Atomic Force Microscopy Adhesion Mapping: Revealing Assembly

Sep 10, 2013 - Atomic Force Microscopy Adhesion Mapping: Revealing Assembly Process in Inorganic Systems ... and different characterization techniques...
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Atomic Force Microscopy Adhesion Mapping: Revealing Assembly Process in Inorganic Systems Pichitchai Pimpang,†,‡ Ahmad Sabirin Zoolfakar,†,§ Duangmanee Wongratanaphisan,‡ Atcharawon Gardchareon,‡ Emily P. Nguyen,† Serge Zhuiykov,∥ Supab Choopun,*,‡ and Kourosh Kalantar-zadeh*,† †

School of Electrical and Computer Engineering, RMIT University, Melbourne, VIC 3001, Australia Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, and ThEP center, CHE, Bangkok 10400, Thailand § Faculty of Electrical Engineering, Universiti Teknologi MARA, 40450 Shah Alam, Malaysia ∥ Materials Science and Engineering Division, CSIRO, Highett, VIC 3190, Australia ‡

S Supporting Information *

ABSTRACT: There are still many unknowns regarding assembly processes. In this work, we demonstrate the capability of atomic force microscopy (AFM) adhesion mapping in revealing the conditions that promote the light-induced assembly of nanoparticles (NPs) on nanostructured surfaces in inorganic systems, both in macroand nanodomains. Gold (Au) NPs and zinc oxide (ZnO) nanostructures are employed as the model materials, and different characterization techniques are used for extracting the relationship between the materials’ crystallinity, stoichiometry, and morphology as well as surface adhesion mapping information. The light-induced assembly of Au NPs is associated with the attraction forces between the opposite surface charges of the NPs and preferential ZnO sites, which can be identified by adhesion mapping. We show that the yield of Au nanoclusters assembled onto the ZnO surface depends on the crystallinity and stoichiometry of ZnO and is not due to the roughness of the surface. The presented experiments demonstrate that AFM adhesion mapping can be used as an invaluable tool for predicting the strength and directions of assembly processes.

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“pull-off” (adhesion) force is measured as the minimum tension required for removing the tip from the sample surface. The nature of the forces probed can be associated with many different types of attraction forces including van der Waals and electrostatic forces, taking into consideration the measured value of the pull-off forces.8,9 In addition, the combined magnitude of the forces is affected by the surface topography and depends on factors such as the surface energy characteristics of the tip and the sample. AFM adhesion mapping can be applied to a wide variety of materials and has great potential for investigating their surface heterogeneity.10 To date, AFM adhesion mapping has been used for mapping the surface heterogeneity of materials such as polymers10,11 and metal oxides8,9 and mapping the interactions between proteins.12 In this work, we report our findings about the nature of lightinduced assembly gold (Au) nanoclusters onto zinc oxide (ZnO) nanostructures and associate them to the adhesion property of ZnO. Au and ZnO are used as model inorganic materials in our experiments due to access to comprehensive

INTRODUCTION Bottom up assembly processes are frequently used in the synthesis of films with various granular morphologies. The assembly processes are a combination of self- and directed assembly pathways, which compete with each other to result in the formation of the final structure. Self-assembly is the process where a disordered system of pre-existing components forms an organized structure or pattern without the exertion of any external force,1 while directed assembly is the process where a force, such as chemical, mechanical, electromagnetic (light included), and thermal forces, is incorporated to form desired structures. Both self- and directed assemblies play important roles in the synthesis of materials as they allow for “bottom up” approaches and novel material designs. Some materials include molecular crystals, colloids, lipid bilayers, Langmuir−Blodgett films, and phase-separated polymers.2−7 However, many aspects of assembly routes are still unknown. For many assembly processes of inorganic systems, the roles of surface energy, stoichiometry, and crystallinity are still unclear and debatable. We believe that atomic force microscopy (AFM) adhesion mapping can be a great tool for revealing these and other assembly process and addressing such unknowns. The adhesion map of a surface is obtained by bringing the AFM tip in contact with the sample and then retracting it. The © 2013 American Chemical Society

Received: June 24, 2013 Revised: September 5, 2013 Published: September 10, 2013 19984

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Figure 1. SEM top view images of ZnO nanostructured thin films obtained at different annealing temperatures of: (A) 400, (B) 450, (C) 500, and (D) 550 °C with the scale bar of 500 nm.

