Structural, Optical, and Electrical Properties of in Situ Synthesized ZnO

Article pubs.acs.org/JPCC

Structural, Optical, and Electrical Properties of in Situ Synthesized ZnO−CuPc Nanocomposites Manoranjan Ghosh,*,† N. Padma,*,† R. Tewari,‡ and A. K. Debnath† †

Technical Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India Materials Science Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India

‡

ABSTRACT: Nanocomposites of copper phthalocyanine (CuPc) and ZnO nanoparticles (NPs) have been grown in situ in a colloidal solution of CuPc using zinc acetate as precursor and sodium hydroxide as precipitating agent. n-Type ZnO NPs form a network with p-type CuPc by donating electrons as evidenced by various techniques. Increase in NaOH concentration produces larger ZnO nanostructures (NS) with higher aspect ratio (length/diameter) that influences the aggregation state of the CuPc. The drop-cast films of pure CuPc show stacking in both H and J aggregation modes which changes due to composite formation. A significant blue-shift in the Q-band corresponding to the J aggregated mode (∼30 nm) indicates a change in the aggregation state of CuPc molecules from slipped facial to cofacial stacking. A defect related emission from ZnO shows a blueshift with a reduced intensity, confirming the formation of ZnO nanorods that are firmly attached to the CuPc in the composite. Further, current−voltage (I−V) characteristics of nanocomposites exhibit rectification ratio of about 20 at 2 V which is absent in the individual constituent materials. Photoluminescence lifetime as well as I−V measurement confirms an intimate mixing of the constituent materials through the electron transfer reaction. The broadening of the Q bands in the absorption spectrum due to a change in the aggregation may increase the light harvesting capacity of the composite material for photovoltaic applications.

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INTRODUCTION Recently, inorganic−organic composites have attracted much attention because of their potential application as active materials in gas sensors,1 field effect transistors,2 and especially as new photovoltaic materials.3−5 Hybrid structures are a powerful way of exploiting the advantages of inorganic semiconductors such as broad range of light absorption, particle size induced tunability of optical band gap, effective transport of charge carriers and that of organic polymers or small molecules such as low cost, plasticity, flexibility, easy chemical modification, and solution processability. The absorption spectra of most of the organic materials fall in the visible or infrared region, whereas inorganic semiconductors like TiO2, ZnO, etc., exhibit a strong absorption in the UV region. The combined absorption in the hybrid structures therefore covers a wide spectral range, thereby enhancing the efficiency of photocarriers generation. Hybrid structures are employed as simple planar multilayer junctions,6,7 disordered bulk heterojunctions,8,9 (where the two materials are mixed to form composite materials), ordered bulk heterojunctions10,11 and dye-sensitized solar cell.12 Several methods have been employed to form hybrid composite materials like electrochemical deposition,3 coevaporation,9 simple physical mixing of the two materials in solution,8 ball milling,4 etc. In the above-mentioned hybrid structures the extent to which these materials can mix and form a composite plays a significant role since efficient charge separation, and transport is needed at the inorganic (acceptor)−organic (donor) interface.13 Thus, nanocomposites having better © 2013 American Chemical Society

intimacy of contacts between individual components will ensure improved interface and hence power conversion efficiency of these devices. ZnO is a human friendly, easily obtainable wide band gap inorganic semiconductor that exhibits n-type conductivity in bulk form. Phthalocyanine (Pc) is a popular organic dye which is chemically and thermally stable and used as a p-type material in heterojunction devices. Although promising for investigation, there are limited reports available on fabricating ZnO−CuPc composite material.3,8 Further, in these reports ZnO nanoparticles (NPs) are separately prepared and later added into the solution of organic materials, reducing the chance of intimate mixing. In the present study ZnO−CuPc composites were synthesized in situ by growing ZnO nanostructures (NS) in a colloidal solution of CuPc. It is seen that aggregation of CuPc assisted by ZnO NPs above certain size plays a crucial role in determining the functional properties of the composite. It has been observed earlier that ZnO NPs below 10−15 nm synthesized by the current method show a p-type conductivity14,15 which could be limiting the efficient electron transfer with the p-type CuPc, although smaller NPs offer a larger interface. The method prescribes a way to control the size of ZnO NPs in situ to achieve uniform mixing and better contact with the CuPc that leads to various changes in the structural and optical properties of the composite material. Received: June 17, 2013 Revised: October 17, 2013 Published: December 17, 2013 691

dx.doi.org/10.1021/jp4059609 | J. Phys. Chem. C 2014, 118, 691−699

The Journal of Physical Chemistry C

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Table 1. Structural Properties of Select Composites by TEM and XRD size and morphology of ZnO by TEM

size and orientation of ZnO by XRD

samples

NaOH conc (mM)

