Alexander Generalov - Academia.edu (original) (raw)

Papers by Alexander Generalov

The f-driven temperature scales at the surfaces of strongly correlated materials have increasingl... more The f-driven temperature scales at the surfaces of strongly correlated materials have increasingly come into the focus of research efforts. Here, we unveil the emergence of a two-dimensional Ce Kondo lattice, which couples ferromagnetically to the ordered Co lattice below the P-terminated surface of the antiferromagnet CeCo2P2. In its bulk, Ce is passive and behaves tetravalently. However, because of symmetry breaking and an effective magnetic field caused by an uncompensated ferromagnetic Co layer, the Ce 4f states become partially occupied and spin-polarized near the surface. The momentum-resolved photoemission measurements indicate a strong admixture of the Ce 4f states to the itinerant bands near the Fermi level including surface states that are split by exchange interaction with Co. The temperature-dependent measurements reveal strong changes of the 4f intensity at the Fermi level in accordance with the Kondo scenario. Our findings show how rich and diverse the f-driven properties can be at the surface of materials without f-physics in the bulk.

Journal of Physics: Conference Series, 2022

BiPd is a noncentrosymmetric superconductor with Dirac-like surface states on both (010) and ( 0 ... more BiPd is a noncentrosymmetric superconductor with Dirac-like surface states on both (010) and ( 0 1 ¯ 0 ) faces. The Dirac cone on (010) surface is intense and appears at 0.66 eV binding energy. These states have drawn much attention due to contradictory reports on dimensionality and the momentum of these Dirac fermions. We have studied the properties of these Dirac fermions using varied photon energies and different experimental conditions. The behavior of the Dirac cone is found to be two-dimensional. In addition, we found few more surface states appearing at higher binding energies compared to the Dirac cone.

Physical Review B, 2021

We present a combined experimental and theoretical study of the two-dimensional electron states a... more We present a combined experimental and theoretical study of the two-dimensional electron states at the iridium-silicide surface of the antiferromagnet GdIr 2 Si 2 above and below the Néel temperature. Using angleresolved photoemission spectroscopy (ARPES) we find a significant spin-orbit splitting of the surface states in the paramagnetic phase. By means of ab initio density-functional-theory (DFT) calculations we establish that the surface electron states that reside in the projected band gap around the M point exhibit very different spin structures which are governed by the conventional and the cubic Rashba effect. The latter is reflected in a triple spin winding, i.e., the surface electron spin reveals three complete rotations upon moving once around the constant energy contours. Below the Néel temperature, our ARPES measurements show an intricate photoemission intensity picture characteristic of a complex magnetic domain structure. The orientation of the domains, however, can be clarified from a comparative analysis of the ARPES data and their DFT modeling. To characterize a single magnetic domain picture, we resort to the calculations and scrutinize the interplay of the Rashba spin-orbit coupling field with the in-plane exchange field, provided by the ferromagnetically ordered 4 f moments of the near-surface Gd layer.

Advanced Electronic Materials, 2021

New temperature scales and remarkable differences from bulk properties have increasingly placed t... more New temperature scales and remarkable differences from bulk properties have increasingly placed the surfaces of strongly correlated f materials into the focus of research activities. Applying first‐principles calculations and angle‐resolved photoelectron spectroscopy measurements, a strong Rashba effect and spin‐split surface states at the CeIn surface of the heavy‐fermion superconductor CeIrIn5 are revealed. The unveiled 4f‐derived electron landscape is remarkably distinct for surface and bulk Ce implying the existence of novel temperature scales near the surface region in this material. These results show that ab initio calculations can reliably predict the unusual electronic and spin structure of surfaces of strongly correlated 4f systems where Rashba spin‐orbit‐coupling phenomena emerge. It is suggested that the structural blocks of such materials can be combined with magnetically active layers for engineering of novel nanostructural objects with appropriate substrates where the...

