C60 on Nanostructured Nb-Doped SrTiO3(001) Surfaces - The

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C60 on Nanostructured Nb-Doped SrTiO3(001) Surfaces Chao Lu,† Erkuang Zhu,† Yadi Liu,† Zhongyuan Liu,† Yafeng Lu,‡ Julong He,† Dongli Yu,† Yongjun Tian,† and Bo Xu*,† State Key Laboratory of Metastable Materials Science and Technology, Yanshan UniVersity, Qinhuangdao, Hebei 066004, China, and Northwest Institute for Nonferrous Metal Research, P.O. Box 51, Xi’an, Shaanxi 710016, China ReceiVed: NoVember 5, 2009; ReVised Manuscript ReceiVed: January 24, 2010

Nanostructured Nb-doped SrTiO3(001) surfaces were investigated with STM, and the surface patterns for observed nanostructures were assigned. Sequential C60 deposition onto these nanostructured templates reveals distinct growth modes, including discrete small C60 islands on the c(4 × 2) reconstruction surface, parallel one-dimensional C60 chains on (6 × 2) dilines, C60 double chains on (8 × 2) trilines, epitaxial C60 close packed adlayers over (11 × 2) tetralines, and two-dimensional ordered C60 dimer arrays on (7 × 6) waffles. These structural diversities mainly stem from the relatively strong adsorbate-substrate interactions as well as the surface topography demands. The nanostructured oxide surfaces as templates thus have great potential in molecular nanoarchitecture. 1. Introduction The adsorption and assembly of molecules on surfaces play a vital role in the molecular based bottom-up nanotechnology.1-4 The functionalities and performances of such molecular nanostructures rely on the properties of the constituting molecular species and how they are organized. A comprehensive understanding of the assembly process of organic molecules on surface as well as the pathway to control it will contribute to the fabrication and optimization of these nanostructures. Usually, the noncovalent interactions between molecules, such as hydrogen bonding,5-7 van der Waals interaction,8,9 and dipole-dipole interaction,10 are exploited to assist the molecular assembly process. Nanostructured templates with suitable chemical characteristics and preformed surface patterns can also benefit the molecular assembly process.5,11-15 The design of the template surface is of great importance for molecular area-selective assembly.16 Depending on the local stoichiometry, metal oxide surfaces demonstrate different interactions to molecules, and undercoordinated metal cations can work as a strong adsorption site for molecules.17-19 The SrTiO3(001) surface, extensively examined with the motivation of making atomically smooth surfaces on which to grow thin films,20-24 is a representative with such locally modified surface structures.25 A rich variety of reconstruction surfaces have been observed on SrTiO3(001), which are highly related to the sample preparation strategies such as surface etching or sputtering, as well as annealing conditions (temperature and duration).25-35 The nonstoichiometric SrTiO3(001) nanostructured surfaces are good candidates to investigate the template influence on molecular assembly.12-14,36 In our experiments, the nanostructured SrTiO3(001) surfaces were prepared according to the strategy of Castell et al.28-30 The surface patterns for these structures were calibrated with respect to a Si(111)-(7 × 7) sample using scanning tunneling microscopy (STM). Sequential depositions of C60 onto distinct SrTiO3(001) templates revealed how the surface topographies, * Corresponding author. E-mail: [email protected]. † Yanshan University. ‡ Northwest Institute for Nonferrous Metal Research.

