2001 - 2005

Deformation of a ''rigid'' molecule in self-assembled nanostructures
H. Tang, C. Coudret, T. Maroutian, R. Berndt, J. Phys. Chem. B 109, 24031 (2005)

Abstract:
A simple spirobifluorene molecule with pseudotetrahedral structure was investigated for its supposed conformational resilience upon adsorption. Through deposition at room temperature of this molecule on a Cu(111) surface and subsequent observation at 5 K with an ultrahigh vacuum scanning tunneling microscope, this ''rigidity'' upon physisorption is confirmed. However, an unexpected chemisorbed state was also found with the molecules arranged in trimers. The unique coexistence of physisorbed and chemisorbed states on the same substrate is thus demonstrated at the early stage of self-assembly.

[BibTeX]
@article{HTA_JPCB05,
  author = {Hao Tang and Ch. Coudret and T. Maroutian and R. Berndt},
  title = {Deformation of a ''rigid'' molecule in self-assembled nanostructures},
  journal = {J. Phys. Chem. B},
  volume = {109},
  pages = {24031--24034},	
  year = {2005},
  url = {http://dx.doi.org/10.1021/jp0539728},
  doi = {http://doi.org/10.1021/jp0539728}
}
 
 
Surface state electron dynamics of clean and adsorbate-covered metal surfaces studied with the scanning tunnelling microscope
J. Kröger, L. Limot, H. Jensen, R. Berndt, S. Crampin, E. Pehlke, Prog. Surf. Sci. 80, 26 (2005)

Abstract:
Using low-temperature scanning tunnelling microscopy and spectroscopy we have studied the dynamics of surface state electrons confined to vacancy islands on Ag(111) and localised at single magnetic and non-magnetic atoms adsorbed on Ag(111) and Cu(111). The line width of confined electronic states is found to be only weakly affected by the actual geometry of the vacancy island. A corresponding model shows that lossy boundary scattering is the dominant lifetime-limiting process in the vacancies studied.
We present a corrected analysis of the spatial decay of electron interference patterns, leading to a more consistent description of the Ag(111) surface state lifetime than was previously the case. A scanning tunnelling spectroscopy study of single adsorbed atoms is presented. By means of an extended Newns-Anderson model an observed resonance is interpreted in terms of an adsorbate-induced bound state split off from the bottom of the surface-state band. A preliminary line shape analysis of the bound state indicates that adsorbed atoms can modify the surface-state lifetime.

[BibTeX]
@article{JKR_PSS05,
  author = {J. Kröger and L. Limot and H. Jensen and R. Berndt and S. Crampin and E. Pehlke},
  title = {Surface state electron dynamics of clean and adsorbate-covered metal surfaces studied with the scanning tunnelling microscope},
  journal = {Prog. Surf. Sci.},
  volume = {80},
  pages = {26--48},	
  year = {2005},
  url = {http://dx.doi.org/10.1016/j.progsurf.2005.10.002},
  doi = {http://doi.org/10.1016/j.progsurf.2005.10.002}
}
 
 
Role of elastic scattering in electron dynamics at ordered alkali overlayers on Cu(111)
C. Corriol, V. Silkin, D. Sánchez-Portal, A. Arnau, E. Chulkov, P. Echenique, T. von Hofe, J. Kliewer, J. Kröger, R. Berndt, Phys. Rev. Lett. 95, 176802 (2005)

Abstract:
Scanning tunneling spectroscopy of p(2x2) Cs and Na ordered overlayers on Cu(111) reveals similar line widths of quasi two-dimensional quantum well states despite largely different binding energies. Detailed calculations show that 50% of the line widths are due to electron-phonon scattering while inelastic electron-electron scattering is negligible. A frequently ignored mechanism for ordered structures, i.e., enhanced elastic scattering due to Brillouin zone back folding, contributes the remaining width.

