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

Research Highlights of the CRC 951

 

Demonstration of the key substrate-dependent charge transfer mechanisms between monolayer MoS2 and molecular dopants

 

Tuning the Fermi level (EF) in two-dimensional transition metal dichalcogenide (TMDC) semiconductors is crucial for optimizing their application in hybrid inorganic/organic (opto-)electronic devices. Doping by molecular electron acceptors and donors has been suggested as a promising method to achieve EF-adjustment. The authors of this CRC 951 research highlight demonstrate that the charge transfer (CT) mechanism between TMDC and molecular dopant depends critically on the electrical nature of the substrate as well as its electronic coupling with the TMDC. Using angle-resolved ultraviolet and X-ray photoelectron spectroscopy, they reveal three fundamentally different, substrate-dependent CT mechanisms between the molecular electron acceptor F6TCNNQ and a MoS2 monolayer. Depending on the insulating, semi-metallic, and metallic properties of the substrate, as well as the coupling strength between the 2D TMDC and the substrate, these are:


(1) Insulating substrate: Direct CT occurs between the 2D TMDC valence levels or gap states to the LUMO level of the molecular acceptor, resulting in p-type doping of the 2D TMDC.


(2) Weakly coupled (semi-) metallic substrate: Indirect CT occurs from the substrate to the dopant molecule, through the MoS2 monolayer.


(3) Strongly coupled metallic substrate: CT occurs from the 2D TMDC-metal hybridized frontier level to the LUMO level of the molecular acceptor.


Their results demonstrate that any substrate that acts as charge reservoir for dopant molecules can prohibit factual doping of a TMDC monolayer. On the other hand, the three different CT mechanisms can be exploited for the design of advanced heterostructures, exhibiting tailored electronic properties in (opto-)electronic devices based on two-dimensional semiconductors.

 

For more information see:

S. Park, T. Schultz, X. Xu, B. Wegner, A. Aljarb, A. Han, L.-J. Li, V. C. Tung, P. Amsalem, N. Koch

Communications Physics 2, 109 (2019)

 

DOI: 10.1038/s42005-019-0212-y

Research Highlight Park

 

 

FIGURE: Schematics of the mechanisms of charge transfer (CT) between MoS2/substrate and F6TCNNQ.
a Direct CT (MoS2/sapphire, 2D transition metal dichalcogenides (TMDC)/insulator),
b indirect CT (MoS2/highly oriented pyrolytic graphite (HOPG), 2D TMDC/weakly interacting conductor),
c orbital hybridization with CT (MoS2/Au, 2D TMDC/strongly interacting conductor).


 

Gap states induce soft Fermi level pinning upon charge transfer at ZnO/molecular acceptor interfaces

 

The understanding and control of the electronic structure at the organic/inorganic interface is of utmost importance for the fabrication of HIOS. The deposition of strong molecular electron acceptors onto ZnO induces a substantial work function (ϕ) increase due to electron transfer from the inorganic semiconductor to the molecules. The ϕ increase results from two mechanisms: (i) a change of the surface band bending within ZnO and (ii) an interface dipole between the inorganic surface and the negatively charged acceptors. The molecule adsorption induced upward band bending in ZnO is, however, found to be limited to a few 100 meV, while the ϕ increase is significantly larger (up to 2.8 eV). The authors of this CRC 951 research highlight elucidate the origin of limited upward band bending by revealing a notable gap state density-of-states (GDOS) using high-sensitivity photoemission spectroscopy. Upon acceptor-induced upward band bending, the GDOS with a wide energy distribution becomes increasingly unoccupied. This, in turn, makes the interface dipole dominant and limits the ZnO surface band bending changes due to a “soft” pinning at the Fermi level.

 

For more information see:

R. Schlesinger, F. Bussolotti, J. Yang, S. Sadofev, A. Vollmer, S. Blumstengel, S. Kera, N. Ueno, N. Koch

Phys. Rev. Materials 3, 074601 (2019)

 

DOI: 10.1103/PhysRevMaterials.3.074601

Research Highlight Schlesinger

 

 

FIGURE: Evolution of (a) the work function (φ) and (b) the acceptor adsorption-induced ZnO band bending changes upon F4TCNQ and HATCN adsorption on different faces of ZnO. The surface band bending change is determined from the core level shifts (ΔE) in the O1s and Zn3p spectra. The observed range of different core level shift sizes from different measurement series is indicated by error bars (for therespective highest molecular coverage Θ only). For all samples the work function shift (Δφ) by far surpasses the core level shift, implyingthe existence of a significant complementary interface dipole (ΔφID).