Lesker Company, 99.98%) for 15 min. The thickness of the films was measured to be approximately 300 nm. The films were then immersed in a 1.0 × 10−4 M of HAuCl4 solution in DI water (pH of 4). These ZnO nanostructured films were irradiated to 1 mW/cm2 of UV-A light intensity (wavelength = 365 nm) for 30 min. HAuCl4 solution was prepared by dissolving gold sheets using aqua regia (HCl:HNO3, 3:1). Finally, the obtained Au nanoclusters/ZnO nanostructured thin films were cleaned with distilled water and dried at room temperature. The morphology, stoichiometry, and crystallinity of the nanostructured thin films were assessed using scanning electron microscopy (SEM, JEOL JSM-7001F), X-ray diffraction (XRD, Rigaku miniflex II), AFM (Bruker MultiMode 8 with PF QNM), X-ray photoelectron spectroscopy (XPS, Thermo Scientific), and UV−vis spectroscopy (Varian Cary 50).

reports on their physiochemical properties. ZnO is an n-type wide-bandgap semiconductor of the II−VI semiconductor group. ZnO nanostructures have been shown to be incorporated into electronic devices, chemical sensors, solar cells, catalysts, photoluminescence-based devices, thermoelectric systems, and photocatalysis.13−24 It is well-known that the fast recombination of electron−hole pairs limits ZnO performance in the aforementioned systems.25,26 This issue can be addressed by modifying the surface with noble metal nanoparticles (NPs), such as Au, as they improve both charge separation and interfacial charge transfer kinetics.25−30 In addition, Au NPs modified ZnO exhibits surface plasmon resonance in the visible region, depending on the size and morphology of the Au NPs.31 A thorough understanding regarding the properties of the substrates onto which the selected materials to be assembled is needed, where in our case ZnO is the substrate and Au NPs are the assembled materials. The ZnO nanostructured thin films are fully characterized to reveal those properties, and their assembly mechanism in the macrodomain, i.e., stoichiometry and crystal phase of the ZnO films. The study of specific interaction sites on the surface of ZnO is also necessary to predict the light-induced assembly mechanism of Au NPs onto its surface in the nanodomain and to identify the preferential adhesion sites. Considering the aforementioned points, we investigate the assembly of Au NPs onto ZnO in this work by assessing the crystal phase of ZnO, its stoichiometry, and morphological properties and relate these properties to the characteristics obtained using AFM adhesion mapping.

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RESULTS AND DISCUSSION SEM images of the ZnO nanostructured films after annealing at 400, 450, 500, and 550 °C are presented in Figure 1. ZnO nanostructures appear to form connected beads or short rods, branched into their ZnO nanocluster bases. The dimensions of ZnO nanostructures increase by elevating the annealing temperatures, and the average size of ZnO nanocrystallites increased from 18.7 to 21.6 nm (Table S1, Supporting Information). The crystal phases of ZnO nanostructured films were assessed using XRD patterns and XPS spectra (Figure 2). XRD patterns show the ZnO films are highly crystalline with preferred orientation of (002) in all cases (Figure 2i). The preferred (002) orientation strongly suggests single crystallinity and that the majority of nanostructures preferably grows along the c-axis.32−34 This can be correlated with the presence of polar surfaces of the wurtzite ZnO structure. It can be seen that the ZnO film annealed at 450 °C had the strongest crystallinity, showing the maximum intensity of the preferred (002) orientation. This film also shows the maximum texture coefficient, Tc(002), value (Table S1, Supporting Information). XPS analysis is used for obtaining the compositions of ZnO

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EXPERIMENTAL SECTION To investigate the adhesion properties of ZnO and its effect on the formation and assembly of Au NPs, ZnO nanostructured thin films were prepared by thermal oxidation of zinc thin films on glass substrates under normal atmospheric conditions at different annealing temperatures of 400, 450, 500, and 550 °C for 20 h. To form ZnO films, zinc thin films were deposited by thermal evaporation using 0.2 g of zinc wires (Zn, Kurt J. 19985

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force applied to the tip is produced by electrostatic charges. The system simultaneously obtains the topographical (Figure 3i) and adhesion mapping (Figure 3(ii)) images of the surfaces.

