Thomas A. Jung

Many interesting properties of porphyrins and phthalocyanines emerge around the interaction of their central metal atom with ligands. This plays a key role in bio-chemistry, in particular in the oxygen binding to the heme. Different porphyrin substituents have been used in the past to systematically modify the physico-chemical properties and the steric accessibility of such ligation. A whole category of substituted porphyrins, the picket-fence porphyrins [1, 2] had been synthesized, for example, after the 3D structure of hemoglobin had been identified. [3]

We report the realization of a two‐dimensional analog of the archetypical metal‐organic framework UiO‐66 as a Langmuir monolayer, constructed from preformed metal oxo clusters and amphiphilic terephthalic acid ligands at the air–water interface. Structural changes of the metal oxo clusters in aqueous solution were elucidated to clarify their role in interfacial self‐assembly at the water surface. Langmuir balance studies demonstrated that self‐assembly with specifically designed organic linkers proceeds through a coordination‐driven process. The resulting films were characterized by complementary techniques to determine their morphology, thickness, and composition. Based on these findings, a packing model of the monolayer was proposed. Finally, we extended this coordination‐driven self‐assembly strategy beyond aqueous systems to form, for the first time, monolayers at the air– N , N ‐dimethylformamide interface.

The interplay between spin and charge can give rise to remarkable quantum states of matter. A celebrated example is the Kondo effect, which occurs when localized magnetic impurities are screened by itinerant electrons. While significant advances have been made in probing the Kondo effect in systems consisting of magnetic impurities adsorbed on conventional bulk metals, its manifestation on unconventional metals with strong many-body interactions, particularly down to atomic-layer thickness, has hitherto remained unexplored. Here we investigate the charge and spin interaction between magnetic cobalt phthalocyanine (CoPc) molecules acting as spin-bearing Kondo impurities and a single, substrate supported layer of the charge-density-wave (CDW) metal H-NbSe₂. Remarkably, we present unambiguous Kondo signatures on certain adsorption sites. We identify four distinct configurations depending on the position of the Co²⁺ ion relative to the atomic lattice and the CDW superlattice of NbSe₂. We show precise control of the Kondo effect by reversible repositioning of the CoPc molecule between the different positions. Our scanning tunneling microscopy (STM) and spectroscopy (STS) measurements further reveal the symmetry breaking of the Kondo resonance, indicating a pronounced magnetic anisotropy in CoPc/NbSe₂. More importantly, this interaction can induce local magnetism into the nonmagnetic NbSe₂ layer, offering new possibilities for tailoring spin textures.

We report the realization of a two-dimensional analog of the archetypical metal–organic framework UiO-66 as a Langmuir monolayer, constructed from preformed metal oxo clusters and amphiphilic terephthalic acid ligands at the air–water interface. Structural changes of the metal oxo clusters in aqueous solution were elucidated to clarify their role in interfacial self-assembly at the water surface. Langmuir balance studies demonstrated that self-assembly with specifically designed organic linkers proceeds through a coordination-driven process. The resulting films were characterized by complementary techniques to determine their morphology, thickness, and composition. Based on these findings, a packing model of the monolayer was proposed. Finally, we extended this coordination-driven self-assembly strategy beyond aqueous systems to form, for the first time, monolayers at the air–N,N -dimethylformamide interface.

The emergence of complexity during self-assembly of simple molecular building blocks is an important aspect in the synthesis of nanoarchitectures from supramolecular functional units. In particular, two-dimensional nanostructures are important from the point-of-view of technological applications. Here, a remarkably complex on surface network is observed to form spontaneously from a single molecular module (porphyrin) having multiple site-specific conformations. The interplay of different physicochemical interactions at the surface contributes to the site-specific symmetry breaking of the porphyrin conformation, and was investigated at different substrates. Molecular conformational flexure, relocation in the corrugated surface potential, interactions with surface state electrons, and last but not least mutual intermolecular binding by hydrogen bonding at different elevations above the substrate are critical elements. We discuss the possibility of surfaces and interfaces causing quasidegeneracy of molecular configurations in supramolecular self-assembly, and the adsorbate-adsorbent interface as the driver for this system to behave counterintuitively to equilibrium thermodynamics.