 

 

Unraveling the electronic properties of lead halide perovskites with surface photovoltage in photoemission studies

 

The tremendous success of metal-halide per-ovskites, especially in thefield of photovoltaics, has triggered asubstantial number of studies in understanding their optoelectronic properties. However, consensus regarding the electronic properties of these perovskites is lacking due to a huge scatter in the reported key parameters, such as workfunction (Φ) and valence band maximum (VBM) values. The authors of this CRC 951 research highlight demonstrate that the surface photovoltage (SPV) is a key phenomenon occurring at the perovskite surfaces that feature a non-negligible density of surface states, which is more the rule than an exception for most materials understudy. With ultraviolet photoelectron spectroscopy (UPS) and Kelvin probe. The authors of this CRC 951 research highlight elucidate that even minute UV photonfluxes are sufficient to induce SPV and shift the perovskite Φ and VBM by several 100 meV compared to dark. By combining UV and visible light, they establish flat band conditions at the surface of four important perovskites, and find that all are p-type in the bulk, despite a pronounced n-type surface character in the dark. The present findings highlight that SPV effects must be considered in all surface studies to fully understand perovskites’ photophysical properties

 

For more information see:

F. Zu, C. M. Wolff, M. Ralaiarisoa, P. Amsalem, D. Neher, N. Koch

ACS Appl. Mater. Interfaces 11, 21578 (2019)

 

DOI: 10.1021/acsami.9b05293

Research Highlight Zu

 

 

FIGURE: Summarized Φ and VBM values (the latter evaluated on linear and logarithmic intensity plots) for different excitation fluxes for (a) FAMA, (b) CsFAMA.


 

 

Direct observation of state-filling at hybrid tin oxide/organic interfaces▼

 

Electroluminescence (EL) spectra of hybrid charge transfer states at metal oxide/organic type-II heterojunctions exhibit bias-induced spectral shifts. The reasons for this phenomenon have been discussed controversially and arguments for either electric field-induced effects or the filling of trap states at the oxide surface have been put forward. The authors of this CRC 951 research highlight combine the results of EL and photovoltaic measurements to eliminate the unavoidable effect of the series resistance of inorganic and organic components on the total voltage drop across the hybrid device. For SnOx combined with the conjugated polymer MeLPPP [ladder type poly-(para-phenylene)], they find a one-to-one correspondence between the blueshift of the EL peak and the increase of the quasi-Fermi level splitting at the hybrid heterojunction, which they unambiguously assign to state filling. The data is resembled best by a model considering the combination of an exponential density of states with a doped semiconductor.

 

For more information see:

U. Hörmann, S. Zeiske, S. Park, T. Schultz, S. Kickhöfel, U. Scherf, S. Blumstengel, N. Koch, D. Neher

Appl. Phys. Lett. 114, 183301 (2019)

 

DOI: 10.1063/1.5082704

Research Highlight Hörmann

 

 

FIGURE: EL spectra of an SnOx/MeLPPP hybrid device, recorded at stepwise increased current densities [62.5–312.5 mA/cm2]. The blueshift of the EL peak with increasing current density is clearly visible.

 

 


 

Pulsed thermal deposition of binary and ternary transition metal dichalcogenide monolayers and heterostructures▼

 

 

The application of transition metal dichalcogenides (TMDCs) as inorganic component in hybrid inorganic/organic systems (HIOS) for opto-electronic, photonic, or valleytronic devices requires the growth of continuous TMDC monolayers, heterostructures, and alloys of different materials in a single process. The authors of this CRC 951 research highlight present a facile pulsed thermal deposition method which provides precise control over the number of layers and the composition of two-dimensional systems. The method is based on pulsed direct resistive heating and sublimation of the metal elements. The amount of the deposited material as well as the composition is precisely controllable by the pulse length and pulse sequence.