Figure 2. (i) XRD patterns and (ii) XPS spectra of O1s of ZnO nanostructured thin films annealed at different temperatures of: (A) 400, (B) 450, (C) 500, and (D) 550 °C. Figure 3. AFM: (i) topographical and (ii) adhesion mapping images of ZnO nanostructured thin films obtained at different annealing temperatures of: (A) 400, (B) 450, (C) 500, and (D) 550 °C with the scale bar of 500 nm.

nanostructured films. To remove carbon contamination on the surface of the ZnO films, the surface is initially sputter etched using the Ar-ion accelerated to 200 eV for 30 s. Noticeable peaks at 1021.3 ± 0.2 and 529.9 ± 0.1 eV (S1, Supporting Information) that can be associated to zinc and oxygen appear, respectively.35−37 The O 1s signals shown are resolved into two peaks by a Lorentzian distribution fitting as shown in Figure 2ii. The peak with low binding energy, O1s(1), corresponds to O2− on the normal wurtzite structure of ZnO.38 Another peak, O1s(2), centered at 531.6 eV is attributed to O2− in the oxygendeficient regions within the matrix of ZnO.39 Thus, the intensity of the high binding energy is related to the amount of oxygen deficiency within the ZnO structure. It is found that the ZnO nanostructured film annealed at 450 °C has the minimum oxygen deficiency and the most perfect stoichiometry, thus resulting in the strongest surface polarity. The AFM adhesion mapping is carried out on the surface of all ZnO nanostructured films. Using surface energy models, it is predicted that the adhesion is proportional to the tip end radius.40 In our case, the tip is made of silicon nitride (8 nm), and the end radius is coated with 20 nm thick iridium/ platinum. The existence of such a metal coating assures that the

The surface roughness of the ZnO nanostructures is obtained from the topographical images in each case (Figure 3i). The surface roughness is augmented by increasing the annealing temperature, and the root-mean-square (RMS) value increased from 54 to 85 nm. The adhesion mapping is determined by the contrast of the adhesion force region between tip approach and retracing during the scanning surface of the ZnO films. The adhesion force is mainly associated with electrostatic interaction8 between the tip and the ZnO surface. Hence, the adhesion force is measured using voltage. The stronger adhesion regions are displayed as higher voltage areas in the images, and the adhesion intensity is calculated using ImageJ software. It is found that the adhesion intensity of ZnO nanostructured films obtained at 400 and 450 °C is approximately >1.5 times higher than those films obtained at 500 and 550 °C. In addition, ZnO nanostructured films obtained at 450 °C exhibit maximum adhesion intensity (S4, Supporting Information). These 19986

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Figure 4. SEM top view images of Au NPs/ZnO nanostructured thin films prepared using ZnO films annealed at: (A) 400, (B) 450, (C) 500, and (D) 550 °C with the scale bar of 500 nm. Inset: high-magnification SEM images with the scale bar of 100 nm.

dimensions in the range of 25−40 nm. The yields of Au nanoclusters assembled onto ZnO nanostructured thin films obtained at different annealing temperatures are shown in Figure 5. It is found that the maximum yield of Au nanoclusters