We present a remarkably complex on-surface network that forms spontaneously from a single porphyrin module taking up multiple site-specific conformations. Based on this observation we analyze the different physico-chemical interactions at surfaces contributing to the symmetry breaking of the porphyrin in its conformational adaptation. We compare the binding motifs observed in 3D crystal structures obtained from X-ray diffraction with high-resolution scanning tunneling microscopy and spectroscopy data and chemical information from x-ray photoelectron spectroscopy. Density functional theory of the molecular modules on different substrates evidence that the complex interplay of different possible interactions on the flexible molecule is the cause of the complexity observed: Molecular conformational flexure facilitates the site-specific adaption of the molecule and its relocation to the most energetically favored site on the corrugated surface potential. Further, the molecule interacts with the surface state electrons in the underlying substrate and via intermolecular binding through hydrogen bonds linking molecular units at different elevations above the substrate surface. Notably also, the molecule is showing conformational diversity on different substrates, i.e., on the different surface facets of Au(111). In conclusion we discuss surfaces and interfaces as a possible cause for the emergence of quasi degeneracy of molecular configurations in supramolecular self-assemblies. The adsorbate-adsorbent interface is recognized as the driver for this system to behave counterintuitively to equilibrium thermodynamics. Fig. 1) Tetrakis(3,4,5-trimethoxyphenyl)porphyrin forms spontaneous assemblies of molecular modules with up to three conformers co-existing on the same substrate, Ag(111) in this case. Conformational identification by Scanning Tunneling Microscopy reveals the site-specific conformational adaptation mechanism which is recognized as the key element for conformational diversity and a pre-requisite for the formation of more complex supramolecular structures – like the Kagome network shown here. Figure 1

In single layer 2D materials the reduced dimensionality of the electronic system can be probed in its pure form. In the present work we use on-surface coordination networks interacting with 2D surface states of atomically clean coinage metal surfaces to fabricate periodic arrays of quantum wells and explore their properties. Angle resolved photoelectron spectroscopy shows that this system comprises a super-band formed by the periodic superposition of the 2D periodic quantum well states in analogy to the emergence of bands from the periodic arrangements of atoms [1-2]. The quantum wells in this periodic array are comprised by ~1nm sized pockets containing a localized electronic state below E_F that can be modified by the presence of adsorbates therein. By choosing Xe as the adsorbate, we can control the occupancy of the pores overall by precise dosing of the adsorbate, but also by scanning tunneling microscope repositioning of individual atoms [3]. Each quantum box exhibits 12 filling levels, which incrementally perturb the underlying quantum well state via Pauli repulsion and thereby disturbing the 2D periodicity of the system. We investigate the local and non-local electronic effects by gradual filling – which is tunable from 1 to 12 [4] which goes far beyond what has been achieved by e.g. substitutional doping of graphene and other 2D materials. Our approach demonstrates that self-assembled two-dimensional quantum box architectures may serve as quantum state analogues of breadboards commonly employed in electronic circuitry allowing for the hardware programming of electronic states and the exploration of functional electronic states. References: Lobo Checa et al., Science, 325, 300-303 (2009) M. Matena et al., Physical Review B, 90, 125408-125415 (2014) S. Nowakowska et al., Nature Communications, 6, 6071 (2015) S. Nowakowska et al., Small, 12, 3757–3763 (2016)

