Influence of Seeding Layers on the Morphology, Density, and Critical

Article pubs.acs.org/JPCC

Influence of Seeding Layers on the Morphology, Density, and Critical Dimensions of ZnO Nanostructures Grown by Chemical Bath Deposition Alexander M. Lockett, P. John Thomas,* and Paul O’Brien* School of Chemistry, The University of Manchester, Oxford Road, Manchester M139PL, U.K. ABSTRACT: The role of the seeding layer on the form of nanostructured ZnO films grown by chemical bath deposition is studied. We find that the method by which the seeding layer is deposited, the crystallite size, and the thickness have a significant effect on the morphology, density, and the critical dimensions of the ZnO nanostructures subsequently formed. The morphologies obtained include highly ordered, dense nanorod arrays; long rods, unusually growing along the a axis; and three-dimensional clusters.

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INTRODUCTION Zinc oxide is a II−VI semiconductor with a wide, direct band gap (3.37 eV) and a large exciton binding energy (60 meV). Nanocrystalline ZnO materials have been widely studied for high-technology applications such as in photovoltaic devices,1−3 light-emitting diodes,4,5 photodetectors,6,7 and gas sensors.8−10 The electrical and optical properties of nanocrystalline ZnO make it a viable alternative to TiO2 in Grätzel-type photovoltaic devices.11 A variety of anisotropic ZnO nanostructures such as stars,12,13 helices,14 rods,15−17 tubes,18 and other complex structures19,20 have been reported. Hexagonal ZnO nanorods, in particular, have attracted considerable attention2,4−6,8−11 because of their sensing ability. Sputtering,21,22 pulsed laser deposition,23 electrochemical deposition,4 metal organic chemical vapor deposition,24 spray pyrolysis,25 and chemical bath deposition (CBD)26 have all been used to deposit high quality films of ZnO. Among these methods, chemical bath deposition is particularly attractive as it is simple, versatile, and does not require high temperatures, vacuum, and/or expensive equipment.27 Control can be exercised on the morphology of the deposits by the use of templates. For example, high density nanorod arrays can be obtained by using nanostructured thin films as seeding layers.28 Critical seeding layers to yield nanorod arrays of ZnO by CBD have been obtained using a number of methods. The crucial role of the seeding layer (SL) in the deposition of nanorods was first highlighted by us.26 Saha and co-workers exemplified this recently by using a sol−gel method to prepare seeding layers29 and obtained ordered arrays of ZnO nanorods with diameters in the range of 90−140 nm. Despite wide ranging studies, the specific role of a seeding layer cannot be established by comparison as often different conditions and reagents have been used to grow the nanorod overlayers. Herein, continuing our interest in ZnO,26,28,30−35 we carry out a systematic study, whereby seeding layers grown by different methods have been used as templates for subsequent chemical © 2012 American Chemical Society

bath deposition studies under standard deposition conditions. The role of the seeding layer can therefore be ascertained by direct comparison of the resulting ZnO structures. Seeding layers have been obtained using aerosol assisted chemical vapor deposition (AACVD), radio frequency magnetron sputtering, and sol−gel growth, as well as by spin coating of quantum dots.

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EXPERIMENTAL SECTION Seeding Layer Depositions. SLs were usually grown on glass microscopy slides from Fisher Scientific, which were cut into 1 cm2 squares. Deposition by AACVD was carried out on 1 cm2 Si/SiO2 (100) wafers. Prior to the growth of the SLs, the glass substrates were cleaned by ultrasonication in a 1:1 mixture of ethanol and water for 15 min and air-dried. Si wafers were similarly cleaned using a 1:1 mixture of acetone and water. Sol−Gel Growth. The sol−gel was prepared and transferred to glass substrates following previous reports.30,36 Briefly, zinc acetate dihydrate (Zn(ac)2·2H2O) (17.8 g, 0.08 mol) was dissolved in n-propanol (120 cm3) by refluxing the liquid for 20 min. The solution was then cooled to room temperature and tetramethylammonium hydroxide (TMAH) (25% in MeOH, 36 cm3) was added to obtain a transparent ZnO coating sol (∼0.5 mol dm−3). The sol was then deposited onto glass substrates by dip-coating followed by sintering at 400 °C for 2 min in a furnace. Aerosol Assisted Chemical Vapor Deposition. A 2-necked round-bottom flask containing a solution of (Zn(ac)2 2H2O) (0.01 M) in methyl acetate was placed atop an aerosol generator (piezoelectric modulator of a Pifco ultrasonic humidifier, model 1077). A carrier gas (Ar) with a flow rate of 2.5 L min−1 was employed to sweep the aerosol to the furnace chamber, held at 700 °C. Deposition was carried out on Received: November 18, 2011 Revised: March 2, 2012 Published: March 22, 2012 8089

dx.doi.org/10.1021/jp211121d | J. Phys. Chem. C 2012, 116, 8089−8094

The Journal of Physical Chemistry C

Article

characteristic of a thin layer of ZnO. Stronger reflections were obtained following deposition by CBD (see Figure 1).

