J. Phys. Chem. C 2008, 112, 6887-6890
6887
Molecular Growth Determined by Surface Domain Patterns John Åhlund,† Katharina Nilson,† Pål Palmgren,‡ Emmanuelle Go1 thelid,†,§ Joachim Schiessling,† Mats Go1 thelid,§ Nils Mårtensson,†,⊥ and Carla Puglia*,† Department of Physics and Materials Science, Uppsala UniVersity, Box 530, S-751 21 Uppsala, Sweden, Materialfysik, MAP, ICT, Kungliga Tekniska Ho¨gskolan, Electrum 229, S-164 40 Stockholm, Sweden, Department of Chemical Engineering, Ma¨lardalens UniVersity, Box 325, S-631 05 Eskilstuna, Sweden, and MAX-lab, UniVersity of Lund, Box 118, S-221 00 Lund, Sweden ReceiVed: December 12, 2007; In Final Form: February 15, 2008
The growth of iron phthalocyanine (FePc) on InSb(001) c(8 × 2) at submonolayer coverage has been investigated with scanning tunneling microscopy (STM). FePc adsorbs flat centered on the In rows both at 70 K and at room temperature (RT). However, the shapes of the two-dimensional molecular islands are fundamentally different; while the RT growth results in chainlike structures along the [110] direction, as already observed for other Pc’s adsorbed on the same surface, the islands are prolonged along [1h10], i.e., perpendicular to the substrate rows, at 70 K. These observations are explained on the basis of a recently observed new surface phase at low temperature, resulting in structural domains on the surface. The molecular growth front follows the propagating domain boundary that freezes at low temperature.
Introduction Phthalocyanines (Pc’s) and porphyrines form an interesting class of macrocyclic molecules due to their many possible applications, such as in solar cells,1 gas and radiation sensors,2 catalytical systems,3-8 for epoxidation of alkenes,9,10 or in fuel cells.11-21 Pc’s are planar organic molecules that consist of four pyrrole units linked in a circular manner by nitrogen bridges and surrounded by four benzene rings, with either a metal atom (metal phthalocyanine, MePc, C32H16N8Me) or two hydrogen atoms in the center (metal-free Pc, H2Pc, C32H16N8H2). InSb(001) reconstructs in a c(8 × 2) structure after sputtering and annealing. This relatively complex surface comprises subsurface dimers and top layer In rows along the [110] direction, with a spacing of about 18 Å.22-24 Recently, a lowtemperature STM study by Goryl et al.25 has shown additional structures interpreted as a surface wave. The In rows have been shown to be the adsorption site for different Pc’s.26-30 At room temperature (RT) PbPc adsorbs on the indium rows that guide the molecular diffusion, resulting in a monolayer (ML) structure dominated by one-dimensional, chainlike ordering along the [110] direction.27 The same type of adsorption and growth has been identified for H2Pc,28 CuPc,29 and ZnPc30 by RT STM investigations. Our room temperature results show that also FePc’s adsorb on the In rows, in line with the literature.26-30 The very mobile FePc at RT did not allow detailed imaging at low coverage, but the near-monolayer structure is similar to previous results with chainlike growth along the [110] direction. Low-temperature measurements from about 0.2 ML to about 1 ML displayed two-dimensional molecular islands prolonged in the [1h10] direction. The growth pattern follows the substrate phase * Corresponding author: e-mail
[email protected], Ph +46 18 471 36 00, Fax +46 18 471 35 24. † Uppsala University. ‡ Kungliga Tekniska Ho ¨ gskolan. § Ma ¨ lardalens University. ⊥ University of Lund.
