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

Research Highlights of the CRC 951

 

Limitations of Particle-Based Spasers

 

The authors of this CRC 951 research highlight present a semiclassical analytic model for spherical core-shell surface plasmon lasers. Within this model, the widely used one-mode approximations are dropped in favor of fully electromagnetic Mie theory. This allows for incorporation of realistic gain relaxation rates that so far are massively underestimated. Especially, higher order modes can undermine and even reverse the beneficial effects of the strong Purcell effect in such systems. The published model gives a clear view on gain and resonator requirements, as well as on the output characteristics.

These insights should help experimenters design particle-based spasers with significantly improved efficiencies and estimate their performance. The insights can be also used one to one, when it comes to attempts to compensate for losses in metamaterials. This model can be suitably enhanced by numerical approaches for determining “quasinormal modes” for complex plasmonic nanoparticles. So, it provides a rather easy tool to use for computing minimum requirements for a given design and will thus be extremely helpful for future experimental works. In addition, the presented semiclassical model may provide the basis for full quantum-optical treatment of spaser action.

 

For more information see:

G. Kewes, K. Herrmann, R. Rodríguez-Oliveros, A. Kuhlicke, O. Benson, K. Busch

Phys. Rev. Lett. 118, 237402 (2017)

 

DOI: 10.1103/PhysRevLett.118.237402

Research Highlight Kewes_Herrmann

 

FIGURE: Physical model of the gain medium and loss processes for a spaser that operates with emitters and the dipolar resonances of a metal sphere. For more details please see the publication text.

 

Research Highlight Kewes_Herrmann

 

FIGURE: Stationary solution of the rate equations: (a),(b) without (dfree = 0.1 nm) and (c),(d) with a 2 nm spacing layer between the emitters and the sphere surface. For more details please see the publication text.


 

Strong Coupling between Surface Plasmon Polaritons and Molecular Vibrations

 

The authors of this CRC 951 research highlight have demonstrated the strong coupling of surface plasmon polaritons (SPPs) and molecular vibrations in an organic/inorganic plasmonic hybrid structure consisting of a ketone-based polymer deposited on top of a silver layer. Attenuated-total-reflection spectra of the hybrid reveal an anticrossing in the dispersion relation in the vicinity of the carbonyl stretch vibration of the polymer with an energy splitting of the upper and lower polariton branch up to 15 meV. The splitting is found to depend on the molecular layer thickness and saturates for micrometer-thick films.

This new electromagnetic or vibrational eigenstate is not only interesting from a fundamental point of view but also of direct practical relevance. It has been demonstrated that hybridization of microcavity photons and excitons in organic compounds offers the possibility to modify chemical reaction rates. Recently, the same concept has been also applied to molecular vibrations. By coupling to SPPs instead of microcavity photons, this approach can be transferred into a large-area and geometrically simple setting. The scenario is not restricted to the stretch vibration of the carbonyl group, and any infrared active vibrational transition with a strong optical dipole moment could be targeted. The modification of intramolecular relaxation rates is also of immediate interest for optoelectronic functionality. Moreover, vibrational energy, otherwise localized on a single molecule or side group, can now be coherently transported along the surface for triggering electronic processes like exciton dissociation.

 

For more information see:

H. Memmi, O. Benson, S. Sadofev, S. Kalusniak

Phys. Rev. Lett. 118, 126802 (2017)

 

DOI: 10.1103/PhysRevLett.118.126802

Research Highlight Memmi

 

FIGURE: TM-polarized attenuated-total reflection spectra of a poly(vinyl-methylketone)/silver (PVMK/silver) hybrid structure in the strong coupling regime. For more details please see the publication text.

 

Research Highlight Memmi

 

FIGURE: Dispersion relation of a PVMK/silver hybrid structure in the strong coupling regime. For more details please see the publication text.


 

Metal−Semiconductor Nanoparticle Hybrids Formed by Self-Organization: A Platform to Address Exciton−Plasmon Coupling

Hybrid nanosystems composed of excitonic and plasmonic constituents can have different properties than the sum of the two constituents, due to the exciton-plasmon interaction. In this recent CRC 951 research highlight, a flexible model system based on colloidal nanoparticles that can form hybrid combinations by self-organization is shown.

The system allows the authors to tune the interparticle distance and to combine nanoparticles of different sizes and thus enables a systematic investigation of the exciton-plasmon coupling by a combination of optical spectroscopy and quantum-optical theory. A strong influence of the energy difference between exciton and plasmon, as well as an interplay of nanoparticle size and distance on the coupling is experimentally observed.

The authors develop a full quantum theory for the luminescence dynamics and discuss the experimental results in terms of the Purcell effect. As the theory describes excitation as well as coherent and incoherent emission, possible quantum optical effects are also considered. A good agreement of the observed and the calculated luminescence dynamics induced by the Purcell effect is found. This also suggests that the self-organized hybrid system can be used as platform to address quantum optical effects.

 

For more information see:

C. Strelow, T. S. Theuerholz, C. Schmidtke, M. Richter, J.-P. Merkl, H. Kloust, Z. Ye, H. Weller, T. F. Heinz, A. Knorr, H. Lange

Nano Lett. 18, 4811 (2016)

 

DOI: 10.1021/acs.nanolett.6b00982

Research Highlight Strelow

 

FIGURE: Sketch of the model and basic processes. The coupled system is driven coherently by an exciting laser pulse. The QD exciton (between conduction and valence band) is subject to dephasing and exchanges energy with the AuNP, which looses energy by damping. The whole system couples to the far field, where energy is dissipated radiatively. This contribution corresponds to the observed PL signal.

 


 

Structure of p‑Sexiphenyl Nanocrystallites in ZnO Revealed by High-Resolution Transmission Electron Microscopy

 

In this CRC 951 research highlight, the structure of para-sexiphenyl (6P) nanocrystallites embedded in ZnO single crystals is resolved by cross-sectional high-resolution transmission electron microscopy (HRTEM) combined with image contrast simulations and X-ray diffraction measurements.

The hybrid structures are prepared by subsequent physical vapor deposition of 6P on ZnO(1010) templates followed by overgrowth with ZnO. Application of ultramicrotomy for HRTEM specimen preparation and imaging under different focus conditions provides direct access to the atomic and molecular structure of the hybrid interface and the organic inclusion.

The hybrid stacks reveal a high structural perfection. The 6P nanocrystallites maintain a structure as in the bulk crystal. Individual 6P lattice planes can be traced up to the lateral and top interfaces with ZnO, indicating that all interfaces are defined on an atomic/molecular level.

