January 2012
UBAS: Within the last quarter of 2011, the first reporting period of our joint project, we started with a joint meeting in Krakow in the end of October. E. Meyer, R. Pawlak and Th. Glatzel participated from Basel. Christoph Werle started working on the project in Basel full time. The electrospray deposition setup for the existing UHV-STM/AFM system was planned.
UJ: During the first quarter the most important activity was the organization of the Kick–off meeting. The Meeting took place 24.10.2011 in the conference room of the Jagiellonian Library in Krakow. The participants from both parties taking part in the project discussed the prospects and the workplan. Additionally, we also hosted Prof. Edwin Constable from Departament of Chemistry, University of Basel who is a renowned chemists and expert in the field of synthesis organic compounds used for construction of solar cells. Prof. Constable also gave a public talk for the students and employees of the Faculty Physics, Astronomy and Applied Computer Science of the Jagiellonian University.
Additionally the head of a AFM/STM microscope which will be used in the realization of the project was chosen and purchased.
April 2012
UBAS: All parts for the electrospray deposition setup for the UHV-STM/AFM system were ordered and all parts arrived in time. TiO2 (anatase and rutile) samples were ordered and have to be mounted and cleaned for UHV sample preparation. Porphyrin molecules were analyzed on a Cu(111) surface by tuning fork based AFM/STM measurements. The measurements show, that we have now full control on the single atomic level on the tip-molecule interaction, which is mandatory for further experiments. Mass spectroscopy during UHV evaporation was performed for DA-molecules mentioned in the last report (4-11).
UJ: Since the STM/AFM head is not yet delivered, the activity connected with the adaptation of the UHV system for the STM/AFM system is stalled. It will be resumed immediately after the delivery. Despite the promising results of the experiments with the starphene molecules the search for the new molecules, more interesting from point of view of photovoltaics are continued.
July 2012
UBAS: The electrospray deposition setup for the UHV-STM/AFM system was mounted and tested. Following the paper from C. Satterley et al. (Nanotechnology, 2007, 18, 455304) we started depositing C60/C70 by the new system directly onto a UHV cleaned Cu(111) surface. Obviously the cleanliness of the system is an important requirement which was optimized during the last period. The measurements show, that we can now deposit and image submonolayers of C60/C70 which is the first step towards more complex molecular structures.
UJ: The new STM/AFM head was bought and tested. The ongoing work connected with the integrations of the STM/AFM head into the existing UHV system are continued.The intensive experimentd concerning the hydrogen passivated Ge(001) were continued. Since the defects of the surface structure seem to play the crucial role in t he adsorption process, a lot of attention were paid to them, and especially to single and dimer dangling bonds. At the same time experiments with the evaporation of staphene molecules onto the germanium substrate were continued.
October 2012
UBAS: The electrospray deposition setup for the UHV-STM/AFM system was continuously improved. By depositing C60/C70 onto a UHV cleaned Cu(111) surface we optimized the system regarding layer thicknesses and cleanness. The cleanliness of the deposition turned out to be a major problem and we added a pumping stage to acquire clean surfaces. We succeeded to deposit a new porphyrin based molecule which is not able to evaporate thermally.Between 11th and 13th of October 2012 the meeting of the members of the project took place in Basel. The group members from both parties involved in the Project discussed the progress of the project and decided upon the workplan for the next year.
UJ: Intensive efforts connected with the integration of the new STM/AFM with the existing UHV system were undertaken. The new microscope chamber was designed manufactured and tested for leaks. Also the new system of mounting and transfer of samples, compatible with the Omicron standard was created.
Experiments concerned with growth and adsorption of the molecules on semiconductor and insulating surfaces were carried out. Among the tested systems are PTCDA/Ge(001), PTCDA/Ge(001):H, DBH/Ge(001), DBH/Ge(001):H, DBB /Ge(001), DBB/Ge(001):H, QUAD/KBr.Preliminary experiments concerned with evaporation of organic molecules on TiO2 surfaces were carried out. First tests with evaporation of DBH and DBB molecules on rutile surfaces (11) and (001) respectively were carried out.