Physical Review Letters, 2020

Physics of the Solid State, 2018

Physical Review B, 2017

Using resonant angle-resolved photoemission spectroscopy and electron band-structure calculations... more Using resonant angle-resolved photoemission spectroscopy and electron band-structure calculations, we explore the electronic structure and properties of Sm atoms at the surface and in the bulk of the antiferromagnet SmRh 2 Si 2. We show that the Sm atoms reveal weak mixed-valent behavior both in the bulk and at the surface. Although trivalent 4f emission strongly dominates, a small divalent 4f signal near the Fermi energy can be clearly resolved for surface and bulk Sm atoms. This behavior is quite different to most other Sm-based materials which typically experience a surface valence transition to a divalent state of Sm atoms at the surface. This phenomenon is explained in analogy to the isostructural Ce compound, where strong 4f hybridization stabilizes mixed-valent ground state both in the bulk and at the surface, and which were described in the light of the single-impurity Anderson model. Implications for other RERh 2 Si 2 (RE = rare-earth elements) compounds are discussed.

Carbon, 2017

The thermal stability and the structural evolution of graphene grown on Fe(110) has been studied ... more The thermal stability and the structural evolution of graphene grown on Fe(110) has been studied upon annealing in ultra-high vacuum conditions (UHV, P ≤ 10−9 mbar) and in the presence of gaseous ethylene at a pressure of ∼10−6 mbar by grazing incidence X-ray diffraction. It was observed that upon annealing at temperatures below 630 °C, graphene on Fe is thermally stable. Exposure to ethylene at these temperatures promotes the formation of graphene while inhibiting its deterioration. Annealing graphene/Fe(110) at temperatures above 630 °C results in a fast degradation of graphene followed by carburization of the sample, that is the irreversible formation of various iron carbides, with the most common phases being Fe3C (cementite) and Fe7C3 (Eckstrom-Adcock carbide). Annealing of the carburized sample does not result in the formation of a detectable graphitic structure.

The Journal of Physical Chemistry C, 2013

Graphite fluorides with different structural types (C y F) n (y = 2.5, 2, and 1) and room tempera... more Graphite fluorides with different structural types (C y F) n (y = 2.5, 2, and 1) and room temperature graphite fluorides were studied by solid state NMR and NEXAFS. Data extracted from those two techniques are complementary, providing information about the C−F bonding and the hybridization character of the carbon atom valence states. The comparison of data obtained by different methods such as NMR, Raman, and X-ray absorption leads to similar conclusions regarding the chemical bonding in fluorographites. Several major configurations of fluorinated graphites are discussed, that is, planar sheets with mainly sp 2 hybridization in room temperature graphite fluorides and corrugated sheets with sp 3 hybridization in covalent high temperature graphite fluoride. Different references such as highly oriented pyrolytic graphite (HOPG), graphitized carbon nanodiscs (graph-CNDs) and nanodiamonds (NDs) have also been investigated for comparison.

Nature Communications, 2014

Carrying a large, pure spin magnetic moment of 7 mB per atom in the half-filled 4f shell, divalen... more Carrying a large, pure spin magnetic moment of 7 mB per atom in the half-filled 4f shell, divalent europium is an outstanding element for assembling novel magnetic devices in which a two-dimensional electron gas may be polarized due to exchange interaction with an underlying magnetically-active Eu layer. Here we show that the Si-Rh-Si surface trilayer of the antiferromagnet EuRh 2 Si 2 bears a surface state, which exhibits an unexpected and large spin splitting controllable by temperature. The splitting sets in below B32.5 K, well above the ordering temperature of the Eu 4f moments (B24.5 K) in the bulk, indicating a larger ordering temperature in the topmost Eu layers. The driving force for the itinerant ferromagnetism at the surface is the aforementioned exchange interaction. Such a splitting may also be induced into states of functional surface layers deposited onto the surface of EuRh 2 Si 2 or similarly ordered magnetic materials with metallic or semiconducting properties.

Carbon, 2012

CuX@SWCNT (X = Cl, Br, I) nanostructures were prepared by capillary filling of 1.4-1.6 nm single-... more CuX@SWCNT (X = Cl, Br, I) nanostructures were prepared by capillary filling of 1.4-1.6 nm single-walled carbon nanotubes (SWCNT) with copper halides. The structure of CuX@SWCNT (X = Cl, Br, I) represents a distorted two-layer hcp of halogen atoms arranged along the SWCNT. The EXAFS and the high angle angular dark field (HAADF) HRTEM data indicate that Cu is partially coordinated by C. According to the optical absorption, valence band photoemission spectroscopy and work function measurements, a Fermi level (FL) downshift as compared with the initial value for the nanotubes and a corresponding charge transfer from the nanotubes to the 1D crystals is observed for CuX@SWCNT nanostructures. The FL shift increases in the sequence CuI < CuBr < CuCl due to an increase of the electron affinity for the halogen atoms. The XPS data confirm the acceptor effect of copper halides and indicate that metallic and semiconducting nanotubes behave differently. Raman spectroscopy performed under electrochemical charging allowed estimation of the value of charge transfer between the nanotube walls and the intercalated 1D crystal. The X-ray absorption and emission spectra for carbon and copper thresholds revealed a new energy level composed of the carbon 2p z and copper 3d-orbitals. This indicates the Cu-C bonding, which in line with the structural HAADF HRTEM and EXAFS data.