 

The versatility of the method is demonstrated on ternary monolayers of Mo1-xWxS2 and on heterostructures combining metallic TaS2 and semiconducting MoS2 layers. The fabricated ternary monolayers cover the entire composition range of x=0…1 without phase separation. The TMDCs grow in a layer-by-layer fashion resulting in homogeneous films. Bandgap engineering and control over the spin–orbit coupling strength are demonstrated by absorption and photoluminescence spectroscopy. Vertical heterostructures are grown without intermixing. The formation of clean and atomically abrupt interfaces is evidenced by high-resolution transmission electron microscopy. Since both the metal components and the chalcogen are thermally evaporated, complex alloys and heterostructures can thus be prepared in future HIOS.

 

For more information see:

N. Mutz, T. Meisel, H. Kirmse, S. Park, E. List-Kratochvil, N. Koch, C. T. Koch, S. Blumstengel, S. Sadofev

Appl. Phys. Lett. 114, 162101 (2019)

 

DOI: 10.1063/1.5088758

Research Highlight Mutz

 

 

FIGURE: (a) PL spectra of Mo1-xWxS2 monolayers with varying compositions excited at 2.82 eV.
(b) Cross-sectional HRTEM image of the metallic TaS2 and semiconducting MoS2 heterostructure.

 

 


 

Surface termination dependent work function and electronic properties of Ti3C2Tx MXene▼

 

MXenes, an emerging family of 2D transition metalcarbides and nitrides, have shown promise in various opto-electronic applications. The physical properties of MXenes are known to be strongly dependent on their surface terminations. The authors of this CRC 951 research highlight investigated the electronic properties of Ti3C2Tx for different surface terminations, as achieved by different annealing temperatures, with the help of photoelectron spectroscopy, inverse photoelectron spectroscopy, and density functional theory calculations. They find that fluorine occupies solely the face-centered cubic adsorption site, whereas oxygen initially occupies at least two different adsorption sites, followed by a rearrangement after fluorine desorption at high annealing temperatures. The measured electronic structure of Ti3C2Tx showed strong dispersion of more than 1 eV, which they conclude to stem from Ti−O bonds by comparing it to calculated band structures. The authors further measured the work function of Ti3C2Tx as a function of annealing temperature and found that it is in the range of 3.9−4.8 eV, depending on the surface composition. A comparison of the experimental work function to detailed density functional theory calculations shows that the measured value is not simply an average of the work function values of uniformly terminated Ti3C2 surfaces but that the interplay between the different surface moieties and their local dipoles plays a crucial role.

 

For more information see:

T. Schultz, N. C. Frey, K. Hantanasirisakul, S. Park, S. J. May, V. B. Shenoy, Y. Gogotsi, N. Koch

Chem. Mater. (2019)

 

DOI: 10.1021/acs.chemmater.9b00414

Research Highlight Schultz

 

FIGURE: Work function values as a function of annealing temperature and work functions obtained from DFT for different surface terminations, calculated by using the real surface stoichiometry obtained from XPS (blue) and obtained by averaging the work functions of purely terminated Ti3C2O2, Ti3C2F2, Ti3C2OH2, and Ti3C2 surfaces, weighted by the experimentally determined stoichiometry (red). For exact composition at each temperature, check the supporting information of this work.

 

 


 

State-of-Matter-Dependent Charge-Transfer Interactions between Planar Molecules for Doping Applications▼

 