in combination with adsorbate-substrate and intermolecular interactions, yield distinct molecular growth architectures. 2. Experimental Section The experiments were performed with a SPECS Aarhus VTSTM system operated at room temperature with a base pressure better than 1 × 10-10 mbar. Single crystals of SrTiO3 doped with 0.7 wt % Nb with polished (001) surfaces were supplied by KMT Co., China. The as-received samples were sputtered with Ar+ (1.5 kV, 2 µA, and 15 min) and subsequently annealed to 900-1200 °C for different surface topographies. The temperatures were measured with an optical pyrometer through a viewport. C60 (Aldrich) was deposited from a Knudsen cell (Createc) at 350 °C to the room temperature samples in a conjoint preparation chamber with pressure maintained around 1 × 10-9 mbar. The deposition rate for C60 is 0.1 ML/min. All reported STM images were collected with constant current mode. 3. Results The nanostructured surfaces on SrTiO3(001) were previously studied by Castell et al.28-30 In their experiments, different annealing conditions were chosen after Ar+ sputtering of the SrTiO3(001) to produce a series of nanostructures: linear nanostructures with (6 × 2) and (9 × 2) surface patterns after 30 min annealing at 850 and 950 °C, respectively; linear nanostructures with a (12 × 2) surface pattern after repeated annealing at 1000 °C; highly ordered two-dimensional (2d) arrays of nanosized trenches on the surface with an (8 × 6) periodicity after repeated annealing at 950 °C. These structures were named as dilines, trilines, tetralines, and waffles by Castell.29 The same nomenclature is used hereinafter. We followed the sample preparation conditions of Castell in our experiments and produced similar surface nanostructures. During our STM measurements, the scanning distortion was carefully calibrated with respect to a Si(111)-(7 × 7) sample measured with the same scanning tip using the sample holder. All the STM images reported here are after the calibration.

10.1021/jp910556c  2010 American Chemical Society Published on Web 02/09/2010

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Figure 1. (a) STM image (V ) 1.5 V, I ) 0.1 nA) of a c(4 × 2) reconstruction SrTiO3(001) surface. The inset shows a high-resolution image with the unit cell marked. (b) STM image (V ) 1.4 V, I ) 0.13 nA) of the nanostructured SrTiO3(001) surface with dilines, trilines, and waffle structures.

Samples that were sputtered and sequentially annealed to 1200 °C for 2 h possessed a c(4 × 2) reconstruction surface as shown in Figure 1a. An STM image of this surface with atomic resolution is shown in the inset, where a unit cell is marked. The flat terraces of SrTiO3 can span as large as 100 nm separated by straight steps. The unit cell high steps are preferentially along the lattice vector directions of the (001) surface. Sputtering and sequentially annealing samples to 950 °C for 2 h would lead to a surface similar to what is shown in Figure 1b. Linear features (nanolines) and 2d ordered arrays of nanometer-size trenches (waffles) are prevailing over the whole surface. Prolonging (or shortening) the annealing time would lead to the increase (or decrease) of the fractional area of the waffle phase on the surface. A high-resolution STM image of dilines and trilines is shown in Figure 2a. Each diline consists of two parallel rows constituted with atoms or atom complexes. The protrusions in these two rows are arranged in either a square configuration or a zigzag configuration.28 Each triline consists of three parallel rows with different periodicities. The center row, with two times as many protrusions as those in the outer rows, is topographically higher than the outer rows by 0.03 nm. Two line profiles with the same length and direction are drawn in Figure 2a to clarify the structural parameters. One line profile is scanning along a triline, and the other is scanning across triline and diline structures that are oriented perpendicular to the former triline. It is clear that the periodicity along the trilines is 2a, and the widths of the diline and triline are 6a and 8a, respectively. Here a ) 0.3905 nm is the lattice constant of the SrTiO3 crystal. The diline thus has the same (6 × 2) pattern as assigned before. The triline structure possesses an (8 × 2) surface pattern, which is more compact than the (9 × 2) triline structure reported by Castell.29 Two rotational domains of the waffle structure, A and B, are presented in Figure 2b. The line profile spanning seven repeat units along the short axis direction in domain A has the same length as the line profile spanning six repeat units along the long axis direction in domain B. The aspect ratio of the unit cell is thus determined to be 6:7. The length of the short axis equals 24a/4 ) 6a determined from the two identical green lines in Figure 2b. Thus, a (7 × 6) surface pattern is assigned to the waffle structure, which is also more compact than the (8 × 6) waffle structure studied previously by Castell et al.29 To confirm