ZnAc:CuPc wt ratio

diameter (nm)

length (nm)

size (nm)

I(002)/I(101)

ZC1 ZC2 ZC3 ZC4 ZC5 ZC6

20 40 80 100 300 400

19 8 19 8 19 10.5

5−6 10−12

10−15

10−15

20−30

18−20

30−40

9 10 10 14.5 18

0.55 0.70 1.01 0.63 0.66

Thick films of the composite materials were prepared by drop-casting the washed solution on cleaned fused quartz/glass substrates for absorption measurement and silicon substrates for photoluminescence (PL) measurements. Grazing incidence (angle fixed at 0.1°) X-ray diffraction (XRD) measurements were carried out on similar films on glass substrates using Cu Kα radiation using a RIGAKU analytical X-ray diffraction system (XRD 3003-TT). The size and morphology of the constituent materials were investigated by transmission electron microscoy (TEM). Vibrational spectra were recorded by Fourier transform infrared spectroscopic studies (FTIR) (Bruker make VERTEX 80V system) in the attenuated total reflectance (ATR) mode. The optical absorption spectra were collected by a UV−vis spectrophotometer (model JASCO V530 Japan). Mg Kα (1253.6 eV) source and a DESA-150 electron analyzer (Staib Instruments, Germany) were employed for XPS measurements. The binding energy scale was calibrated to adventitious carbon (C 1s) line of 284.6 eV. Photoluminescence (PL) measurement was performed by a spectrofluorometer from Edinburg Instrument. For electrical measurements, thick film of ZnO−CuPc nanocomposites were deposited on ITO above which CuPc film of thickness about 20 nm was deposited by a thermal evaporation technique in a vacuum chamber. Control devices such as ITO/ZnO nanoparticulate/20 nm thick evaporated CuPc (bilayer structure), ITO/ZnO nanoparticulate, and ITO/evaporated CuPc were also fabricated. Device structures were completed by thermally evaporating gold top contact electrodes of size 500 μm × 500 μm through a shadow mask. Both CuPc film and gold electrodes were thermally evaporated at a base pressure of 1.5 × 10−5 mbar in a vacuum chamber. Electrical measurements were carried out in normal ambience and under room light conditions using a Keithley voltage source/picoammeter (model 6487).

More importantly, the synthesized nanocomposite in this work forms rectifying junction with improved rectification ratio compared to that of a simple bilayer structure between p-type CuPc and n-type ZnO which is a prerequisite for various device applications. Aggregated Pcs, mainly revealed by their corresponding absorption spectrum, are useful in enhanced absorption of light and hence important for use in photovoltaic devices. Aggregation of Pcs in different solvents and with different concentrations is common, but limited studies are available on aggregation assisted by inorganic NPs.16,17 Sulfonated zinc Pc is reported to be interacting with ZnO when electrochemically codeposited, resulting in a blue-shift in its absorption spectrum.18 To our knowledge there are no reports on the study of ZnO-assisted aggregation of unsubstituted CuPc. In this work we investigate aggregation of CuPc induced by ZnO nanostructures of different sizes through optical techniques.

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EXPERIMENTAL SECTION Earlier efforts on fabricating ZnO−CuPc composite material were mainly focused on the physical mixture of ZnO and CuPc in a solution.8 For improved mixing, ZnO NS in this study have been grown in a colloidal solution of CuPc. The ZnO NP is commonly synthesized by dissociation of Zn(CH3COO)2· 2H2O (ZnAc) in a basic medium at temperatures about 60−75 °C.19 Three precursors, namely commercial CuPc powder (from Sigma-Aldrich), zinc acetate dihydrate (ZnAc) from eMERCK, and NaOH, have been used without any purification for the growth. To obtain ZnO−CuPc composite material, first NaOH solution of required concentration was prepared in 8 mL of ethanol. A specific amount of CuPc powder and Zn(CH3COO)2·2H2O (Table 1) were added in the NaOH solution and sonicated for about 1 h. Then the mixture of all precursors was heated without stirring at 70−75 °C for 1 h. A proper mixing of the two materials can be verified by a change in the color of the solution from strong blue to greenish. The final solution is washed repeatedly by methanol. NaOH is used as a precipitating agent and maintains the basic environment of the solution which plays a pivotal role in determining the size and shape of the synthesized ZnO nanostructures. The weight ratio of ZnAc and CuPc also plays an important role in the formation of the composite. Therefore, two sets of samples were prepared. NaOH concentration was varied with a fixed ZnAc:CuPc weight ratio to check the effect of size and morphology of ZnO NPs on the composites. Further, ZnAc:CuPc weight ratio was varied keeping NaOH concentration fixed to study the effect of this ratio on functional properties of the composites. Growth conditions of selected samples are described in Table 1. Pure ZnO NPs was also synthesized using the same procedure without mixing the CuPc.