two substrates, one at the hot zone, and another upstream, at the medium-hot zone near the exit of the chamber. The temperature of the latter was estimated to be 500 °C. A high growth rate of 1700 nm/h has been previously reported for this deposition.37 Hence, the deposition time was limited to 10 min. Surface profilometeric studies yielded thicknesses of ∼280 nm for substrates at the hot zone and 150 nm for substrates at the medium-hot zone. Spin-cast Quantum Dots. ZnO QDs were prepared following the method by Mulvaney and co-workers.40 Briefly, (Zn(ac)2 2H2O) (50 mL, 20 mM) was dissolved in 99.8% ethanol and cooled to 4 °C using a refrigerated water bath and a double-walled reaction vessel. To this solution, tetramethylammonium hydroxide (TMAH; 25 wt %, in methanol) (0.85 mL) was injected via a motorized syringe pump at a rate of 0.05 mL min−1 to obtain a translucent colloid. The diameters of the quantum dots were determined from the absorption spectra following the method described by sarma and co-workers.38 A drop of this colloid was spin coated on to the substrate spun at 2000 rpm for 120 s. Once coated, the substrates were dried at room temperature. Radio Frequency Sputtering. RF sputtering was carried out in Ar atmosphere in an Emitech K675X high resolution large chamber sputter coater with Lesker ZnO target. Films with thicknesses of 10 and 20 nm were produced by controlling the sputtering time. A more detailed description of the experimental setup and the characterization of thin films has been published previously.39 Chemical Bath Deposition. The substrates with SLs were cleaned by repeated washing, followed by ultrasonication in ethanol and deionized water for 15 min. After drying at room temperature, the substrates were fixed to a clean glass microscope slide with a double-sided adhesive carbon tape and used for CBD. A bath was made, in double-walled jacketed vessel, using a solution of (Zn(ac) 2 2H2 O) (40 mL, 0.025 M) and hexamethylenetetramine (HMT, 40 mL 0.025 M). The pH of this solution was adjusted to 5, by dropwise addition of acetic acid, and the solution was heated to 92.5 °C. The substrates were introduced in the heated bath at the point of onset of visible turbidity. After a growth time of one hour, the substrates were removed, thoroughly washed with deionized water, and allowed to dry at room temperature before characterization. In order to ensure that the results obtained are reliable, two or three repeat runs of depositions were carried out. We found that the products from the original and repeat runs were identical. Material Characterization. The films on glass or Si substrates were used as such for X-ray diffraction studies. X-ray diffraction was carried out using Cu Kα radiation on a Bruker AXS D8 advance diffractometer. The samples were coated with a thin film of thermally evaporated carbon, to facilitate imaging by scanning electron microscopy. The surface morphologies of the ZnO films were examined by field emission scanning electron microscopy using a Philips XL30 microscope. Energy dispersive X-ray analysis was carried out in the same instrument.

Figure 1. Powder X-ray diffraction patterns of ZnO nanostructures grown by chemical bath deposition on seeding layers obtained using (a) sputtered templates with a thickness of 20 nm; (b) sol−gel method; (c) aerosol assisted chemical vapor deposition, substrate at high temperature zone; (d) spin coating of solution prepared quantum dots; (e) sputtered templates with a thickness of 10 nm; (f) aerosol assisted chemical vapor deposition, substrate at medium-hot temperature zone; and (g) standard diffraction pattern ICDD card No. 361451.

The positions of the peaks are in good agreement with those corresponding to the wurtzite form of ZnO (ICDD No. 361451). The diffraction patterns indicate that the samples are crystalline consisting of pure hexagonal ZnO. The relative intensities of the Bragg peaks in some of the patterns indicate strong orientation along specific directions. Relatively intense (0002) peaks (2θ ≈ 34°) are seen in X-ray patterns corresponding to deposits grown on 20 nm sputtered (20 nm S) sol−gel and high temperature AACVD SLs (Figure 1a−c, respectively), suggesting that the deposits grow preferentially along the [0001] direction. The observed degree of orientation, is greatest in the 20 nm S sample, followed by the overlayers on the sol−gel template and finally the high temperature AACVD sample. ZnO grown on spin coated quantum dot seed layers (Figure 1d) presents an enhanced (1010) peak, while the thinner sputtered (10 nm S) seeds (Figure 1e) yield (1010) and (1011) peaks more intense than (0002), indicating preferred growth in directions other than [0001]. In these cases, the overall degree of orientation is lower. Diffraction patterns also show that ZnO grown on the medium temperature AACVD seeding layer (Figure 1f) is not oriented. The significant differences in orientation is indeed unexpected, as chemical bath deposition was carried out under identical conditions in all cases. FESEM images of the samples with enhanced (0002) peaks are shown in Figure 2. In each case, ordered arrays of nanorods oriented roughly perpendicular to the substrate are seen. Previous studies have established that ZnO deposited near equilibrium conditions grows along the [0001] direction resulting in a relatively intense (0002) peak.28,30,35 When rods such as the ones observed herein adopt this growth mode, their c axis is perpendicular to the substrate surface.28,30,35 The