our assignment, we checked the length of the double arrow marked in Figure 1b, which spans 16 repeat units along the long axis direction of the waffle structure as well as 11 trilines and 4 dilines. Our assignments give a consistent value of 112a. The surface patterns for dilines, trilines, and waffle structure observed in our studies are thus assigned as (6 × 2), (8 × 2), and (7 × 6), respectively. Sometimes a linear structure with broader center rows was observed during STM measurements, as marked with the blue arrows in Figure 3b. We presume it is a structure similar to that previously observed by Castell et al. as the tetralines.29,36 It possesses an (11 × 2) surface pattern according to our assignment. C60 molecules were deposited onto these distinct SiTrO3 structures, and the template effects of these structures on C60 growth were investigated. On the c(4 × 2) reconstruction surface, C60 molecules nucleate into randomly distributed small islands, as shown in Figure 3a. These islands usually are formed with tens of C60 molecules, and the island growth process is highly impeded on the c(4 × 2) surface. There is no apparent relationship between C60 close packing directions and the SrTiO3(001) surface. Although it seems that the step edge is still a favored bonding site for C60 (most of the step edges are decorated with molecules), growth from the step edge shows no advantage over the open terrace. This growth mode is clearly in contrast to what happens on the Ag surface.37 An STM image of the SrTiO3(001) surface with typical nanostructures (dilines, trilines, tetralines, and waffles) after C60 deposition is presented in Figure 3b. Some features can be revealed from this image. C60 molecules prefer to anchor into the nanotrenches in the waffles. After the depletion of available nanotrenches, other adsorption sites over dilines, trilines, and tetralines are sequentially occupied. However, C60 molecules on dilines are less stable than those on trilines and tetralines, and transient movements of C60 molecules on dilines were observed. When C60 molecules are deposited onto the diline domains, they tend to locate on the seam sites between neighboring dilines and avoid sitting directly on the dilines, as shown in Figure 4a. The distribution of C60 molecules at these seams is stochastic. With increasing C60 coverage, molecules form quasi onedimensional (1d) chains defined by the diline surface pattern with irregular nearest neighbor distances. Time consecutive STM

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Figure 2. (a) STM image with atomic resolution of dilines and trilines. (b) STM image of two rotational waffle domains. The line profiles shown in the right are marked with corresponding colors in the STM images. Tunneling parameters are V ) 0.8 V, I ) 0.2 nA, and V ) 1.4 V, I ) 0.1 nA, for (a) and (b), respectively.

Figure 3. STM images of C60 on distinct SrTiO3(001) templates. (a) c(4 × 2) reconstruction surface and (b) nanostructure surfaces with dilines, trilines, tetralines, and waffle structures. The tetralines are marked with the blue arrows in (b). Tunneling parameters are V ) 1.5 V, I ) 0.1 nA, and V ) 1.4 V, I ) 0.1 nA, for (a) and (b), respectively.

measurements suggest that C60 molecules are hopping between available seam sites. C60 molecules demonstrate an improved regularity on the third template surface, trilines, compared with the diline template.

In Figure 4b, C60 molecules preferentially assemble into double chains on top of the trilines. Each C60 chain sits between two neighboring rows of the triline. The nearest neighbor distance along the chain is 1.0 nm. In addition, less organized C60

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Figure 4. Representative STM images of C60 on (a) dilines, (b) trilines, and (c) mixed trilines and tetralines. (d) Schematic molecular packing models of C60 on trilines and tetralines. The packing differences on trilines and tetralines are emphasized with the yellow ovals and red square, respectively, in (c). Tunneling parameters are V ) 1.4 V, I ) 0.1 nA, V ) 1.4 V, I ) 0.1 nA, and V ) 1.6 V, I ) 0.14 nA, for (a), (b), and (c), respectively.

molecules attached to the double chain were observed in the experiments. More C60 double chains were produced with increasing molecular coverage, as shown in Figure 4c. A rainbow color scheme is applied to this image for better contrast. The double chain character of C60 growth on the triline is reserved. While the space between two neighboring double chains is enough to accommodate more molecules, C60 molecules show much less regularity in these areas, as marked with yellow ovals in Figure 4c. A close packing growth mode of C60 is difficult to realize on SrTiO3 trilines. We do observe a close packed C60 area in Figure 4c, as marked with the red square. However, the underneath SrTiO3 structure is tetralines instead of trilines. Magnified images of fullerene assembly over trilines and tetralines with typical line profiles are presented in the Supporting Information (S1). Schematic packing models of C60 on trilines and tetralines are drawn in Figure 4d. A detailed explanation of the difference between these two templates will be presented in the Discussion. The last template, the waffle structure, is a highly ordered 2d array of nanometer-size trenches associated with a (7 × 6) periodicity. After C60 deposition, the original empty trenches are preferentially filled with molecules, as shown in Figure 5. Each trench can host at most two molecules to form a C60 dimer.