December 2012
We investigated the self assembly of TTF-dppz (Tetrathiafulvalene-Fused Dipyridophenazine, Fig. a)) [1], a donor-acceptor molecules, on metal substrates and insulating thin films by combined tuning fork based UHV-AFM/STM at 77K and 5K, respectively. The TTF-dppz molecules are functionalized with cyano-groups which yields in a better sticking of the molecules, especially for deposition on insulating substrates [2]. First attempts of deposition on Cu(111) revealed that the molecules are assembled in a disordered phase (b). After post-annealing of the surface at 100°C a highly ordered self-assembly can be found, consisting of molecular wires (d), a network with six-fold symmetry(c), small crystallites (c) and step edge decoration. Most probably the molecules decompose during the post-annealing, involving Cu-adatoms. The fact, that TTF-dppz molecules do not form such ordered structures on a Ag(111)-substrate, strengthens our presumption. Further investigations comprise the study of TTF-dppz on thin films and the co-adsorption of the two single compounds dppz and TTF.In this section the latest results of the project will be described.
Simultaneously, experiments concerning the growth CuPc on the passivated Ge(001) was performed. Unfortunately the result is negative – the molecules do not exhibit any kind of organization, and hence there is no means of control of their morphology. Therefore, the new molecular system is being searched.
[1] C. Jia et al., Chem. Eur. J., 13 (2007)
[2] T. Trevethan et al., Small, 7, 9 (2011)
Summary of results in 2012
We have invested the DA molecule TTF-dppz (Tetrathiafulvalene-Fused Dipyridophenazine) [C. Jia et al., Chem. Eur. J., 13 (2007) ] on metal substrates. To resolve the problem of decomposition of the molecule we performed mass spectroscopy measurements in the group of R. Möller in Duisburg, Germany. They provide a mass spectrometer with a mass range of 1000u which enabled us to analyze the molecules while evaporating them. The molecules is expected to have a mass of 629u, nevertheless we haven’t found an indication for it after UHV evaporation. Instead only several peaks with lower mass (32, 34, 76, 118) have been found which might be related to fragments of the molecule (Fig. 1a). Therefore electrospray deposition might avoid the fragmentation of the molecule in future experiments.
Fig 1 . a) mass spectrum of TTF-dppz molecules; b) a porphyrine molecule in two different configurations
Furthermore, we investigated the mechanical properties of free-based porphyrins confined on Cu(111) in the saddle conformation via combined AFM/STM measurements of force and current with sub-Ångström precision (Fig. 1b). We observed a reliable method of creating Cu-coordinated bonds between specific end-groups of the molecules and the AFM tip and could apply site-dependent vertical forces on the molecular structure. In this way we induced localized deformation which leads to a mechanically-induced rotation of the molecules in a reproducible manner. Such systematic studies might also open the way for mechanically-driven manipulation techniques or local force-induced chemical reactions at the atomic scale. More details have to be evaluated within the next several month.
Subsequently, we have performed the first test with our molecular electrospray in UHV and succeed to image molecules on a metallic surface with a NCAFM at room temperature. The chosen molecules for the experiments come from a mixed powder of C60(80%) and C70(20%) and the sample is Cu(111). The solution we used, is a mixture of Toluene and Acetonitrile (4:1), which is the same solution than in [C. J. Satterley et al., Nanotechnology, vol. 18, no. 45, p. 455304, Nov. 2007]. In this solution, the molecule concentration is not precisely known. Different exposure time and annealing of the sample with molecules were performed. For an deposition time of 2 minutes, followed by an annealing of the sample at 200°C for 1h, it has been possible to image some molecules on the surface. Large scale topography (Fig. 2) images shows different area on the Cu(111) surface. Figure 3 is a higher resolution, close to a step-edge of Fig. 2, where an organized island of C60(C70) is observed.
Fig. 2: Topography (500nm)², NCL cantilever
Fig. 3: Topography (50nm)², NCL cantilever
Figures 4 and 5 show higher resolution images of the molecular island observed in Fig 3. As seen on these different figures, we succeed to image molecules deposited on the Cu(111) surface with the molecular electrospray.
During these first experiments different parameters appear to be very important for the quality of the spray deposition. Therefore, the solution type, the molecular concentration and the position of the different capillary need to be tuned to evaluate their impact on the deposition. The cleanliness of the different stages of the molecular spray is also very important and different capillary and syringe should be used to always have a clean spray deposition.
