J. Phys. Chem. C 2009, 113, 11643–11646
11643
Gold Nanoparticle Arrays Fabricated on a Silicon Substrate Covered with a Covalently Bonded Alkyl Monolayer by Electroless Plating Combined with Scanning Probe Anodization Lithography† Hiroyuki Sugimura,* Shinichiro Nanjo, Hikaru Sano, and Kuniaki Murase Department of Materials Science and Engineering, Kyoto UniVersity, Sakyo, Kyoto 606-8501, Japan ReceiVed: February 15, 2009; ReVised Manuscript ReceiVed: April 7, 2009
Hexagonal nanoholes, in which an array of Au nanoparticles was located, were successfully fabricated on the basis of scanning probe anodization lithography. The lithography has been conducted to draw nanopatterns on a hexadecyl self-assembled monolayer covalently bonded to a Si substrate through Si-C bonds. Using a conductive atomic force microscope (AFM) probe as a point contact electrode, current was locally injected into the monolayer-Si sample. The monolayer was decomposed on the location where current injected. Subsequently, at this point, the substrate Si was anodized resulting in the formation of an oxide nanodot. Next, the oxide nanodot was selectively etched with HF in order to make a nanohole while the surrounding monolayer remaining unetched. As a result, a nanohole was formed at the current injected point. Employing the remained monolayer as an etch mask, the nanohole was further etched with NH4F in order to prepare a well-defined hydrogen-terminated surface in the nanohole. The nanohole became hexagonal due to the anisotropic nature of Si etching with NH4F. The lateral size of the fabricated nanohexagonal holes were in the range of 180 ∼ 320 nm. Finally, electroless plating was applied to the sample. Au nanoparticles were selectively nucleated on the surface of the nanohole’s bottom and walls. The particles with a diameter less than 10 nm were well-separated each other with a nucleation density of 150,000 µm-2. The NH4F etching was successful in order to control the nucleation manner for the fabrication of the Au nanoparticle array. Introduction Metallic nanoparticles have attracted increasing attention as crucial building blocks for constructing nanomaterials through a bottom-up approach. Such nanomaterials are expected to perform a variety of functions adaptable in, for example, electronic, photonic, magnetic, chemical, and biological applications.1,2 Among various metal nanoparticles, those made of gold are of primary importance due to their photonic applications based on plasmonic activities of gold. Furthermore, gold (Au) has the advantage that its surface can be selectively modified with an organic monolayer having a particular function through chemical affinities of gold toward some types of functional groups. The arrangement of gold nanoparticles into a two-dimensional micropattern is of special interest, since such micropatterns of Au nanoparticles are potentially applicable to combinatorial microarrays for chemical and biological screening and surface plasmonic microdevices. Self-assembling is a powerful approach in order to fabricate two-dimensional periodic aggregates of Au nanoparticles with a high regularity.3 However, when we intend to use such an Au nanoparticle array as an functional element of a microdevice, the array must be located on a particular position in the microdevice. Namely, there is a need for the technology to arrange nanoparticles along a defined pattern without periodicity. This requirement cannot be satisfied by self-assembly alone but can be fulfilled with a help of a lithographic technique. Indeed, the patterned deposition of Au nanoparticles has been successfully attained onto microtemplates fabricated by a lithographic method, e.g., photolithography and microcontact printing.4-7 †
Part of the “Hiroshi Masuhara Festschrift”. * To whom correspondence should be addressed.