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RESULTS Interaction between the constituents such as inorganic nanostructures (ZnO) and organic molecules like Pcs lead to change in the structural and optical properties of the composites. These changes arise mainly because of the tendency of Pc’s to readily aggregate due to their planar geometry, causing a strong van der Waals interaction among monomer molecules. The type of arrangement of individual molecules during the aggregation, i.e., head-to-head or head-totail stacking, primarily decides the properties of the composite materials. Formation and Structural Properties of ZnO−CuPc Composites by TEM and XRD. Figure 1 shows the XRD pattern of commercial CuPc powder and that of different ZnOCuPc nanocomposites. X-ray reflections of CuPc powder match with its β-phase structure which is highly polycrystalline in nature. The XRD pattern of ZnO−CuPc nanocomposites 692



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Figure 2. (a) TEM micrographs of ZnO−CuPc nanocomposites for NaOH concentration 20 mM (ZC1). (b) HRTEM image of (a). (c) HRTEM image of pure ZnO nanoparticles.

ZnO NPs obtained through similar growth procedure. As the NaOH concentration increases from 40 to 100 mM, the size of the NS increases and anisotropic growth along the [002] direction becomes prominent. The bigger ZnO rods hold the CuPc structures together, thus forming a ZnO−CuPc network as evident from the TEM micrographs (Figure 3a,b,d). The

Figure 1. XRD patterns of commercial CuPc powder, ZnO nanoparticles (size ∼ 5 nm) and ZnO−CuPc nanocomposites. Stable composite is formed by stacking of molecules with certain d-spacing.

confirms the presence of two distinct phases, viz., wurtzite ZnO (indexed in Figure 1) and β phase of CuPc. It can be seen that the polycrystalline nature of CuPc powder is reduced in the composite materials. Aggregation or stacking of CuPc molecules takes place with certain d-spacing, and the preferentially oriented molecules exhibit fewer number of Xray reflections compared to that of polycrystalline powder. The orientations of CuPc molecules for different mixing level of constituent materials are not similar as evident from the X-ray diffraction results. Reflections corresponding to different planes of β-CuPc dominate for various nanocomposites synthesized. ZnO nanostructures, the other constituent, show variation in size and shape depending on the precursors concentrations. The average grain size of the ZnO nanostructures has been determined by the Scherrer formula and listed in Table 1. Full width at half-maxima (fwhm) of a single peak is considered to reduce the error in comparative sizes. The grain size thus determined is found to increase with concentration of precipitating agent (NaOH) and ZnAc.20 Further, the relative intensity of the (002) peak increases with the increase in the NaOH concentration. It indicates preferential growth of nanostructures toward [002] direction which is found to be the easy axis of growth for ZnO (c-axis). The preferential growth may be attributed to the difference in surface energies and growth rate for different faces of ZnO nanocrystals. The enhanced OH− ion concentration modifies the surface energies in such a way that accelerate the growth toward [002] direction. With an increase in the zinc acetate concentration, the available space for one-dimensional growth may become limited and formation of square-pillar-like morphologies having bigger sizes are more probable.20 As a result, the relative intensity of (002) peak saturates beyond a certain concentration of ZnAc. The evolution of size and morphology of ZnO NS in the process of composite formation can be visualized by TEM study. For the lower concentration of NaOH, ZnO NPs with a uniform average size of 5 nm are obtained (Figure 2a,b). Here, there is hardly any physical attachment between ZnO and CuPc. The CuPc structures exist separately with a few ZnO dots on the wall. Figure 2c shows the HRTEM image of pure

Figure 3. (a) Dark-field TEM micrographs of ZnO−CuPc nanocomposites synthesized using higher NaOH concentration viz. 100 mM (ZC4). Dark and bright field HRTEM of (a) are shown in (b) and (d) respectively. The pristine ZnO nanorods grown by similar conditions (without using CuPc) are shown in (c).

