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Adsorption of organic molecules on the TiO2(011) surface: STM study
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10.1063/1.3593403
/content/aip/journal/jcp/134/22/10.1063/1.3593403
http://aip.metastore.ingenta.com/content/aip/journal/jcp/134/22/10.1063/1.3593403
View: Figures

Figures

Image of FIG. 1.
FIG. 1.

Schematic view of studied molecules: (a) CuPc, (b) PTCDA, and (c) VL (top and perspective), respectively.

Image of FIG. 2.
FIG. 2.

TiO2(011)-(2×1) surface: (a) STM empty-state image, bright dots could be attributed to hydroxyl groups, dark spots indicate oxygen vacancies within zigzag rows, (b) high resolution STM image with clearly visible zigzag pattern (both STM images recorded with bias voltage +2.0V and tunneling current 2pA), (c) full structural model (top and side view), (d) schematic simplified structural model, i.e., only oxygen atoms belonging to outstanding rows are indicated.

Image of FIG. 3.
FIG. 3.

CuPc molecules on the TiO2(011) surface at very low coverage (0.025 ML): (a) a typical STM scan; (b) molecules decorating step edges (I) and domain boundaries (II), molecule (III) located at the step running along surface rows (only two isoindole rings are visible); (c) submolecularly resolved STM image of a CuPc molecule (empty states) and a scheme of the molecule. All scans: bias voltage +2.0V, tunneling current 2pA.

Image of FIG. 4.
FIG. 4.

Adsorption geometries of CuPc molecules on the TiO2(011) substrate (coverage: 0.06 ML): (a) submolecularly resolved empty state STM image, squares—molecules adsorbed on oxygen zigzag rows and circles—molecules adsorbed in between the zigzag rows (see text), (b) schematic illustration of stable adsorption geometries on terraces, (c) schematic image of a CuPc molecule and two orbitals forming the LUMO, (d) STM images of molecules exhibiting two-fold symmetry and their schematic illustration, (e) schematic illustration of an STM appearance of a CuPc molecule. All scans: bias voltage +2.0V, tunneling current 2pA.

Image of FIG. 5.
FIG. 5.

STM images of CuPc molecules at moderate coverage (0.1 ML). Mobile molecules are manipulated by STM during scanning and appear as illusory lines, some remaining molecules (marked with circles) stay intact. Panels differ in the scan direction. All scans: bias voltage +3.0V, tunneling current 2pA.

Image of FIG. 6.
FIG. 6.

STM scans showing 1 ML of CuPc molecules assembled on the TiO2 surface: (a) typical empty state image, (b) high resolution image with clearly visible surface reconstruction rows, (c) image of a one-molecule wide line attached to the surface step, (d) submolecular resolved images of CuPc molecules in the layer, (e) structural model of the wetting layer, grey lines indicate surface reconstruction zigzag rows. All scans: bias voltage +3.0V, tunneling current 2pA.

Image of FIG. 7.
FIG. 7.

STM images of CuPc molecules adsorbed on the TiO2(011) surface (1.3 ML coverage), panels (a) and (b) show sample before annealing, two distinct phases are observed, panels (c) and (d) illustrate sample after 2 hour annealing at 150 °C, only phase II is visible, the amount of molecule dislocations, vacancies, and defects are decreased after annealing, panels (e) and (f) show sample after two-hour annealing at 200 °C, when completely new phase III is formed with up-right molecules. All scans: bias voltage +3.0V, tunneling current 2pA.

Image of FIG. 8.
FIG. 8.

Molecular structures of the second layer (coverage: 1.3 ML), (a) phase II, (b) phase I with several vacancies in up-right molecular rows, (c) phase I with fully occupied up-right molecular rows, (d) phase III obtained after two-hour 200 °C annealing, (e), (f), and (g) structural models of phase II, I, and III, respectively. All scans: bias voltage +3.0V, tunneling current 2pA.

Image of FIG. 9.
FIG. 9.