domains closely and shows that the molecular diffusion along the surface is controlled by the surface domain pattern. Experimental Section RT and LT STM measurements have been carried out on an Omicron variable temperature STM at the Department of Physics and Materials Science in Uppsala. The instrument consists of a separate chamber for the STM connected to a preparation chamber equipped with sputtering and annealing facilities and a LEED/Auger instrument. An evaporation chamber was connected to the preparation chamber. In addition, RT measurements have been performed at the Royal Institute of Technology (KTH) in Stockholm. The STM images presented here are taken in the constant current mode. Using positive (negative) bias voltage, the unoccupied (occupied) states are probed. The length scale in the images is calibrated using the spacing between the indium rows23 of 18.3 Å. The InSb(001) samples were cut from a wafer supplied by Wafer Technology Ltd., UK. The wafer was oriented within (0.1° to the (001) plane and was n-type doped with Te ((2.33.6) × 1015 cm-3). The surface was cleaned by cycles of Ar+ sputtering and annealing to 670 K (Uppsala) and 700 K (Stockholm). The clean substrate showed a c(8 × 2) LEED pattern. The STM images display large terraces up to several hundred nanometers wide. In the evaporation chamber, a tantalum pocket (Uppsala) and a quartz glass tube (Stockholm) containing FePc (Aldrich 90% purity) were used for deposition after being thoroughly degassed. The molecules have been sublimated onto the InSb(001) surface by heating the pocket up to 650 K. The substrate has been kept at room temperature (RT) during deposition. LT measurements have been performed at 70 K. Results and Discussion STM images of FePc/InSb(001) taken at RT always showed streak noise along the indium rows due to mobile molecules.
10.1021/jp711680q CCC: $40.75 © 2008 American Chemical Society Published on Web 04/09/2008
6888 J. Phys. Chem. C, Vol. 112, No. 17, 2008
Åhlund et al.
Figure 2. 9 nm × 9 nm (-2.9 V, 0.27 nA) STM image of as deposited FePc on InSb cooled to about 70 K. A structural model has been fitted above one of the FePc molecules. The model has been geometrically optimized according to ref 32. Additional circular protrusions are marked with arrows. The substrate orientation is indicated.
Figure 1. (A) 58 nm × 58 nm image of FePc on InSb (-2.2 V, 0.27 nA) recorded at 70 K. (B) is a close-up (21.7 nm × 16.5 nm) of (A) as indicated by the square. The contrast is adjusted to highlight structures in the substrate. These structures form darker and brighter areas on the surface, predominantly elongated in [1h10] direction. Two circular features (1 and 2) and a stripelike feature (3) are marked with numbered arrows. (C) is the same as (B) but with a different contrast enhancement. The isolated molecules show a five-lobe structure, and the molecular pair to the right has four lobes each and shares an additional protrusion between the two molecules. The additional protrusions are indicated by arrows.
Cooling the system to 70 K (LT) improved the quality of the images considerably. At such a temperature only slight molecular motion was observed. In Figure 1A, we show low-coverage images of FePc/ InSb(001) acquired at LT. Bright features with almost quadratic
shape identify molecules. The bright lines in the [110] direction of the bare substrate reflect surface Sb dangling bonds. Depending on the bias the substrate changes appearance, and for some voltages the Sb atoms, geometrically located between and slightly lower than the In rows, appear higher.31 At this low coverage the molecules are dispersed over the imaged area, and no long-range ordering is observed. However, a clear pairing of the adsorbed molecules appears. Even the short molecular chains (see bottom of Figure 1A) are constituted by pairs indicating that short-range molecule-molecule interaction plays a quite significant role in the initial adlayer formation. All molecules have the