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Tunable Ag Nanowires Grown on Cu(110)-Based Templates Thomas Brandstetter, Thorsten Wagner, Daniel R. Fritz, and Peter Zeppenfeld* Institute of Experimental Physics, Johannes Kepler University Linz, Altenberger Strasse 69, A-4040 Linz, Austria
ABSTRACT Metallic nanostructures have unique properties concerning, for instance, their optical excitation and the propagation of plasmons. These properties are strongly correlated with the geometry of the nanostructures. We present a method to tune and even invert the length-to-width ratio of Ag nanowires grown on Cu(110)-based templates by molecular beam epitaxy. While the dimension of the nanowires along the [001] surface direction is determined by surface strain effects, the template structure limits the growth along the orthogonal [110] direction. The resulting nanowires are analyzed by scanning tunneling microscopy, and a statistical evaluation of the data is presented that confirms the high uniformity of the resulting nanowires. SECTION Nanoparticles and Nanostructures
chemisorbed on clean Cu(110), these islands are (2 1)O reconstructed.1,3 They are elongated along the [001] direction and only confined by the terrace width. As oxygen chemisorption at elevated temperatures leads to large (110) terraces separated by high step bunches,9,10 the typical length of such a CuO island can be larger than 1 μm. The Ag, on the other hand, is located between the CuO islands and forms—depending on the Ag coverage—a Ag/Cu surface alloy or a Ag wetting layer.5,6 By varying the amounts of oxygen and/or Ag, it is possible to tune the dimensions of the structure. As an example, Figure 1b shows a structure that was prepared by exposing the Cu(110) surface to 1 L of oxygen at 660 K and subsequently depositing only a few percent of a ML of silver at the same temperature. In this case, the Ag-rich areas as well as the CuO islands are very narrow, forming a regular array of stripes. In the present letter we demonstrate the potential of these new structures as templates for the growth of Ag nanowires at room temperature. The growth of such nanowires on clean Cu(110) has been studied in detail by Zhao et al.11 and is known to be of the Stranski-Krastanov type, i.e., the silver initially formes a complete, monatomic wetting layer from which the nanowires start to grow. Figure 2 shows a Cu(110)-based template similar to the one presented in Figure 1a after additional deposition of about three MLs of silver at 300 K. The structure of the template, i.e., the oval shapes of the original CuO islands is essentially preserved upon Ag deposition at room temperature. Nevertheless, two completely different types of Ag growth are observed on the CuO islands and on the Ag-rich areas in between.
B
ecause of their unique physical and chemical properties, low dimensional structures have attracted attention in the science community. The confinement of electrons within a one-dimensional nanowire, for example, gives rise to novel densities of states not encountered in bulk matter. One way to create such structures is to grow them on single crystalline surfaces in a bottom-up process based on selforganization. Therefore, a fundamental understanding of the growth process is indispensable. Especially, the formation of one-dimensional nanostructures is often based on stress that is induced by a mismatch between substrate and deposited material. Thereby, the stress limits the width of the growing structure, whereas there is usually no means to control the length. In the present paper we demonstrate that coadsorption of oxygen and silver on the Cu(110) surface can be used to create a template that allows one to tune the length-to-width ratio of Ag nanowires. The method even enables us to invert that ratio, i.e., to “turn” the nanowires by 90°. Although oxygen chemisorption1-3 as well as silver deposition4-7 on the Cu(110) surface are well-investigated systems, so far only little is known about the combination of the two. In a recent scanning tunneling microscopy (STM) study, we reported on a self-organized pattern formed on Cu(110) upon partial oxygen chemisorption and subsequent deposition of a submonolayer of silver at 660 K.8 The high surface mobility at this temperature enables a phase separation between oxygen and silver where the oxygen gathers in large CuO islands. An example for such a structure is shown in Figure 1a. It was prepared under ultrahigh vacuum (UHV) conditions by exposing the clean Cu(110) surface to 0.3 L of oxygen at 660 K and subsequently depositing about half a monolayer (ML) of silver at the same temperature. As all STM data presented in this letter, the image was recorded in situ and at room temperature. The CuO islands cover about 1/3 of the surface and appear dark in the STM image. As for the case of oxygen
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Received Date: January 18, 2010 Accepted Date: February 24, 2010 Published on Web Date: March 05, 2010
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Figure 1. STM images (3 3 μm2) of the phase separation between oxygen and silver on the Cu(110) surface. The characteristic dimensions of the CuO islands (dark) and the Ag/Cu areas (bright) depend on the amount of preadsorbed oxygen and silver. The structures were prepared upon exposure of the clean Cu(110) surface to 0.3 L of oxygen and subsequent deposition of about half a ML of silver (a) and after exposure to 1 L of oxygen and subsequent deposition of a few percent of a ML of silver (b) at 660 K.