Applied Surface Science, 2013

Abstract The atomic and electronic structure of the graphene/Al/Ni(1 1 1) system is studied via c... more Abstract The atomic and electronic structure of the graphene/Al/Ni(1 1 1) system is studied via combination of LEED and spectroscopic methods (X-ray absorption, core-level and valence-band photoelectron spectroscopy), respectively. These data demonstrating the (2 × 2) overstructure and the decoupling of the graphene layer from the substrate (with respect to graphene/Ni(1 1 1)) are in very good agreement with DFT calculations supporting nearly free-standing state of graphene in this system (with small n-doping). The perspectives of application of Al-based intercalation-like systems are discussed.

Applied Physics Letters, 2013

In this letter, we report an easy and tenable way to tune the type of charge carriers in graphene... more In this letter, we report an easy and tenable way to tune the type of charge carriers in graphene, using a buried layer of AlBr3 and its derivatives on the graphene/Ir(111) interface. Upon the deposition of AlBr3 on graphene/Ir(111) and subsequent temperature-assisted intercalation of graphene/Ir(111) with atomic Br and AlBr3, pronounced hole doping of graphene is observed. The evolution of the graphene/Br-AlBr3/Ir(111) system at different stages of intercalation has been investigated by means of microbeam low-energy electron microscopy/electron diffraction, core-level photoelectron spectroscopy, and angle-resolved photoelectron spectroscopy.

Scientific Reports, 2018

We investigate the bottom-up growth of N = 7 armchair graphene nanoribbons (7-AGNRs) from the 10,... more We investigate the bottom-up growth of N = 7 armchair graphene nanoribbons (7-AGNRs) from the 10,10′-dibromo-9,9′-bianthracene (DBBA) molecules on Ag(111) with the focus on the role of the organometallic (OM) intermediates. It is demonstrated that DBBA molecules on Ag(111) are partially debrominated at room temperature and lose all bromine atoms at elevated temperatures. Similar to DBBA on Cu(111), debrominated molecules form OM chains on Ag(111). Nevertheless, in contrast with the Cu(111) substrate, formation of polyanthracene chains from OM intermediates via an Ullmann-type reaction is feasible on Ag(111). Cleavage of C–Ag bonds occurs before the thermal threshold for the surface-catalyzed activation of C–H bonds on Ag(111) is reached, while on Cu(111) activation of C–H bonds occurs in parallel with the cleavage of the stronger C–Cu bonds. Consequently, while OM intermediates obstruct the Ullmann reaction between DBBA molecules on the Cu(111) substrate, they are required for the f...

Journal of Materials Chemistry C

Charge transfer complex (CPX) formation at a donor–acceptor interface reduces the amount of Fermi... more Charge transfer complex (CPX) formation at a donor–acceptor interface reduces the amount of Fermi-level pinning induced interfacial charge transfer.

npj Quantum Materials

Ultra-violet angle-resolved photoemission spectroscopy (UV-ARPES) was used to explore the tempera... more Ultra-violet angle-resolved photoemission spectroscopy (UV-ARPES) was used to explore the temperature dependence of the Ce-4f spectral responses for surface and bulk in the antiferromagnetic Kondo lattice CeRh2Si2. Spectra were taken from Ce- and Si-terminated surfaces in a wide temperature range, and reveal characteristic 4f patterns for weakly (surface) and strongly (bulk) hybridized Ce, respectively. The temperature dependence of the Fermi level peak differs strongly for both cases implying that the effective Kondo temperature at the surface and bulk can be rather distinct. The greatly reduced crystal–electric-field (CEF) splitting at the surface gives reason to believe that the surface may exhibit a larger effective Kondo temperature because of a higher local-moment effective degeneracy. Further, the hybridization processes could strongly affect the 4f peak intensity at the Fermi level. We derived the k-resolved dispersion of the Kondo peak which is also found to be distinct due...