ZnO nanostructures grown by a similar route without adding CuPc are shown in Figure 3c for comparison. The size and morphology of constituent materials for different growth conditions are listed in Table 1. The aforesaid observations helped us to explain the formation of the stable ZnO−CuPc network in the solution. As discussed later, the intimate contact between the two components arises due to electron transfer from n-type ZnO to 693



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Figure 4. (a) UV−vis absorption spectra of plain CuPc and ZnO−CuPc nanocomposites. The excitonic peaks of ZnO NPs are indicated by an arrow. Positions of Q1 and Q2 bands as a function of NaOH concentration and ZnAc:CuPc weight ratio are displayed in (b) and (c), respectively. The absorbance ratio of Q2 and Q1 bands depends on ZnAc:CuPc weight ratio as shown in (d).

For phthalocyanines, the absorption due to π−π transitions occur in the region between 550 and 750 nm called as Qband.23 The absorption spectrum of a drop-cast film of plain CuPc dispersed in ethanol (0.5 mM concentration) shows two peaks corresponding to Q-band, i.e., at 648 nm (Q1) and 748 nm (Q2) (Figure 4a). The CuPc was also dissolved in dimethyl sulfoxide (DMSO), and the Q-band peaks were observed at the same positions (data not shown). Since the monomer peak generally appears near 690 nm,24 the Q2 band here is redshifted and the Q1 band is blue-shifted with respect to the monomer peak. This indicates the formation of both J and H aggregates for the plain CuPc film. The intensity of the Q2 band is higher than that of Q1, indicating more number of molecules is involved in the J aggregation than H aggregation. Upon complexation with ZnO both Q1 and Q2 bands shift toward blue side and relative intensity of Q2 with respect to Q1 band reduces (Figure 4). This implies that CuPc molecules are arranging themselves in a cofacial manner such that the H aggregation is increased at the cost of reduced J aggregation. The effect of various growth parameters on the aggregation behavior of CuPc in the composites has been studied by optical absorption measurements. Figure 4b shows a blue-shift in both Q1 and Q2 bands for varying NaOH concentrations with a fixed ZnO−CuPc weight ratio. Similar variation has been observed for varying ZnO−CuPc weight ratio but fixed NaOH concentration (Figure 4c). As the number of molecules in the H aggregate increases and J aggregate decreases, the gap between ground state and allowed electronic excited state increases, leading to the blue-shift in both Q2 and Q1 bands. The absorbance ratio of Q2 and Q1 band also found to vary systematically with the ZnAc:CuPc weight ratio (Figure 4d). There is an optimum weight ratio of ZnAc:CuPc (20:1) when the H aggregated CuPc molecules are maximum in number. A large broadening of Q1 band with shoulders appearing around 635 and 610 nm is seen. Under different precursor’s concentrations, either of these two shoulders becomes predominant, and the peak position of the same is considered.

p-type CuPc, the effect of which is subdued in ZnO nanosphere of smaller in size. In addition, ZnO can spread into the CuPc layer over the long lengths as the size and aspect ratio of the nanostructures increases, thereby forming a dense network. The bonding of the complex becomes stronger when denser network is formed by increasing the precursor concentrations. A change in optical properties of the individual components is also seen due to the alteration of electronic configuration by the complex formation. Aggregation Behavior of CuPc Investigated by UV− vis Spectroscopy. There is a systemic change in the aggregation state of CuPc molecules due to the composite formation. Aggregation is usually depicted as a coplanar association of rings progressing from monomer to dimer and higher order complexes through hydrogen bonds, van der Waals and electrostatic interactions, and hydrophobic effects.16 Pcs are usually strongly aggregating because their planar molecular geometry allows significant π−π interactions between molecules.17 In the aggregated state the electronic structure of the complexed phthalocyanine rings are modified, as predicted by molecular exciton theory based on excited state resonance interaction between molecules.21,22 The transition dipole moments of the monomers are strongly coupled. Depending on the stacking type or the angle between these coupled transition dipole moments and the line joining the center of the molecules, shift in the absorption bands is seen toward lower or higher wavelength side compared to that of monomers.22 When a pair of molecules interact noncovalently with each other, the ground state of the molecules remain localized but the exciton splitting of the excited states takes place, lowering the degeneracy of the excited state compared to uncoupled monomer. If the molecules are stacked head-to-head, H aggregates are formed causing blue-shift while head-to-tail stacking results in J-type aggregates reflected by the red-shift in the absorption spectrum, following allowed transitions from ground state to the corresponding excited electronic state.21 694