Further evaluation of the HRTEM images reveals peculiarities of 6P growth on ZnO(1010). The common 6P beta-phase coexists here with the rarely reported gamma-phase.

The ZnO surface topography, on the other hand, is critical for the structural perfection of 6P. Although conformal growth is observed, ZnO step edges induce characteristic stacking faults in 6P nanocrystallites.

 

For more information see:

H. Kirmse, M. Sparenberg, A. Zykov, S. Sadofev, S. Kowarik and S. Blumstengel

Cryst. Growth Des. 16, 2789 (2016)

 

DOI: 10.1021/acs.cgd.6b00109

Research Highlight Kirmse

 

FIG. 1: (a−e) Representative selection of cross-sectional HRTEM images of 6P nanoaggregates embedded in a ZnO(1010) hybrid stack. The deposited amount of 6P corresponds to a nominal layer thickness of 2.5 nm. The dark lattice fringes of the 6P nanoaggregates correspond to the (100) lattice planes. The images are recorded at an underfocus of df = −1000 nm except for the inset of (c), which is obtained at a Scherzer defocus of df = −43 nm. The scale bar in the inset corresponds to a length of 2 nm.

 


 

Tuning the work function of GaN with organic molecular acceptors

 

In this CRC 951 research highlight the capability of two molecular organic acceptors (HATCN and F6-TCNNQ) to tune the work function (Φ) of intrinsically doped GaN(0001) and for comparison of intrinsically doped ZnO(0001) is demonstrated and the fundamental physical phenomena involved are unraveled. The ability to adjust the GaN work function with molecular interlayers at will could lead to a wide manifold of device realizations, e.g., high-frequency and sensor applications or hybrid light-emitting diodes with a wide emission energy range.

Using ultraviolet and x-ray photoelectron spectroscopy it was demonstrated, that the work function of GaN can be increased substantially, and ΔΦ achieved was 1.5 and 1.8 eV with HATCN and F6-TCNNQ, respectively.

The Φ increase has two contributions: one is the formation of an interface dipole (ΦID) at the inorganic-organic interface, the other is a surface band bending modification inside the inorganic semiconductor (ΦBB). From these two contributions, ΔΦBB is found to be significantly lower for GaN compared to ZnO (0.35 vs. 1 eV).

A model that relates the relative contribution of ΔΦBB to the underlying material parameters, i.e., the inorganic semiconductor doping level and the surface-state density, is presented. The lower the doping level is, the lower the surface-state density needs to be to reduce the band bending contribution to ΔΦ. This limits the maximum value of Φ tuning for GaN with molecular acceptors.

The method of Φ tuning with molecular acceptors is robust against surface order variations. For highly ordered surfaces essentially the same ΔΦ was found as for surfaces that did not exhibit any low-energy electron diffraction (LEED) pattern.

 

For more information see:

T. Schultz, R. Schlesinger, J. Niederhausen, F. Henneberger, S. Sadofev, S. Blumstengel, A. Vollmer, F. Bussolotti, J.-P. Yang, S. Kera, K. Parvez, N. Ueno, K. Müllen, and N. Koch

Phys. Rev. B 93, 125309 (2016)

 

DOI: 10.1103/PhysRevB.93.125309

Research Highlight Schultz

 

FIG. 1: (a) and (b) Valence and (c) and (d) secondary electron cutoff spectra of sputtered samples without [(a) and (c) disordered] and with [(b) and (d) ordered] subsequent annealing. Different shapes of spectra are due to different analyzers used; valence spectra vertically offset for clarity. Surface order was evidenced by LEED. (e) Zoom of near-EF region of pristine GaN and ZnO on linear (solid curves) and logarithmic (dashed curves) scales, emphasizing the intensity in the gap region. Spectra are normalized for comparison.

FIG. 2: Schematic energy-level diagrams of ZnO with (a) F6-TCNNQ and (b) HATCN, of (c) GaN with F6-TCNNQ, and (d) HATCN. The faint gray lines and values on the inorganic side indicate the levels before acceptor deposition.

 


 

Ultrafast Nonlinear Response of Bulk Plasmons in Highly Doped ZnO Layers

 

Understanding the plasmon dynamics in inorganic systems is vital for the development of hybrid plasmonic systems. In this recent CRC 951 research highlight we present the first ultrafast optical study of longitudinal “bulk” plasmons in a two-layer, highly n-doped ZnO:Ga system.

The thickness of the top layer is well below the skin depth exhibiting one polariton branch with an electric field component perpendicular to the surface. The bottom layer acts as a metallic mirror, so that the superposition of the incoming and reflected beam results in an electric field component normal to the sample surface, which interacts with the bulk plasmons in the top layer. Via linear reflection spectroscopy, the bulk plasmon for this sample structure was determined to 360 eV.

Figure 1(a) shows a contour plot of the femtosecond pump-probe data. Figure 1(b) shows cross sections through the plot for fixed delay times between pump and probe beam. A nonlinear plasmon response, manifested in a femtosecond plasmon redshift is observed. In addition, Figure 2 shows the time-resolved absorption changes for the measured bulk plasmon at fixed probe frequencies for different incident pump fluences plotted as a function of pump-probe delay. The absorption changes rise within the time resolution of the experiment and vanish completely beyond a 2 ps delay. With increasing pump fluence, the signal amplitudes begin to saturate.

By theoretical calculations the bulk plasmon dynamics can be explained by intraband excitation of electrons by the pump pulse. The initial nonequilibrium electron distribution thermalizes into a hot Fermi distribution within the time resolution of the experiment and cools back to lattice temperature by emission of LO phonons and later of acoustic phonons. The redshift can be explained by transient increase of the ensemble-averaged electron mass in the strongly heated distribution of conduction band electrons and the concomitantly reduced plasma frequency in the hot electron plasma.

The spectral position of the bulk plasmon can be tailored via the electron density. In combination with the strong transient nonlinearity of the system, there is a strong potential for a wide range of applications in ultrafast plasmonics.

 

For more information see:

T. Tyborski, S. Kalusniak, S. Sadofev,
F. Henneberger, M. Woerner, T. Elsaesser

Phys. Rev. Lett. 115, 147401 (2015)

 

DOI:10.1103/PhysRevLett.115.147401

FIG. 1: (a) Contour plot of the femtosecond pump-probe data. The change of absorbance ΔA is plotted as a function of probe energy (abscissa) and pump-probe delay (ordinate). (b) Cross sections through the contour plot for fixed delay times.

FIG. 2: Time-resolved absorption changes for the measured bulk plasmon at fixed probe frequencies plotted for different incident pump fluences as a function of pump-probe delay.