outcomes are curious, as the overall surface adhesion increase shows trend opposite to that of increasing surface roughness. It appears that the overall adhesion intensities of the surface of the ZnO films are associated with the strength of their growth along the c-axis and the reduction in oxygen deficiency. ZnO is a II−VI compound with an intrinsic electrical dipole moment, creating a polar surface. It seems that strong polarity, as a result of better stoichiometry and more enhanced c-axis growth, of the films obtained at 400 and 450 °C resulted in stronger surface adhesions. The strong polarity of the surface of these films hence induces a stronger electrostatic force at the iridium/platinum AFM tip end. These outcomes are in agreement with Gourianova et al. who also showed that the adhesion force is correlated with the strength of the polar surface of the sample and AFM probe.41 To demonstrate the efficiency of AFM adhesion mapping for revealing the favorable sites in the assembly process, Au NPs were formed on the surface of the ZnO nanostructured films via the light-induced assembly process that is explained in the Experimental Section. There are various techniques to deposit Au NPs onto the ZnO surface, such as hydrothermal routes, pulsed laser deposition, electro-deposition, spray pyrolysis, sputtering, and photodeposition.9,15,42−48 Among them, photodeposition is highly favorable for our adhesion mapping experiments as it is simple and highly controllable for the formation of Au NPs onto ZnO and requires no surface modification. No surface modification is especially important in adhesion mapping experiments as it allows for the association of the Au NP adhesion sites to the surface map obtained. Additionally, photodeposition is advantageous as it allows relatively controlled amounts of Au NPs, with fairly small dimensions, to be loaded onto ZnO.30,49−51 Figure 4 shows that Au NPs exhibit clustered growth around ZnO nanostructures with dimensions less than 100−150 nm, where the bright areas in the SEM images are attributed to Au. Particularly, it seems likely that Au NPs have a preferential growth to encapsulate individual ZnO nanostructures. These Au nanoclusters are comprised of several Au NPs with

Figure 5. Yield of Au nanoclusters assembled onto ZnO nanostructured thin films annealed at different temperatures (assessed using four SEM images in each case). The normalized Au nanoclusters yield is calculated as the number of Au nanoclusters divided by the total number of Au nanoclusters and no clustered NPs.

is obtained for the ZnO film annealed at 450 °C which has the highest crystallinity and most perfect stoichiometry, which resulted in the strongest surface polarity. This is also in agreement with the surface adhesion mapping experiments (Figure 3) that show the strongest electrostatic forces onto the AFM tip induced by the 450 °C film. Interestingly, the increase in the surface roughness of the ZnO films (which is the highest for films obtained at 550 °C) does not directly contribute to the increase of the yield of Au nanoclusters (which is the lowest for the same films). 19987

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The growth mechanism model for Au nanoclusters assembled onto ZnO nanostructures is demonstrated in Figure 7. The mechanism is proposed by taking images of the same sites on the surface of the ZnO nanostructured films before and after the assembly of Au nanoclusters. First, negatively AuCl4− ions are attracted onto the polar surface of the ZnO that has the lowest surface energy.53−56 As previously discussed, adhesion mapping shows that these sites are the corners between nanostructures. When ultraviolet (UV) is irradiated onto ZnO, photogenerated electron−hole pairs are formed as49

For understanding the mechanism of assembly in the nanodomain and to reveal the map of local adhesion regions on the surface of ZnO nanostructures, two zoomed in images of the ZnO sample annealed at 450 °C are presented in Figure 6:

hν

− + ZnO → e(CB) + h(VB)

(1)

Then, AuCl4− ions are reduced to Au by the photogenerated electrons30,49,57,58 − 3e(CB) + AuCl−4 → Au(near ZnO surf) + 4Cl−

(2)

Next, the initial Au nuclei are formed and are condensed into Au NPs.54 The Au NPs are further attracted onto the corners of ZnO nanostructures by the nature of the minimized surface energy59,60 and the surface polarity54,61−64 forming Au nanoclusters. This mechanism proceeds until the Au clusters block the light. During this process, the generated holes, in the presence of Au/ZnO, photocatalyze H2O to produce oxygen gas and H+ according to57,58 + 4h(VB) + 2H 2O → 4H+ + O2

(3)

Without applying any voltage, the rate of this chemical equation is slow.58 As a result, the oxygen evolution is slow, and the H+ produced does not considerably alter the pH of the acidic environment. The adhesion maps show that the least rough surfaces of 400 and 450 °C films have the highest adhesion. Using adhesion mapping (in the microlevel) the largest surface voltages are seen for these surfaces, which is associated to the strongest surface attraction forces. On the macrolevel, both XPS and XRD show that the highest stoichiometry and crystallinity are obtained for 400 and 450 °C films, which produce the strongest surface polarity. This suggests that the strongest polarity produces the strongest surface adhesion. The outcomes are interesting as the surface roughness does not increase the surface adhesion, and the surface attraction forces play a much more important role in the presented case.

