Selected recent publications

H. Mönig, S. Amirjalayer, A. Timmer, Z. Hu, L. Liu, O. Díaz Arado, M. Cnudde, C.A. Strassert, W. Ji, M. Rohlfing and H. Fuchs
Quantitative assessment of intermolecular interactions by atomic force microscopy imaging using copper oxide tips
Nature Nanotechnology 13, 371–375 (2018)

© Macmillan Publishers Limited, part of Springer Nature

Atomic force microscopy is an impressive tool with which to directly resolve the bonding structure of organic compounds. The methodology usually involves chemical passivation of the probe-tip termination by attaching single molecules or atoms such as CO or Xe. However, these probe particles are only weakly connected to the metallic apex, which results in considerable dynamic deflection. This probe particle deflection leads to pronounced image distortions, systematic overestimation of bond lengths, and in some cases even spurious bond-like contrast features, thus inhibiting reliable data interpretation. Recently, an alternative approach to tip passivation has been used in which slightly indenting a tip into oxidized copper substrates and subsequent contrast analysis allows for the verification of an oxygen-terminated Cu tip. Here we show that, due to the covalently bound configuration of the terminal oxygen atom, this copper oxide tip (CuOx tip) has a high structural stability, allowing not only a quantitative determination of individual bond lengths and access to bond order effects, but also reliable intermolecular bond characterization. In particular, by removing the previous limitations of flexible probe particles, we are able to provide conclusive experimental evidence for an unusual intermolecular N–Au–N three-centre bond. Furthermore, we demonstrate that CuOx tips allow the characterization of the strength and configuration of individual hydrogen bonds within a molecular assembly.

D. Ji, T. Li, Y. Zou, M. Chu, K. Zhou, J. Liu, G. Tian, Z. Zhang, X. Zhang, L. Li, D. Wu, H. Dong, Q. Miao, H. Fuchs, W. Hu
Copolymer dielectrics with balanced chain-packing density and surface polarity for high-performance flexible organic electronics
Nature Communications 9, 2339 (2018)

© Macmillan Publishers Limited, part of Springer Nature

The ever-increasing demand for flexible electronics calls for the development of low-voltage and high-mobility organic thin-film transistors (OTFTs) that can be integrated into emerging display and labeling technologies. Polymer dielectrics with comprehensive and balanced dielectric properties (i.e., a good balance between their insulating characteristics and compatibility with organic semiconductors) are considered particularly important for this end. Here, we introduce a simple but highly efficient strategy to realize this target by using a new type of copolymer as dielectrics. Benefiting from both high chain packing density guaranteeing dielectric properties and surface polarity optimizing molecular packing of organic semiconductors, this rationally designed copolymer dielectric endows flexible OTFTs with high mobility (5.6 cm2 V1 s−1), low operating voltage (3 V) and outstanding stability. Further, their applicability in integrated circuits is verified. The excellent device performance shows exciting prospects of this molecular-scale engineered copolymer for the realization of plastic high-performance integrated electronics.

L. Liu, H. Klaasen, A. Timmer, H.-Y. Gao , D. Barton, H. Mönig , J. Neugebauer, H. Fuchs, A. Studer
α-Diazo Ketones in On-Surface Chemistry
J. Am. Chem. Soc. 140, 6000–6005 (2018)

© American Chemical Society

Polymerization of a biphenyl bis α-diazo ketone on Cu(111) and Au(111) surfaces to provide furandiyl bridged poly-para-phenylenes is reported. Polymerization on Cu(111) occurs via initial N2 fragmentation leading to Cu-biscarbene complexes at room temperature as polymeric organometallic structure. At 135 °C, carbene coupling affords polymeric α,β-unsaturated 1,4-diketones, while analogous alkene formation on the Au(111) surface occurs at room temperature. Further temperature increase leads to deoxygenative cyclization of the 1,4-diketone moieties to provide alternating furandiyl biphenyl copolymers on Cu(111) (165 °C) and Au(111) (240 °C) surfaces. This work shows a new approach to generate Cu-biscarbene intermediates on surfaces, opening the pathway for the controlled generation of biphenyl copolymers.