The molecules constituting the dimer are separated by 1.2 nm, while the distance between neighboring dimers is defined by the waffle template. The apparent height of the molecule is about 0.4 nm, much less than the height of C60 on the c(4 × 2) reconstruction surface (0.8 nm), indicating that molecules are anchored into the trenches. Two line profiles are overlapped in Figure 5, suggesting that C60 molecules inside the trenches are fixed at specific adsorption sites determined by the waffle structure. A close-up observation of the waffle structure reveals an internal fine structure of the trench, as seen in Figure 2b. Thus the formation of the C60 dimer is attributed to the waffle template constraint instead of the intermolecular interaction. The same constraint was previously reported for [email protected],14 4. Discussion The distortion of STM images often complicates interpretations and limits the information one can extract from STM measurements. It is very important to determine the accurate structural parameters for understanding the surface nanostructures with reasonable models and necessary theoretical calculations. During our experiments, the two main components accounting for the distortion, drift and scaling, were carefully

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Figure 5. STM image of 2d C60 dimer arrays on the waffle surface. The overlapped line profiles are marked with corresponding colors in the STM image. Tunneling parameters is V ) 1.4 V, I ) 0.1 nA.

examined on a Si(111)-(7 × 7) sample using the sample holder and scanning tip. STM images were then calibrated to remove the drift and scaling effects. Moreover, the line profiles in Figures 2 and 5 are chosen with the same direction to further eliminate the distortion from drift and scaling. The surface patterns assigned to the observed SrTiO3 nanostructures are believed to be accurate and consistent with each other. Also, the distinct C60 growth modes on trilines and tetralines provide further proof to our assignment, as discussed later in this paper. In our C60/SrTiO3 systems, the C60 intermolecular interactions are based on the van der Waals interaction and are of the same nature on different templates. The diversity of the self-assembly structures on the substrates comes from the subtle balance between intermolecular and adsorbate-substrate interactions as well as other effects, such as the surface topography. On the relatively flat c(4 × 2) surface, the formation of the randomly distributed small C60 islands can be attributed to the joint contributions from intermolecular and adsorbate-substrate interactions. While the attractive intermolecular interactions assist C60 molecules in the nucleation process, the strong interactions between C60 molecules and the substrate highly reduce the molecular mobility on the c(4 × 2) reconstruction surface. The growth of C60 islands is thus impeded, leading to the small island size of several nanometers. Moreover, the c(4 × 2) surface possesses two perpendicular domains with typical sizes of a few nanometers and many defects (S2 in the Supporting Information), which may exclude the epitaxial relationship between C60 small islands and the SrTiO3 surface. In contrast to the apparent height of ca. 0.5 nm on Ag, the C60