Figure 6. AFM: (i) “topographical image” and (ii) “adhesion mapping image” of ZnO nanostructured thin films annealed at 450 °C with the scale bar of 200 nm. Ovals added for comparison.

(i) “topographical image” and (ii) “adhesion mapping image”. Surprisingly, the strongest adhesion sites are seen in the corners between the ZnO nanostructures, especially at the interior corners between subsequent steps (i.e., trough) (Figure 6 − the ovals). The step corner effect on the growth of nanoclusters has also been reported previously.52 As such, the AFM adhesion mapping provides invaluable information regarding local adhesion regions in nanodomains.

Figure 7. Schematic diagram, together with the relevant images, presents the assembly of Au nanoclusters onto ZnO nanostructures. 19988

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(7) Klajn, R.; Bishop, K. J. M.; Grzybowski, B. A. Light-Controlled Self-Assembly of Reversible and Irreversible Nanoparticle Suprastructures. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 10305−10309. (8) Zoolfakar, A. S.; Ahmad, M. Z.; Rani, R. A.; Ou, J. Z.; Balendhran, S.; Zhuiykov, S.; Latham, K.; Wlodarski, W.; Kalantar-zadeh, K. Nanostructured Copper Oxides as Ethanol Vapour Sensors. Sens. Actuators, B: Chem. 2013, 185, 620−627. (9) Zoolfakar, A. S.; Rani, R. A.; Morfa, A. J.; Balendhran, S.; O’Mullane, A. P.; Zhuiykov, S.; Kalantar-zadeh, K. Enhancing the Current Density of Electrodeposited ZnO-Cu2O Solar Cells by Engineering Their Heterointerfaces. J. Mater. Chem. 2012, 22, 21767− 21775. (10) Eaton, P.; Estarlich, F. F.; Ewen, R. J.; Nevell, T. G.; Smith, J. R.; Tsibouklis, J. Combined Nanoindentation and Adhesion Force Mapping Using the Atomic Force Microscope: Investigations of a Filled Polysiloxane Coating. Langmuir 2002, 18, 10011−10015. (11) Jogikalmath, G.; Stuart, J. K.; Pungor, A.; Hlady, V. Adhesion Mapping of Chemically Modified and Poly(Ethylene Oxide)-grafted Glass Surfaces. Colloid Surf. A 1999, 154, 53−64. (12) Bhushan, B.; Tokachichu, D. R.; Keener, M. T.; Lee, S. C. Morphology and Adhesion of Biomolecules on Silicon based Surfaces. Acta Biomater. 2005, 1, 327−341. (13) Ozgur, U.; Alivov, Y. I.; Liu, C.; Teke, A.; Reshchikov, M. A.; Dogan, S.; Avrutin, V.; Cho, S. J.; Morkoc, H. A Comprehensive Review of ZnO Materials and Devices. J. Appl. Phys. 2005, 98, 041301. (14) Wang, Z. L. Zinc Oxide Nanostructures: Growth, Properties and Applications. J. Phys.: Condens. Matter 2004, 16, R829−R858. (15) Sadek, A. Z.; Choopun, S.; Wlodarski, W.; Ippolito, S. J.; Kalantar-zadeh, K. Characterization of ZnO Nanobelt-based Gas Sensor for H2, NO2, and Hydrocarbon Sensing. IEEE Sens. J. 2007, 7, 919−924. (16) Martinson, A. B. F.; Elam, J. W.; Hupp, J. T.; Pellin, M. J. ZnO Nanotube based Dye-Sensitized Solar Cells. Nano Lett. 2007, 7, 2183− 2187. (17) Walia, S.; Weber, R.; Balendhran, S.; Yao, D.; Abrahamson, J. T.; Zhuiykov, S.; Bhaskaran, M.; Sriram, S.; Strano, M. S.; Kalantar-zadeh, K. ZnO based Thermopower Wave Sources. Chem. Commun. 2012, 48, 7462−7464. (18) Yang, Y.; Pradel, K. C.; Jing, Q. S.; Wu, J. M.; Zhang, F.; Zhou, Y. S.; Zhang, Y.; Wang, Z. L. Thermoelectric Nanogenerators Based on Single Sb-Doped ZnO Micro/Nanobelts. ACS Nano 2012, 6, 6984− 6989. (19) Jasieniak, J.; MacDonald, B. I.; Watkins, S. E.; Mulvaney, P. Solution-Processed Sintered Nanocrystal Solar Cells via Layer-byLayer Assembly. Nano Lett. 2011, 11, 2856−2864. (20) Morfa, A. J.; Kirkwood, N.; Karg, M.; Singh, T. B.; Mulvaney, P. Effect of Defects on the Behavior of ZnO Nanoparticle FETs. J. Phys. Chem. C 2011, 115, 8312−8315. (21) Suchea, M.; Christoulakis, S.; Moschovis, K.; Katsarakis, N.; Kiriakidis, G. ZnO Transparent Thin Films for Gas Sensor Applications. Thin Solid Films 2006, 515, 551−554. (22) Lee, S.; Bae, S. H.; Lin, L.; Yang, Y.; Park, C.; Kim, S. W.; Cha, S. N.; Kim, H.; Park, Y. J.; Wang, Z. L. Super-Flexible Nanogenerator for Energy Harvesting from Gentle Wind and as an Active Deformation Sensor. Adv. Funct. Mater. 2013, 23, 2445−2449. (23) Wang, X.; Song, J.; Liu, J.; Wang, Z. L. Direct-current Nanogenerator Driven by Ultrasonic Waves. Science 2007, 316, 102− 105. (24) Zoolfakar, A. S.; Ab Kadir, R.; Rani, R. A.; Balendhran, S.; Liu, X.; Kats, E.; Bhargava, S. K.; Bhaskaran, M.; Sriram, S.; Zhuiykov, S.; et al. Engineering Electrodeposited ZnO Films and their Memristive Switching Performance. Phys. Chem. Chem. Phys. 2013, 15, 10376− 10384. (25) Kochuveedu, S. T.; Oh, J. H.; Do, Y. R.; Kim, D. H. SurfacePlasmon-Enhanced Band Emission of ZnO Nanoflowers Decorated with Au Nanoparticles. Chem.Eur. J. 2012, 18, 7467−7472. (26) Sun, L.; Zhao, D.; Song, Z.; Shan, C.; Zhang, Z.; Li, B.; Shen, D. Gold Nanoparticles Modified ZnO Nanorods with Improved Photocatalytic Activity. J. Colloid Interface Sci. 2011, 363, 175−181.