 


 

Zinc oxide modified with benzylphosphonic acids as transparent electrodes in regular and inverted organic solar cell structures

 

The work function of untreated ZnO of about 4.3 eV causes injection barriers to almost all conventional organic semiconductors and is poorly reproducible due to the physisorption of contaminations. One way to alter the WF and, at the same time, passivate the ZnO surface is the attachment of a selfassembled monolayer (SAM) consisting of polar molecules. Figure 1 shows how by employing substituted benzyl phosphonic acids (BPA) as well as pyrimidine phosphonic acid (PyPA) with different dipole moments (in brackets) of the head group, the work function of well-definied ZnO (0001), ZnO (000-1), ZnO(10-10) surfaces can be altered from about 4.1 eV to almost 5.7 eV. The work function of polycrystalline sol-gel derived ZnO can be even tuned over a range of more than 2.3 eV.

These modified sol-gel processed ZnO films have been applied as transparent cathode (using ABPA as SAM) in inverted, and as transparent anode (using 3FMBPA as SAM) in regular P3HT:PCBM solar cell structures. A benchmark reference cell on ITO/PEDOT:PSS in a regular structure was also prepared for comparison. The resulting J-V-curves under illumination (full line) and in the dark (dashed line) are shown in Figure 2 and the respective performance parameters of the best performing devices in the table below. The power conversion efficiency (PCE) of the inverted structure distinctly exceeds the reference value of the benchmark cell. Notably, also the regular solar cell displays a high PCE and fill factor, comparable to that of the reference cell, highlighting the electronic grade of the SAM-modified ZnO electrodes.

As these SAM-modified ZnO layers can be coated via solution-based methods on virtually any electrode material, the presented approach paves the way to ITO-free devices without the need for including electrolyte buffer layers.

 

For more information see:

Ilja Lange, Sina Reiter, Juliane Kniepert, Fortunato Piersimoni, Michael Pätzel, Jana Hildebrandt, Thomas Brenner, Stefan Hecht, and Dieter Neher

Appl. Phys. Lett. 106, 113302 (2015)

 

DOI: 10.1063/1.4916182

FIG. 1: Surface potential as function of the molecular head group dipole moment of three single crystalline and sol-gel processed ZnO surfaces covered with the molecular SAMs displayed below.

FIG. 2: J-V-curves of all investigated P3HT:PCBM solar cells with different electrodes and architectures under illumination (full line) and in the dark (dashed line). Below: Device parameters of the best performing solar cells.

 


 

A scale-bridging modeling approach for anisotropic organic molecules at patterned semiconductor surfaces

 

Hybrid systems consisting of organic molecules at inorganic semiconductor surfaces are gaining increasing importance as thin film devices for optoelectronics. The efficiency of such devices strongly depends on the collective behavior of the adsorbed molecules.

In this recent HIOS research highlight, we propose a novel, coarse-grained model addressing the condensed phases of a representative hybrid system, that is, para-sexiphenyl (6P) at zinc-oxide (ZnO). Within our model, intermolecular interactions are represented via a Gay-Berne potential (describing steric and van-der-Waals interactions) combined with the electrostatic potential between two linear quadrupoles. Similarly, the molecule-substrate interactions include a coupling between a linear molecular quadrupole to the electric field generated by the line charges characterizing ZnO(10-10).

To validate our approach, we perform equilibrium Monte Carlo simulations, where the lateral positions are fixed to a 2D lattice, while the rotational degrees of freedom are continuous. We use these simulations to investigate orientational ordering in the condensed state. We reproduce various experimentally observed features such as the alignment of individual molecules with the line charges on the surface, the formation of a standing uniaxial phase with a herringbone structure, as well as the formation of a lying nematic phase

 

For more information see:

Nicola Kleppmann and Sabine H. L. Klapp,

J. Chem. Phys. 142, 064701 (2015)

 

DOI: 10.1063/1.4907037

FIG. 1: Sketches of a 6P molecule with different degrees of sophistication: (a) atomistic model with the blue (white) parts representing C-(H-)atoms, (b) charge distribution with negative π-orbitals above and below the molecule and positively charged H-atoms around the edge, and (c) representation as an uniaxial ellipsoid.

FIG. 2: Nematic order parameter of the fully interacting system including all types of molecule-molecule (GB, QQ) and molecule-substrate (LJ, QS) interactions. Ordering states: I: upright uniaxial order of the long molecular axes combined with a T-shaped ordering of their quadrupoles , III: disorder within the x-z-plane, and IV: planar uniaxial order in the x-y-plane. Parts (b)-(d) show snapshots at a =0.4nm (I), a= 1.0 nm (III) and a = 4.0 nm (IV), respectively.

 


 

Calculating optical absorption spectra of thin polycrystalline organic films: Structural disorder and site-dependent van der Waals interaction

 

In this recent HIOS research highlight, a new approach for calculating the optical absorption spectra of organic polycrystalline thin films on inorganic substrates (here PTCDI on KBr(100)) is presented. In particular, a novel relation for molecular excitation energy shifts due to dispersion effects of the environment is derived. The obtained formulas allow for the determination of site-dependent level shifts of a molecule in a given nanoscale environment.

The calculated and experimental Im(ϵ) spectra for different PTCDI coverages are shown in Figure 1a and 1b respectively. Both the calculated and experimental 1 ML spectra are red-shifted compared to the 0.1 ML spectra with increasing coverage. The calculations in this publication indicate that this shift is due to dispersion interaction. The line shape suggests that the spectrum is a superposition of spectra of a perfectly grown crystalline phase (feature at ∼2.1 eV) and a more loosely packed, rather disordered phase (broad band between 2.3 and 2.8 eV).

To model the experimental spectra, an additional disordered phase is considered consisting of small crystallites with a reduced packing density (see figure 2) The average is taken over a mixture of ordered and disordered phases and. Agreement with the experimental spectrum is obtained when assuming that the PTCDI film consists of 53% of the ordered phase.

These results explain the absorption features as disorder induced dispersive effects due to nonresonant excitonic coupling.

 

For more information see:

J. Megow, T. Körzdörfer, T. Renger, M. Sparenberg, S. Blumstengel, F. Henneberger, and V. May,
J. Phys. Chem. C 119, 5747 (2015).

 

DOI: 10.1021/acs.jpcc.5b01587

 

FIG. 1: Computed absorption line shapes (Fig. 1a) and measured (Fig. 1b) absorption line shapes of different thicknesses of a PTCDI film. The calculations in this publication indicate that the red-shift is due to dispersion interaction.