same orientation with respect to the substrate. At the step edge molecular-like structures are seen. In contrast to the appearance of molecules on the terraces, no lobe structure can be resolved. This observation suggests a slight deformation of the molecules due to the substrate interaction at the step or that the molecules adsorb with a tilt angle with respect to the surface plane. In addition to the molecules, small features are seen on the substrate at LT. In the close-up in Figure 1B these appear as circular protrusions located on top of the In rows, indicated by arrow number 1. Protrusions between the Sb rows are also seen indicated in Figure 1B by arrow number 2. Another, rather weak, structure elongated in the [110] direction is indicated by arrow number 3. On a larger scale these later features agglomerate and form areas of different brightness, as enclosed by dashed lines in Figure 1B. These areas are elongated predominantly in the [1h10] direction. Similar structures have recently been observed in STM experiments for InSb(001) at 77 K by Goryl et al.,25 interpreted as a charge density wave or perhaps two coexisting surface structures. At this LT a majority of the molecules show also a bright circular additional protrusion between two of the benzene rings, on top of the In row, the molecules appearing as pentagons (Figure 1C). These additional protrusions, seen for single molecules as well as within the molecular pairs/chains, are marked with arrows in Figure 1C and Figure 2. Other Pc’s
Molecular Growth Determined by Surface Domain Patterns
J. Phys. Chem. C, Vol. 112, No. 17, 2008 6889
Figure 4. 35 nm × 35 nm. STM image of FePc/InSb taken at RT (-2.15 V, 0.08 nA). The In rows of the substrate are resolved. An arrow indicates the streak noise seen along the indium rows due to high-mobility molecules. In the figure molecules without (A) and with (B) a five-lobe structure and molecular pairs (white ellipse) are marked to evidence the similarity to the LT data shown in previous figures.
Figure 3. (A) Images at 70 K. The surface orientations are indicated in the images. The molecules are seen as quadratic bright objects: 95 nm × 95 nm (-2.7 V, 0.27 nA); the coverage is about 0.5 ML. (B) 95 nm × 95 nm STM (3.0 V, 0.25 nA); the coverage is about 0.9 ML. The FePc molecules form two-dimensional islands following the substrate domain pattern.
adsorbed on InSb c(8 × 2) at RT show two additional protrusions, giving a hexagonal shape.28 The structures were assigned to an adsorption-induced enhanced density of states on In atoms adjacent to the adsorbed Pc’s. The extra protrusions seen in our images could therefore be related either to adsorbed molecules (Pc’s28,30 and/or possibly hydrogen32) that induce a modification of the surface density of states or to substrate defects, which would constitute a preferential adsorption site for the molecules. At this point we cannot be completely sure about the origin of these protrusions since we see them both isolated on the surface or in connection to adsorbed FePc’s molecules. However, the number of protrusions increases with coverage, favoring that these kinds of protrusion are induced by the adsorbate (FePc’s) or possible hydrogen. In Figure 2 we show an image, taken at LT, at higher magnification in order to investigate the orientation of molecules on the substrate rows. Each molecule basically appears as four
lobes with a bright center. The four lobes are due to the isoindole units of the molecules while the intensity at the center is due to the central Fe ion. In FePc the Fe 3d orbital gives a major contribution to the states close to the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular levels.33-35 The molecular structure is fitted on top of a molecule in Figure 2. The molecular center is located on an In row. The single molecules are oriented with two isoindole units at each side of the row. The characteristic undistorted four-lobe shape in the STM images shows that the molecules are adsorbed