Controlling the electrical conductivity of organic semiconductors is a key asset for organic electronics, nowadays realized mostly by molecular dopants. Two doping mechanisms have been reported – charge-transfer complex (CTC) and ion pair (IPA) formation. However, their occurrence depending on molecular structure, energy levels, and structure of thin films remains elusive. Here, we study p-type doping of the planar organic semiconductor dibenzotetrathiafulvalene (DBTTF) in combination with the electron acceptors tetracyanonaphthoquinodimethane (TCNNQ) and hexafluorotetracyanonaphthoquinodimethane (F6TCNNQ) as planar dopants. The conductivity of DBTTF films increases by more than two orders of magnitude upon doping with F6TCNNQ and only slightly with TCNNQ. The highest conductivity is reached at about 10 mol % dopant concentration as a result of two counteracting effects: (1) increasing carrier concentration and (2) reduced carrier mobility due to the growing density of structural defects. We identified two different CTCs for DBTTF:TCNNQ blends and both types of charge-transfer interactions (CTC and IPA) in films of DBTTF doped with F6TCNNQ from absorption measurements. No signature of the charge-transfer interaction is found for DBTTF and TCNNQ in solution, whereas IPA formation only is observed for DBTTF and F6TCNNQ. Many-body perturbation theory calculations of the electronic and optical properties of one-dimensional stacks complement the experimental data and help in understanding the behavior of CTCs. The degree of charge transfer turns out to be higher for the DBTTF:F6TCNNQ complexes than for DBTTF:TCNNQ, as derived from the C≡N stretching mode softening in infrared absorption. We discuss the different fundamental semiconductor–dopant interactions in solution as compared to the solid state with the aid of the state-of-matter-dependent energy levels of the materials. The presence of both charge-transfer mechanisms in the material combinations investigated here gives us access to their doping efficiency, which is higher for IPA than for CTC formation. Avoiding the CTC formation by structural imperfections seems to be a way to increase the doping efficiency for crystalline materials. The determination of energy levels both in solution and in thin films is beneficial for understanding charge-transfer behavior.

 

For more information see:

P. Beyer, D. Pham, C. Peter, N. Koch, E. Meister, W. Brütting, L. Grubert, S. Hecht, D. Nabok, C. Cocchi, C. Draxl, A. Opitz

Chem. Mater. 31, 1237 (2019)

 

DOI: 10.1021/acs.chemmater.8b01447

Research Highlight Beyer

 

 

Research Highlight Futscher

 

FIGURE: Ball-and-stick visualization of the mixed stacks for DBTTF:TCNNQ (left column) and DBTTF:F6TCNNQ (right column) in top and side views as used in calculations. (Carbon: gray; nitrogen: blue; sulfur: yellow; fluorine: cyan; and hydrogen: white). The dimer included in the unit cell and replicated in the stacking direction by periodic boundary conditions is framed by the red box. Arrows indicate the intermolecular distances. The lattice parameters are 6.72 and 6.62 Å for DBTTF:TCNNQ and DBTTF:F6TCNNQ stacks, respectively.

 

 


 

Uncovering the (un-)occupied electronic structure of a buried hybrid interface▼

 

The energy level alignment at organic/inorganic (o/i) semiconductor interfaces is crucial for any light-emitting or -harvesting functionality. Essential is the access to both occupied and unoccupied electronic states directly at the interface, which is often deeply buried underneath thick organic films and challenging to characterize. We use several complementary experimental techniques to determine the electronic structure of p -quinquephenyl pyridine (5P-Py) adsorbed on ZnO(1 0   −1 0). The parent anchoring group, pyridine, significantly lowers the work function by up to 2.9 eV and causes an occupied in-gap state (IGS) directly below the Fermi level EF. Adsorption of upright-standing 5P-Py also leads to a strong work function reduction of up to 2.1 eV and to a similar IGS. The latter is then used as an initial state for the transient population of three normally unoccupied molecular levels through optical excitation and, due to its localization right at the o/i interface, provides interfacial sensitivity, even for thick 5P-Py films. We observe two final states above the vacuum level and one bound state at around 2 eV above EF, which we attribute to the 5P-Py LUMO. By the separate study of anchoring group and organic dye combined with the exploitation of the occupied IGS for selective interfacial photoexcitation, this work provides a new pathway for characterizing the electronic structure at buried o/i interfaces.

 

For more information see:

S. Vempati, J.-C. Deinert, L. Gierster, L. Bogner, C. Richter, N. Mutz, S. Blumstengel, A. Zykov, S. Kowarik, , Y. Garmshausen, J Hildebrandt, S. Hecht, J. Stähler

J. Phys.: Cond. Matt. 31, 094001 (2019)

 

DOI: 10.1088/1361-648X/aaf98a

Research Highlight Vempati

 

FIGURE: (a) Schematic of the energy levels of pyridine in the gas phase (right) and the naively expected alignment at the pyridine/ZnO(1 0   −1 0) interface (left) where the molecular levels shift rigidly due to vacuum level alignment and the build-up of an interfacial dipole. Inset: the pyridine molecule. (b) Photoelectron spectrum of the region around EF (blue). The combined system exhibits an IGS, which is highlighted after subtracting the secondary electron background of the 1PPE spectrum (red). Inset: work function reduction ΔΦ of up to  −2.9 eV from the bare ZnO(1 0   −1 0) to full monolayer (ML) coverage. Reprinted from [37], with the permission of AIP Publishing.