[BibTeX]
@article{CCO_PRL05,
  author = {C. Corriol and V.M. Silkin and D. Sánchez-Portal and A. Arnau and E.V. Chulkov and P.M. Echenique and T. von Hofe and J. Kliewer and J. Kröger and R. Berndt},
  title = {Role of elastic scattering in electron dynamics at ordered alkali overlayers on Cu(111)},
  journal = {Phys. Rev. Lett.},
  volume = {95},
  pages = {176802},	
  year = {2005},
  url = {http://link.aps.org/abstract/PRL/v95/p176802},
  doi = {http://doi.org/10.1103/PhysRevLett.95.176802}
}
 
 
Resonator design for surface electron lifetime studies using scanning tunneling spectroscopy
S. Crampin, H. Jensen, J. Kröger, L. Limot, R. Berndt, Phys. Rev. B 72, 035443 (2005)

Abstract:
We derive expressions for the lossy boundary-scattering contribution to the linewidth of surface electronic states confined with atomic corrals and island resonators. Correcting experimentally measured linewidths for these contributions along with thermal and intrumental broadening enables intrinsic many-body lifetimes due to electron-electron and electron-phonon scattering to be determined. In small resonators lossy-scattering dominates linewidths whilst different scaling of widths and separations cause levels to merge in large resonators. Our results enable the design of resonators suitable for lifetime studies.

[BibTeX]
@article{SCR_PRB05,
  author = {S. Crampin and H. Jensen and J. Kröger and L. Limot and R. Berndt},
  title = {Resonator design for surface electron lifetime studies using scanning tunneling spectroscopy},
  journal = {Phys. Rev. B},
  volume = {72},
  pages = {035443},	
  year = {2005},
  url = {http://link.aps.org/abstract/PRB/v72/p035443},
  doi = {http://doi.org/10.1103/PhysRevB.72.035443}
}
 
 
Atom Transfer and Single-Adatom Contacts
L. Limot, J. Kröger, R. Berndt, A. Garcia-Lekue, W. Hofer, Phys. Rev. Lett. 94, 126102 (2005)

Abstract:
The point contact of a tunnel tip approaching towards Ag(111) and Cu(111) surfaces is investigated with a low temperature scanning tunneling microscope. A sharp jump-to-contact, random in nature, is observed in the conductance. After point contact, the tip-apex atom is transferred to the surface, indicating that a one-atom contact is formed during the approach. In sharp contrast, the conductance over single silver and copper adatoms exhibits a smooth and reproducible transition from tunneling to contact regime. Numerical simulations show that this is a consequence of the additional dipolar bonding between the homoepitaxial adatom and the surface atoms.

[BibTeX]
@article{LLI_PRL05_2,
  author = {L. Limot and J. Kröger and R. Berndt and A. Garcia-Lekue and W.A. Hofer},
  title = {Atom Transfer and Single-Adatom Contacts},
  journal = {Phys. Rev. Lett.},
  volume = {94},
  pages = {126102},	
  year = {2005},
  url = {http://link.aps.org/abstract/PRL/v94/p126102},
  doi = {http://doi.org/10.1103/PhysRevLett.94.126102}
}
 
 
Electron dynamics in vacancy islands: Scanning tunneling spectroscopy on Ag(111)
H. Jensen, J. Kröger, R. Berndt, S. Crampin, Phys. Rev. B 71, 155417 (2005)

Abstract:
The dynamics of Ag(111) surface state electrons confined to nanoscale hexagonal and triangular vacancy islands are investigated using scanning tunneling spectroscopy. The lifetimes of quantised states with significant amplitude near the centers of the vacancies are weakly affected by the geometry of the confining cavity. A model that includes the dependence of the lifetime on electron energy, vacancy size, step reflectivity, and the phase coherence length describes the results well. For vacancy islands with areas in the range of 40-220 nm^2 lossy scattering is the dominant lifetime-limiting process. This result and a corrected analysis of published experimental data improve the consistency of experimental and calculated surface-state lifetimes.

[BibTeX]
@article{HJE_PRB05,
  author = {H. Jensen and J. Kröger and R. Berndt and S. Crampin},
  title = {Electron dynamics in vacancy islands: Scanning tunneling spectroscopy on Ag(111)},
  journal = {Phys. Rev. B},
  volume = {71},
  pages = {155417},	
  year = {2005},
  url = {http://link.aps.org/abstract/PRB/v71/p155417},
  doi = {http://doi.org/10.1103/PhysRevB.71.155417}
}
 
 
Phase coherence length and quantum interference patterns at step edges
S. Crampin, J. Kröger, H. Jensen, R. Berndt, Phys. Rev. Lett. 95, 029701 (2005)

Abstract:
A Comment on the Letter by P. Wahl et al., Phys. Rev. Lett. 91, 106802 (2003). The authors of the Letter offer a Reply.

