Ge(001) before and after gold evaporation and annealing
Kernel of the 4-probe microscope (left), crushed tip on Si surface (middle), two STM tips over gold structures on Ge(001) surface

Joint Polish-Swiss Research Programme PSRP-085/2010

„Molecular assemblies on semiconductors and insulating surfaces”

under the Polish-Swiss Research Programme.
Project supported by a grant from Switzerland through the Swiss Contribution to the enlarged European Union.
Polish title: „Struktury molekularne na powierzchniach półprzewodnikowych i izolujących”, project no: PSPB-085/2010 for years: 2011-2016.

The main aim of the project is to investigate processes taking place around the molecular assemblies formed on insulating and semiconducting substrate under irradiation by photons. The molecular assemblies grown either by evaporation or by electro-spray deposition will be examined by scanning probe methods, especially non contact atomic force microscopy (NC-AFM) and Kelvin probe force microscopy (KPFM) in order to determine dependence of the electrical properties of the assemblies of their morphology, and exploit that dependence to control the electrical properties of the assemblies. Within the project a number of molecule/substrate systems will be tested in order to find the most suitable ones for examination of the evolution of excitation in the assemblies induced by the incoming light. As the result we hope to gain deeper understanding of charge evolution and transport in the assembly which is crucial in many fields of the nanotechnology and research related to development of light-harvesting media

Scientific quality:
Electrical properties of organic molecules at nanoscale have been investigated intensively using several methods such as break-junction method [1] or STM [2]. Despite the undisputed success of both methods, their requirements make it often difficult to transfer the results to more application-related systems. They are limited to investigations of molecules interacting with metallic electrodes (or substrates in the case of STM) which in the context of both molecular electronics and organic solar cells are of limited interest. Much more interesting is the direct investigation of the behaviour of the charge distribution in the molecules placed on wide-bandgap materials. The only method which can connect high lateral resolution (even down to a scale of a single molecule) on insulators or wide-gap semiconductors with a sensitivity to locale charges is noncontact atomic force microscopy (nc-AFM) combined with Kelvin probe force microscopy (KPFM) [3].

For molecular electronics the interest in wide-gap semiconductors and insulators as substrates for molecular growth is fueled by the requirement that that surface has to play two, contradictory roles: on the one hand it has to immobilize molecules, but on the other hand the interaction between the molecular core and the surface has to be weak so that it does not damage the projected electronic function of the molecule. Certainly, conductive substrates renders the electronic function of an adsorbed molecule since typically an electron transfer between the molecules and the metallic substrate occurs, which alternate the shape of the molecular orbitals. The shift to wide-gap materials: passivated semiconductors and insulators was not easy, since reduction of interactions led to difficulties in anchoring molecules on surfaces. Some solutions of that problems were put forward, such as using nanostructured surfaces, however, only recently, it was demonstrated that by the proper design of the molecules they can be successfully anchored on the surface of an ionic crystal [4].

As for the organic solar cells, the required electronic structure of substrates is dictated by the energetics of a molecule acting as a donor of electrons. The lowest unoccupied molecular orbital (LUMO) of the molecule has to be close in energy to conduction band of the substrate in order to facilitate the electron transfer between them. Since the maximal voltage of a solar cell cannot be bigger than the bandgap of the light-harvesting molecule, it is clear why wide-gap materials, such as TiO2 are used. Therefore, a tool is needed to investigate electronic properties of molecular assemblies on insulating materials. Due to its high spatial resolution nc-AFM/KPFM is our method of choice.

KPFM was originally proposed by Nonnenmacher and co-workers [5] as the extension of capabilities of the nc-AFM. It is based on the method of measuring the contact potential difference (CPD) proposed by Lord Kelvin back in 1898. In KPFM, the two electrodes are the tip and the investigated sample. While the bias between the electrodes is varied by a known frequency, the analysis of the response of the cantilever to which the tip is mounted allows for determination of the CPD between the tip and the sample. The method was successfully employed to investigate various systems, including those relevant for photovoltaics. However, only recently the performance and the resolution of this systems allows the measurement of molecular assemblies with high resolution and connecting their electrical and optical properties with the molecular structure [6]. Similarly, only recently the connection between the CPD measured by KPFM and the irradiation by visible light was established [7-10]. While Hoppe et al. could distinguish optical active areas of an organic polymer based solar cell from inactive ones by surface photovolatge measurements, Burke and co-workers were able to determine the dependence between the shift of the local CPD under irradiation on three possible structures of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) molecules on a nanostructured KBr surface. Sadewasser and co-workers presented the connection of the KPFM with surface photovoltage spectroscopy (SPS). However, while the precision of determining the surface photovoltage (i.e. the difference between material work function with and without irradiation) is very high, their spatial resolution does not allow for molecular-scale measurements.

[1] N.J. Tao, Nature Nanotechnology 1 (2006) 173.
[2] L. Lafferentz et. al. Science 323 (2009) 1193.
[3] L. Gross et al., Science 324, 1428 - 1431, (2009).
[4] B. Such et al. ACS Nano 4 (2010) 3429.
[5] M. Nonnenmacher et al. Appl. Phys. Lett. 58 (1991) 2921.
[6] E. Meyer and Th. Glatzel, Science 324, (2009), 1397.
[7] H. Hoppe et al., Nano Lett. 5, 269-274, (2004). Th. Glatzel et al., Jap. J. Appl. Phys. 44, 5370-5373, (2005).
[8] S.A. Burke et al. Adv. Mater. 21 (2009) 1.
[9] F. Streicher et al. Rev. Sci. Instrum. 80 (2009) 013907.
[10] S. Saderwasser et al. J. Vac. Sci. Techol. B 28 (2010) C4D29.

October 2011 – June 2016

Project partners:

  • Jagiellonian University, Kraków, Poland - Beneficiary
  • University of Basel, Basel, Switzerland

Project webpage

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