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region of 400−550 cm−1. Peaks in the region of 1300−1600 cm−1 of pure ZnO film are due to impurities such as methanol and zinc acetate used while synthesizing ZnO NPs. For plain CuPc film, peaks corresponding to ν(C−H) out-of-plane bending and ν(C−N) stretching modes at 732 and 782 cm−1, respectively, indicate the presence of CuPc in β phase.26 The maximum intensity is observed for peak at 732 cm−1, which is considered as reference peak in this study. In-plane bending and stretching modes of tetrapyrrolic macrocycle and porphyrin ring systems of CuPc are mainly observed at 1067, 1089, 1120, 1166, 1288, 1333, 1420, 1508, and 1584 cm−1 with certain relative intensity with respect to the intensity of reference peak.27 The FTIR spectrum of nanocomposites showed significant variations in the relative intensity of these peaks with respect to the peak at 730 cm−1 due to a change in the chemical environment.28 Mainly, the peak corresponding to inplane skeletal vibrations of Pc macrocycle shows an enhanced relative intensity. Drastic increase in relative intensity of peak at 1420 cm−1 corresponding to in-plane stretching vibration of pyrrole ring is observed along with shift in frequency to lower wavenumbers to 1403 cm−1. A weak peak at 1018 cm−1 corresponding to in-plane skeletal stretching mode becomes prominent after the interaction with ZnO. Peaks below 700 cm−1 (620, 637, and 648 cm−1) originating most probably from vibrations in benzene ring in interaction with pyrrole ring,29 which are unnoticeable in plain CuPc spectrum also show an increase in relative intensity. All these variations clearly indicate that ZnO interacts with CuPc through a charge transfer mechanism occurring directly between Pc macroring and ZnO nanostructure. X-ray Photoelectron Spectroscopy Study. X-ray photoelectron spectroscopy (XPS) measurements of pure ZnO, pure CuPc, and ZnO−CuPc composite (ZC4) films have been carried out in order to identify chemical interaction between CuPc and ZnO. Parts a, b, c, and d of Figure 6 show the XPS spectra of O 1s, N 1s, Zn 2p, and Cu 2p, respectively, for ZnO, CuPc, and composite films indicated on the graph. The O 1s peak for pure ZnO is observed at 531.3 eV, which is shifted by ∼0.8 eV to higher binding energy in the case of ZnO−CuPc composites. It indicates withdrawal of electron from oxygen sites in ZnO. On the other hand, the N 1s peak of CuPc corresponding to the Cu−N bond originally observed at 398.5 eV (Cu−N)30 is shifted by about 0.9 eV to lower binding energy while that corresponding to the C−N bond is shifted by only about 0.3 eV for the composites. Peaks corresponding to copper are also shifted by about 0.4 eV to lower binding energy. Broadly, the results confirm increase of electron density in CuPc. The electron from surface oxygen site of ZnO may access the empty orbital of Cu in CuPc. Thus, n-type ZnO interacts with p-type CuPc by electron transfer process. A similar kind of interaction has been observed in composite films comprising of ZnO nanowires and polypyrrole (PPy).31 Photoluminescence Study. The intimate contact between the constituent materials can be established through photoluminescence measurements as well. The excitation and emission spectra of the composites and its constituent materials are shown in Figure 7. The excitation spectra of ZnO−CuPc composites (for emission at 500 nm) show two distinct peaks as shown in Figure 7a. The peak around 360 nm originates from the band edge absorption by ZnO which shifts from 357 to 371 nm due to an increase in the size of ZnO during composite formation. The other maximum in the excitation spectra around 270 nm does not show much variation with