A.Timmer, H. Moenig, M. Uphoff, O. Díaz Arado, S. Amirjalayer, H. Fuchs
Site-specific adsorption of aromatic molecules on a metal/ metal oxide phase boundary
Nano Letters Just Accepted Manuscript

Nano-structured surfaces are ideal templates to control the self-assembly of molecular structures towards well-defined functional materials. To understand the initial adsorption process, we have investigated the arrangement and configuration of aromatic hydrocarbon molecules on nano-structured substrates composed of an alternating arrangement of Cu(110) and oxygen-reconstructed stripes. Scanning tunneling microscopy reveals a preferential adsorption of molecules at oxide phase boundaries. Non-contact atomic force microscopy experiments provide a detailed insight into the preferred adsorption site. By combining sub-molecular resolution imaging with density functional theory calculations, the interaction of the molecule with the phase boundary was elucidated, excluding a classical hydrogen bonding. Instead, a complex balance of different interactions is revealed. Our results provide an atomistic picture for the driving forces of the adsorption process. This comprehensive understanding enables developing strategies for the bottom-up growth of functional molecular systems using nano-templates.

Q. Shen, H.Y. Gao, H. Fuchs
Frontiers of on-surface synthesis: From principles to applications
Nano Today 13, 77-96 (2017)
Abstract   Download full article

© wwu

On-surface synthesis is the bottom-up construction of covalent bonds between molecular building blocks, which has been greatly developed during the past decade. Dozens of reactions have been successfully realized and scrutinized on various surfaces with the help of surface science techniques combined with theoretical calculations. Functional nanoarchitectures such as one-dimensional nanowires, nanoribbons and two-dimensional nanonetworks have been constructed on surfaces and explored in several potential applications. In fact, the generation of multilevel nanostructures will play a key role in future soft nanoscience and technologies due to their emergent properties ranging far beyond those of the individual molecules building them up. In this review, we discuss selected examples of important processes in on-surface synthesis developed in recent years and summarize them under the following aspects:

  1. on-surface reactions in a category of different carbon types;
  2. techniques applied in on-surface synthesis;
  3. on-surface synthesized functional nanostructures; and
  4. potential applications of on-surface synthesized nano-materials.

The review concludes with a perspective of the future development of on-surface synthesis.

H.-Y. Gao, P. A. Held, S. Amirjalayer, L. Liu, A. Timmer, B. Schirmer, O. Díaz Arado, H. Mönig, C. Mück-Lichtenfeld, J. Neugebauer, A. Studer, H. Fuchs
Intermolecular On-Surface σ‑Bond Metathesis
J. Am. Chem. Soc. 139 (20), pp 7012–7019 (2017)

J. Am. Chem. Soc., 2017, 139 (20), pp 7012–7019
© American Chemical Society

Silylation and desilylation are important functional group manipulations in solution-phase organic chemistry that are heavily used to protect/deprotect different functionalities. Herein, we disclose the first examples of the σ-bond metathesis of silylated alkynes with aromatic carboxylic acids on the Ag(111) and Au(111) surfaces to give the corresponding terminal alkynes and silyl esters, which is supported by density functional theory calculations and further confirmed by X-ray photoelectron spectroscopy analysis. Such a protecting group strategy applied to on-surface chemistry allows self-assembly structures to be generated from molecules that are inherently unstable in solution and in the solid state. This is shown by the successful formation of self-assembled hexaethynylbenzene at Ag(111). Furthermore, it is also shown that on the Au(111) surface this σ-bond metathesis can be combined with Glaser coupling to fabricate covalent polymers via a cascade process.