npj Quantum Materials

The development of materials that are non-magnetic in the bulk but exhibit two-dimensional (2D) m... more The development of materials that are non-magnetic in the bulk but exhibit two-dimensional (2D) magnetism at the surface is at the core of spintronics applications. Here, we present the valence-fluctuating material EuIr 2 Si 2 , where in contrast to its nonmagnetic bulk, the Si-terminated surface reveals controllable 2D ferromagnetism. Close to the surface the Eu ions prefer a magnetic divalent configuration and their large 4f moments order below 48 K. The emerging exchange interaction modifies the spin polarization of the 2D surface electrons originally induced by the strong Rashba effect. The temperature-dependent mixed valence of the bulk allows to tune the energy and momentum size of the projected band gaps to which the 2D electrons are confined. This gives an additional degree of freedom to handle spin-polarized electrons at the surface. Our findings disclose valence-fluctuating rare-earth based materials as a very promising basis for the development of systems with controllable 2D magnetic properties which is of interest both for fundamental science and applications.

The Journal of Physical Chemistry C

On-surface synthesis has emerged in the last decade as a method to create graphene nanoribbons (G... more On-surface synthesis has emerged in the last decade as a method to create graphene nanoribbons (GNRs) with atomic precision. The underlying premise of this bottomup strategy is that precursor molecules undergo a well-defined sequence of inter-and intramolecular reactions, leading to the formation of a single product. As such, the structure of the GNR is encoded in the precursors. However, recent examples have shown that not only the molecule, but also the coinage metal surface on which the reaction takes place, plays a decisive role in dictating the nanoribbon structure. In this work, we use scanning probe microscopy and X-ray photoelectron spectroscopy to investigate the behavior of 10,10′-dichloro-9,9′-bianthryl (DCBA) on Ag(111). Our study shows that Ag(111) can induce the formation of both seven-atom wide armchair GNRs (7-acGNRs) and 3,1-chiral GNRs (3,1-cGNRs), demonstrating that a single molecule on a single surface can react to different nanoribbon products. We additionally show that coadsorbed dibromoperylene can promote surface-assisted dehydrogenative coupling in DCBA, leading to the exclusive formation of 3,1-cGNRs.

The f-driven temperature scales at the surfaces of strongly correlated materials have increasingl... more The f-driven temperature scales at the surfaces of strongly correlated materials have increasingly come into the focus of research efforts. Here, we unveil the emergence of a two-dimensional Ce Kondo lattice, which couples ferromagnetically to the ordered Co lattice below the P-terminated surface of the antiferromagnet CeCo2P2. In its bulk, Ce is passive and behaves tetravalently. However, because of symmetry breaking and an effective magnetic field caused by an uncompensated ferromagnetic Co layer, the Ce 4f states become partially occupied and spin-polarized near the surface. The momentum-resolved photoemission measurements indicate a strong admixture of the Ce 4f states to the itinerant bands near the Fermi level including surface states that are split by exchange interaction with Co. The temperature-dependent measurements reveal strong changes of the 4f intensity at the Fermi level in accordance with the Kondo scenario. Our findings show how rich and diverse the f-driven properties can be at the surface of materials without f-physics in the bulk.

Journal of Physics: Conference Series, 2022

BiPd is a noncentrosymmetric superconductor with Dirac-like surface states on both (010) and ( 0 ... more BiPd is a noncentrosymmetric superconductor with Dirac-like surface states on both (010) and ( 0 1 ¯ 0 ) faces. The Dirac cone on (010) surface is intense and appears at 0.66 eV binding energy. These states have drawn much attention due to contradictory reports on dimensionality and the momentum of these Dirac fermions. We have studied the properties of these Dirac fermions using varied photon energies and different experimental conditions. The behavior of the Dirac cone is found to be two-dimensional. In addition, we found few more surface states appearing at higher binding energies compared to the Dirac cone.