 

 


 

Electronic properties of hybrid organic/inorganic semiconductor pn-junctions ▼

 

Hybrid inorganic/organic semiconductor heterojunctions are candidates to expand the scope of purely organic or inorganic junctions in electronic and optoelectronic devices. Comprehensive understanding of bulk and interface doping on the junction's electronic properties is therefore desirable. In this work, we elucidate the energy level alignment and its mechanisms at a prototypical hybrid pn-junction comprising ZnO (n-type) and p-doped N,N'-di(1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine (α-NPD) as semiconductors, using photoelectron spectroscopy. The level alignment can be quantitatively described by the interplay of contact-induced band and energy level bending in the inorganic and organic component away from the interface, and an interface dipole due to the push-back effect. By adjusting the dopant concentration in α-NPD, the position of the frontier energy levels of ZnO can be varied by over 0.5 eV and that of α-NPD by over 1 eV. The tunability of this pn-junction's energy levels evidences the substantial potential of the hybrid approach for enhancing device functionality.

 

For more information see:

M. H. Futscher, T. Schultz, J. Frisch, M. Ralaiarisoa, E. Metwalli, M. V. Nardi, P. Müller-Buschbaum, N. Koch

Phys. Condens. Matter 31, 064002 (2018)

 

DOI: 10.1088/1361-648X/aaf310

Research Highlight Futscher

 

FIGURE: Schematic energy level diagrams at the hybrid interface between (a) α-NPD and ZnO(0 0 0 1) and (b) α-NPD p-doped with 2 vol% F6TCNNQ and ZnO(0 0 0 1). Gray shaded energy bands indicate the energy levels of ZnO(0 0 0 1) before molecular adsorption.

 

Research Highlight Futscher

 

FIGURE: Dependence of the work function (Φ), the HOMO onset (ΔHOMO) and core level position of Zn2p3/2, O1s, C1s, and N1s as a function of F6TCNNQ doping concentration (Ndop) in α-NPD films on ZnO(0 0 0 1). All molecular films have a nominal thickness of 5 nm (see SI figure S9 for corresponding PES spectra).

 

 


 

Electronic and Optical Excitations at the Pyridine/ZnO(10-10) Hybrid Interface ▼

 

By combining all‐electron density‐functional theory with many‐body perturbation theory, a prototypical inorganic/organic hybrid system, composed of pyridine molecules that are chemisorbed on the nonpolar ZnO( 10-10) surface is investigated. The G0W0 approximation is employed to describe its one‐particle excitations in terms of the quasiparticle band structure, and the Bethe–Salpeter equation is solved to obtain the absorption spectrum. The different character of the constituents leads to very diverse self‐energy corrections of individual Kohn–Sham states, and thus the G0W0 band structure is distinctively different from its DFT counterpart, that is, many‐body effects cannot be regarded as a rigid shift of the conduction bands. The nature of the optical excitations at the interface over a wide energy range is explored and it is shown that various kinds of electron‐hole pairs are formed, comprising hybrid excitons and (hybrid) charge‐transfer excitations. The absorption onset is characterized by a strongly bound bright ZnO‐dominated hybrid exciton. For the selected examples of either exciton type, the individual contributions from the valence and conduction bands are analyzed and the binding strength and extension of the electron‐hole wavefunctions are discussed.

 

For more information see:

O. Turkina, D. Nabok, A. Gulans, C. Cocchi, C. Draxl

Hybrid Interface Adv. Theory Simul., 1800108 (2018)

 

DOI: doi: 10.1002/adts.201800108

Research Highlight Turkina

 

FIGURE: Top left: side view of a pyridine monolayer on the nonpolar ZnO(10-10) surface along the [1-210] direction. The unit cell is indicated by the black rectangle, with vacuum extending in the [10-10] direction. Top right: side view along the [0001] direction. Bottom left: top view indicating the lattice vectors of the unit cell. Bottom right: surface Brillouin zone of Py/ZnO(10-10), with the k‐path used in the band structure marked in purple.