Figure 3. Dimensions of the Ag nanowires grown upon deposition of about three MLs of silver at room temperature on clean Cu(110) (blue squares), a Cu(110)-based template as shown in Figure 2 (red circles), and a Cu(110)-based template as shown in Figure 4 (green triangles). The dotted line marks the transition from [110]- to [001]-oriented nanowires. The insets show 700 700 nm2 STM images of the different types of nanowires.
(blue squares). On clean Cu(110), where nothing constrains the growth of the nanowires along the [110] direction, their widths lie in a narrow range between 15 and 25 nm. This “natural” width is probably defined by the strain field induced by the pseudomorphic growth of the Ag nanowires on the Cu(110) substrate.11 The corresponding model assumes that the Ag nanowires grow in registry with the [110] oriented troughs of the Cu(110) substrate, forming a distorted (110) structure, as found by Taylor et al.5 The restriction of the nanowires in length (i.e., along the [110] direction), however, leads to an increase in width, suggesting that the strain induced limitation of the width is overcome when Ag atoms are hindered from attaching to the ends of a wire. The mobility of silver on the (2 1)O reconstructed Cu(110) surface is known to be smaller by about 2 orders of magnitude12 as compared to silver deposited on clean Cu(110) and probably negligible for the temperatures and time scales of our experiments. Therefore, we assume that the silver deposited onto a CuO island remains on top of the island and only contributes to the polycrystalline Ag layer. On the other hand, the silver landing on the areas between the CuO islands is assumed to first complete the Ag wetting layer and then contribute to the growth of the nanowires. This assumption is confirmed by the observation, that the height of the polycrystalline Ag layer with respect to the Ag wetting layer increases linearly with the amount of deposited silver, as shown in the inset in Figure 2b. The difference between bare copper and oxygen reconstructed areas at a nominal silver coverage of 1 ML is 0.15 nm, which actually coincides with the height of the additional CuO layer.3 The main message of Figure 2 is, however, that the Ag nanowires only grow on the areas between the CuO islands. Since the islands are elongated along the [001] direction and the wires grow along the orthogonal [110] direction, the dimension of a wire can be tuned by the spacing between two neighboring CuO islands. By carefully choosing the parameters for the preparation, templates with very narrow Ag-rich areas, as the one presented in Figure 1b, can be
Figure 2. STM images at different scales (left: 1.5 1.5 μm2, right 0.2 0.2 μm2) of a Cu(110)-based template after additional deposition of about three MLs of silver at 300 K. Ag nanowires grow on the original Ag/Cu areas of the template, whereas the CuO islands are overgrown by a polycrystalline Ag layer. The template was prepared by exposing the Cu(110) surface to 0.5 L of oxygen at 660 K and subsequently depositing about half a ML of silver at the same temperature. The numbers mark (1) a CuO island overgrown by polycrystalline Ag, (2) the Ag wetting layer, and (3) a Ag nanowire. The inset in panel b shows the averaged height of the polycrystalline layer with respect to the Ag wetting layer as a function of the amount of silver deposited at 300 K. The dashed line is a linear fit to the data.