Physical Review B, 2021

We present a combined experimental and theoretical study of the two-dimensional electron states a... more We present a combined experimental and theoretical study of the two-dimensional electron states at the iridium-silicide surface of the antiferromagnet GdIr 2 Si 2 above and below the Néel temperature. Using angleresolved photoemission spectroscopy (ARPES) we find a significant spin-orbit splitting of the surface states in the paramagnetic phase. By means of ab initio density-functional-theory (DFT) calculations we establish that the surface electron states that reside in the projected band gap around the M point exhibit very different spin structures which are governed by the conventional and the cubic Rashba effect. The latter is reflected in a triple spin winding, i.e., the surface electron spin reveals three complete rotations upon moving once around the constant energy contours. Below the Néel temperature, our ARPES measurements show an intricate photoemission intensity picture characteristic of a complex magnetic domain structure. The orientation of the domains, however, can be clarified from a comparative analysis of the ARPES data and their DFT modeling. To characterize a single magnetic domain picture, we resort to the calculations and scrutinize the interplay of the Rashba spin-orbit coupling field with the in-plane exchange field, provided by the ferromagnetically ordered 4 f moments of the near-surface Gd layer.

Advanced Electronic Materials, 2021

New temperature scales and remarkable differences from bulk properties have increasingly placed t... more New temperature scales and remarkable differences from bulk properties have increasingly placed the surfaces of strongly correlated f materials into the focus of research activities. Applying first‐principles calculations and angle‐resolved photoelectron spectroscopy measurements, a strong Rashba effect and spin‐split surface states at the CeIn surface of the heavy‐fermion superconductor CeIrIn5 are revealed. The unveiled 4f‐derived electron landscape is remarkably distinct for surface and bulk Ce implying the existence of novel temperature scales near the surface region in this material. These results show that ab initio calculations can reliably predict the unusual electronic and spin structure of surfaces of strongly correlated 4f systems where Rashba spin‐orbit‐coupling phenomena emerge. It is suggested that the structural blocks of such materials can be combined with magnetically active layers for engineering of novel nanostructural objects with appropriate substrates where the...

Physical Review Letters, 2020

Physics of the Solid State, 2018

Physical Review B, 2017

Using resonant angle-resolved photoemission spectroscopy and electron band-structure calculations... more Using resonant angle-resolved photoemission spectroscopy and electron band-structure calculations, we explore the electronic structure and properties of Sm atoms at the surface and in the bulk of the antiferromagnet SmRh 2 Si 2. We show that the Sm atoms reveal weak mixed-valent behavior both in the bulk and at the surface. Although trivalent 4f emission strongly dominates, a small divalent 4f signal near the Fermi energy can be clearly resolved for surface and bulk Sm atoms. This behavior is quite different to most other Sm-based materials which typically experience a surface valence transition to a divalent state of Sm atoms at the surface. This phenomenon is explained in analogy to the isostructural Ce compound, where strong 4f hybridization stabilizes mixed-valent ground state both in the bulk and at the surface, and which were described in the light of the single-impurity Anderson model. Implications for other RERh 2 Si 2 (RE = rare-earth elements) compounds are discussed.

Carbon, 2017

The thermal stability and the structural evolution of graphene grown on Fe(110) has been studied ... more The thermal stability and the structural evolution of graphene grown on Fe(110) has been studied upon annealing in ultra-high vacuum conditions (UHV, P ≤ 10−9 mbar) and in the presence of gaseous ethylene at a pressure of ∼10−6 mbar by grazing incidence X-ray diffraction. It was observed that upon annealing at temperatures below 630 °C, graphene on Fe is thermally stable. Exposure to ethylene at these temperatures promotes the formation of graphene while inhibiting its deterioration. Annealing graphene/Fe(110) at temperatures above 630 °C results in a fast degradation of graphene followed by carburization of the sample, that is the irreversible formation of various iron carbides, with the most common phases being Fe3C (cementite) and Fe7C3 (Eckstrom-Adcock carbide). Annealing of the carburized sample does not result in the formation of a detectable graphitic structure.

The Journal of Physical Chemistry C, 2013

Graphite fluorides with different structural types (C y F) n (y = 2.5, 2, and 1) and room tempera... more Graphite fluorides with different structural types (C y F) n (y = 2.5, 2, and 1) and room temperature graphite fluorides were studied by solid state NMR and NEXAFS. Data extracted from those two techniques are complementary, providing information about the C−F bonding and the hybridization character of the carbon atom valence states. The comparison of data obtained by different methods such as NMR, Raman, and X-ray absorption leads to similar conclusions regarding the chemical bonding in fluorographites. Several major configurations of fluorinated graphites are discussed, that is, planar sheets with mainly sp 2 hybridization in room temperature graphite fluorides and corrugated sheets with sp 3 hybridization in covalent high temperature graphite fluoride. Different references such as highly oriented pyrolytic graphite (HOPG), graphitized carbon nanodiscs (graph-CNDs) and nanodiamonds (NDs) have also been investigated for comparison.