FIG. 2: Full black line: Experimental spectrum for a 4.9 ML film of PTCDI on KBr. Dashed black line: Calculated spectrum of a nanocrystalline film of nominally 5 ML thickness. Agreement with the experimental spectrum is obtained when assuming that the PTCDI film consists of 53% of the ordered phase

 

 

 


 

Efficient light emission from inorganic and organic semiconductor hybrid structures by energy-level tuning

 

The unfavourable energy level offset in hybrid inorganic/organic structures (HIOS) is an inherent obstacle for efficiently exploiting such structures for light emitting applications. In this publication (see below), the CRC 951 shows how by introducing a [RuCp*mes]+ interlayer between ZnO and a triply spiroannulated ladder-type quarterphenyl (L4P-sp3) the energy levels of this hybrid structures can be optimized leading to an increment of the radiative emission yield by a factor of seven.

Figure 1 exhibits the energy level diagrams of L4P-sp3 without interlayer (1a) and with [RuCp*mes]+ interlayer (1b) on Zn-terminated ZnO(0001). The energy values (in eV) were determined by UV-photoelectron spectroscopy and are referenced to the Fermi level. In Fig. 1a energy level alignment (ELA) is of type-II with a final offset between the respective filled/empty frontier levels of 1.1 eV. The type-II ELA rather facilitates charge transfer that quenches light emission.

Introducing the [RuCp*mes]+ as interlayer (Fig. 1b), the organic semiconductor levels rigidly shift in energy with respect to those of the inorganic component with an offset as little as 0.1 eV. This type-I ELA is favorable for energy transfer and subsequent light emission. This is shown in Figure 2 which exhibits the PL spectra of a hybrid structure consisting of a ZnO/Zn0.9Mg0.1O QW structure overgrown with 3 nm L4P-sp3 (blue) and of an equivalent hybrid structure, but with a [RuCp*mes]+ interlayer between QW structure and L4P-sp3 (green). By introducing the interlayer the molecular emission of the L4P-sp3 increases by a factor of seven.

This clearly shows the great potential of HIOS and is a major step towards HIOS based light-emitting applications. 

 

For more information see:

R. Schlesinger, F. Bianchi, S. Blumstengel, C. Christodoulou, R. Ovsyannikov, B. Kobin, K. Moudgil, S. Barlow, S. Hecht, S. R. Marder, F. Henneberger,and N. Koch, Nature Communications 6 (2015)

DOI: 10.1038/ncomms7754

 

FIG. 1. Energy level diagrams of a) L4P-sp3 without interlayer and b) with [RuCp*mes]+ interlayer on Zn-terminated ZnO(0001). Energy values are referenced to the Fermi level and in eV. The offset between the L4P-sp3 and ZnO energy levels is highlighted in red. The L4P-sp3 LUMO region is shaded with a gradient to represent uncertainties due to the unknown transport gap.

FIG. 2. PL spectra of hybrid a structure consisting of a ZnO/Zn0.9Mg0.1O QW structure overgrown with 3 nm L4P-sp3 (blue) and of an equivalent hybrid structure, but with a [RuCp*mes]+ interlayer between QW structure and L4P-sp3 (green).

 

 

 


 

Nanotubular J-Aggregates and Quantum Dots Coupled for Efficient Resonance Excitation Energy Transfer

 

Resonant coupling between distinct excitons in organic supramolecular assemblies and inorganic semiconductors is supposed to offer an approach to optoelectronic devices. Here, we report on colloidal nanohybrids consisting of self-assembled tubular J-aggregates decorated with semiconductor quantum dots (QDs) via electrostatic self-assembly. The role of QDs in the energy transfer process can be switched from a donor to an acceptor by tuning its size and thereby the excitonic transition energy while keeping the chemistry unaltered.

QDs are located within a close distance (<4 nm) to the J-aggregate surface, without harming the tubular structures and optical properties of the J-aggregates. The close proximity of J-aggregates and QDs allows the strong excitation energy transfer coupling, which is around 92% in the case of energy transfer from the QD donor to the J-aggregate acceptor (Fig. 1) and approximately 20% in the reverse case. The very strong coupling between the inorganic semiconductors and the tubular J-aggregates is promising for the search for coherent coupling effects, which are expected for such systems within the strong coupling regime.

This system provides a model of an organic–inorganic light-harvesting complex using methods of self-assembly in aqueous solution, and it highlights a route toward hierarchical synthesis of structurally well-defined supramolecular objects with advanced functionality. Due to the well-defined structural composition, a detailed insight into the photophysical properties of such organic-inorganic hybrid systems can be obtained, which will be helpful for the understanding of hybrid inorganic-organic systems in general.

For more information see:

Y. Qiao, F. Polzer, H. Kirmse, E. Steeg, S. Kühn, S. Friede, S. Kirstein, J.P. Rabe

ACS Nano 9 (2015), 1552-1560

DOI: 10.1021/nn506095g

 

FIG. 1. Optical spectra of J-aggregate/QD-535 nanohybrids to illustrate FRET from QD-535 to the J-aggregate.

(a) Absorption and emission spectra of C8S3 J-aggregate nanotubes (λex = 535 nm) and QD-535 (λex = 380 nm), normalized to maximum value.

(b) PL spectra of the J-aggregate/QD-535 nanohybrids, as well as QD-535 and J-aggregate controls (λex = 380 nm), recorded at comparable respective concentrations and identical conditions. The inset shows the absorbance spectrum of the nanohybrids (red solid line) and the J-aggregate control (blue dashed line) in semi-logarithmic scale.

(c) Time-resolved decay of PL emission at 535 (10 nm in J-aggregate/QD-535 nanohybrids, as well as QD-535 and J-aggregate controls, both curves normalized to maximum intensity.

(d) Time-resolved decay of PL emission at 600 (3 nm in J-aggregate/QD-535 nanohybrids and J-aggregate control, normalized to maximum intensity.

 

 

 


 

Surface Modification of ZnO(0001)−Zn with Phosphonate-Based Self-Assembled Monolayers: Binding Modes, Orientation, and Work Function

 

We used partially fluorinated alkyl and aromatic phosphonates as model systems with similar molecular dipole moments to form self-assembled monolayers (SAMs) on the Zn-terminated ZnO(0001) surface. The introduced surface dipole moment allows tailoring the ZnO work function to tune the energy levels at the inorganic−organic interface to organic semiconductors, which should improve the efficiency of charge injection/extraction or exciton dissociation in hybrid electronic devices.