When a more precise control in dimension of the nanoparticle arrangement is required, one candidate is the use of electron beam lithography.8,9 Another promising way is scanning probe lithography in which scanning probe microscopy (SPM) is employed as a lithographic tool10-13 for the fabrication of templates. Scanning probe lithography has been recognized as a convenient tool for nanolithography. Indeed, nanoscale templates have been fabricated by scanning probe lithography and successfully applied in order to arrange metal nanoparticles.14-18 In order to fabricate metallic nanoarrays, nucleation of metal nanoparticles from selected sites based on electroless plating19 is another promising approach, besides immobilization of presynthesized nanoparticles onto a template. Although, metallic nanostructures have been successfully fabricated via scanning probe lithography followed by an electroless plating process so far,20-22 the density of metal nucleation sites has been controlled so as to be sufficiently high for forming continuous metal nanolines rather than attaining the density to form arrays consisting of isolated nanoparticles. Here we report on a fabrication method for Au nanoparticle arrays on Si substrate by means of site-specific electroless plating. Two-dimensional patterns have been drawn on a silicone (Si) substrate covered with an alkyl self-assembled monolayer (SAM) using this SAM as a resist film for scanning probe anodization lithography. The surface where isolated Au nanoparticles nucleate is prepared through a two-step chemical etching process following the lithography. Experimental Section Cleaned Si(111) substrates (cut from a phosphorus-doped n-type Si wafer with a resistivity of 10∼50 Ω cm) were first etched in 5% HF for 5 min at room temperature. Next, the
10.1021/jp901470q CCC: $40.75 2009 American Chemical Society Published on Web 05/05/2009
11644
J. Phys. Chem. C, Vol. 113, No. 27, 2009
Figure 1. Schematic illustration of the fabrication process for Au nanoparticle arrays. (a) HD-SAM/Si sample, (b) scanning probe anodization lithography, and (c) chemical process for attaining areaselective Au deposition.
substrates placed in dark were treated in 40% NH4F for 30 s at a temperature of 80 °C in dark. Through these treatments, the surface oxide layer on each of the Si substrates was removed and the substrate surfaces were terminated with hydrogen. A SAM of the hexadecyl form (tSi-C16H33) was fabricated on some of the hydrogen-terminated Si substrates (Si(111)-H) through a thermal activation process as previously described.23 A Si(111)-H sample was immersed in neat 1-hexadecene (Tokyo Kasei Co Ltd.) heated at 180 °C for 2 h with a continuous flow of nitrogen. Then, the treated sample was rinsed thoroughly using hexane, ethanol, and ultra pure water in that order. The water contact angle of the sample treated with 1-hexadecene was found to be 109°, suggesting the formation of a conformationally ordered monolayer. Nanostructures were fabricated on a Si substrate covered with a hexadecyl SAM (Si-HD) by scanning probe anodization lithography. As schematically illustrated in Figure 1b, a positive bias was applied to the Si substrate, while an electrically conductive probe of an atomic force microscope (AFM) served as a counter electrode was in contact with the sample surface. An AFM probe made of Si coated with Rh (SII NanoTechnology, Inc., SI-DF3-R) was used for anodization. The sample patterned by scanning probe anodization was etched in 5% HF for 5 min at room temperature and, then, treated in 40% NH4F for 30 s at a temperature of 80 °C. Si(111)-H samples and the Si-HD sample nanopatterned through the procedures described above were immersed in an electroless gold plating solution (Kojundo Chemical Laboratory Co., K-24S) at room temperature for an appropriate time. The samples were rinsed with pure water and then blown dry. The samples were characterized by the same AFM used for patterning and a field-emission scanning electron microscope (FESEM, JEOL Ltd., JSM-6500F). Results and Discussion Prior to experiments on scanning probe lithography, we have studied Au nucleation behavior on the Si(111)-H surface prepared by HF etching followed by NH4F etching. Panels a-d in Figure 2 show surface AFM images of the Si(111)-H
Sugimura et al.
Figure 2. AFM images of Si(111)-H surfaces treated in the Au electroless plating bath. Each sample was immersed in the bath for (a) 5, (b) 60, (c) 300, and (d) 3600 s, respectively.