[BibTeX]
@article{SCR_PRL05,
  author = {S. Crampin and J. Kröger and H. Jensen and R. Berndt},
  title = {Phase coherence length and quantum interference patterns at step edges},
  journal = {Phys. Rev. Lett.},
  volume = {95},
  pages = {029701},	
  year = {2005},
  note = {(Comment)},
  url = {http://link.aps.org/abstract/PRL/v95/p029701},
  doi = {http://doi.org/10.1103/PhysRevLett.95.029701}
}
 
(Comment)
 
Rastertunnelmikroskopie an chiralen Molekülen
M. Böhringer, R. Berndt, W.-D. Schneider, Chem. unserer Zeit 39/5, 326 (2005)

Abstract:
Mit dem Rastertunnelmikroskop (RTM) können Atome und Moleküle auf Oberflächen direkt abgebildet und gezielt manipuliert werden. Damit ermöglicht das RTM eine Chemie auf Nanometerskala basierend auf Experimenten an Einzelmolekülen. Am Beispiel des 1-Nitronaphthalin beschreiben wir Elemente einer Stereochemie an Einzelmolekülen. 1-Nitronaphthalin nimmt bei Adsorption auf der Au(111) Oberfläche eine chirale Konfiguration an. Hochaufgelöste RTM-Bilder spiegeln die asymmetrische elektronische Struktur der Moleküle wider und ermöglichen die Bestimmung der absoluten Konfiguration jedes einzelnen Moleküls in komplexen Überstrukturen. Auf diese Weise lassen sich stereochemische Phänomene wie strukturelle übergänge von chiralen in achirale Phasen unmittelbar beobachten. Schließlich zeigen wir, dass chirale Strukturen mit dem RTM auch gezielt manipuliert werden können und trennen eine racemische Mischung chiraler 1-Nitronaphthalin-Aggregate in die enantiomerenreinen Komponenten.

[BibTeX]
@article{MBO_CIUZ05,
  author = {M. Böhringer and R. Berndt and W.-D. Schneider},
  title = {Rastertunnelmikroskopie an chiralen Molekülen},
  journal = {Chemie in unserer Zeit},
  volume = {39/5},
  pages = {326--335},	
  year = {2005},
  url = {http://www3.interscience.wiley.com/cgi-bin/abstract/112099633/ABSTRACT},
  doi = {http://doi.org/10.1002/ciuz.200400338}
}
 
 
Adsorption geometry of PTCDA on 2H-NbSe_2
J. Kröger, H. Jensen, T. Jürgens, T. von Hofe, J. Kuntze, R. Berndt, Appl. Phys. A 81, 1285 (2005)

Abstract:
Low-temperature scanning tunnelling microscopy of submonolayers of PTCDA adsorbed on 2H-NbSe_2 reveals an adsorbate layer that is neither commensurate with the substrate lattice nor with the charge density wave periodicity. The observed smallness of deviations from the PTCDA bulk state unit cell dimensions indicate that PTCDA film formation is dominated by intermolecular interactions rather than by interaction between PTCDA and 2H-NbSe_2.

[BibTeX]
@article{JKR_APA05,
  author = {J. Kröger and H. Jensen and Th. Jürgens and Th. von Hofe and J. Kuntze and R. Berndt},
  title = {Adsorption geometry of PTCDA on 2H-NbSe_2},
  journal = {Appl. Phys. A},
  volume = {81},
  pages = {1285},	
  year = {2005},
  url = {http://www.springerlink.com/openurl.asp?genre=article&id=doi:10.1007/s00339-004-3039-6},
  doi = {http://doi.org/10.1007/s00339-004-3039-6}
}
 
 
Surface-State Localization at Adatoms
L. Limot, E. Pehlke, J. Kröger, R. Berndt, Phys. Rev. Lett. 94, 036805 (2005)

Abstract:
Low-temperature scanning tunneling spectroscopy of magnetic and non-magnetic metal atoms on Ag(111) and on Cu(111) surfaces reveals the existence of a common electronic resonance at an energy below the binding energies of the surface states. Using an extended Newns-Anderson model, we assign this resonance to an adsorbate-induced bound state, split off from the bottom of the surface-state band, and broadened by the interaction with bulk states. A line shape analysis of the bound state indicates that Ag and Cu adatoms on Ag(111) and Cu(111), respectively, decrease the surface-state lifetime, while a cobalt adatom causes no significant change.