As shown in the upper left part of Figure 2b, the (2 1)O reconstruction of the CuO islands is overgrown by a rough Ag layer. The amplitude of the corrugation is about 0.2 nm, and the same structure is observed after Ag deposition at 300 K on the fully (2 1)O reconstructed Cu(110) surface. In agreement with the findings of the photoemission study by K€ urpick et al.,12 this phase is interpreted as polycrystalline Ag. On the areas between the CuO islands, however, the deposited silver forms three-dimensional nanowires oriented along the [110] direction. These nanowires are very similar to those grown on clean Cu(110).7,11 The difference is, however, that they only grow on the Ag phase of the template. This means, that their length is confined by the width of the Ag rich areas between the original CuO islands. As shown in Figure 3, the widths of these confined nanowires (red circles) are distributed over a much wider range than the widths of isolated nanowires grown on the clean Cu(110) surface
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EXPERIMENTAL SECTION The experiments were performed in an UHV system with a base pressure around 5 10-10 mbar. The vacuum chamber is equipped with an argon ion gun for sample preparation, an evaporator (Focus EFM3) for Ag deposition, an STM system (Omicron VT XA), and a photoelectron emission microscope (Focus IS-PEEM). All STM data presented in this paper were taken at room temperature. Sample cleanliness was checked by low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) using a four-grid LEED/AES system. The calibration of the evaporator was also done with AES. To this end, the AES signal of Ag was measured as a function of evaporation time, and the first kink in the signal was defined as the completion of the Ag ML.4 As substrate we used a polished Cu(110) surface with a nominal miscut below 0.1°. The sample temperature was measured via a thermocouple attached to the manipulator in the vicinity of the sample transfer plate on which the crystal was mounted. The sample could be heated up to ∼1000 K via resistive heating on the manipulator. The Cu(110) surface was prepared by argon ion sputtering with an ion energy of 900 eV and subsequent annealing at 900 K for 5 min. The molecular oxygen was dosed via a leak valve. A dose of 1 L was achieved by opening quickly the leak valve so that the pressure in the preparation chamber rises to 1.3 10-7 mbar. The valve was kept open for 10 s and then closed as fast as possible.
Figure 4. (a) STM image (750 750 nm2) of the novel, [001]oriented Ag nanowires grown upon deposition of about three MLs of silver at 300 K on the Cu(110)-based template shown in Figure 1b. (b) Cross sections along the [110] direction (top) and the [001] direction (bottom). Positions are indicated by the dotted lines in Figure 4a.
created. In this case, the typical dimension of the Ag-rich areas along the [110] direction is about 30 nm. Figure 4a shows the same template after deposition of about three MLs of silver at 300 K. The extension of the nanowires along the [110] direction, measured at half of their height, is confined to about 20 nm (see height profile on the left side of Figure 4b). As mentioned above, the extension along the orthogonal [001] direction of these confined nanowires is increased with respect to the width of isolated nanowires grown on the clean Cu(110) surface. In the case of Figure 4a, the dimension along the [001] direction even exceeds the dimension along the [110] direction, leading to a novel type of Ag nanowires whose long axis is oriented along the [001] direction. The green triangles in Figure 3 show a statistical evaluation of the dimensions of these [001]-oriented nanowires. Along [110] they are all confined to about 20 nm, whereas along [001] we observe a broad distribution ranging from 10 to about 100 nm. Height profiles along the [001] direction, like the one shown on the right side of Figure 4b, reveal a nonuniform height. As mentioned above, for nanowires grown on clean Cu(110), there is a strain induced, “natural” width of about 20 nm along the [001] direction, and it seems plausible to assume that similar strain fields are involved in the formation of the novel, [001]-oriented nanowires. Accordingly, the height profile in Figure 4b (right side) is interpreted in terms of three pseudomorphically grown Ag crystallites coalesced into a single Ag nanowire. Note that all data points to the right of the dotted line in Figure 3 represent nanowires with the long axis oriented along [110], whereas those on the left represent nanowires whose long axis is aligned along the orthogonal [001] direction. In summary, we have shown that the structures formed upon the phase separation between oxygen and silver on the Cu(110) surface are useful templates to manipulate the growth of Ag nanowires. They allow us to tune the length-to-width ratio of the nanowires in a wide range and even change their orientation by 90°. While on the clean Cu(110) surface Ag nanowires grow along the [110] direction, on our template we can enforce a growth along the orthogonal [001] direction even though in both cases the wires directly grow from the Ag/Cu(110) interface.
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AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail: peter.
[email protected]. Phone: þþ43-732-2468-8510. Fax: þþ43732-2468-8509.
ACKNOWLEDGMENT This work was financially supported by the Austrian Science Fund (FWF) under Contract No. NFN-S9002-N20.
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