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RESULTS AND DISCUSSIONS Optically transparent overlayers were obtained in areas covered by the SLs except in the case of overlayers grown on AACVD SLs, which were translucent. X-ray diffraction patterns of seeding layers consisted of broad low intensity peaks 8090



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In this context, the ability of the seeding layer to affect surface density is significant. SEM images of ZnO overlayers grown on QD SLs show dense nanorods (see Figure 3a). However, in contrast to the

Figure 3. FESEM images of ZnO nanostructures deposited by chemical bath deposition on seeding layers using (a) spin coating of solution prepared quantum dots and (b) sputtered templates with a thickness of 10 nm. Insets of panels a and b show corresponding low resolution images.

nanorods in Figure 2, which are all perpendicular to the substrate, the rods herein subtend a broader range of angles, though they remain principally upright. ZnO overlayers grown on 10 nm thick sputter SL comprise of microscopic starburst structures made up of thick nanorods. The outgrowths are sparsely scattered throughout the substrate. The most significant variation in morphology is seen in the case of ZnO deposited on the seeds grown at the medium-hot zone by AACVD. The SEM images in Figure 4 show microscopic globular aggregates, with diameters between 12−15 μm, distributed all over the substrate. The ordered nanorod arrays such as the ones seen in Figure 2, have been previously seen when SLs are employed to nucleate ZnO growth in CBD reactions.15−17,26 The SL promotes the growth of ZnO along the high surface energy [0001] plane, leading to growth of rods along the c axis, with the [0001] basal plane. In the case of this study, some of the SLs have facilitated nanorod array growth. However, the

Figure 2. FESEM images of ZnO nanostructures deposited by chemical bath deposition on seeding layers using (a) sputtered templates with a thickness of 20 nm (inset shows an image obtained at a tilt of 45°); (b) sol−gel method; and (c) high temperature aerosol assisted chemical vapor deposition. Insets of panels b and c show corresponding lower resolution images.

SLs affect the density of the nanorod arrays on the substrate, with the highest density being observed in the most oriented case (Figure 2a) and the lowest seen with the least oriented nanorod arrays (Figure 2c). Some applications such as the use of nanorods in dye-sensitized solar cells require tailoring of surface density as the ability of the dye to interpenetrate the semiconducting material is critically dependent on the density. 8091



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the substrate.30 The AACVD method is initiated from sporadic seeding across the surface of the substrate, leading to nonuniform distribution of deposits.37 The effects of this distribution of the seeds is apparent in the SEM images of the overlayers. The SG templates result in highly ordered, slender, and dense nanorod arrays, consistent with a template that is uniform, dense, and nanocrystalline. The less dense, stouter nanorods obtained atop AACVD deposited seeds are less uniformly spread across the surface. The sputtered seeds present an intermediate between both the SG and AACVD deposited ones, yielding a pure and relatively uniform layer.39 The effects of such a layer is best seen in the case of the 20 nm S seeds, which result in a highly ordered, dense nanorod array similar to that of the SG. The change in morphology from nanorod arrays to starburst structures is seen when the thin sputtered seeding layer (10 nm S) is employed. It is possible that the out growths result from nucleation at surface defect sites created by etching, a process that generally accompanies sputtering, especially at low film thicknesses. However, given the copious quantities of the starburst structures, we believe that it is more likely that the 10 nm thick film is simply too thin to facilitate rod growth. Figure 5 shows starburst structures that adorn defect sites as well as

Figure 4. FESEM images of ZnO overlayers deposited by chemical bath deposition on seeding layers obtained using substrates left at the medium-hot zone of the AACVD reactor. Inset shows a lower resolution image.