Most high-efficiency all-perovskite tandem solar cells use a "superstrate" configuration, integrating a wide-bandgap (WBG) top subcell and a narrow bandgap (NBG) bottom subcell. However, this structure suffers oxidative degradation due to easily air-exposable bottom NBG subcells. A "substrate" structure offers improved stability for tandems by encapsulating the NBG top subcell with the air-stable WBG bottom subcell. However, conventional WBG perovskite solar cells (PSCs) using water-based NiO_x interlayers hinder their fabrication on top of the NBG subcells in an inert atmosphere. To overcome this, we developed a nonaqueous NiO_x nanoparticle dispersion, enabling interlayer fabrication inside a glove box. The blade-coated NiO_x interlayer facilitated the formation of a densely packed 2PACz (2-(9H-carbazol-9-yl)ethyl]phosphonic acid) monolayer hole transporting layer (HTL). The energetically aligned 2PACz molecules reduced minority carrier recombination at the NiO_x/perovskite interface. As a result, the fully scalable 1.77 eV WBG PSCs employing a NiO_x/2PACz hybrid HTL delivered a champion power conversion efficiency of 17.4%.

Reactive adsorbates in on-surface reactions and in heterogeneous catalysis on different substrates are of emerging importance. This is for the crucial roles they take in reaction pathways and for their decisive influence on reaction kinetics. [1] The thermally activated 2D mobility [2] of reaction precursors and reactive adatoms as well as their spatial and temporal coincidence at reaction sites is of mechanistic importance for many interface-specific reactions. In our work, we use the well-studied metalation reaction of H₂-porphyrins to investigate the action of adatoms on metallic and passivated surface substrates. Porphyrin metalation has been established on atomically clean metallic substrates [3] as well as on metal-oxides [4] and metallic surfaces modified by oxygen [5]. We perform spectro-microscopy correlation experiments combining X-ray Photoelectron Spectroscopy, Ultraviolet Photoelectron Spectroscopy, Low Energy Electron Diffraction and Scanning Tunnelling Microscopy on Cl- and N terminated Cu(001). Thereby we find extended layers of adsorbate-induced superstructures that have decisive and contrasting impact on the reactivity of the surface, as well as on molecular self-assembly. The O and N termination of Cu facilitates the metalation reaction and self-assembled domains of CuTPP are formed at room temperature. Cl termination on the contrary, fully inhibits the metalation reaction and causes 2HTPP to assemble into small ‘magic’ clusters. References [1] Waiz et al., Nature 541, 68–71 (2017). 10.1038/nature20782 [2] Ahsan et al., Nanoscale, 2019, 11, 4895. 10.1021/acs.jpcc.3c03325 [3] Auwärter et al., ChemPhysChem 8 , 250 (2007). 10.1002/cphc.200600675 [4] Wang, C. et al Chem Commun 50, 8291 (2014). 10.1039/C4CC02919D [5] Nowakowski et al., Chem. Commun., 49, 2347 (2013). 10.1039/c3cc39134e [6] Nowakowski et al., ChemistrySelect 2016, 5, 891 –895. 10.1002/slct.201600215 Figure 1

Quantum spin liquids (QSLs) are in a quantum disordered state that is highly entangled and has fractional excitations. As a highly sought-after state of matter, QSLs were predicted to host spinon excitations and to arise in frustrated spin systems with large quantum fluctuations. Here we report on the experimental observation and theoretical modeling of QSL signatures in monolayer 1T-NbSe₂, which is a newly emerging two-dimensional material that exhibits both charge-density-wave (CDW) and correlated insulating behaviors. By using scanning tunneling microscopy and spectroscopy (STM/STS), we confirm the presence of spin fluctuations in monolayer 1T-NbSe₂ by observing the Kondo resonance as monolayer 1T-NbSe₂ interacts with metallic monolayer 1H-NbSe₂. Subsequent STM/STS imaging of monolayer 1T-NbSe₂ at the Hubbard band energy further reveals a long-wavelength charge modulation, in agreement with the spinon modulation expected for QSLs. By depositing manganese-phthalocyanine (MnPc) molecules with spin S = 3/2 onto monolayer 1T-NbSe₂, new STS resonance peaks emerge at the Hubbard band edges of monolayer 1T-NbSe₂. This observation is consistent with the spinon Kondo effect induced by a S = 3/2 magnetic impurity embedded in a QSL. Taken together, these experimental observations indicate that monolayer 1T-NbSe₂ is a new promising QSL material.