with the molecular plane parallel to the surface. This local adsorption pattern is the same as for PbPc, SnPc, and H2Pc. Figure 3 shows LT images obtained at 0.5 and 0.9 ML FePc coverage. The general impression is a stripelike arrangement of molecules perpendicular to the In rows. At 0.5 ML the mean island size is 13-20 molecules. For higher coverages the islands have, to a large extent, merged into two-dimensional islands elongated along the [1h10] direction. All previous studies including PbPc’s,27 H2Pc’s,28 CuPc,29 and ZnPc’s30 on InSb(001) have reported molecular chain structures along the [110] direction. A plausible explanation is a relation between molecular adsorption and the substrate domain pattern observed at LT. It is also worth noticing that, even at these higher coverages, a great number of molecules tend to appear pairwise on the substrate. For all coverages bright areas of different shape and size are occasionally seen in the images. The origin of these features is not clear. It might be due to impurities or to possible 3-dimensional island growth. However, the latter seems unlikely since our data in general indicate a layer-by-layer growth. A result from RT imaging near-monolayer coverage is shown in Figure 4. There are both similarities and differences with respect to the LT data. Even if the quality of the images is perturbed by mobile molecules (streak noise along rows as indicated by a black arrow in Figure 4), it is still observed that molecules adsorb pairwise. One such pair is indicated by a white ellipse. Again pentagonal molecules are clearly visible in this
6890 J. Phys. Chem. C, Vol. 112, No. 17, 2008 image (B). A molecule lacking the additional protrusion is marked A. However, most importantly the RT adsorption structure consists of molecular chains along [110] similar to previous results. This result suggests that the molecular adsorption organization at LT is a direct consequence of the domain pattern on the bare substrate. Conclusions We have studied the growth of FePc on InSb(001) at room and low temperatures. In agreement with the vast majority of previous studies on similar systems, we find that FePc adsorb centered on the indium rows with two isoindole units on each side. We further observe that a significant amount of the FePc molecules arrange pairwise on the indium rows of the substrate for both RT and LT samples. A bright additional protrusion appears frequently on adjacent In atoms between benzene rings, both at single molecules and between molecular pairs. The growth patterns of two-dimensional islands are different at LT and RT. While it follows the general trend with molecular chains along surface In rows at room temperature, it is follows closely the surface domain pattern at LT. This clearly shows that the freezing of these domains upon cooling also freezes the molecular layer. Thus, these substrate waves control the growth of the adsorbed layer. Acknowledgment. This work has been supported by the Swedish Foundation for Strategic Research (SSF), the Swedish Research Council (VR) and the Knut and Alice Wallenberg Foundation (KAW), the Go¨ran Gustafsson Foundation, and the Carl Trygger Foundation. References and Notes (1) Conboy, J. C.; Olsson, E. J.; Adams, D. M.; Kerimo, J.; Zaban, A.; Gregg, B. A.; Barbara, P. F. J. Phys. Chem. B 1998, 102, 4516. (2) Trometer, M.; Even, R.; Simon, J.; Dubon, A.; Laval, J.-Y.; Germain, J. P.; Maleysson, C.; Pauly, A.; Robert, H. Sens. Actuators B 1992, 8, 129. (3) Grennberg, H.; Ba¨ckall, J. E. Acta Chem. Scand. 1993, 47, 506. (4) Groves, J. T.; Mayers, R. S. J. Am. Chem. Soc. 1983, 105, 5791. (5) Campbell, L. A.; Kodadek, T. J. Mol. Catal. A: Chem. 1996, 113, 293. (6) Sorokin, A. B.; Tuel, A. Catal. Today 2000, 57, 45. (7) Berner, S.; Biela, S.; Ledung, G.; Gogoll, A.; Ba¨ckvall, J.-E.; Puglia, C.; Oscarsson, S. J. Catal. 2006, 244, 86. (8) Sorokin, A. B.; Tuel, A. Catal. Today 2000, 57, 45. (9) Groves, J. T.; Mayer, R. S. J. Am. Chem. Soc. 1983, 105, 5791.
