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Lithographic Fabrication of Model Systems in Heterogeneous Catalysis and Surface Science Studies Michael X. Yang,† David H. Gracias, Peter W. Jacobs, and Gabor A. Somorjai* Materials Sciences Division and Department of Chemistry, Lawrence Berkeley National Laboratory, University of California at Berkeley, Berkeley, California 94720 Received July 3, 1997. In Final Form: January 21, 1998 Lithographic technologies are applied to fabricate model systems for surface science and heterogeneous catalysis studies. An ordered metal nanocluster array fabricated on oxide substrates is also an ideal model system of supported industrial catalysts. Taking advantage of an ordered nanocluster array fabricated by electron beam lithography, the thermal and chemical stability of supported silver catalysts are examined in both oxidizing and reducing conditions. In reducing conditions, the supported silver nanoparticles are stable up to ∼700 °C. In oxidizing conditions, however, the silver nanoparticles are oxidized below 200 °C, and conglomerate to micrometer-size amorphous clusters ∼400 °C. The supported nanocluster sample can also be adapted to study reactivity of supported metal catalysts, as confirmed by measurement of ethylene hydrogenation turnover rates on platinum nanoparticle samples. Lithographic technologies can also fabricate model systems for other surface science research. A nanometer scale pattern is created on a poly(methyl methacrylate) (PMMA) surface by electron beam lithography. The sample is adapted to test a recent development in nanotribology, in which surface elastic modulus (hardness) is determined by a modified atomic force microscope. In addition, lithographically fabricated supported nanostructures are used to image the AFM tip (thereby determining the radius of curvature of the tip), which is a critical parameter for the quantification of surface mechanical properties such as elastic modulus. Finally, taking advantage of the uniform height profile of lithographically fabricated nanostructures, ion sputtering yield can be determined by the reduction of nanostructure height as a function of ion exposure.
1. Introduction: Methodology There has been an increasing interest in the surface science of nanometer scale supported structures. It is based on the extensive research of uniform and/or flat surfaces. The complexity of research increases drastically with the dimension of the surface reducing from infinity for a uniform surface (single crystal) to the nanometer domain (small clusters). Regardless of the recent revolutionary development in scanning probe microscopies and transmission electron spectroscopy, at the current stage many nanoscale surface science issues cannot be addressed unless the subjects are significantly simplified by an introduction of model systems. Design and application of model systems have been a persistent theme in surface science studies. For example, in a so-called surface science approach to heterogeneous catalysis, amorphous industrial metal catalysts have been substituted by single-crystal metal surfaces with uniform structures. The information of reaction kinetics and mechanisms obtained in single-crystal studies has served as guidelines in understanding the real industrial catalytic process.1 Lithographic technologies are superior for the fabrication of model nanostructures. Lithographic fabrication of nanostructures is the cornerstone of semiconductor industry. A standard lithographic process is shown in Figure 1. With a collimated electron or photon beam, ordered patterns are produced on a resist (in most cases a polymeric material) layer coated on the substrate. The exposed polymer is then selectively removed, while the unexposed portions of polymer layer remain intact on the * Corresponding author. Fax: 510-643-9668. E-mail: somorjai@ garnet.berkeley.edu. † Current address: Applied Materials, Inc., 3100 Bowers Av, m/s 0205, Santa Clara, California 95054. (1) Somorjai, G. A. Surf. Sci. 1994, 299/300, 84.
Figure 1. Schematic diagram of lithographic fabrication of nanostructures.
substrate. This is followed by a materials deposition step, usually in a line-of-sight thermal evaporation process. The final step in the lithographic process is to remove the remaining polymer on the surface, along with the thin film materials covering the polymer layer. In this fashion, materials are placed on the substrate precisely at positions registered by the photon or electron beam exposure. Lithographic fabrication has a number of advantages over other nanofabrication techniques. First of all, lithographic technologies are universal and are capable of depositing a variety of materials on essentially any flat surface. Beyond the resolution limit, the dimension and the shape of the supported nanostructures can be tailored at will. In addition, the fabricated supported structures have uniform height, which is determined by the thin film thickness in the materials deposition step. At last but not least, with photolithography or electron beam lithography, nanoscale contrasts can be created on semiconductor and/or insulator surfaces. Taking advantage of lithographic fabrication, we can design model systems for nanoscale surface science studies. We present a few examples in the following
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Figure 2. Scanning electron microscopy (SEM) micrographs of silver nanoclusters (Pattern A) deposited on an oxidized silicon surface after annealing in 1 atm of hydrogen at the indicated temperature for 1 h.