Figure 3. Heights and densities of the nucleated Au nanoparticles on Si(111)-H.
samples treated in the Au plating solution. As can be seen in Figure 2a, the sample surface has a characteristic feature corresponding to Si(111) surfaces prepared with the NH4F etching, namely, a surface consisting of atomically flat terraces with a (111) surface separated with a monatomic step of nearly 0.3 nm in height, since, in this etching solution, the Si etching rate to the [111] direction is much slower than the other directions. Close inspection of Figure 2a shows that small protruding dots are present at certain sites located on the step edges. Panels b-d in Figure 2 demonstrate that the size of the protruding dots increases with the increase in the immersion time in the plating solution. Based on X-ray photoelectron spectroscopy, the deposited dots are confirmed to be made from Au through an increase in photoelectron signal from Au with the increase in the plating time. We have evaluated the size of the Au nanoparticles based on their heights rather than their diameters, since the diameters of these particles are close to a tip diameter of the AFM probe used for imaging, lateral features of the dots are inevitably enlarged in the images and, consequently, not accurate due to the so-called tip effect. As summarized in Figure 3, the Au nanoparticle height estimated form cross sections of the AFM images increases monotonically with the plating time. The particle height markedly increases from 4 to 10 nm. It is interesting that, in the plating time range between 5 and 300 s, the heights of the Au nanoparticles on each of the samples are quite uniform, although their average
Gold Nanoparticle Arrays
J. Phys. Chem. C, Vol. 113, No. 27, 2009 11645
Figure 4. Scanning probe anodization of the HD-SAM/Si sample. (a) Summary of experimental conditions: applied voltages and duration times. (b) Topographic AFM images of the sample anodized under the condition indicated on the left side.
height increases with the plating time. On the contrary, the change in the Au nanoparticle density is not straightforward as demonstrated in Figure 3. At the initial stage of Au plating, the density increases from about 400 µm-2 at a plating time of 5 s to over 600 µm-2 at a plating time of 60 s. However, when the plating time is prolonged, the density decreases down to near 500 µm-2 at a plating time of 300 s. This is most likely caused through fusing of nucleated Au nanoparticles. We have successfully fabricated Au nanoparticle arrays on Si substrates with controlled size and density. In addition to be noted, the nucleated Au nanoparticles are located on regulated positions, namely, on the parallel lines which correspond to the monatomic step lines of the Si(111)-H surface. It is considered that, in the plating bath used for the experiment, the Au nucleation first starts through the galvanic displacement of Si atoms of the substrate with Au complex ions in the solution. Then, the Au nuclei grow further to be larger particles by the catalytic mechanism. It is natural that the galvanic displacement reaction proceeds more readily at the step edge that on the flat terrace surface. Consequently, on the Si-Si(111)-H surface, the electroless plating Au nanoparticles align on the step edge lines. If we can control the location and density of the nucleation sites, that is, those of the step edges, we can expect to fabricate Au nanoparticle arrays with an intentional design. We applied our scanning probe lithography technique for this goal. The AFM-based anodization condition is summarized in Figure 4a. Dot features were fabricated with a substrate bias voltage in the range of 6∼10 V and a bias duration time of 3 ms to ∼10 s. Figure 4b shows an AFM topographic image of the fabricated dot features. As clearly seen in the image, the bias-applied regions are protruded from the surrounding unmodified regions. These protruded dot features are considered to be formed on the basis of the local anodization mechanism proceeding in the adsorbed water nanocolumn formed at an SPM-robe/sample junction in an atmosphere humidified to some extent.24 When current is injected locally into an organic monolayer covered Si substrate from an SPM probe, the monolayer is gradually degraded followed by the anodization of the substrate Si.25-27 Finally, the monolayer is decomposed and a protruded feature is formed at the probe-sample junction because of the volume expansion of the anodized area accompanied with oxidation of Si to Si oxide. This anodic behavior of the HD-SAM/Si substrate is schematically illustrated in Figure 1b. As summarized in Figure 5, the dot heights and diameters roughly increase with the bias duration time and bias voltage. The minimum feature size in this case is ca. 80 nm, probably depending on the probe tip size. As can be seen in Figure 5, the height and diameter of the anodized dots is not sufficiently controllable, but is scattered
Figure 5. Heights and diameters of the anodized dots as functions of the bias duration time at each of the bias voltages.