Åhlund et al. (10) Campbell, L. A.; Kodadek, T. J. Mol. Catal. A: Chem. 1996, 113, 293. (11) Wiesener, K.; Ohms, D.; Neumann, V.; Franke, R. Mater. Chem. Phys. 1989, 22, 457. (12) Lefe`vre, M.; Dodelet, J. P.; Bertrand, P. J. Phys. Chem. B 2002, 106, 8705. (13) Lefe`vre, M.; Dodelet, J. P.; Bertrand, P. J. Phys. Chem. B 2000, 104, 11238. (14) Anson, F. C.; Shi, C.; Steiger, B. Acc. Chem. Res. 1997, 30, 437. (15) Schulenburg, H.; Stankov, S.; Schu¨nemann, V.; Radnik, J.; Dorbandt, I.; Fiechter, S.; Bogdanoff, P.; Tributsch, H. J. Phys. Chem. B 2003, 107, 9034. (16) Zhang, L.; Zhang, J.; Wilkinson, D. P.; Wang, H. J. Power Sources 2006, 156, 171. (17) Zhao, F.; Harnisch, F.; Schro¨der, U.; Scholz, F.; Bogdanoff, P.; Herrmann, I. Electrochem. Commun. 2005, 7, 1405. (18) Zhang, L.; Song, C.; Zhang, J.; Wang, H.; Wilkinson, D. P. J. Electrochem. Soc. 2005, 7, 1405. (19) Arihara, K.; Mao, L. Q.; Liddell, P. A.; Marino-Ochoa, E.; Moore, A. L.; Imase, T.; Zhang, D.; Sotomura, T.; Ohsaka, T. J. Electrochem. Soc. 2004, 151, A2047. (20) Jahnke, H.; Scho¨nborn, M.; Zimmermann, G. Top. Curr. Chem. 1976, 61, 133. (21) Jasinski, R. J. Electrochem. Soc. 1965, 112, 526. (22) Schweitzer, M. O.; Leibsle, F. M.; Jones, T. S.; McConville, C. F.; Richardson, N. V. Surf. Sci. 1993, 280, 63. (23) Kumpf, C.; Marks, L. D.; Ellis, D.; Smilgies, D.; Landemark, E.; Nielsen, M.; Feidenhans’l, R.; Zegenhagen, J.; Bunk, O.; Zeysing, J. H.; Su, Y.; Johnson, R. L. Phys. ReV. Lett. 2001, 86, 3586. (24) Varekamp, P. R.; Bjo¨rkqvist, M.; Go¨thelid, M.; Karlsson, U. O. Surf. Sci. 1996, 350, L221. (25) Goryl, G.; Boelling, O.; Godlewski, S.; Kolodziej, J. J.; Such, B.; Szymonski, M. Surf. Sci. 2007, 17, 3605. (26) Salomon, E.; Angot, T.; Papageorgiou, N.; Layet, J.-M. Surf. Sci. 2005, 596, 74. (27) Angot, T.; Salomon, E.; Papageorgiou, N.; Layet, J.-M. Surf. Sci. 2004, 572, 59. (28) Palmgren, P.; Priya, B. R.; Niraj, N. P. P.; Go¨thelid, M. J. Phys.: Condens. Matter 2006, 18, 10707. (29) Tekiel, A.; Goryl, M.; Szymonski, M. Nanotechnology 2007, 18, 475707. (30) Nilson, K.; Palmgren, P.; Åhlund, J.; Schiessling, J.; Go¨thelid, E.; Mårtensson, N.; Puglia, C.; Go¨thelid, M. Surf. Sci. 2008, 602, 452. (31) McConville, C. F.; Jones, T. S.; Leibsle, F. M.; Driver, S. M.; Noakes, T. Q.; Schweitzer, M. O.; Richardson, N. V. Phys. ReV. B 1994, 50, 14965. (32) Åhlund, J.; Nilson, K.; Schiessling, J.; Kjeldgaard, L.; Berner, S.; Mårtensson, N.; Puglia, C.; Brena, B.; Nyberg, M.; Luo, Y. J. Chem. Phys. 2006, 125, 034709. (33) Storchark, V. G.; Eshchenko, D. G.; Brewer, J. H.; Cottrell, S. P.; Lichti, R. L. Phys. ReV. B 2006, 73, 81203 (R). (34) Lu, X.; Hipps, K. W. J. Phys. Chem. B 1997, 101, 5391. (35) Liao, M.-S.; Scheiner, S. J. Chem. Phys. 2001, 114, 9780. (36) Yoshida, H.; Tsutsumi, K.; Sato, N. J. Electron Spectrosc. Relat. Phenom. 2001, 121, 83.