 

Research Highlight Turkina

 

FIGURE:The reciprocal‐space representation shows the contributions of individual bands to the exciton as colored circles along a k path. The size of the circle is proportional to the transition weight; the color code indicates the band character going from red (ZnO) over shades of purple (hybridized states) to blue (Py). The real‐space representation shows the probability density of finding the hole component of the e–h wavefunction given a fixed position of the electron (upper right panel) and vice versa (lower right panel). The electron (hole) probability distribution is depicted in orange (green) with the corresponding hole (electron) position marked by a black circle.

 

 


 

Microstructure and Elastic Constants of Transition Metal Dichalcogenide Monolayers from Friction and Shear Force Microscopy ▼

 

Optical and electrical properties of 2D transition metal dichalcogenides (TMDCs) grown by chemical vapor deposition (CVD) are strongly determined by their microstructure. Consequently, the visualization of spatial structural variations is of paramount importance for future applications. This study demonstrates how grain boundaries, crystal orientation, and strain fields can unambiguously be identified with combined lateral force microscopy and transverse shear microscopy (TSM) for CVD‐grown tungsten disulfide (WS2) monolayers, on length scales that are relevant for optoelectronic applications. Further, angle‐dependent TSM measurements enable the fourth‐order elastic constants of monolayer WS2 to be acquired experimentally. The results facilitate high‐throughput and nondestructive microstructure visualization of monolayer TMDCs and insights into their elastic properties, thus providing an accessible tool to support the development of advanced optoelectronic devices based on such 2D semiconductors.

 

For more information see:

X. Xu, T. Schultz, Z. Qin, N. Severin, B. Haas, S. Shen, J. N. Kirchhof, A. Opitz, C. T. > Koch, K. Bolotin, J. P. Rabe, G. Eda, N. Koch

Adv. Mater. 2018, 1803748 (2018)

 

DOI: 10.1002/adma.201803748

Research Highlight Xu

 

FIGURE: Single‐grain flake under strain field. a) LFM image of an as‐grown flake on SiO2/Si substrate showing diffuse contrast characteristics of strain field. b) Line profile of the marked line in (a), showing deflection signal of 5–10 mV difference in the location with diffuse contrast. c) TSM image of the same as‐grown flake in (a) showing no contrast. d) PL intensity map of a typical flake showing heterogeneity in photoluminescence as influenced by strain field (color‐scale in counts). e) PL spectra of the spots marked in (d), showing fluorescence quenching with red shift as an effect of strain. f) LFM image of the flake shown in (a) and (b) after transfer onto another SiO2/Si substrate. Scale bar: 10 µm

 

Research Highlight Xu

 

FIGURE: Multigrain flakes. a) Reflection microscopy and LFM images of the multigrain flakes forming the shape of “mountain,” “butterfly,” and “fishtail,” respectively. The domain angle α (angle between two neighboring edges) and GB angle β (angle between the GB and the neighboring domain edge) are schematically illustrated in blue and white, respectively. b) TSM image of the “butterfly”‐shape multigrain flake. Scale bar: 20 µm.

 

 


 

Subtle Fluorination of Conjugated Molecules Enables Stable Nanoscale Assemblies
on Metal Surfaces ▼

 