sections, covering surface characterization, materials processing and heterogeneous catalysis. The application in heterogeneous catalysis studies is discussed in section 2, with subjects ranging from thermal and chemical stability to reactivity of supported metal catalysts. The applications of lithographic technologies in other surface science studies are then mentioned briefly in section 3, including nanotribology (section 3.1) and ion sputtering (section 3.2). 2. Lithographic Fabrication of Model Supported Catalysts Most industrial catalysts are small metal or metal alloy particles deposited on oxide substrates. In conventional chemical routes, catalysts are prepared from solution by impregnation and coprecipitation. One of the major challenges in catalyst fabrication and catalysis studies is to deposit tailored metal clusters on oxide surface with an ordered structure. This can be accomplished by lithographic technologies. An ordered supported metal nanocluster array is an ideal model system of industrial supported catalysts. Changes in nanoparticle size, structure, composition and distribution on the supports can be monitored under catalytic reaction conditions, i.e., under high gas pressure (∼1 atm) of reactants and high temperature (up to 1000 °C). In addition, the size dependence and substrate sensitivity of catalytic reactions can be investigated. 2.1. Thermal Stability of Silver Clusters on Oxide Supports. The stability of metal particles deposited on oxide substrates is a fundamental issue in catalysis studies. The optimal catalytic reaction conditions are determined in part by the thermal and chemical stability of supported metal catalysts. Unfortunately, structural changes of conventional industrial catalysts are difficult to investigate, due to a broad distribution of metal particle size and a random distribution of particles on the substrate. In contrast, lithographic fabrication can provide a well-defined metal nanocluster array, with a precise control of the particle size and spacing. The uniform nanocluster arrays can be characterized by a variety of surface microscopy techniques. One example is the thermal stability of supported silver catalysts.2 Silver is an excellent catalyst for a number of industrial catalytic reactions, among which the partial oxidation of ethylene to ethylene oxide is arguably the (2) Yang, M. X.; Jacobs, P. W.; Yoon, C.; Muray, L.; Anderson, E.; Attwood, D. A.; Somorjai, G. A. Catal. Lett. 1997, 45, 5.
most important.3 Since silver is vulnerable to oxidation, one of the issues in reaction engineering is the thermal stability of supported silver catalysts. To study the thermal stability of supported silver catalysts, we fabricated an ordered array of nanoscale silver clusters on oxidized silicon and aluminum surfaces by electron beam lithography. Silica (silicon oxide) and alumina (aluminum oxide) are two common oxide substrates for catalysts. In our studies, they were mimicked by oxidized silicon and aluminum surfaces. Silicon surfaces were oxidized by dry oxidation, while aluminum surfaces were oxidized in concentrated sulfuric acid. The oxide overlayers were characterized by Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). The thickness of the oxide layer is ∼500 Å for silicon and ∼30 Å for aluminum surface. Silver clusters were fabricated in two sizes. Pattern A has a matrix of 100 nm silver clusters with an interparticle distance of ∼ 2 µm and a particle height of ∼ 20 nm. Four micrometer scale markers are deposited around the cluster array (Figure 2). Pattern B contains a pair of 750 nm × 750 nm × 36 nm squares separated by 250 nm and the twin feature is repeated with a ∼10 µm periodicity (Figure 5). The silver nanoclusters were characterized by highresolution optical microscopy, scanning electron microscopy (SEM), scanning Auger microscopy (SAM), and atomic force microscopy (AFM). The thermal stability of silver nanoclusters was studied under both reducing and oxidizing conditions. Hydrogen and air are representative reducing and oxidizing agents. The results obtained in these two conditions are discussed first, followed by a summary of the nanocluster stability in other chemical conditions (vacuum, N2, O2, C2H4, and O2/C2H4/N2 mixture). 2.1.1. Thermal Stability in Hydrogen. The thermal stability of silver clusters deposited on oxidized silicon and aluminum was first examined in an ambience of hydrogen. Figure 2 displays SEM micrographs of pattern A deposited on an oxidized silicon surface. The pictures were collected after annealing the sample in 1 atm of hydrogen at the indicated temperature for 1 h. As shown in Figure 2, the silver cluster array remains intact up to 680 °C, and no migration of silver clusters is observed. After an annealing at 730 °C, the entire cluster array vanishes, which is due to silver evaporation from the surface. (3) Dever, J. P.; George, K. F.; Hoffman, W. C.; Soo, H. In KirkOthmer Encyclopedia of Chemical Technology; John Wiley & Sons: New York, 1994; Vol. 9.
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Figure 3. Optical microscopy pictures of silver nanoclusters (pattern A) on SiOx after annealing in 1 atm of air at the indicated temperatures for 1 h. A fraction of silver cluster array is displayed with part of a micron scale marker.