to some extent. In order to improve the accuracy and reproducibility of nanofabrication, scanning probe lithography in the constant current mode in which the probe-injecting current is regulated by properly adjusting the applied voltage with a constant current circuit28,29 will be helpful. This sample was further treated by the series procedures of HF etching, NH4F etching, and Au electroless plating as described in the experimental section. The plating was prolonged for 300 s. The HD-SAM has an excellent durability to HF etching,30 the anodized Si oxide is selectively etched while the surrounding HD-SAM surface is free from etching, when treated with the HF solution, as schematically illustrated in Figure 1c. After the treatment, hexagonal holes are appeared on the sample as shown in an FE-SEM image of Figure 6a. The position of each hole corresponds to that of each anodized dot as shown in panels a and b in Figure 4. These hexagonal features are assumed to be formed due to the anisotropic nature of NH4F etching where the etch rate to [111] direction is slower than to the other directions, such as [100] and [110]. An enlarged FESEM image shown as Figure 6b demonstrates that minute Au nanoparticles are nucleated on the bottom and walls of the hexagonal hole corresponding to that indicated at the upper left of the FE-SEM image of Figure 6a. The size of the etched hole is laterally enlarged to 320 nm from its original anodized dot size of 160 nm. In the case of the minimum anodized dot of 80 nm in diameter, the etched hexagonal hole size is about 180 nm. The diameters of the Au nanoparticles are less than 10 nm. This particle size is consistent with the height of the Au nanoparticles formed on a Si(111)-H surface by electroless plating for the same deposition time for 300 s. It was confirmed that, through the two-step etching procedure, a minute hole in nanoscale with its bottom having a Si(111)-H surface of a proper nucleation site density was successfully formed at the AFM-anodized area on the HD-SAM/Si sample.
11646
J. Phys. Chem. C, Vol. 113, No. 27, 2009
Sugimura et al. combined with scanning probe anodization lithography using an alkyl SAM covalently bonded to the Si substrate as a resist film. The Au nanoparticles consisting of the arrays were electrolessly nucleated on the bottom of a hexagonal nanohole fabricated chemical etching in HF and NH4F. The sizes of the hexagonal holes were in the range of 180∼320 nm. These nanohexagon holes modified with an Au nanoparticle array are expected to be served as reaction nanovessels with plasmonic functions. Acknowledgment. This work was supported by KAKENHI (Grant-in-Aid for Scientific Research) No. 19049010 on Priority Area “Strong Photons-Molecules Coupling Fields (470)” from the Ministry of Education, Culture, Sports, Science and Technology of Japan. References and Notes
Conclusion
(1) Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem 2000, 1, 18. (2) Kneipp, K.; Kneipp, H.; Kneipp, J. Acc. Chem. Res. 2006, 39, 443. (3) Kanehara, K.; Kodzuka, E.; Teranishi, T. J. Am. Chem. Soc. 2006, 128, 13084. (4) Vossmeyer, T.; Delonno, E.; Heath, J. R. Angew. Chem., Int. Ed. 1997, 36, 1080. (5) He, H. X.; Zhang, H.; Li, Q. G.; Zhu, T.; Li, S. F. Y.; Liu, Z. F. Langmuir 2000, 16, 3846. (6) Chen, C.-F.; Tzeng, S.-D.; M.