Nature Communications, 2014

Carrying a large, pure spin magnetic moment of 7 mB per atom in the half-filled 4f shell, divalen... more Carrying a large, pure spin magnetic moment of 7 mB per atom in the half-filled 4f shell, divalent europium is an outstanding element for assembling novel magnetic devices in which a two-dimensional electron gas may be polarized due to exchange interaction with an underlying magnetically-active Eu layer. Here we show that the Si-Rh-Si surface trilayer of the antiferromagnet EuRh 2 Si 2 bears a surface state, which exhibits an unexpected and large spin splitting controllable by temperature. The splitting sets in below B32.5 K, well above the ordering temperature of the Eu 4f moments (B24.5 K) in the bulk, indicating a larger ordering temperature in the topmost Eu layers. The driving force for the itinerant ferromagnetism at the surface is the aforementioned exchange interaction. Such a splitting may also be induced into states of functional surface layers deposited onto the surface of EuRh 2 Si 2 or similarly ordered magnetic materials with metallic or semiconducting properties.

Carbon, 2012

CuX@SWCNT (X = Cl, Br, I) nanostructures were prepared by capillary filling of 1.4-1.6 nm single-... more CuX@SWCNT (X = Cl, Br, I) nanostructures were prepared by capillary filling of 1.4-1.6 nm single-walled carbon nanotubes (SWCNT) with copper halides. The structure of CuX@SWCNT (X = Cl, Br, I) represents a distorted two-layer hcp of halogen atoms arranged along the SWCNT. The EXAFS and the high angle angular dark field (HAADF) HRTEM data indicate that Cu is partially coordinated by C. According to the optical absorption, valence band photoemission spectroscopy and work function measurements, a Fermi level (FL) downshift as compared with the initial value for the nanotubes and a corresponding charge transfer from the nanotubes to the 1D crystals is observed for CuX@SWCNT nanostructures. The FL shift increases in the sequence CuI < CuBr < CuCl due to an increase of the electron affinity for the halogen atoms. The XPS data confirm the acceptor effect of copper halides and indicate that metallic and semiconducting nanotubes behave differently. Raman spectroscopy performed under electrochemical charging allowed estimation of the value of charge transfer between the nanotube walls and the intercalated 1D crystal. The X-ray absorption and emission spectra for carbon and copper thresholds revealed a new energy level composed of the carbon 2p z and copper 3d-orbitals. This indicates the Cu-C bonding, which in line with the structural HAADF HRTEM and EXAFS data.

Applied Surface Science, 2013

Abstract The atomic and electronic structure of the graphene/Al/Ni(1 1 1) system is studied via c... more Abstract The atomic and electronic structure of the graphene/Al/Ni(1 1 1) system is studied via combination of LEED and spectroscopic methods (X-ray absorption, core-level and valence-band photoelectron spectroscopy), respectively. These data demonstrating the (2 × 2) overstructure and the decoupling of the graphene layer from the substrate (with respect to graphene/Ni(1 1 1)) are in very good agreement with DFT calculations supporting nearly free-standing state of graphene in this system (with small n-doping). The perspectives of application of Al-based intercalation-like systems are discussed.

Applied Physics Letters, 2013

In this letter, we report an easy and tenable way to tune the type of charge carriers in graphene... more In this letter, we report an easy and tenable way to tune the type of charge carriers in graphene, using a buried layer of AlBr3 and its derivatives on the graphene/Ir(111) interface. Upon the deposition of AlBr3 on graphene/Ir(111) and subsequent temperature-assisted intercalation of graphene/Ir(111) with atomic Br and AlBr3, pronounced hole doping of graphene is observed. The evolution of the graphene/Br-AlBr3/Ir(111) system at different stages of intercalation has been investigated by means of microbeam low-energy electron microscopy/electron diffraction, core-level photoelectron spectroscopy, and angle-resolved photoelectron spectroscopy.

