on the right side of each nanorod is clearly confirmed. If we consider also the distribution in Figure 6b there is a further confirmation of the asymmetric distribution of the organic molecule. In fact, while the distribution of Zn is symmetrically shaped, the Ti distribution is practically on the right side of the NR position. The compositional, together with the morphological analysis, clearly demonstrates the directional sensitization of the nanostructures. From the point of view of the optical properties, the mentioned evidence that the spectra of ZnO sensitized by TiOPc is different from the spectra of TiOPc alone is demonstrated by the emission band at 430 nm that is completely quenched. Considering the ZnO and TiOPc band energies, the ZnO conduction band lies at 5.6 eV with respect to the vacuum,36 while the TiOPC LUMO is located at 3.8 eV37 (i.e., 1.8 eV above). The injection of TiOPc photoexcited charge in the oxide can hence take place and the exciton no longer recombines radiatively into the molecule.38 Moreover, the thickness of the TiOPc layer (figure 4) is about 10 nm, thus comparable with the typical exciton diffusion length in organic semiconductors.39 This means that the sensitization is effective and the charge is injected from organic molecule into the oxide nanostructures.39 This is particularly promising for photovoltaic applications of this organic−inorganic hybrid system. The second feature emerging is that the emission of sensitized NRs is less intense then the one observed for bare ZnO NRs. This can be ascribed to the action of the TiOPc layer that absorbs both the laser incident radiation as well as the ZnO luminescence emission.24 All together these observations demonstrate the ability to selective synthesize a single NR structure with a controlled and homogeneous deposition of TiOPc, choosing the side to be covered without any need of selective surface treatments of chemical modifications of the organic molecule. We believe that these results pave the way to a novel directional sensitization process, where two or more molecular species can simulta-

Figure 7. Photoluminescence spectra of nanorod of ZnO nanorods on glass substrate (green, letter C), TiOPc on glass substrate (blue, letter B), and ZnO nanorods synthesized by directional growth of TiOPc on glass (red, letter A).

As observed previously (see Figure 3), the SuMBD deposition of TiOPc on the sample at 90° shows all the common characteristics of the sensitization of nanostructure by a molecular flow. The NRs are uniformly covered in a symmetric and continuous way and the TiOPc film is regular, without any directional effect. As expected a sensitization of ZnO NRs with SuMBD deposition in 90° configuration could be useful to prepare solar cell interface, in solid state or liquid ionic DSSC, or could be used as active material in advanced gas sensing devices. This is a good approach, as in many other cases, where the sensitization/functionalization could be accomplished by a single molecular species, without any possibility of producing a multiple and selective sensitization of the ZnO surface that would be of great interest for the formation of multiple active interfaces with different functionalities. This is simply due to the fact that the nondirectional growth fully covers the ZnO nanostructured surface and after growth processes, if possible, would clearly be cumbersome. This limitation is elegantly overcome by the SuMBD sensitization of TiOPc realized in 45° configuration; as shown in Figure 5, a critical change in the sensitization has been in fact realized. Due to the high directionality of the supersonic beam the ZnO NRs are uniformly covered only on 8227



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neously sensitize some nanosystem, each one of them covering selectively a portion of the NR. This is very promising for use in sensitized solar cell applications, to improve the absorption on different spectral regions: by choosing different molecules with different complementary absorption spectra, one could in principle fully absorb the whole solar spectrum. In the framework of the solar cell applications, our approach would resolve basic questions concerning, for example, the issues of the limited exciton diffusion in dyes, where the distance between the molecule and the oxide nanostructure becomes crucial. Instead of stacking multiple layers like in multijunction cells, as is commonly done, it will be possible to design a different cell structure, where the complementary absorbing molecules are deposited in spatially different regions of the NRs surface. Moreover, the directional functionalization of the same NR with different molecules could be applied in gas-sensing devices, improving the sensitivity and selectivity of the device, with a differential response on each side of the structure. Many other applications, including biomedicine, may benefit from a controlled directional functionalization of the nanostructures with different organic active materials deposited side-by-side on each nanostructure.

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CONCLUSIONS Aligned ZnO nanorods have been grown by vapor phase deposition directly on aluminum-doped ZnO thin film (AZO). We demonstrated the ability to sensitize selectively a specific area of the nanorods by fully exploiting the high directionality and hyperthermal kinetic energy of a supersonic molecular beam of TiOPc molecules. We also showed that the specific area to be sensitized could be simply selected by the angle of incidence of the beam with respect to the major axis of the NRs. The chemical quantitative distribution analysis of Zn and Ti confirmed the selective deposition only on the chosen side of the nanorod. PL spectroscopy revealed a complete quenching of the organic emission in contact with ZnO nanorods, indicating a charge transfer from the organic to the oxide and confirming the effective sensitization of the nanohybrid material. We envisage that the described selective and directional deposition could be fruitfully used to realize multiple sensitization with different molecules on each side of the nanostructure, paving the way to innovative applications in hybrid photovoltaics and sensing.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +39 0521 269217. Fax: +39 0521 269206. Present Address §

Institut fur Physik, Humboldt-Universitat zu Berlin, Newtonstrasse 15, 12489 Berlin, Germany. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank The Provincia Autonoma di Trento and CARITRO foundation on DAFNE Project for financial support and Claudio Corradi and Marco Pola for technical support.

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REFERENCES

(1) Pearce, M. E.; Melanko, J. B.; Salem, A. K. Pharm. Res. 2007, 24, 2335−2350. 8228



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Directionally Selective Sensitization of ZnO Nanorods by TiOPc: A

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