[BibTeX]
@article{LLI_PRL05,
  author = {L. Limot and E. Pehlke and J. Kröger and R. Berndt},
  title = {Surface-State Localization at Adatoms},
  journal = {Phys. Rev. Lett.},
  volume = {94},
  pages = {036805},	
  year = {2005},
  url = {http://link.aps.org/abstract/PRL/v94/p036805},
  doi = {http://doi.org/10.1103/PhysRevLett.94.036805}
}
 
 
Color View of Atomic Highs and Lows in Tunneling Induced Light Emission
G. Hoffmann, T. Maroutian, R. Berndt, Phys. Rev. Lett. 93, 076102 (2004)

Abstract:
Based on a detailed experimental study of light emission stimulated with a scanning tunneling microscope, we put forward a consistent picture for the atomic-scale contrasts observed to date on noble metal surfaces. Divergent contrasts near various atomic steps and conflicting interpretations of light emission from a model atomic grating, (2x1) reconstructed Au(110), are accounted for. The light intensity modulation results from different spatial distributions of the local density of final states in the elastic and inelastic tunneling channels.

[BibTeX]
@article{GHO_PRL04,
  author = {G. Hoffmann and Th. Maroutian and R. Berndt},
  title = {Color View of Atomic Highs and Lows in Tunneling Induced Light Emission},
  journal = {Phys. Rev. Lett.},
  volume = {93},
  pages = {076102},	
  year = {2004},
  url = {http://link.aps.org/abstract/PRL/v93/p076102},
  doi = {http://doi.org/10.1103/PhysRevLett.93.076102}
}
 
 
Stark Effect of the Au(111) and Cu(111) Surface States
J. Kröger, L. Limot, H. Jensen, R. Berndt, P. Johansson, Phys. Rev. B 70, 033401 (2004)

Abstract:
We present a low-temperature scanning tunneling spectroscopy study of the Au(111) and of the Cu(111) surface states showing that their binding energy increases when the tip approaches towards the surface. This result, supported by a one-dimensional model calculation and by a comparison to existing photoemission spectroscopy measurements, confirms the existence of a tip-induced Stark effect as previously reported for Ag(111) [L. Limot et al., Phys. Rev. Lett. 91, 196801 (2003)], and suggests that this effect is a general feature of scanning tunneling spectroscopy.

[BibTeX]
@article{JKR_PRB04,
  author = {J. Kröger and L. Limot and H. Jensen and R. Berndt and P. Johansson},
  title = {Stark Effect of the Au(111) and Cu(111) Surface States},
  journal = {Phys. Rev. B},
  volume = {70},
  pages = {033401},	
  year = {2004},
  url = {http://link.aps.org/abstract/PRB/v70/p033401},
  doi = {http://doi.org/10.1103/PhysRevB.70.033401}
}
 
 
Chiral structures of lander molecules on Cu(100)
J. Kuntze, X. Ge, R. Berndt, Nanotechnology 15, S337 (2004)

Abstract:
Supramolecular assemblies of lander molecules (C_90H_98) on Cu(100) are investigated with low-temperature scanning tunnelling microscopy. The energetically most favourable conformation of the adsorbed molecule is found to exist in two mirror symmetric enantiomers or conformers. At low coverage the molecules align in enantiomerically pure chains along the chiral directions [012] and [021]. The arrangement is proposed to be mainly governed by intermolecular van der Waals interaction. At higher coverages, the molecular chains arrange into chiral domains, for which a structural model is presented.