The broadening of the H aggregated peak (Q1) indicates a disorder in the aggregation due to the stacking of CuPc molecules at different orientations influenced by ZnO. The presence of ZnO NS in the composites can also be confirmed by the band edge absorption near the UV region (∼360 nm). The characteristic excitonic peak is observed near the onset of the absorption spectrum that shifts toward a higher wavelength due to the formation of the composite (Figure 4a). It indicates a decrease in the band gap due to an increase in the size of ZnO NS in the composite material as the NaOH concentration increases. Size of the nanostructure for a given band gap value has been calculated using the Brus equation.25 The maximum shift in the band gap of ZnO nanostructures in the composite materials is found to be 0.15 eV [from 3.4 eV (365 nm) to 3.55 eV (349 nm)]. The corresponding size estimated from the Brus equation shows a variation from 8 to 18 nm that agrees well with that obtained from XRD and TEM investigation. A similar change in size and shape of ZnO NS due to a change in the NaOH concentration has been observed in earlier reports.20 The observations reported above indicate two determining factors in the process of composite formation. The change in NaOH concentration has an indirect effect on the size, shape, and type of conductivity of the synthesized ZnO that finally determines the charge transfer between the two constituents. As pointed out earlier, a lower NaOH concentration produces p-type ZnO nanospheres. The CuPc is also known to exhibit a p-type conductivity. Therefore, electron transfer between ZnO nanosphere and CuPc is not efficient. A limited charge transfer may still occur due to the existing energy difference between conduction bands of ZnO (4.1 eV) and LUMO of CuPc (3.2 eV). This is revealed by a very small shift in the Q1 and Q2 bands (Figure 4b). On the other hand, ZnO nanorods have a reduced positive surface charge and exhibit an n-type conductivity. Thus, stable composite is formed wherein an efficient charge transfer occurs between ZnO nanorod and ptype CuPc, manifested by a large shift in the Q1 and Q2 bands, reaching to almost as high as 35 nm. Second, total weight and relative weight ratio of ZnAc and CuPc control the densification and availability of constituent materials. Increase in total weight and arriving at an optimum weight ratio of precursors (indicated in Figure 4d) also increase the possibility of making physical attachment between the constituent materials. Investigation of Vibrational Modes by FTIR. FTIR spectra of pure CuPc, pure ZnO, and nanocomposites (ZC3) are shown in Figure 5. Individual peak corresponding to ZnO could not be assigned due to strong noise observed in the

Figure 5. FTIR spectra of ZnO nanoparticles, CuPc, and ZnO−CuPc (ZC3) nanocomposites. 695



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Figure 6. XPS spectra of (a) O 1s, (b) N 1s, (c) Zn 2p, and (d) Cu 2p for ZnO, CuPc, and composite films indicated in the graph.

the ZnO−CuPc composite material which was not present in the pure ZnO and CuPc. Table 2. Peak Positions Extracted from Photoluminescence Data by Gaussian Fitting CZ0 (nm) CuPc ZnO ZC1 ZC4 ZC6

402 402 406

C1 (nm)

C2 (nm)

C3 (nm)

413

437

462

424 421 422

442 442 445

493 487 486

Z1 (nm)

Z2 (nm)

535 526 516 509

601 580 537 558

On the other hand, pure ZnO NPs of size 5−20 nm show a defect related emission in the green-yellow region (500−600 nm) (Figure 7c). It has been suggested that the broad defect emission may arise due to the presence of oxygen vacancies having two different charge states.34 Signature of other kinds of defects such as zinc vacancy at the surface of ZnO NPs prepared by this route has also been probed by positron annihilation spectroscopy.35 However, photoluminescence measurement on ZnO NPs showing defect emission within the range of 480−580 nm has been explained well by the presence of singly and doubly charged oxygen vacancies.20,34 Thus, the defect related visible luminescence from ZnO can be fitted by two Gaussians Z1 and Z2 due to the contribution arising from two kinds of defect species, viz. doubly (Vo2+) and singly (Vo+) charged oxygen vacancies, respectively.34 As the size increases, Z1 and Z2 show a red-shift in nanosphere but a blue-shift in nanorods.20 In this study both Z1 and Z2 show a significant blue-shift due to composite formation (Figure 7d,e and Table 2). This observation confirms the formation of bigger ZnO nanorods in the composites as the NaOH concentration increases. It has been also suggested that the relative contribution of Z1 and Z2 depends on the morphology of the ZnO NS (Figure 7f).34 The intensity ratio Z2/Z1 gradually decreases as the shape of the nanostructures changes from spherical to rod-like morphology.20 Intensity ratio of Z2/