Scientific Reports, 2018

We investigate the bottom-up growth of N = 7 armchair graphene nanoribbons (7-AGNRs) from the 10,... more We investigate the bottom-up growth of N = 7 armchair graphene nanoribbons (7-AGNRs) from the 10,10′-dibromo-9,9′-bianthracene (DBBA) molecules on Ag(111) with the focus on the role of the organometallic (OM) intermediates. It is demonstrated that DBBA molecules on Ag(111) are partially debrominated at room temperature and lose all bromine atoms at elevated temperatures. Similar to DBBA on Cu(111), debrominated molecules form OM chains on Ag(111). Nevertheless, in contrast with the Cu(111) substrate, formation of polyanthracene chains from OM intermediates via an Ullmann-type reaction is feasible on Ag(111). Cleavage of C–Ag bonds occurs before the thermal threshold for the surface-catalyzed activation of C–H bonds on Ag(111) is reached, while on Cu(111) activation of C–H bonds occurs in parallel with the cleavage of the stronger C–Cu bonds. Consequently, while OM intermediates obstruct the Ullmann reaction between DBBA molecules on the Cu(111) substrate, they are required for the f...

Journal of Materials Chemistry C

Charge transfer complex (CPX) formation at a donor–acceptor interface reduces the amount of Fermi... more Charge transfer complex (CPX) formation at a donor–acceptor interface reduces the amount of Fermi-level pinning induced interfacial charge transfer.

npj Quantum Materials

Ultra-violet angle-resolved photoemission spectroscopy (UV-ARPES) was used to explore the tempera... more Ultra-violet angle-resolved photoemission spectroscopy (UV-ARPES) was used to explore the temperature dependence of the Ce-4f spectral responses for surface and bulk in the antiferromagnetic Kondo lattice CeRh2Si2. Spectra were taken from Ce- and Si-terminated surfaces in a wide temperature range, and reveal characteristic 4f patterns for weakly (surface) and strongly (bulk) hybridized Ce, respectively. The temperature dependence of the Fermi level peak differs strongly for both cases implying that the effective Kondo temperature at the surface and bulk can be rather distinct. The greatly reduced crystal–electric-field (CEF) splitting at the surface gives reason to believe that the surface may exhibit a larger effective Kondo temperature because of a higher local-moment effective degeneracy. Further, the hybridization processes could strongly affect the 4f peak intensity at the Fermi level. We derived the k-resolved dispersion of the Kondo peak which is also found to be distinct due...

npj Quantum Materials

The development of materials that are non-magnetic in the bulk but exhibit two-dimensional (2D) m... more The development of materials that are non-magnetic in the bulk but exhibit two-dimensional (2D) magnetism at the surface is at the core of spintronics applications. Here, we present the valence-fluctuating material EuIr 2 Si 2 , where in contrast to its nonmagnetic bulk, the Si-terminated surface reveals controllable 2D ferromagnetism. Close to the surface the Eu ions prefer a magnetic divalent configuration and their large 4f moments order below 48 K. The emerging exchange interaction modifies the spin polarization of the 2D surface electrons originally induced by the strong Rashba effect. The temperature-dependent mixed valence of the bulk allows to tune the energy and momentum size of the projected band gaps to which the 2D electrons are confined. This gives an additional degree of freedom to handle spin-polarized electrons at the surface. Our findings disclose valence-fluctuating rare-earth based materials as a very promising basis for the development of systems with controllable 2D magnetic properties which is of interest both for fundamental science and applications.

The Journal of Physical Chemistry C

On-surface synthesis has emerged in the last decade as a method to create graphene nanoribbons (G... more On-surface synthesis has emerged in the last decade as a method to create graphene nanoribbons (GNRs) with atomic precision. The underlying premise of this bottomup strategy is that precursor molecules undergo a well-defined sequence of inter-and intramolecular reactions, leading to the formation of a single product. As such, the structure of the GNR is encoded in the precursors. However, recent examples have shown that not only the molecule, but also the coinage metal surface on which the reaction takes place, plays a decisive role in dictating the nanoribbon structure. In this work, we use scanning probe microscopy and X-ray photoelectron spectroscopy to investigate the behavior of 10,10′-dichloro-9,9′-bianthryl (DCBA) on Ag(111). Our study shows that Ag(111) can induce the formation of both seven-atom wide armchair GNRs (7-acGNRs) and 3,1-chiral GNRs (3,1-cGNRs), demonstrating that a single molecule on a single surface can react to different nanoribbon products. We additionally show that coadsorbed dibromoperylene can promote surface-assisted dehydrogenative coupling in DCBA, leading to the exclusive formation of 3,1-cGNRs.