[BibTeX]
@article{JKU_NTEC04,
  author = {J. Kuntze and X. Ge and R. Berndt},
  title = {Chiral structures of lander molecules on Cu(100)},
  journal = {Nanotechnology},
  volume = {15},
  pages = {S337--S340},	
  year = {2004},
  url = {http://www.iop.org/EJ/abstract/0957-4484/15/6/003},
  doi = {http://doi.org/10.1088/0957-4484/15/6/003}
}
 
 
Kondo effect and surface state electrons
L. Limot, R. Berndt, Appl. Surf. Sci. 237, 572 (2004)

Abstract:
We have used low temperature scanning tunneling spectroscopy and atomic manipulation to study the role of surface-state electrons in the Kondo effect of an isolated cobalt atom adsorbed on Ag(1 1 1). We show that the observed Kondo signature remains unchanged in close proximity of a monoatomic step, where the local density of states of the surface-state electrons is strongly perturbed. This result suggests a minor role for surface-state electrons in the Kondo effect of the cobalt adatom, compared to bulk electrons. A possible explanation for our findings is presented.

[BibTeX]
@article{LLI_ASS04,
  author = {L. Limot and R. Berndt},
  title = {Kondo effect and surface state electrons},
  journal = {Appl. Surf. Sci.},
  volume = {237},
  pages = {572--576},	
  year = {2004},
  url = {http://dx.doi.org/10.1016/j.apsusc.2004.07.023},
  doi = {http://doi.org/10.1016/j.apsusc.2004.07.023}
}
 
 
Decay of electronic excitations at metal surfaces
P. Echenique, R. Berndt, E. Chulkov, T. Fauster, A. Goldmann, U. Höfer, Surf. Sci. Rep. 52, 219 (2004)

Abstract:
Recent experimental work has reached an accurate determination of surface-state linewidth by scanning tunneling spectroscopy, photoemission and directly in the time domain by two-photon photoemission. The concomitant progress in the theoretical description of surface-state decay has led to an excellent agreement between theory and experiment for low-index noble-metal surfaces. The understanding of the behavior of s-p-like surface states and image-potential states on these surfaces constitutes the basis for the application to more complicated systems.

[BibTeX]
@article{PEC_SSR04,
  author = {P.M. Echenique and R. Berndt and E.V. Chulkov and Th. Fauster and A. Goldmann and U. Höfer},
  title = {Decay of electronic excitations at metal surfaces},
  journal = {Surf. Sci. Rep.},
  volume = {52},
  pages = {219--317},	
  year = {2004},
  url = {http://dx.doi.org/10.1016/j.surfrep.2004.02.002},
  doi = {http://doi.org/10.1016/j.surfrep.2004.02.002}
}
 
 
Stark Effect and Scanning Tunneling Spectroscopy of the Ag(111) Surface State
L. Limot, T. Maroutian, P. Johansson, R. Berndt, Physica acta polonica A 104, 185 (2003)

Abstract:
Low-temperature scanning tunneling spectroscopy is used to study the Ag(111) surface state over an unprecedented range of junction resistances. The presence of the tip causes a shift of the surface state towards higher binding energies, increasingly stronger as the resistance decreases. A one-dimensional model calculation reproduces this observation and provides a connection to existing photoemission spectroscopy data. Implications of the effect on STS studies are discussed.

[BibTeX]
@article{LLI_PAPA03,
  author = {L. Limot and T. Maroutian and P. Johansson and R. Berndt},
  title = {Stark Effect and Scanning Tunneling Spectroscopy of the Ag(111) Surface State},
  journal = {Physica acta polonica A},
  volume = {104},
  pages = {185--192},	
  year = {2003},
  url = {http://przyrbwn.icm.edu.pl/APP/ABSTR/104/a104-3-4-185.html},
  doi = {http://doi.org/10.12693/APhysPolA.104.185}
}
 
 
Theory of STM-induced light emission from Na overlayers on Cu
P. Johansson, G. Hoffmann, R. Berndt, Proc. Int'l conf. Scanning Tunneling Microscopy/Spectroscopy, Eindhoven, 21.-25. Juli 2003, AIP Conf. Proc. 696, pp. 37-44 (2003)

Abstract:
We present experimental and theoretical results for light emission from a scanning tunneling microscope probing the metallic quantum well system consisting of a Na overlayer on Cu(111). The experimental results can be summarised as follows: (i) At low bias voltages a broad, plasmon-mediated spectrum of emitted light is seen. (ii) Once the bias voltage U is large enough that electrons can be injected from the tip into the second QWS, the spectrum becomes sharper and is centered at a photon energy h ν corresponding to the energy difference between the second and first QWS, E_2 - E_1. (iii) Finally, and most strikingly, photons with this energy (E_2 - E_1) are observed, albeit with lower intensity, also when eU eU with quantum yield and power-law variation of the intensity with the tunnel current in good agreement with experiment.