Figure 7. (a) Room temperature excitation spectra of nanocomposites and its individual components recorded after fixing the emission monochromator at 500 nm. The PL spectra collected after excitation at 270 nm of (b) pure CuPc and (c) pure ZnO and ZnO−CuPc composite materials [(d) and (e)] are displayed as indicated on the graph. (f) Intensity of various components of the defect related visible emission from ZnO nanostructures.

composite formation and attributed to the CuPc. Since ZnO also absorbs at 270 nm, the PL spectra of the composites were recorded after excitation at 270 nm. Pure α-CuPc shows violet emission (438 nm) after excitation at 360 nm32 and also known to emit in the infrared region (1050−1300 nm) of the spectrum when excited around 900 nm.33 In this study CuPc exhibits a violet emission in the range of 400−475 nm peaking at 413, 437, and 462 nm (Figure 7b). These violet bands from CuPc show a noticeable red-shift with nanocomposite formation (Table 2). In addition, a new peak around 405 nm appears for 696



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weight of the components and the decay time constants τi (i = 1, 2, 3) of the transients, are given in Table 3. The first observation that we make is that there is actually a decrease in all the decay time constants on composite formation. The major component τ3 shows significant change in the lifetime. The component τ2 exhibits marginal change due to composite formation. The lifetime of the fastest component of ZnO (τ1) has minor contribution but reduces significantly due to composite formation. The overall contribution of multiple relaxation pathways determine the lifetime of a single state by taking into account the rate of radiative and nonradiative decay to either valence or defect states. The lifetime of an impurityand/or defect-bound exciton state is reported to be 1−2 ns.36 So, τ1 can be attributed to the exciton state bound to surface defect, giving rise to emission near 380 nm. As discussed in the previous section, two charge states of the oxygen vacancies Vo+ and Vo2+ in ZnO NPs are responsible for emission near 500 (Z1) and 550 nm (Z2) respectively. Relatively short lifetime 10−15 ns can be ascribed to the Vo+ as suggested by the earlier reports.37 This component is intrinsic in nature and immune to the change in chemical environment.34 It should be noted that a very long-lived component also exists in the defect emission from ZnO NPs. Very high lifetime (∼50−100 ns) can arise due to the recombination at Vo2+ state which is very sensitive to the change in ambient environment.38 As seen from the Table 3, A2 decreases but A3 increases significantly after composite formation. It indicates that Vo+ transforming to Vo2+ by donating an electron to the attached CuPc. The reduction in τ3 can be ascribed to the increase in nonradiative defect center at the surface of ZnO NPs under the influence of CuPc. p−n Junction Diode Behavior of ZnO−CuPc Nanocomposite Film. As discussed in the previous section, ZnO− CuPc nanocomposites form p−n junctions in a stable composite due to efficient electron transfer between the two constituents. Thus, the current (I) vs voltage (V) characteristics of ZnO−CuPc heterojunction will show rectification properties similar to a p−n diode. The ZnO−CuPc nanocomposite film exhibits rectification ratio of about 20 at 2 V. The device structure ITO/ZnO−CuPc nanocomposite film/CuPc (20 nm)/Au is used for I−V measurement where forward bias corresponds to negative bias given to ITO (Figure 9). The

Z1 is also directly linked with the surface charge of the nanostructures.34 As confirmed by zeta potential and Kelvin probe measurements, the spherical ZnO NPs have a positive surface charge and positive contact potential difference.14,34 As the diameter and aspect ratio of the NS increase, the positive surface charge reduces along with the intensity ratio of Z2/Z1 and negative contact potential difference is created. Finally, due to a large surface area, small spherical NPs show a p-type conductivity influenced by external factors. But bigger nanorods of high aspect ratio exhibit intrinsic n-type conductivity.14,15 During the process of composite formation by varying precursor concentrations, there is a gradual change in aspect ratio from ∼1 (spherical) to 1.5 (rod) of ZnO nanostructures. Thus, efficient electron transfer is possible between n-type ZnO nanorods and p-type CuPc molecules in a stable composite. The reduction in the absolute intensity of ZnO NS (both Z1 and Z2) as it forms stable composites with CuPc (Figure 7f) is an evidence of the p−n junction formed between the constituents. An efficient electron transfer between the constituent materials forming p−n junction results in subdued recombination event required for the generation of PL. In this situation, stable composite is created by the intimate contact between ZnO nanorods and CuPc. On the other hand, for a lower concentration of NaOH, mostly p-type ZnO nanospheres are synthesized which does not show an efficient electron transfer to the p-type CuPc. Thus, the stable network between ZnO nanosphere and CuPc is not realized. Photoluminescence Lifetime Study. ZnO NPs show near band edge (NBE) UV emission due to free exciton (FE) recombination whereas the visible emission originates from the surface related defect states. We have observed that the characteristics of defect related visible emission from ZnO NPs change due to composites formation. The same can also be seen from the photoluminescence lifetime measurement. Since composite formation is mainly surface phenomena, only defect emission from ZnO near 550 nm is considered for lifetime study. The decay curves of ZnO and ZnO−CuPc composites have been resolved into three components by deconvolution fitting of the data with the relation I(t) = ∑i=1−3Ai exp(−t/τi) (Figure 8). The parameters Ai (i = 1, 2, 3), which give the