Lu et al. height on the SrTiO3 flat surface is 0.8 nm. This can be attributed to different electronic states of C60 on the SrTiO3 surface than on Ag. C60 molecules demonstrate distinct growth modes on diline, triline, and tetraline structures. After the deposition, C60 molecules tend to avoid the top sites and form a quasi 1d chain at the seam sites with a stochastic intermolecular distance on the diline template, while they can sit on top of trilines (tetralines) and form a double chain (close packed) structure with a well-defined intermolecular distance along the chain on the triline (tetraline) template, as shown in S1 in the Supporting Information. The two constituting rows in dilines as well as the outer rows in trilines and tetralines have similar topographic heights. The center row in a tetraline is higher than the outer rows by 0.2 nm, consistent with the half unit cell height in SrTiO3. In trilines, the periodicity of the center row is 2 times that of the outer row and the center row is higher than the outer rows by 0.03 nm (Figure 2a). This 0.03 nm height difference is bias-voltage dependent due to a distinct local density of states.29 It is plausible that the center rows in trilines and tetralines are constituted with different atom species than the outer rows and diline rows, resulting in stronger interactions with C60 molecules. The alignment of the C60 close packing direction along the center row would maximize this interaction. Moreover, the dilines are less ordered than trilines and tetralines. These structural differences may contribute to the distinct regularities of C60 structures on these templates. It is noted that Er3N@C80 also shows a similar preference in packing direction on trilines, where rows of fullerenes are grouped into stripes along the triline directions.14 At high C60 coverage, the density of the C60 double chains increases on the triline template and more C60 molecules can be introduced into the areas between two neighboring double chains, as seen in Figure 4c. These additional molecules are less ordered and are located closer to either one of the C60 double chains. On the other hand, C60 molecules can form close packed structures on the tetraline template. The difference in C60 packing modes on these two nanostructured surfaces can be understood by considering the periodicities across the nanolines. The width of triline and tetraline are 8a and 11a, respectively. For a close packed C60 structure, the distance between neighboring rows is 0.866 nm, denoted as b. Supposing close packed C60 structures are formed on trilines and tetralines with one of the C60 close packing directions along the nanolines, a lattice mismatch in C60 films would be produced in the direction across the nanolines. The lattice mismatch would be (4b - 8a)/4b ≈ 10% on the triline template, while it would be (5b - 11a)/5b ≈ 0.8% on the tetraline template. As a result, the epitaxial 2d close packing growth of C60 is prohibited on trilines while it is possible on tetralines. This lattice mismatch difference accounts for the distinct C60 growth modes on trilines and tetralines, as demonstrated in Figure 4c. We also noticed an interesting feature in C60 close packed structure on the tetraline template: the C60 rows parallel to the tetraline direction have alternative heights (the blue line profile in S1 in the Supporting Information). It is speculated that the stress due to the lattice mismatch can be released by lifting every other row of C60. Here the successful explanation of molecular packing modes on trilines and tetralines further proves the correction of our surface pattern assignments. The distinct growth modes of C60 molecules on trilines and tetralines (the large lattice mismatch on trilines and topography modulation on tetralines) are schematically emphasized in Figure 4d.

C60 on Nanostructured SrTiO3(001) Surfaces Compared with the growth of C60 on the c(4 × 2) reconstruction surface, the surface topographies and adsorbate-substrate interactions play a more important role in guiding C60 growth on these nanoline templates. The orientations of C60 quasi 1d chains, double chains, and close packed islands on the surface are determined by the underneath SrTiO3 structures. The nearest neighbor distance of C60 molecules in C60 double chains and close packed islands is still 1.0 nm and is controlled by the intermolecular interactions. On the waffle template, the trench sites provide the largest bonding areas between anchored C60 molecules and the substrate, leading to improved stability. The C60 assembly process is now completely dictated by the surface topography: all the molecules are preferentially settled down inside the trenches, forming the dimer array with the periodicity defined by the waffles. For this template, the intermolecular interactions are overwhelmed by the molecule-substrate interactions and the template topographic features. Moreover, this C60 2d grid structure exhibits an improved thermal stability. It can survive to temperatures as high as 400 °C, showing a great advantage over similar structures patterned on self-assembled molecular templates.15,38-41 5. Conclusion By choosing different sample annealing conditions, distinct nanostructured SrTiO3(001) surfaces were fabricated. The surface patterns for these structures were assigned after STM image calibration. The nanostructured surfaces as templates have been investigated for sequential C60 deposition. Five distinct C60 patterns, each corresponding to a particular template, can be formed on strontium titanate surfaces. The arrangement of the fullerene molecules is tuned by the underneath template, and reflects the delicate balance between the intermolecular and molecular-substrate interactions as well as the template topography demands. These ordering molecular patterns are of potential applications in a variety of nanotechnologies, such as quantum dots, 1d molecular wires, and quantum information processing technologies. Our observations demonstrate the effectiveness of nanostructured oxide surfaces as templates in controlling molecular assembly. Acknowledgment. This work has been supported by the National Science Foundation of China under 50871097 and the National Key Project for Basic Research under 2008CB617503. Supporting Information Available: Magnified images of C60 molecules assembled over trilines and tetralines with typical line profiles; high resolution STM image of c(4 × 2) surface showing two coexisting perpendicular domains as well as the defects. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Whitesides, G. M.; Boncheva, M. Proc. Natl. Acad. Sci. U. S. A. 2002, 99, 4769. (2) Lehn, J. Proc. Natl. Acad. Sci. U. S. A. 2002, 99, 4763.

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