[BibTeX]
@inproceedings{PJO_ICSTM03,
  author = {P. Johansson and G. Hoffmann and R. Berndt},
  title = {Theory of STM-induced light emission from Na overlayers on Cu},
  booktitle = {Proc. Int'l conf. Scanning Tunneling Microscopy/Spectroscopy},
  series = {AIP Conf. Proc.},
  volume = {696},
  pages = {37--44},
  address = {Eindhoven},
  period = {21.-25. Juli 2003},	
  year = {2003}
}
 
 
Superstructures and coincidences of a thin oxide film on a metallic substrate: A STM study
T. Maroutian, S. Degen, C. Becker, K. Wandelt, R. Berndt, Phys. Rev. B 68, 155414 (2003)

Abstract:
An alumina thin film, grown at 1000 K on a Ni_3 Al(111) substrate, is investigated using low temperature scanning tunneling microscopy (LT-STM). Depending on the (positive) voltage applied to the sample, two superstructures are imaged. Using tunneling spectroscopy, this effect is traced back to a lateral variation of the local density of states (LDOS) within the apparent unit cell of the oxide thin film. In the vicinity of the conduction band edge, the density of empty states for tunneling electrons is locally enhanced over characteristic points of the unit cell. A coincidence lattice between film and metallic substrate, with a unit cell of same size and symmetry, is proposed as the origin of this modulation of the oxide electronic structure.

[BibTeX]
@article{TMA_PRB03,
  author = {T. Maroutian and S. Degen and C. Becker and K. Wandelt and R. Berndt},
  title = {Superstructures and coincidences of a thin oxide film on a metallic substrate: A STM study},
  journal = {Phys. Rev. B},
  volume = {68},
  pages = {155414},	
  year = {2003},
  url = {http://link.aps.org/abstract/PRB/v68/p155414},
  doi = {http://doi.org/10.1103/PhysRevB.68.155414}
}
 
 
Hole dynamics in a quantum-well state at Na/Cu(111)
E. Chulkov, J. Kliewer, R. Berndt, V. Silkin, B. Hellsing, S. Crampin, P. Echenique, Phys. Rev. B 68, 195422 (2003)

Abstract:
Theoretical calculations and scanning-tunneling spectroscopy measurements of the hole lifetime broadening, τ ^-1, in a quantum-well state for 0.95 and 1.0 monolayer of Na on Cu(111) are reported. A new model potential is proposed for calculating quantum-well states in a monolayer on metal surfaces. The inelastic electron-electron contribution, τ ^-1 _e-e , is evaluated within the GW approximation by using eigenfunctions and eigenenergies obtained with this model potential. The electron-phonon contribution, τ ^-1 _e-ph, is computed by employing Debye and Einstein models as well as a first-principle ultrasoft pseudopotential method. The obtained theoretical results are in excellent agreement with experimental data, both showing a surprisingly large difference in the lifetime broadening for 0.95 and 1.0 monolayers which is attributed mostly to changes in the electron structure.

[BibTeX]
@article{ECH_PRB03,
  author = {E.V. Chulkov and J. Kliewer and R. Berndt and V.M. Silkin and B. Hellsing and S. Crampin and P.M. Echenique},
  title = {Hole dynamics in a quantum-well state at Na/Cu(111)},
  journal = {Phys. Rev. B},
  volume = {68},
  pages = {195422},	
  year = {2003},
  url = {http://link.aps.org/abstract/PRB/v68/p195422},
  doi = {http://doi.org/10.1103/PhysRevB.68.195422}
}
 
 
Surface State Stark Shift in a Scanning Tunneling Microscope
L. Limot, T. Maroutian, P. Johansson, R. Berndt, Phys. Rev. Lett. 91, 196801 (2003)

Abstract:
We report a quantitative low-temperature Scanning Tunneling Spectroscopy (STS) study on the Ag(111) surface state over an unprecedented range of currents (50 pA to 6 μ A) through which we can tune the electric field in the tunnel junction of the microscope. We show that in STS a sizeable Stark effect causes a shift of the surface state binding energy E_0. Data taken are reproduced by a one-dimensional potential model calculation, and are found to yield a Stark-free energy E_0 in agreement with recent state-of-the-art Photoemission Spectroscopy measurements.