Fig. 4: Topography (10nm)² (NCL cantilever, A=10nm, Δf=-4Hz )
Fig. 5: Topography (after a tip change) (8nm)² (NCL cantilever, A=10nm, Δf=-5Hz )
During the following experiments we have further tested test of the molecular electrospray setup in UHV and succeed to deposit long porphyrin molecules on a Cu(111) surface and imaged them by nc-AFM at room temperature. The chosen molecules where synthesized in the group of F. Diederich (ETHZ) and contain two Zn-porphyrin cores with cyanophenyl end-groups. We have chosen this molecule because a thermal evaporation of the molecule in UHV will result in a decomposition. Fig. 6 shows the chemical structure, as well as two AFM images of the molecular layer. Also during these experiments different parameters (solvent, pressure, cleanness) appeared to be very important for the quality of the spray deposition. Therefore, the solution type, the molecular concentration and the pumping stages need to be further tuned to improve the deposition process.
Figure 6: Structure of the used double Zn-porphyrin molecule. nc-AFM images of the molecule deposited on Cu(111) at room temperature show clear features of the intact molecules. The deposition parameters have to be further improved.
Figure 7: a) Topography of a KBr single crystal measured by AFM at room temperature. b) Frequency shift of the second resonance of a bimodal-AFM measurement of double porphyrin molecules on KBr deposited by electrospray.
Following the success to deposit long porphyrin molecules on a Cu(111) surface and to image them by nc-AFM at room temperature (RT). We continued with the same type of molecules however, exchanged the substrate to an ionic crystal, namely KBr. The crystal was cleaved in UHV and the molecules have been deposited by the electrospray setup from solution. The setup was improved by adding a lens and a third pumping stage to further reduce the surface contamination by the solvent. Fig. 7 shows a topographical image of the clean substrate as well as a high resolution image of some molecules on KBr in UHV at RT. Since the sticking coefficient of the molecules on the KBr substrate is very low all the deposition parameters had to be adjusted. For imaging we used bimodal-AFM allowing a very sensitive detection of the interaction forces without moving the molecules below the tip apex.
Figure 8: a) Large scale tuning fork STM/AFM measurement of CuPc molecules deposited on NaCl/Cu(111). b) Zoom in of a single CuPc molecule on NaCl at 4K.
Subsequently, we evaporated thermally CuPc molecules onto a monolayer of NaCl on a Cu(111) substrate. This was done in a low-temperature tuning-fork AFM/STM system. The molecule is a well known compound used in organic solar cells as an electron donator. Since the electron transfer process is still unknown and of general interest in the field of solar energy and molecular electronics we started addressing this issue. Fig. 8 shows STM images taken at 4K with the tuning-fork system. The single parts of the molecule are clearly visible and can now be addressed by specific spectroscopic measurements. We plan for the next quarter the analysis of the molecules by bias- and force-spectroscopy.
Fig 9. NC-AFM image of a PTCDA island grown on Ge(001):H
Since we are also interested in the growth of molecular assemblies on semiconductors, we have studied the growth of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) on a clean and passivated by a single hydrogen layer surface of Ge(001). For analysis of both systems (PTCDA/Ge(001) and PTCDA/Ge(001):H) scanning tunneling microscopy and spectroscopy (STM & STS) were used. On bare Ge(001) surface PTCDA molecules did not organize into any regular pattern due to rather strong molecule-substrate interactions. Clean surface of germanium is reactive and therefore prevents migration of molecules at the surface even at elevated temperatures, as high as 500 C. No ordered assemblies are observed. In contrast, at the hydrogenated surface the migration is facilitated and molecule-molecule interaction starts to be important leading to Volmer-Weber growth of the multi-layer islands. The molecules in the islands are ordered similarly to the molecular crystal, i.e. they form herring bone ordered layers (fig 9). Both α and β phases of bulk molecular crystal are present.
2014
At the beginning of 2014, we gathered our best STM images and published them in form of a calendar.
July 2014
Lukasz Zajac worked at the Department of Physics of the University of Basel from 1.4.-31.7.2014. His research was related to the joint project of the Jagiellonian University Krakow in Poland and the University of Basel in Switzerland within the framework of the Swiss Contribution to the enlarged European Union, contract number PSPB-085/2010. He prepared dye sensitized solar cells under various environmental conditions with natural dyes and characterized the resulting devices by current-voltage,quantum efficiency and impedance spectroscopy measurements. The results will be evaluated and summarized for a joint publication.
5-(4- Carboxyphenyl)-10,15,20-triphenyloporphyrin-Zn(II) on the TiO2 (011) surface
CuPC islands on the porphyrin molecular layer assembled on TiO2(011)