Two partially fluorinated phosphonates, namely an alkyl and an aromatic phosphonate, were used to modify the surface work function of ZnO(0001)−Zn single crystals. The unmodified and phosphonic acid (PA)-modified ZnO surfaces were comprehensively studied by contact angle, SFM, high-resolution XPS, UPS, and XAS measurements. Both UPS and XAS data were supported by additional DFT calculations to unambiguously identify the contributions of the PA molecules to the experimental spectra. By combining all these methods, we could not only iteratively optimize the quality of molecular interlayers with uniform surface coverage but also derive a complete picture of bonding and arrangement of the phosphonates with respect to the ZnO surface. We show that for the aromatic SAM the interaction between neighboring molecules is strong enough to drive the formation of a more densely packed monolayer with a higher fraction of bidentate binding to ZnO, whereas for the alkyl SAM a lower packing density was found with a higher fraction of tridentate binding.

Therefore, this work provides a detailed understanding of the factors impacting the work function modification of the technologically relevant Zn-terminated polar ZnO surface via phosphonate-based SAMs.

 

For more information see:

M. Timpel, M. V. Nardi, S. Krause, G. Ligorio, C. Christodoulou, L. Pasquali, A. Giglia, J. Frisch, B. Wegner, P. Moras, N. Koch

Chem. Mater. 26, 5042-5050 (2014)

DOI:10.1021/cm502171m

FIG. 1. O 1s core level spectra (background subtracted) of the (a) unmodified (lower panel) and F13OPA-modified ZnO surface (upper panel), and (b) unmodified (lower panel) and pCF3PhPA-modified ZnO surface (upper panel). (c) Schematic illustration of the surface - OH groups and possible PA binding modes on the ZnO surface, which are attributed to O 1s core level components in parts a and b.

 

FIG. 2. Plots of the relative σ*- and π*-orbital intensities as a function of the photon incidence angle ΨC. The solid curve corresponds to the best fit of the intensity evolution for (a) F13OPA-, and (b) pCF3PhPA-modified ZnO surface with molecule tilt angle of 26° and 28°, respectively, referred to the surface normal.

 

 


 

Broadband linear high-voltage amplifier for radio frequency ion traps

 

We developed a linear high-voltage amplifier for small capacitive loads consisting of a high-voltage power supply and a transistor amplifier. With this cost-effective circuit including only standard parts sinusoidal signals with a few volts can be amplified to 1.7 kVpp over a usable frequency range at large signal response spanning four orders of magnitude from 20 Hz to 100 kHz under a load of 10 pF.

Figure 1 depicts the voltage gain over the whole usable frequency range for the three different capacitive loads at the maximum output voltage. For higher frequencies, the performance of the amplifier appears to be even better than shown in Figure 1. In our experiments, the amplifier drives different linear quadrupole ion traps consisting of four cylindrical rods. In contrast to many other traps which hold one pair of opposing electrodes at ground, our trap is symmetrically driven, which makes no difference for the trapping potential but permits the high-voltage in our case.

We tested the performance for different capacitive loads and different output voltages under consideration of the total harmonic distortion, to obtain representative results. To estimate thermal stability we conducted long term measurements under full load and maximum output voltage. As a special feature our amplifier is sustained short-circuit proof, which was tested successfully. With the discussed amplifier we are able to trap single charged nano- and microparticles in different quadrupole ion traps.

 

For more information see:

A. Kuhlicke, K. Palis, O. Benson

Rev. Sci. Instrum. 85, 114707 (2014)

DOI:10.1063/1.4901594

FIG. 1. Measured frequency response curves for an output voltage of 1.7 kVpp.

(a) The voltage gain is shown for capacitive loads of 5 pF (blue dotted), 10 pF (black solid), and 20 pF (red dashed curve). The output voltage is measured via the monitor ports and the measurement amplifiers.

(b) Comparison of the voltage gain measured with the differential high-voltage probe (Testec, TT-SI9010, differential input impedance 20 MΩ ǁ 5 pF) to the voltage gain obtained from the corresponding monitor signals. The differential probe is the only load in this measurement. Dots mark the data points. The lines are for optical guideline only.

 

 

 


 

Distant- and Shape-Dependent Excitation Energy Transfer in Nanohybrid Systems: Computations on a Pheophorbide‑α CdSe Nanocrystal Complex

 

The combination of semiconductor nanocrystals (NCs) and molecules for efficient electronic excitation energy transfer is expected to be a promising ingredient of novel hybrid photovoltaic devices. Here energy transfer from a CdSe NC to the tetrapyrrole-type Pheophorbide-a molecule (Pheo) is studied theoretically.

A comprehensive calculation of EET rates has been presented for transitions from different types of CdSeNCs to a Pheo. To account for the more than thousand atoms forming the investigated NCs, we have used a tight-binding model combined with a CI characterization of the Coulomb correlated electron−hole pairs. As in the Pheo case, this procedure allows us to define atomic-centered transition charges that offer a rather exact way to compute the NC Pheo transfer coupling. Besides this quantity, the used rate expressions account for the multitude of NC exciton levels and their thermal distribution. The introduced thermal-averaged transfer coupling offers a single number to characterize all various exciton levels and couplings. For the considered spherical Cd1159Se1450 NC as well as for the hemispherical and pyramidal NC, the mean coupling is below 1 meV. This small number excludes hybrid state formation and justifies the used Golden-Rule-type rate formula.

According to the small coupling, the characteristic decay time of the NC excitations due to the coupling to the Pheo is ∼5 ns if the Pheo is placed at the surface of the NC. This result is in line with very recent measurements, where EET from a CdSe/SdS/ZnS multishell NC coupled to a single perylene diimide molecule has been reported.

 

 

For more information see:

D. Ziemann, V. May

J. Phys. Chem. Lett. 5, 1203 (2014)

doi:10.1021/jz5003023

 

Fig 1. Real part of the transition charge density ρα0 for the first bright exciton state of the s-NC (a) and of the p-NC (b). The color bar indicates the negative (blue) and positive (red) regions, while a change in color toward light green corresponds to a decreasing value of ρα0. The figures have been generated in using the transition charges qu(α0) smeared across the different atoms u.

 

 

 

 


 

Charge Transfer Absorption and Emission at ZnO/Organic Interfaces

 

We investigate hybrid charge transfer states (HCTS) at the planar interface between α-NPD and ZnO by spectrally resolved electroluminescence (EL) and external quantum efficiency (EQE) measurements.