-H., L.; Gwo, S. Langmuir 2006, 22, 7819. (7) Zhang, X.; Sun, B.; Frien, R. H.; Guo, H.; D., N.; Giessen, H. Nano Lett. 2006, 6, 651. (8) Sugimura, H.; Nakagiri, N. Jpn. J. Appl. Phys. 1995, 34, 3406. (9) Quate, C. F. Surf. Sci. 1997, 386, 259. (10) Wouters, D.; Schubert, U. S. Angew. Chem., Int. Ed. 2004, 43, 2480. (11) Garcia, R.; Martinez, R. V.; Martinez, J. Chem. Soc. ReV. 2006, 35, 29. (12) Sato, T.; G, H. D.; Ahmed, H. J. Vac. Sci. Technol. B 1997, 15, 45. (13) Mendes, P. M.; Jacke, S.; Critchley, K.; Plaza, J.; Chen, Y.; Nikitin, K.; Palmer, R. E.; Preece, J. A.; Evans, S. D.; Fitzmaurice, D. Langmuir 2004, 20, 3766. (14) Zheng, J.; Zhu, Z.; Chen, H.; Liu, Z. Langmuir 2000, 16, 4409. (15) Liu, S.; Rivka Maoz, R.; Jacob Sagiv, J. Nano Lett. 2004, 4, 845. (16) M, F. Z.; Fre´chet, M. J. J. Am. Chem. Soc. 2005, 127, 8302. (17) Tzeng, S.-D.; Lin, K.-J.; Hu, J.-C.; Chen, L.-J.; Gwo, S. AdV. Mater. 2006, 18, 1147. (18) Khatri, O. M.; Han, J.; Ichii, T.; Murase, K.; Sugimura, H. J. Phys. Chem. C 2008, 112, 16182. (19) Yae, S.; Nasu, N.; Matsumoto, K.; Hagihara, T.; Fukumuro, N.; Matsuda, H. Electrochim. Acta 2007, 53, 35. (20) Sugimura, H.; Nakagiri, N. Appl. Phys. Lett. 1995, 66, 1430–1431. (21) Brandow, S. L.; Calvert, J. M.; Snow, E. S.; Campbell, P. M. J. Vac. Sci. Technol. A 1997, 15, 1455. (22) Sugimura, H.; Takai, O.; Nakagiri, N. J. Electroanal. Chem. 1999, 473, 230. (23) Sano, H.; Maeda, H.; Matsuoka, S.; Lee, K.-H.; Murase, K.; Sugimura, H. Jpn. J. Appl. Phys. 2008, 47, 5659. (24) Sugimura, H.; Uchida, T.; Kitamura, N.; Masuhara, H. J. Phys. Chem. 1994, 98, 4352. (25) Sugimura, H.; Nakagiri, N. Langmuir 1995, 11, 3623. (26) Ara, M.; Graaf, H.; Tada, H. Appl. Phys. Lett. 2002, 80, 2565. (27) Han, J.; Lee, K.-H.; Fujii, S.; Sano, H; Kim, Y. J.; Murase, K.; Ichii, T.; Sugimura, H. Jpn. J. Appl. Phys. 2007, 46, 5621. (28) Sugimura, H.; Nakagiri, N. Nanotechnology 1997, 8, A15. (29) Wilder, K.; Quate, C. F. J. Vac. Sci. Technol. B 1999, 17, 3256. (30) Sano, H.; Maeda, H.; Ichii, T.; Murase, K.; Noda, K.; Matsushige, K.; Sugimura, H. Langmuir. appeared as ASPS article (la804080g).
The arrays of Au nanoparticles with a diameter less than 5 nm have been fabricated on a Si substrate by the chemical process
JP901470Q
Figure 6. FE-SEM images of the anodized sample treated in the three step chemical process, that is, HF etching, NH4F etching and electroless Au plating. (a) The hole view of the fabricated features which positions corresponding to those shown in Figure 4. (b) An enlarged view of one hexagonal hole at the upper left in the whole view image. Deposited with Au nanoparticles are seen as bright dots.
Although, the nucleated Au nanoparticles are formed in a well-separated fashion, they do not align linearly different from the nucleation manner demonstrated on the large Si(111)-H surface as shown in Figure 2. Moreover, the particle density roughly estimated from the FE-SEM image of Figure 6b is about 150 000 µm-2 which is almost 500 times greater than that on the Si(111)-H surface with a same plating time of 300 s. One plausible explanation is as follows. The interface between the Si substrate and an anodic oxide feature fabricated by the use of a point contact electrode in scanning probe anodization is not perfectly flat as illustrated in Figure 1c. The interface and, accordingly, the surface appeared after etching are thought to be curved and probably waved. This might cause forming many and randomly distributed steps, namely, nucleation sites, on the Si-H surface of the hole bottom.