Structures built by PTCDA molecules after deposition in 100 (a), 80 (b), and 120 °C (c), respectively. Each molecule is visible as two protrusions [which can have different intensities—see panel (d)]. Growth at 80 °C favors orientation with the longer axis perpendicular to the substrate rows (orientation II); aggregates are not formed. At 100 and 120 °C molecules on the terraces adsorb with the longer axis parallel to the rows (orientation I). Two non-equivalent molecule contrasts are observed: symmetrical (S) and asymmetrical (A). At 100 °C molecular lines are formed. At 120 °C molecules build mostly 2D and 3D aggregates. Panel (d): Both orientation schemes and possible contrast variations. The position of the zigzag pattern of the substrate STM contrast is indicated. Molecule coverage: (a) and (b) 0.05 ML, (c) 0.1 ML. All scans: bias voltage +3.8V, tunneling current 1pA.

Image of FIG. 10.
FIG. 10.

Panels (a) and (b): An example of the surface structure evolution induced by the scanning tip after completion of four scan frames, the scanning process leads to assembly of 1D molecular lines on the terraces (examples are indicated), a detailed structure of the line is shown in panel (c). Panels (d) and (e): In contrast to the molecular lines formed at 100 °C, the molecules in the tip-assembled lines are arranged asymmetrically in an alternating way and they have smaller separation equal to 2.5 unit cell length along the direction (1.36 nm). Molecule coverage: 0.1 ML. All scans: bias voltage +3.8V, tunneling current 1pA.

Image of FIG. 11.
FIG. 11.

PTCDA islands formed at higher submonolayer coverage (0.5 ML) at 100 °C. Two preferential structures are formed: ordered (observed in two symmetrical versions) and unordered islands of 1 ML thickness. Substrate step edges are often covered by 3D aggregates, bias voltage +2.0V, tunneling current 2pA.

Image of FIG. 12.
FIG. 12.

Panel (a): STM appearance of the 2D ordered island with characteristic striped pattern running along the direction (recorded at 2.5 V bias voltage, molecule coverage: 0.5 ML). The inset shows selected surface area filtered by 2D Fourier transform revealing atomic-scale features needed for correction of dimensional errors, bias voltage +2.5V, tunneling current 2pA. Panel (b): Model of well-ordered molecular islands based on STM contrast only with indicated unit cell.

Image of FIG. 13.
FIG. 13.

Panels: (a)–(e) Voltage-dependent STM contrasts of the equivalent area of a 2D ordered island recorded at 2.5, 2.0, 1.5, 1.2, and 1.0 V bias voltages, tunneling current 2pA, large-scale molecule coverage: 0.5 ML. STM patterns recorded at low bias voltages reveal the orientation of the molecules, which has been put on previously established structural model of 2D islands from Figure 12(b).

Image of FIG. 14.
FIG. 14.

Empty-state STM images of VL molecules adsorbed on the TiO2(011) substrate (coverage: 0.1 ML), (a) molecules evaporated on the substrate kept at room temperature, (b) sample annealed at 200 °C after molecule deposition, (c) molecules assembled on the substrate kept at 100 °C during evaporation, white rectangle indicates molecular line, (d) line profiles along (blue line) and across (green line) the VL molecule adsorbed at the step edge. All scans: bias voltage +2.0V, tunneling current 2pA.

Image of FIG. 15.
FIG. 15.

Adsorption geometry of VL molecules, (a) empty state STM image of VL molecules adsorbed on the substrate kept at 100 °C during evaporation (molecule coverage: 0.1 ML), (b) and (c) adsorption position of VL molecules (white lines indicate titanium surface row trenches), (d) structural model of the adsorption geometry. All scans: bias voltage +2.0V, tunneling current 2pA.

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/content/aip/journal/jcp/134/22/10.1063/1.3593403
2011-06-08
2014-04-24
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Adsorption of organic molecules on the TiO2(011) surface: STM study
http://aip.metastore.ingenta.com/content/aip/journal/jcp/134/22/10.1063/1.3593403
10.1063/1.3593403
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