Radiative decay of HCTSs is proven by distinct emission peaks in the EL spectra of such bilayer devices in the NIR at energies well below the bulk α-NPD or ZnO emission. The EQE spectra display low energy contributions clearly red-shifted with respect to the α-NPD photocurrent and partially overlapping with the EL emission. Tuning of the energy gap between the ZnO conduction band and α-NPD HOMO level (Eint) was achieved by modifying the ZnO surface with self-assembled monolayers based on phosphonic acids. We find a linear dependence of the peak position of the NIR EL on Eint, which unambiguously attributes the origin of this emission to radiative recombination between an electron on the ZnO and a hole on α-NPD.

 

By varying the work function of the ZnO layer through use of self-assembled monolayers, we were able to tune the energy of the charge transfer state, as evidenced by a lowering of the EL emission peak energy. We find a strict correlation between the energy maximum of the CTS emission, the energy gap at the interface, determined by UPS, and the open-circuit voltage obtained in photovoltaic devices. The results of this investigation prove for the first time that for hybrid organic−inorganic interfaces those three quantities are proportional to each other.

 

For more information see:

F. Piersimoni, R. Schlesinger, J. Benduhn, D. Spoltore, S. Reiter, I. Lange, N. Koch, K. Vandewal, D. Neher

J. Phys. Chem. Lett. 6, 2015, 500–504

DOI: 10.1021/jz502657z

Fig. 1. Electroluminescence from α-NPD deposited on SAM modified ZnO. Increasing the work function of ZnO correlates with a prominent red shift of the EL emission peak (inset).

 

 

Fig. 2. J−V curves of devices using α-NPD as donor and modified ZnO as acceptor. The open-circuit voltage (Voc) decreases with increasing the ZnO work function. The inset shows Voc as a function of the EL emission maximum.

 


 

Ultrafast Exciton Formation at the ZnO(10-10) Surface

 

We study the ultrafast quasiparticle dynamics in and below the ZnO conduction band using femtosecond time-resolved two-photon photoelectron (2PPE) spectroscopy. Above band gap excitation causes hot electron relaxation by electron-phonon scattering down to the Fermi level EF followed by ultrafast (200 fs) formation of a surface exciton (SX).

The dynamics below EF differ significantly from the ones above. Integration of the 2PPE intensity yields the diamond-shaped markers in Figure 1b. They display an initial drop of intensity. Consequently, hνpump must create additional states below EF leading to an increased 2PPE signal. Comparision of 2PPE spectra of two different excitation densities near the Mott limit of ZnO at different time delays shows that photoinduced changes of surface band bending and charge accumulation layer density cannot be the cause of the observed peak. On the contrary, the decreased intensity matches the expectations for an exciton whose formation probability is reduced by screening of the e-h interaction.

In summary, we present a systematic investigation of the ultrafast electron and exciton dynamics at the ZnO(10-10) surface, showing the formation of a subsurface-bound exciton on fs time scales that exhibits a very large binding energy with respect to the bulk conduction band, resulting in a remarkable stability of this feature. The study thus offers a complete and novel picture of the quasiparticle relaxation at this surface. The existence of a subsurface exciton which is stable with regard to surface modifications is of high relevance for applications of ZnO, e.g., involving FRET. The SX may dominate the dipole-dipole coupling across functional interfaces due to its adjacence to the surface, and it may even persist under non-UHV conditions.

 

For more information see:

J. C. Deinert, D. Wegkamp, M. Meyer, C. Richter, M. Wolf, J. Stähler

Phys. Rev. Lett. 113, 057602 (2014)

DOI:10.1103/PhysRevLett.113.057602

 

 

FIG. 1. (a) Ultrafast electron dynamics at the ZnO surface below the Mott limit as probed by 2PPE spectroscopy. False colors represent photoelectrons created or depleted by hνpump. Hot carriers in the CB relax on fs time scales by optical phonon emission. After a few 100 fs, additional electrons are verified below EF, indicating that the surface exciton has formed.

(b) Pump and probe XC traces at indicated energies above (hot electrons) and below EF (exciton) and the corresponding double exponential fits to the data (solid lines). The instrument response function is represented by the dashed curve.

 


 

Unravelling the multilayer growth of the fullerene C60 in real time

 

Molecular semiconductors are increasingly used in devices, but understanding of elementary nanoscopic processes in molecular film growth is in its infancy. Up to now, there is no organic compound for which even the ‘minimal’ set of the three parameters diffusion barrier, lateral binding energy and Ehrlich–Schwoebel barrier have been simultaneously quantified to describe multilayer molecular growth. 

Here we use real-time in situ specular and diffuse X-ray scattering to study C60 nucleation and multilayer growth. Relating the experimental data to results from KMC simulations, we have been able to determine a consistent set of energy parameters determining the growth kinetics on the molecular level. Our approach yields an effective Ehrlich–Schwoebel barrier of EES=110 meV, diffusion barrier of ED=540meV and binding energy of EB=130 meV (Fig. 1). The combined analysis provides a detailed understanding of C60 in terms of molecular-scale processes. Moreover, our study sheds new light on various dynamical aspects accompanying the growth (Fig. 1). In particular, we show that the colloid-like, short-ranged character of C60 interactions leads to relatively long surface diffusion times before immobilization occurs at existing islands.

Since C60 features aspects of both atomic and colloidal systems, our findings will help to gain insight into island nucleation and surface growth processes for van der Waals-bound molecules between the scales of atomic and colloidal systems. This quantitative, scale-bridging understanding enables predictive simulations and a rational choice of growth conditions, which, together with molecular design and synthesis, ultimately leads to optimized design of functional materials.


For more information see:

S. Bommel, N. Kleppmann, C. Weber, H. Spranger, P. Schäfer, J. Novák, S. V. Roth, F. Schreiber, S. H. L. Klapp, S. Kowarik


Nature Communications 5, 5388 (2014)

DOI:10.1038/ncomms6388

 

 

FIG. 1. The diffusion barrier ED, binding energy EB and Ehrlich–Schwoebel barrier EES determine island nucleation and interlayer transport in multilayer growth. Included are numerical values determined by fitting the experiment using KMC simulations.

 

 

 

FIG. 2. Diffuse X-ray scattering during C60 growth. The diffusely scattered intensity oscillates with the nucleation and coalescence of every layer and exhibits a characteristic peak-splitting ∆qǁ. The latter corresponds to the inverse average island distance, which changes with film thickness.

 


 

Cascade energy transfer versus charge separation in LOPP/ZnO hybrid structures for light-emitting applications

 

Usability of inorganic/organic semiconductor hybrid structures for light-emitting applications can be intrinsically limited by an unfavorable interfacial energy level alignment causing charge separation and nonradiative deactivation. Introducing cascaded energy transfer funneling away the excitation energy from the interface by transfer to a secondary acceptor molecule enables us to overcome this issue.