The authors obtained detailed insight into the intricate interplay between molecules and the substrate as well as the intricate balance of attractive and repulsive interactions that govern the self-assembly of molecules on a metal surface. By exploiting the interplay of noncovalent interactions, complex and stable arrangements can be formed with a minimal input of energy. In this contribution, They studied the structure of assemblies formed by three conjugated molecules with very similar chemical structure, that is, 6P and the analogues o-2F-6P and m-2F-6P with two fluorine atoms each, showed vastly different assembly phenomenology on Ag(111) surface. Fluorination at selected positions of conjugated molecules provides for sufficiently strong, yet nonrigid, H···F bonding capability that enables the formation of stable nanoscale molecular assemblies on a metal surface and steers the assemblies’ structure. For the partially fluorinated molecules, the H···F bond formation propensity turns out as the key factor enabling the formation of stable nanoscale molecular assemblies at very low absolute molecular coverage. The results underline that only with state-of-the-art density functional theory (DFT) modeling and deliberate discrimination of all involved interactions, possible to fully rationalize the experimental findings and provide new insight for advanced self-assembly strategies. The strongest repulsive component in all three systems is the Coulombic repulsion resulting from the minute electron transfer from the metal to each molecule. In fact, replacement of hydrogen atoms by fluorine atoms has been shown to result in substantial changes in the bulk structure of conjugated molecules and polymers, with considerable impact on optical and charge-transport properties. This approach should be generally applicable and will facilitate the construction and study of individual nanoscale molecular assemblies with structures that are not attainable in the high-coverage regime. Furthermore, the insight provided here helps in understanding how fluorine substitution in conjugated molecules and polymers contributes to thin film and bulk structures, which, in turn, will enable realizing organic electronic materials with superior optical and charge-transport properties for electronic and optoelectronic applications.

 

For more information see:

J. Niederhausen, Y. Zhang, F. Cheenicode Kabeer, Y. Garmshausen, B. M. Schmidt, Y. Li, K.-F. Braun, S. Hecht, A. Tkatchenko, N. Koch, S.-W. Hla

J. Phys. Chem. 122, 18902 (2018)

 

DOI: 10.1021/acs.jpcc.8b03398

Research Highlight Niederhausen

 

FIGURE: Fluorination at meta position (left) and ortho position (right).

 

Research Highlight Niederhausen

 

FIGURE: Oriented supramolecular chains at higher coverage. (a,b) Large-area STM images that show self-assembled rows of o-2F-6P and m-2F-6P molecules on Ag(111), respectively. The blue and black lines identify the two surface enantiomers [a: V = 1 V, I = 0.3 nA, 29.8 nm × 28.4 nm scan; b: V = 1 V, I = 0.3 nA, 47 nm × 45 nm scan]. (c) Illustration (for m-2F-6P) showing nonequal epitaxial registries for the molecule on the left (L) and right (R) of a given molecule.

 

 


 

All-optical control and super-resolution imaging of quantum emitters in layered materials ▼

 

Layered van der Waals materials are emerging as compelling two-dimensional platforms for nanophotonics, polaritonics, valleytronics and spintronics, and have the potential to transform applications in sensing, imaging and quantum information processing. Among these, hexagonal boron nitride (hBN) is known to host ultra-bright, room-temperature quantum emitters, whose nature is yet to be fully understood. The authors of this CRC 951 research highlight present a set of measurements that give unique insight into the photophysical properties and level structure of hBN quantum emitters. Specifically, the authors report the existence of a class of hBN quantum emitters with a fast-decaying intermediate and a long-lived metastable state accessible from the first excited electronic state. Furthermore, by means of a two-laser repumping scheme, an enhanced photoluminescence and emission intensity, which can be utilized to realize a new modality of far-field super-resolution imaging, is shown. The findings of the authors expand current understanding of quantum emitters in hBN and show new potential ways of harnessing their nonlinear optical properties in sub-diffraction nanoscopy.

 

For more information see:

M. Kianinia, C. Bradac, B. Sontheimer, F. Wang, T. T. Tran, M. Nguyen, S. Kim, Z.-Q. Xu, D. Jin, A. W. Schell, C. J. Lobo, I. Aharonovich, M. Toth

Nature Comm. 9, Article number: 874 (2018)

 

DOI: 10.1038/s41467-018-03290-0

Research Highlight Sontheimer

 

FIGURE: Fluorescence time trajectories of the emitter sampled into 100 ms bins, under excitation with 708 nm (red) or co-excitation with 708 and 532 nm lasers (blue, green). The corresponding histogram of the photon distribution at each excitation condition is shown on the right.

 

Research Highlight Sontheimer

 

FIGURE: Schematic of the setup used to perform GSD nanoscopy employing excitation lasers with doughnut-shaped intensity profiles.

