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Humboldt-Universität zu Berlin - Mathematisch-Naturwissen­schaft­liche Fakultät - SFB 951 - HIOS

Research Program

Motivation | Topics | Application Perspectives

 

1. Concepts for the controlled assembly and fabrication of HIOS
 

Inorganic semiconductor surfaces are often highly reactive and adsorption of conjugated molecules can result in destruction of their chemical and electronic integrity. Strategies for the non-covalent and covalent assembly of conjugated organic materials on such surfaces must thus be invented. The interface and its electronic structure determine crucially the resulting opto-electronic function, and methods to control these properties have to be found. Novel morphologies for achieving direct and strong coupling between the electronic states in inorganic semiconductors and conjugated organic materials have to be devised.

Furthermore, metal nanostructures with appropriate size, shape, plasmon resonance, and damping have to be prepared and properly arranged with respect to other building blocks, in order to realize geometries that provide strong coupling of the plasmonic excitations to the excited states in organic and/or inorganic materials. The fabrication of well-defined metal/organic hybrid structures with sizes of only a few nanometers is still a great challenge. The availability of all this knowledge is a prerequisite for pursuing the goals of the HIOS project

 

2. Electronic coupling and hybridization of fundamental quantum states of inorganic and conjugated organic materials
 

Opto-electronic function as targeted in this CRC will rely on the direct coupling of the electronic states in the inorganic semiconductor and the conjugated organic material across an interface. Two major aspects of such coupling are:
 

  1. Direct hybridization of the electron wavefunctions in the ground-state: Such new hybrid states will combine the delocalized states of the inorganic semiconductor with the localized states of the conjugated organic material through inorganic/organic chemical bond formation. This hybridization opens up principally new approaches for shaping the energy spectrum of HIOS as well as for the implementation of novel schemes for hybrid materials doping, charge carrier injection or separation. New polaronic states and possibly even two-dimensional metallic phases localized right at the HIOS interface may be formed.
  2. Coupling of optical excitations: Excitonic states - Wannier-Mott-type in the inorganic semiconductor and Frenkel-type in the conjugated organic material - can be coupled by dipole-dipole-type interactions. In its incoherent version, such coupling actuates non-radiative energy transfer, which can be exploited for efficient conversion of excitation density from the inorganic to the organic material. In a coherent mode, hybrid excitons are formed, which are predicted to exhibit unprecedented optical properties such as extremely enhanced non-linear and electro-optical response or optical gain.

 

3. Coupling of excitonic and plasmonic excitations
 

The local electromagnetic field associated with surface-plasmons of metal nanostructures couples with the exciton states in their vicinity. This coupling is significantly more long-ranged than electronic interactions and it is less sensitive to structural perturbations.

Opto-electronic function on the nanoscale can thus be improved in this way, or even new function may be created. Our CRC will attend to two main regimes:
 

  1. Exciton-plasmon coupling in the linear regime: Local field “hot spots” will be utilized to enhance absorption cross-sections and radiative recombination rates of conjugated organic materials, or to enhance non-radiative energy transfer rates between inorganic and organic exciton states up to plasmon-induced hybridization of these states. Plasmon-vibration coupling will be an essential aspect in this context and has to be studied.

  2. Surface-plasmon stimulated emission: Owing to their bosonic nature, surface-plasmons are capable of stimulated emission. Through coupling of confined modes to excitonic dipoles of a nearby material, stimulated plasmonic emission can be achieved by optical pumping. Propagating modes will allow for pumping by electrically generated plasmons. Vice versa, the conversion of stimulated localized plasmons into propagating modes will represent a plasmonic counterpart of a coherent laser beam.

  3. An illustrative example demonstrating both the potential as well as challenges of HIOS is the hy-bridization of Wannier-Mott and Frenkel excitons, as pictured in Fig. 1. The type of hybridization depends on how the organic material interacts with the inorganic semiconductor.

  4. For physisorption, the hybridization is mediated by dipole-dipole interaction [see 1.b) above] and the hybridization energy is small compared to the binding energies of the individual excitons. Considering a configuration, where the Wannier-Mott exciton is confined in a quantum well or dot located beneath the surface, it has been predicted that such a hybrid exciton exhibits, e.g., a third-order optical susceptibility exceeding that of the excitons in the separate materials by several orders of magnitude.

  5. For chemisorbed organic molecules, hybridization can already occur in the electronic ground-state [see 1.a) above] and, consequently, in the excitonic states as well. Here, the concepts of Wannier-Mott and Frenkel excitons are no longer meaningful. Instead, an interfacial hybrid state is formed. The optical properties of such a state are entirely unexplored, both theoretically and experimentally.

  6. In either case, the inorganic and organic energy levels must be (quasi-) resonantly adjusted. The selection of appropriate material pairs is demanding because conjugated organic molecules may change their electronic structure and conformation when brought in contact with the inorganic semiconductor through interfacial chemical interactions. Preliminary work of CRC members has revealed that the associated energy level changes can be as large as 1 eV. Simple models, based on tabulated molecular and inorganic semiconductor ionization and electron affinity levels, to predict the energy levels at HIOS interfaces are doomed to fail.

  7. Conversely, these findings disclose new degrees of freedom that can be utilized for achieving targeted properties and functions. Obtaining control over these degrees of freedom is one dedication of this CRC initiative.

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Fig. 1: One example of HIOS goals:Hybrid excitons

Headline: The dipole-dipole-mediated hybrid exciton (ψ) as quantum mechanical superposition of Wannier-Mott (WM) and Frenkel (F) excitons for physisorbed HIOS. (For strong inorganic/organic interaction, such a two-state superposition is no longer adequate and more complex interfacial hybrid excitons are formed.)
a) Upper part: Schematic representation of the isolated components: inorganic semiconductor (atoms in green) and conjugated organic molecules (blue). The Bohr radius (aWM) of the WM exciton comprises thousands of unit cells, while the relative motion of the F exciton is localized on the molecular scale (aF). Lower part: Electrostatic coupling and chemical reactions between the two materials may change the interface atomic and electronic structure and thus function.
b) Among various new features, an enhancement of the third-order nonlinearity ((3)) by the ratio of the Bohr radii where D denotes the dimensionality of the nanostructure is predicted. Up to now, a WM-F coupling energy of 2 meV has been achieved in non-resonant hybrid structures in preliminary work of CRC members.
c) Hybridization requires (quasi-)resonance between the conduction band minimum (CBM)/valence band maximum (VBM) of the inorganic semiconductor and the lowest unoccupied (LUMO) / highest occupied (HOMO) molecular orbital levels of the conjugated organic material.


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Elements of HIOS research: exemplary structures and functions.

Centre: High-resolution transmission electron microscopy image of a prototypical HIOS [conjugated organic material layer (p-sexiphenyl) embedded between two inorganic semiconductor layers (ZnO)], where hybrid ground or excited state formation (top left; shown for a shorter oligophenyl) should occur at the respective interfaces.

Assembly and cross-linking of molecular building blocks on inorganic semiconductor surfaces (bottom left). Charge separation (top right) and energy transfer (left) at the interface adjusted via energy level alignment tuning. Nanoscale light emission and absorption (bottom right) with HIOS elements controlled and enhanced by metal nanostructures.


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Funded by:  DFG Logo