Competition between the FRET cascade and interfacial exciton dissociation is studied on a HIOS composed of a ZnO/ZnMgO QW covered by a layer  of a L4P-Sp3/L6P blend (Fig. 1).  The QW PL transients in the hybrid and reference part of the “cascade” HIOS provide practically the same transfer time and efficiency as for the non-cascaded version. That is, the first step of the cascade is not modified. The donor role of L4P-Sp3 in the cascade step is verified by  a shortening of its decay time. The PL decay time of L6P in the HIOS is nearly identical to that of the blend on an inert substrate. All these observations imply that the excitation energy is effectively funneled away from the interface.

Charge separation caused by an unfavorable interfacial energy level alignment can be surpassed by cascade FRET where the second step takes place within a blend of suitably adjusted organic components. The resulting cascade does not noticeably reduce the total FRET efficiency. At room temperature, about one third of the excitons primarily excited in the QW are passed to the ladder-type oligo(p-phenylenes) and emit light, whereas maximally one tenth would radiatively recombine in the sole QW. Hybrid FRET can thus effectively bypass non-radiative recombination.


For more information see:

F. Bianchi, S. Sadofev, R. Schlesinger, B. Kobin, S. Hecht, N. Koch, F. Henneberger and S. Blumstengel, Appl. Phys. Lett. 105, 233301 (2014)

DOI:10.1063/1.4903517

FIG. 1. Cascade FRET in a L6P:L4P-Sp3/ZnO/ZnMgO HIOS:

(a) PL spectrum at T=5K. (b) PL transients (T=5K) of the ZnO QW (red) in the reference (i) and in the hybrid part (ii) as well as of L6P (green). The solid lines are fits to the data. Inset: sample layout. (c) PL of the hybrid part (ii) of the cascade HIOS at 300 K.

 


 

Raman study of SP61 adsorbed on oxide surfaces

 

In order to exploit the potential of organic/inorganic semiconductor hybrid structures, control over the structural and electronic properties of the heterointerface is crucial. Interface formation in such hybrid structures is a very complex issue, not least due to dangling bonds, steps, and defects on the inorganic surface. Binding of molecules at such sites can cause modifications of their electronic structure or even their fragmentation resulting in ill-defined interfaces.

In a recent publication of project B9 (Stähler/Wolf) and B4 (Knorr/Rinke/Scheffler) the vibrational properties of SP6 on ZnO(000-1), ZnMgO(000-1), and Al2O3(11-20) substrates were investigated by non-resonant Raman spectroscopy.

Figure 1 shows the Raman spectra of 8 nm SP6 deposited onto the different substrates as well as the Raman response of the bare sapphire substrate itself. All SP6 spectra in Figure 1 are dominated by a band around 1300 cm-1 and a peak at 1600 cm-1. DFT calculations of isolated gas phase molecules allowed the assignment to CC-stretch and ring stretch modes, respectively. Remarkably, the substrate does not influence the vibronic response of the few monolayer thick SP6 film.

The substrate’s impact on the vibrational properties of the first SP6 monolayer is focused on in Figure 2. It shows the Raman spectra of 1.0 nm SP6 layer on ZnO (a), and on Al2O3(11–20) (b, blue). Subtraction of these spectra leads to a trace coinciding with the Raman response of ZnO(000-1) (c). If this this substrate-induced background is subtracted from the spectrum of 8.0 nm SP6/ZnO(000-1) and divided by 8, the resulting trace (b, orange) coincides almost perfectly with the spectrum of 1.0 nm SP6/ Al2O3(11–20). This shows that there is no influence of the substrate on the Raman response of SP6 confirming that the electronic structure of SP6 is not perturbed by interaction with the substrate surface.

For more information see:
J. Stähler, O. T. Hofmann, P. Rinke, S. Blumstengel, F. Henneberger, Y. Li, T. F. Heinz.

Chem. Phys. Lett. 587, 74 (2013)

DOI: 10.1016/j.cplett.2013.08.030

1 2,7-bis(biphenyl-4-yl)-20,70-di-tert-butyl-9,9‘-spirobifluorene

                                        Figure 1

 

                                        Figure 2

 

Fig. 1: Raman spectra of 8 nm SP6 on sapphire, ZnMgO(000-1), and ZnO(000-1). The bottom spectrum depicts the sapphire response for comparison. Inset: SP6 molecule as calculated by DFT.

 

Fig. 2: Comparison of 1.0 nm SP6 (mass equivalent) on ZnO(000-1) and sapphire showing that there is no substrate-induced difference in the response of the molecules. The band between 1050 and 1200 cm-1 results from phonons in the ZnO(000-1) film. The signal of 8.0 nm SP6 on ZnO(000-1) matches the 1.0 nm SP6/Al2O3 (11-20) trace after subtraction of the ZnO(000-1) signal and scaling by a factor of 8 (orange curve).

 


 

Space Charge Transfer in Hybrid Inorganic-Organic Systems

 

To describe charge transfer in hybrid inorganic/organic systems (HIOS), we have developed a first principles approach in project B4 that explicitly includes the global effects of doping (i.e. position of the Fermi level) and the formation of a space-charge layer (see Fig. 1). For the example of tetrafluoro-tetracyanoquinodimethane (F4TCNQ) on the ZnO(000-1) surface (see Fig. 2) we show that the adsorption energy and electron transfer depend strongly on the ZnO doping.

 

Figure 1 illustrates schematically how charge transfer proceeds. Initially, the empty acceptor state of F4TCNQ is situated below the Fermi level. Subsequently charge is transferred to the acceptor state and the resulting dipole lifts the state up to the Fermi level. The dipole has two contributions: a short-ranged part arising from charge rearrangement at the surface (see Fig. 2 b) and a long-ranged part from the build up of a space-charge region. The latter is inversely proportional to the bulk doping concentration. For low doping concentrations band bending alone can lift up the acceptor state to the Fermi level. This reduces the required electron transfer to nearly zero.

The associated work-function changes are large, in agreement with photoemission experiments on the same system (see previous research highlight).

 

For more information see:

Y. Xu, O. T. Hofmann, R. Schlesinger, S. Winkler, J. Frisch, J. Niederhausen, A. Vollmer, S. Blumstengel, F. Henneberger, N. Koch, P. Rinke, M. Scheffler.