[BibTeX]
@article{LLI_PRL03,
  author = {L. Limot and T. Maroutian and P. Johansson and R. Berndt},
  title = {Surface State Stark Shift in a Scanning Tunneling Microscope},
  journal = {Phys. Rev. Lett.},
  volume = {91},
  pages = {196801},	
  year = {2003},
  url = {http://link.aps.org/abstract/PRL/v91/p196801},
  doi = {http://doi.org/10.1103/PhysRevLett.91.196801}
}
 
 
STM-Induced Fluorescence from Na Monolayers on Cu(111)
G. Hoffmann, R. Berndt, P. Johansson, Phys. Low-Dim. Struct. 3/4, 209 (2003)

Abstract:
In the scanning tunneling microscope (STM) light is emitted from Na overlayers on Cu(111). This emission is due to (a) inelastic tunneling into quantum well states of the Na layer and (b) transitions between these quantum well states. Experimental observations and model calculations for 3 ML Na on Cu(111) are presented.

[BibTeX]
@article{GHO_PLDS03,
  author = {G. Hoffmann and R. Berndt and P. Johansson},
  title = {STM-Induced Fluorescence from Na Monolayers on Cu(111)},
  journal = {Phys. Low-Dim. Struct.},
  volume = {3/4},
  pages = {209--220},	
  year = {2003}
}
 
 
Two-Electron Photon Emission from Metallic Quantum Wells
G. Hoffmann, R. Berndt, P. Johansson, Phys. Rev. Lett. 90, 046803 (2003)

Abstract:
Unusual emission of visible light is observed in scanning tunneling microscopy of the quantum well system Na on Cu(111). Photons are emitted at energies exceeding the energy of the tunneling electrons. Model calculations of two-electron processes which lead to quantum well transitions reproduce the experimental fluorescence spectra, the quantum yield, and the power-law variation of the intensity with the excitation current.

[BibTeX]
@article{GHO_PRL03,
  author = {G. Hoffmann and R. Berndt and P. Johansson},
  title = {Two-Electron Photon Emission from Metallic Quantum Wells},
  journal = {Phys. Rev. Lett.},
  volume = {90},
  pages = {046803},	
  year = {2003},
  url = {http://link.aps.org/abstract/PRL/v90/p046803},
  doi = {http://doi.org/10.1103/PhysRevLett.90.046803}
}
 
 
Tunnelling induced fluorescence as a probe of electromagnetic interaction at nanometre proximity
G. Hoffmann, J. Aizpurua, S. Apell, R. Berndt, in Tunnelling induced fluorescence as a probe of electromagnetic interaction at nanometre proximity, V. Agranovich and F. Kajzar (Hrsg.) NATO Science Series II 100, 81-91 (Kluwer, Dordrecht, 2003)
[BibTeX]
@inbook{GHO_NSS03,
  author = {G. Hoffmann and J. Aizpurua and S.P. Apell and R. Berndt},
  chapter = {Tunnelling induced fluorescence as a probe of electromagnetic interaction at nanometre proximity},
  title = {Tunnelling induced fluorescence as a probe of electromagnetic interaction at nanometre proximity},
  editor = {V.M. Agranovich and F. Kajzar},
  series = {NATO Science Series II},
  volume = {100},
  pages = {81--91},
  publisher = {Kluwer, Dordrecht},	
  year = {2003}
}
 
 
Light emission from Na/Cu(111) induced by a scanning tunneling microscope
P. Johansson, G. Hoffmann, R. Berndt, Phys. Rev. B 66, 245415 (2002)

Abstract:
Measurements of light emission from a scanning tunneling microscope probing a Na overlayer on the (111) surface of Cu are reported along with results of a model calculation that essentially agree with the experimental ones. The observed light emission spectra show two characteristic features depending on the bias voltage. When the bias voltage is smaller than the energy difference between the two lowest quantum well states formed outside the Na overlayer the light emission is due to a plasmon-mediated process, while for larger biases light emission is mainly caused by quantum well transitions between the two levels.