dimensions and morphology of the obtained nanorods are strikingly different. A summary is shown in Table 1. The sol− Table 1. Morphologies and Critical Dimensions of the Nanostructures Obtained When Different Seeding Layers Are Used seeding layer deposition method sol−gel spin coated quantum dots sputtering, 20 nm aerosol assisted chemical vapor deposition, high temperature zone sputtering, 10 nm aerosol assisted chemical vapor deposition, medium-hot zone

morphology

average dimensions

nanorod nanorod nanorod nanorod

50 nm 200 nm 120 nm 300 nm

starburst globules

700 nm microscopic

gel deposited seeds (Figure 2b) yield nanorods with the smallest average diameter (50 nm), while the sputtered seeds with thickness of 20 nm (Figure 2a) yield thicker rods (120 nm). In either case, the distribution in diameters is narrow. Longer and even more thicker rods are obtained when QD SLs are employed. Here, rods with average diameters of 200 nm and lengths of up to 1.3 μm are obtained (see Figure 3a). Nanorods with the largest average diameter are those from the high temperature AACVD SLs (Figure 2C) and are ca. 300 nm. In this case, the distribution in diameters is large, ranging from under 200 nm to over 400 nms. The starburst structures from the thinner sputtered seeds are relatively large; the clusters as a whole are approximately 8 μm in diameter, while the rods are roughly 4 μm in length and have a diameter of around 500 nm. Such a structure is not uncommon and has been reported previously41−46 when other methods have been used to obtain SLs. The method of deposition of seeding layers plays a crucial role in determining the growth mode, morphology, and surface density of the subsequently deposited overlayer. The SG method uses a colloidal solution, which is dip-coated to get a uniform coating across the template. Once annealed, a uniform nanocrystalline thin film with surface defects originating from the organic capping agents present before annealing remains on

Figure 5. FESEM images of ZnO overlayers deposited by chemical bath deposition on sputtered seeding layers with thickness of 10 nm. (a) The starburst structures adorn a surface defect; (b) starburst structures on surface with no large scale discernible defects. 8092



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dense nanorod arrays to starburst structures. Similar thin seed structures grown at the medium-hot zone of the AACVD reactor results in globular 3D structures.

smooth areas with no discernible large scale defects. Recently, Gao et al. have been able to obtain ball-shaped 3D ZnO clusters with elaborate texture on sputtered seeds, by employing an Al(III) additive.47 The growth of these clusters have been attributed to the additive. Our observation of the relatively simpler 3D clusters in the case of seeds deposited from the medium-hot zone of the CVD reactor suggests that the seeding layers perhaps have a part to play in initiating the growth of such clusters. Akin, to the sputtered seeds above (10 nm S), these seeds are also perhaps too small to facilitate growth of nanorods. The deposition of QD seeds (by spin coating) was employed to examine if small nanoscale particles could nucleate further nanodimensional architectures. We estimate the diameters of the nanocrystals to be 5.1 nm. To the best of our knowledge, such a seeding layer has not been previously employed for the growth of ZnO nanostructures by CBD. The method with respect to purity is the poorest. Even though it is similar to the SG method, the lack of sintering results in a surface largely covered by an organic capping layer.40 The crystallite structures grown from the QDs was unexpected, in particular the size but also the morphology. The X-ray pattern (Figure 1b) clearly has a dominant (1010) peak, which is unusual and signifies preferred orientation in the a axis. There have been previous reports of (1010) orientated growth of ZnO.48−51 In these cases, the deposition of the ZnO structures on substrates have been carried out using high temperature methods such as spray pyrolysis, chemical vapor deposition, and pulsed laser deposition.49−51 Growth along the a axis is desired for use in application such as surface acoustic wave devices.52 This report is the first observation of such a growth by the CBD process. The increased diameter of the nanorods, in comparison to the dimensions of the quantum dots, could be due to agglomeration of the dots following spin coating. Energy dispersive X-ray analysis yielded Zn/O ratios in the range of 0.98−1.1, indicating that the ZnO deposits obtained are pure. Optical bandgaps obtained were identical to bulk ZnO, indicating that the nanostructures have no strong quantum confinement effects, which is inline with expectations for large ZnO nanostructures.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (P.J.T.); paul. [email protected](P.O.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the University of Manchester, RCUK, and the Engineering and Physical Science Research Council (EPSRC, U.K.) for financial support.

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CONCLUSIONS The influence of the seeding layer on the growth of ZnO crystallites by chemical bath deposition has been studied. We have shown that the method by which the SL is deposited, the crystallite size, and, also, the thickness have a significant effect on the morphology, growth mode, and density, as well as the critical dimensions of the ZnO nanostructures formed under standard chemical bath deposition conditions. Sputtered seeds with thickness of 20 nm resulted in highly ordered, dense nanorod arrays, with diameters of 120 nm and a narrow distribution. Similar arrays consisting of nanorods with diameters of 50 nm were obtained when the sol−gel method was employed for generating the seeds. Ordered nanorod arrays, with diameters of 300 nm and larger distribution have been obtained using seeds grown by the AACVD method. When spin-cast films of ZnO quantum dots made by chemical routes were used as seeding layers, long nanorods with diameters of 200 nm and lengths over 1 μm, with uncommon preferred orientation along (1010) and growth along the a axis, are obtained. Changing the thickness of the seeding layers yield fascinating results. A reduction of 10 nm in the case of sputtered seeding layers resulted in overlayers changing from 8093



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Influence of Seeding Layers on the Morphology, Density, and Critical

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