Phys. Rev. Lett. 111, 226802 (2013)

                                Figure 1

 

                                  Figure 2

 

Fig. 1: Schematic illustration of the electron transfer to acceptor states at a surface or interface of an n-doped semiconductor (middle and right). The development of a space-charge region induces band-bending, which brings the acceptor state closer to the Fermi level (right) In an undoped intrinsic semiconductor (left) no such electron transfer can take place resulting in an empty acceptor state in the band gap.

Fig. 2: Top (a) and side view (b) of F4TCNQ adsorbed on ZnO(000-1) (2 x 1)-H (left) and the adsorption-induced electron density rearrangement for n-doped ZnO (right). Electrons flow from the yellow to blue areas upon adsorption. The electron accumulation region mimics the shape of the lowest unoccupied molecular orbital (LUMO) of the free F4TCNQ molecule.

 


 

Modifying the ZnO workfunction by adsorption of F4TCNQ

 

Hybrid inorganic/organic semiconductor (HIOS) heterojunctions have opened up new opportunities for (opto-) electronic devices due to their potential for combining the favorable properties of two distinct material classes. In project A8 we employ molecular acceptor interlayers to tune the work function of the substrate and thus change the energy level alignment (ELA) between the Fermi-level of the substrate and the energy levels of an organic semiconductor. This allows to adjust the ELA to favor e.g. energy or charge transfer across the interface.

We demonstrate, that the work function of ZnO can be controlled over wide ranges (up to 2.8 eV) by adsorbing the molecular electron acceptor F4TCNQ in the (sub-) monolayer regime. Although this is phenomenologically similar to what was observed for metal surfaces, the mechanism of the Φ increase differs markedly for the inorganic semiconductor.

ΔΦ relies on two complementary mechanisms due to electron transfer to the surface-adsorbed acceptor, i.e., band bending in the inorganic semiconductor and an interface dipole, yielding ΔΦBB and ΔΦID, respectively (see Fig. a). We find, that minute electron transfer is sufficient to induce significant ΔΦs (see Fig. b).

 

 

For more information see:
R. Schlesinger, Y. Xu, O. T. Hofmann, S. Winkler, J. Frisch, J. Niederhausen, A. Vollmer, S. Blumstengel, F. Henneberger, P. Rinke, M. Scheffler and N. Koch.

Physical Review B 87, 155311 (2013).

 

 

a) Evolution of the work function (Φ) as a function of F4TCNQ nominal thickness (ʘ) on the ZnO(000-1)-O and ZnO(0001)-Zn surfaces and relative binding energy shifts ΔE of the O1s and Zn3p core levels upon adsorption of F4TCNQ.

b) N1s core-level spectra of (i) 0.5-Å F4TCNQ on ZnO(0001), (ii) 8-Å F4TCNQ on ZnO(0001), (iii) 0.5-Å  F4TCNQ on ZnO(000-1), and (iv) 60-Å  F4TCNQ on Au. The low-binding energy component in spectrum (iv) is from F4TCNQ chemisorbed on Au with a net electron transfer of approximately 0.3-0.4 eV.

 


 

Tayloring Organic Building Blocks for HIOS

 

To create functional HIOS organic molecules with energy levels matching the inorganic semiconductor are needed. In project A3 such molecular building blocks are being custom-designed to match some basic requirements, such as strong and narrow optical transitions resonant to ZnO, high stability towards (photo)degradation, and vacuum-processability via evaporation. Ladder-type para-phenylenes are a promising class of compounds, which however typically carry solubilizing groups interfering with vacuum-based deposition techniques due to their high molecular weight.

In project A3 a new synthetic route to yield even-numbered ladder-type oligo-para-phenylenes (LOPPs) has been developed. Direct comparison with the structurally related non-bridged oligo-para-phenylenes as well as partially bridged oligofluorenes (for structures see Fig. a) shows that the newly synthesized LOPPs exhibit outstanding photophysical properties, such as sharp and intense optical transitions characterized by narrow absorption bands and very small Stokes-shifts as well as large extinction coefficients and high fluorescence quantum yields (for spectra see Fig. b).

Initial investigations in cooperation with project A5 demonstrate that these materials are applicable to organic molecular beam deposition techniques allowing the controlled generation of organic (ultra)thin films on solid substrate surfaces.

 

 

For more information see:

B. Kobin, L. Grubert, S. Blumstengel, F. Henneberger, S. Hecht,

J. Mater. Chem. 22, 4383 (2012).

 

a) Prepared organic components and variation of their important structural parameters.

b) Spectral properties of p-phenylenes in solution: Absorption (Abs), excitation (Exc), and fluorescence (Flu) spectra of 6P, 3F, and L6P in CH2Cl2 (top). Absorption spectra of 1F, L4P, and L6P in CH2Cl2 (bottom).

 


 

Electrostatic-Field Driven Alignment of Organic Oligomers on ZnO Surfaces

 

In project A5, the physisorption of organic oligomers on the strongly ionic ZnO(1010) surface has been studied in collaboration with Fabio della Sala (NNL, Lecce). In previous experiments (see S. Blumstengel et al., Phys. Chem. Chem. Phys. 12, 11642-11646 (2010)), it has been found that sexiphenyl adsorbs on this surface in a well-defined orientation where the long axis of the flat-lying molecules is aligned perpendicular to the c-direction of the ZnO crystal.

Using first-principles density-functional theory and non-empirical embedding methods, it could be demonstrated that this observation is representative of a generic scenario related to the strong dipolar electrostatic field created by the ZnO surface dimers. The electrostatic molecule-substrate coupling is characterized by a linear relation between the in-plane variation of the interaction energy and the molecular dipole moment induced in vertical direction. Long oligomers with a highly axial π-electron system are aligned along rows of positive electric field. The energies required for reorientation reach some 100 meV.

These findings define a new route towards the realization of highly ordered self-assembled arrays of oligomers/polymers on inorganic semiconductors as well as for the interfacial energy level adjustment.


For more information see:

Fabio della Sala, Sylke Blumstengel, Fritz Henneberger,

Phys. Rev. Lett. 107, 146401 (2011)

 

Electrostatic molecule-substrate interaction energy for biphenyle (2P), sexiphenyl (6P), pentacene (5A), and penta-phenylene-vinylene (5PV) on ZnO(1010).
a) Linear relation between the interaction energy and the molecular dipole moment µz induced in vertical direction. The points are obtained by sampling 1470 different molecular configurations.
b) Interaction energy versus rotation angle Θ defining the orientation of the long molecular axis relative to the direction perpendicular to the (1010) axis.
Inset: Orientation of 6P at the global minimum shown on a colormap of the vertical component Fz of the dipolar electrostatic field generated by the ZnO(1010) surface. Vertical distance from surface is 3.5 Å.

 


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