Figure 8. (a) Time-resolved decay curves of the defect emission near 550 nm from ZnO and ZnO−CuPc composites (ZC4). The instrument response function (IRF) is also shown in the same graph.

Figure 9. I−V characteristics of ZnO−CuPc nanocomposites and its constituent materials indicated on the graph.

Table 3. Photoluminescence Lifetime of ZnO Nanoparticles and ZnO−CuPc Nanocomposites samples

τ1 (ns)

A1 (%)

τ2 (ns)

A2 (%)

τ3 (ns)

A3 (%)

ZnO ZnO−CuPc

3.34 ± 0.26 2.58 ± 0.38

13.04 7.61

14.91 ± 1.28 14.50 ± 2.15

33.81 22.32

63.06 ± 6.90 52.02 ± 4.26

53.14 70.08

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control device ITO/ZnO/CuPc/Au exhibits a rectification ratio of 6 at 2 V, whereas ITO/CuPc/Au and ITO/ZnO/Au do not show any rectification (Figure 9). The CuPc is well-known to establish Ohmic contact with ITO and Au since its highest occupied molecular orbital (HOMO) level is at ∼5.2 eV,39 and therefore no rectification is observed in ITO/CuPc/Au. ZnO is reported to have exhibited ohmic contact with ITO though work functions of Au and ITO are about 5.1 and 4.8 eV, respectively, and the conduction band of ZnO is at 4.1 eV.40 Depending on surface conditions of ZnO, either Ohmic or Schottky contact is known to have formed with Au.41 Absence of rectification in ITO/ZnO/Au predicts that ZnO makes an ohmic contact with Au in the present study. Rectification observed in ZnO and CuPc bilayer or nanocomposite film indicates the formation of p−n junction between n-type ZnO nanorods and p-type CuPc where the barrier at the interface with the built-in potential limits the current flow in the reverse direction. A higher rectification ratio in the nanocomposite films could be attributed to the improved contact between ZnO and CuPc in the nanocomposite structure. Such diode behavior could be well suited for photovoltaic devices.

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CONCLUSIONS A nanocomposite comprising ZnO nanostructures and CuPc molecules has been synthesized in situ for improved mixing and stability. Characteristics of the nanocomposites exhibit significant variations from its constituent materials. In the process of making a stable composite, ZnO nanostructures grow in size and change its shape from spherical to rod-like morphology. Bigger nanorods of ZnO exhibit n-type conductivity and interact with p-type CuPc macrocycle ring through electron transfer process, confirming an intimate contact between the constituents. In addition, a high aspect ratio of ZnO nanorods is advantageous in forming a stable ZnO−CuPc network, thereby modifying the aggregation state of the CuPc. Induced by ZnO nanostructure, the aggregation state of CuPc molecules changes from J to H as confirmed by the significant blue-shift of the Q bands of aggregated CuPc compare to that of plain CuPc film. The CuPc in the composite exhibits a violet emission while ZnO nanostructures show a prominent defect related visible emission along with a weak near band edge emission. The reduction in the intensity of defect emission and its components suggest electron transfer between n-type ZnO nanorods and p-type CuPc. Thus, a p−n junction is realized at the CuPc−ZnO interface showing rectification in the I−V characteristics.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (M.G.). *E-mail [email protected] (N.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.G. is thankful to BRNS for the fellowship. REFERENCES

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Structural, Optical, and Electrical Properties of in Situ Synthesized ZnO

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