 

 


 

Beating the thermodynamic limit with photo-activation of n-doping in organic semiconductors ▼

 

Chemical doping of organic semiconductors using molecular dopants plays a key role in the fabrication of efficient organic electronic devices. Although a variety of stable molecular p-dopants have been developed and successfully deployed in devices, air-stable molecular n-dopants suitable for materials with low electron affinity (EA) are still elusive. Two steps are important to improve the conductivity. Coupling n-dopants to a dimer and activation by a short irradiation with light are the key processes. The authors demonstrate that photo-activation of a cleavable air-stable dimeric dopant can result in kinetically stable and efficient n-doping of host semiconductors, whose reduction potentials are beyond the thermodynamic reach of the dimer’s effective reducing strength. Electron-transport layers doped in this manner are used to fabricate high-efficiency organic light-emitting diodes. The stable dimer itself cannot release a negative charge. The photo-activation of n-doping in materials of (EA) with air-stable organometallic dimers is an exceedingly important step forward for organic electronic devices, allowing circumvention of highly reactive and diffusive electropositive elements such as alkali metals, which have been routinely used to n-dope low-EA electron-transport materials used in OLEDs. UV activation leads to a situation whereby electrons are transferred to lower EA unoccupied levels, resulting in a larger EF shift and an increase of the conductivity by several orders of magnitude. This will improve efficiency and ensure a longer lifetime of such components.

 

For more information see:

X. Lin, B. Wegner, K. M. Lee, M. A. Fusella, F. Zhang, K. Moudgil, B. P. Rand, S. Barlow, S. R. Marder, N. Koch, A. Kahn

Nature Mater. 16, 1209 (2017)

 

DOI: 10.1038/nmat5072

Research Highlight Lin

 

FIGURE: Chemical structures of the host material POPy2 (A) and the dimeric dopant [RuCp∗Mes]2 (D2) with associated cationic monomer.

 

Research Highlight Lin

 

FIGURE: Top: electron transfer from the dimer to the host excited state, followed by cleavage of the cation dimer. Bottom: dimer-to-host electron transfer through an intermolecular charge-transfer (CT) absorption, followed by cleavage of the cation dimer.

 

 


 

Spiro-Bridged Ladder-Type Oligo(para-phenylene)s: Fine Tuning Solid State Structure and Optival Properties ▼

 

In this recent CRC 951 research highlight the authors developed synthetic routes that allow to subsequently replace every pair of symmetry-equivalent alkyl groups in ladder-type quaterphenyl by a spiro-bifluorene group. With an increasing number of spiro groups, the optical gap for absorption and emission slightly decreases, which is disadvantageous with respect to resonant energy transfer with ZnO. Thus, a synthetic route to a para-linked ladder-type quaterphenyl carrying all bridging units on one side of the ribbon was developed, which results in an in-plane bending of the para-phenylene. The optical gap increased compared to the linear molecule, however, the absorption coefficient slightly decreased.

The authors analyzed the influence of different deposition techniques on the solid state structure by X-ray diffraction of single crystals obtained by crystallization from solution as well as sublimation. In the cases of L4P-sp2 and L4P-sp3, it could even be shown that sublimation and crystallization from solution result in different crystal structures, of which the ones from sublimation are obviously more relevant in view of the typically employed vacuum deposition and might be advantageous in terms of application in light-emitting devices.

An increasing number of spiro-bifluorene substituents was found to aid thin-film formation on oxide surfaces, such that the optical properties could be preserved in pure, nondiluted thin films.

Finally, promising spiro-L4P derivatives have been employed in energy-transfer devices, for which highly efficient energy transfer from an inorganic quantum well to the organic layer followed by efficient light emission could successfully be demonstrated.

 

For more information see:

B. Kobin, J. Schwarz, B. Braun-Cula, M. Eyer, A. Zykov, S. Kowarik, S. Blumstengel, S. Hecht

Adv. Funct. Mater. 2017, 1704077 (2017)

 

DOI: 10.1002/adfm.201704077

Research Highlight Kobin

 

FIGURE: Asymmetric units (left) and arrangement of molecules in the crystal (right) of different molecule types. Thermal ellipsoids drawn at 50% probability level, cell edges marked in a: red, b: blue and c: green. For more details please see the publication text.

 

Research Highlight Kobin

 

FIGURE: Absorption (solid lines) and normalized PL (dotted) spectra of final products 10-6 - 10-5 mol L-1 in CH2Cl2. For more details please see the publication text.

 

 


 

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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