CONCLUSION In summary, we have shown the impressive capability of AFM adhesion mapping for revealing the fundamentals of assembly mechanisms in an inorganic system comprised of ZnO nanostructures and Au NPs. ZnO nanostructures were obtained by thermal oxidation of zinc thin films at different annealing temperatures. It was shown that the photosynthesized Au NPs had the strongest assembly onto the ZnO nanostructured film annealed at 450 °C. On the macrodomain, this enhancement could be associated with the strong c-axis growth of ZnO films promoting perfect stoichiometry and thus producing the strongest polarity for such films. In the nanodomain, the AFM adhesion mapping also showed that the interior corners between subsequent steps provided the strongest adhesion sites for the Au NP assembly. The AFM adhesion mapping showed that the surface attraction forces play a more important role in the formation of Au NPs than the surface roughness. The outcomes can be readily extended to reveal self- and directed assembly for many other complex systems. As a result, AFM adhesion mapping provides a great tool to investigate the fundamentals of assembly in future studies.

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ASSOCIATED CONTENT

S Supporting Information *

XPS, SEM, UV−vis spectra, and summary of the crystallite dimensions and the texture coefficients of ZnO nanostructured films. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Phone: (66) 53943375 E-mail: [email protected]. *Phone: (613) 9925 3254. E-mail: [email protected]. au. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Thailand Research Fund through the Royal Golden Jubilee Ph.D. program (Grant No. PHD/ 0157/2552) is acknowledged. P. Pimpang would also like to acknowledge the financial support of the Graduate School, Chiang Mai University.

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dx.doi.org/10.1021/jp406210u | J. Phys. Chem. C 2013, 117, 19984−19990