Figure 4. SEM micrographs of silver nanoclusters (pattern A) on SiOx after annealing in atmosphere at 450 °C for 1 h.
Figure 5. Atomic force microscopy (AFM) images of silver nanoclusters (pattern B) on SiOx (A) before and (B) after annealing at 300 °C in air.
A comparative SEM study of silver nanoclusters deposited on an oxidized aluminum surface yields identical results. In conclusion, the thermal stability of silver nanoclusters is insensitive to the oxide support under atmospheric pressure of hydrogen. 2.1.2. Thermal Stability in Air. Compared with the results under hydrogen, silver nanoclusters have a much lower thermal stability in oxidizing conditions. Figure 3 presents a series of optical microscopy photographs of pattern A deposited on a SiOx substrate after annealing in 1 atm of air at the indicated temperature for 1 h. For the convenience of presentation, only part of the silver cluster array is displayed, along with part of a silver marker deposited around the nanocluster array. Oxidation of the silver nanocluster surface takes place below 200 °C, as indicated from a gradual color change in the photographs. The oxidized silver nanoclusters remain stable up to 350 °C. After an annealing at 400 °C, the silver cluster array is erased. As shown in Figure 4, amorphous silver clusters are scattered on the oxidized
silicon surface. The size of smallest clusters ranges from10-1 to 10 µm. The structural change of silver clusters upon annealing was also monitored by atomic force microscopy (AFM). Figure 5 presents three-dimensional AFM images of pattern B deposited on the SiOx surface. As shown in Figure 5a, the uniform silver clusters fabricated at room temperature are 36 nm high with a surface roughness of ∼ 2 nm. After an annealing in air at 300 °C, the surface morphology is changed drastically. As shown in Figure 5b, while the overall size of the silver pattern remains approximately constant, the silver cluster surface is corrugated, with a height variation of ∼90 nm within a single cluster. Once again, the silver nanoclusters deposited on oxidized aluminum surface exhibit a thermal stability similar to silver nanoclusters fabricated on oxidized silicon surface. The silver nanoparticles are oxidized below 200 °C, and conglomerate to micrometer-size amorphous clusters at ∼400 °C. The thermal stability of silver clusters
Lithographic Fabrication of Model Systems
Figure 6. Scanning electron/Auger microscopy (SAM) analysis of silver nanoclusters (pattern B) on SiOx (A) before and (B) after annealing at 300 °C in air. SEM micrographs of silver clusters are presented on top and SAM scans collected at indicated positions are shown at bottom.
is significantly lower in air than in hydrogen, in which the silver cluster array periodicity stays intact up to ∼700 °C. Despite the similarity in thermal stability for silver clusters fabricated on oxidized silicon and aluminum surfaces, the surface chemical composition of silver nanoclusters is sensitive to the oxide support, especially under oxidizing condition and at elevated temperatures. It is illustrated in spatially resolved chemical composition analysis of the silver nanocluster samples by scanning Auger electron microscopy (SAM). Figure 6 shows SAM scans of pattern B, together with corresponding scanning electron microscopy micrographs. For the fresh silver nanocluster sample fabricated at room temperature (left panel), silicon and oxygen signals are detected on the SiOx substrate (point 1), while the silver signal is observed on the silver cluster (point 2) together with a very small oxygen peak. SEM micrograph displayed in the right panel of Figure 6 confirms a corrugation of silver cluster surface after an annealing at 300 °C in air for an hour. In addition, the SAM survey reveals a silicon buildup on the silver clusters (point 2), which suggests a spillover of the support onto the silver cluster and/or a silication of silver cluster surfaces. Such a buildup of substrate oxide on metal cluster surface, however, is absent for silver clusters deposited on oxidized aluminum surface. Figure 7 presents SAM surveys of silver clusters on an oxidized aluminum surface. After the cluster was annealed at 300 °C for an hour, the aluminum signal is absent on the silver cluster surface (point 2). 2.1.3. Thermal Stability in Other Gas Ambients. The stability of silver clusters was also studied in a vacuum and other gas ambients, including nitrogen, oxygen, and ethylene. In addition, a gas mixture of 10% oxygen and 10% ethylene in nitrogen was employed to mimic the gas composition in industrial ethylene oxidation reactions. The behavior of silver clusters can be divided into two categories based on the gas composition: the oxygen-free agents (vacuum, N2, and H2), and the oxygen-containing agents (O2, air, O2/C2H4/N2 mixtures). In the absence of oxygen, no migration of silver clusters is observed prior to their evaporation at >700 °C. The thermal stability of silver clusters decreases in the presence of oxygen. With increasing annealing temperature in oxygen, silver cluster surface oxidation occurs at