H. Kong, S. Yang, H.-Y. Gao, A. Timmer, J. P. Hill, O. Díaz Arado, H. Mönig, X. Huang, Q. Tang, Q. Ji, W. Liu, H. Fuchs
Substrate Mediated C-C and C-H Coupling after Dehalogenation
J. Am. Chem. Soc. 139 (10), pp 3669–3675 (2017)

J. Am. Chem. Soc., 2017, 139 (10), pp 3669–3675
© American Chemical Society

Intermolecular C–C coupling after cleavage of C–X (mostly, X = Br or I) bonds has been extensively studied for facilitating the synthesis of polymeric nanostructures. However, the accidental appearance of C–H coupling at the terminal carbon atoms would limit the successive extension of covalent polymers. To our knowledge, the selective C–H coupling after dehalogenation has not so far been reported, which may illuminate another interesting field of chemical synthesis on surfaces besides in situ fabrication of polymers, i.e., synthesis of novel organic molecules. By combining STM imaging, XPS analysis, and DFT calculations, we have achieved predominant C–C coupling on Au(111) and more interestingly selective C–H coupling on Ag(111), which in turn leads to selective synthesis of polymeric chains or new organic molecules.

D. Ji, X. Xu, L. Jiang, S. Amirjalayer, L. Jiang, Y. Zhen, Y. Zou, Y. Yao, H. Dong, J. Yu, H. Fuchs, W. Hu
Surface polarity and self-structured nanogrooves collaborative oriented molecular packing for high crystallinity towards efficient charge transport
J. Am. Chem. Soc. 139 (7), pp 2734–2740 (2017)

J. Am. Chem. Soc., 2017, 139 (7), pp 2734–2740
© American Chemical Society

Efficient charge transport in organic semiconductors is essential for construction of high performance optoelectronic devices. Herein, for the first time, we demonstrate that poly(amic acid) (PAA), a facilely deposited and annealing-free dielectric layer, can tailor the growth of organic semiconductor films with large area and high crystallinity toward efficient charge transport and high mobility in their thin film transistors. Pentacene is used as a model system to demonstrate the concept with mobility up to 30.6 cm2 V–1 s–1, comparable to its high quality single crystal devices. The structure of PAA has corrugations with OH groups pointing out of the surface, and the presence of an amide bond further allows adjacent polymer strands to interact via hydrogen bonding, leading to a self-rippled surface perpendicular to the corrugation. On the other hand, the strong polar groups (−COOH/–CONH) of PAA could provide repulsive forces between PAA and pentacene, which results in the vertical orientation of pentacene on the dielectric surface. Indeed, in comparison with its imidized counterpart polyimide (PI), PAA dielectric significantly enhances the film crystallinity, drastically increases the domain size, and decreases the interface trap density, giving rise to superior device performance with high mobility. This concept can be extended to more organic semiconducting systems, e.g., 2,6-diphenylanthracene (DPA), tetracene, copper phthalocyanine (CuPc), and copper hexadecafluorophthalocyanine (F16CuPc), demonstrating the general applicability. The results show the importance of combining surface nanogrooves with the strong polarity in orienting the molecular arrangement for high crystallinity toward efficient charge transport in organic semiconductors.

S. Amirjalayer, A. Martinez-Cuezva, J. Berna, S. Woutersen, W. Buma
Photoinduced Pedalo-Type Motion in an Azodicarboxamide-Based Molecular Switch
Angew. Chem. Int. Ed. 57, 1792-1796 (2018)
Angew. Chem. 130, 1810-1814 (2018)

© Wiley-VCH Verlag

Well‐defined structural changes of molecular units that can be triggered by light are crucial for the development of photoactive functional materials. Herein, we report on a novel switch that has azodicarboxamide as its photo‐triggerable element. Time‐resolved UV‐pump/IR probe spectroscopy in combination with quantum‐chemical calculations shows that the azodicarboxamide functionality, in contrast to other azo‐based chromophores, does not undergo transcis photoisomerization. Instead, a photoinduced pedalo‐type motion occurs, which because of its volume‐conserving properties enables the design of functional molecular systems with controllable motion in a confined space.