[BibTeX]
@article{PJO_PRB02,
  author = {P. Johansson and G. Hoffmann and R. Berndt},
  title = {Light emission from Na/Cu(111) induced by a scanning tunneling microscope},
  journal = {Phys. Rev. B},
  volume = {66},
  pages = {245415},	
  year = {2002},
  url = {http://link.aps.org/abstract/PRB/v66/p245415},
  doi = {http://doi.org/10.1103/PhysRevB.66.245415}
}
 
 
Electromagnetic coupling on an atomic scale
J. Aizpurua, G. Hoffmann, S. Apell, R. Berndt, Phys. Rev. Lett. 89, 156803 (2002)

Abstract:
Light emission from a scanning tunneling microscope is used to investigate the electromagnetic coupling (EMC) of a metal tip and a metal sample. Sub-atomic scale modifications of the tip-sample cavity cause spectral shifts of the fluorescence. We study quantitatively this effect for a monoatomic step on the sample surface in the proximity of the tip. The change of the spectral response is affected by the lateral extension of the localised surface plasmon at the cavity. For sharp tips the EMC is confined to a lateral range of a few nm. The results are consistent with model calculations of the electromagnetic response of an appropriate tip-sample geometry.

[BibTeX]
@article{JAV_PRL02,
  author = {J. Aizpurua and G. Hoffmann and S.P. Apell and R. Berndt},
  title = {Electromagnetic coupling on an atomic scale},
  journal = {Phys. Rev. Lett.},
  volume = {89},
  pages = {156803},	
  year = {2002},
  url = {http://link.aps.org/abstract/PRL/v89/p156803},
  doi = {http://doi.org/10.1103/PhysRevLett.89.156803}
}
 
 
Tunneling induced luminescence from adsorbed organic molecules with submolecular lateral resolution
G. Hoffmann, L. Libioulle, R. Berndt, Phys. Rev. B 65, 212107 (2002)

Abstract:
Light emission from molecule-covered surfaces in the scanning tunneling microscope has repeatedly been reported with optical contrast occurring on a molecular scale. The role of the molecules in the emission is unclear although molecular fluorescence has been suggested. Here, simultaneous optical spectroscopy and spatial mapping of the emission from single hexa-tert-butyl-decacyclene molecules on noble-metal surfaces are combined and submolecular constrast is observed. However, the emission spectra are indicative of plasmon mediated emission of the metal substrate and tip while the molecule merely acts as a spacer, which slightly modifies the plasmon emission.

[BibTeX]
@article{GHO_PRB02,
  author = {G. Hoffmann and L. Libioulle and R. Berndt},
  title = {Tunneling induced luminescence from adsorbed organic molecules with submolecular lateral resolution},
  journal = {Phys. Rev. B},
  volume = {65},
  pages = {212107},	
  year = {2002},
  url = {http://link.aps.org/abstract/PRB/v65/p212107},
  doi = {0.1103/PhysRevB.65.212107}
}
 
 
Conformations of a molecular wire adsorbed on a metal surface
J. Kuntze, R. Berndt, P. Jiang, H. Tang, A. Gourdon, C. Joachim, Phys. Rev. B 65, 233405 (2002)

Abstract:
The conformation changes of a molecular wire induced upon adsorption on a Cu(001) surface are investigated by low-temperature scanning tunneling microscopy. Measured images are compared with calculations using the elastic-scattering quantum chemistry technique, allowing the extraction of molecular adsorption geometries on terraces and at steps. The molecular bonds prove to be very flexible, resulting in different conformations upon adsorption compared to the molecular gas-phase structure.

[BibTeX]
@article{JKU_PRB02,
  author = {J. Kuntze and R. Berndt and P. Jiang and H. Tang and A. Gourdon and C. Joachim},
  title = {Conformations of a molecular wire adsorbed on a metal surface},
  journal = {Phys. Rev. B},
  volume = {65},
  pages = {233405},	
  year = {2002},
  url = {http://link.aps.org/abstract/PRB/v65/p233405},
  doi = {http://doi.org/10.1103/PhysRevB.65.233405}
}
 
 
Quantum Well Luminescence from Metallic Monolayers
G. Hoffmann, R. Berndt, Int. J. Nanosci. 1, 53 (2002)

Abstract:
The emission of visible light is observed from the metallic quantum well system Na on Cu(111) when electrons tunnel from the tip of a scanning tunneling microscope to unoccupied states of the sample. Spectral analyses of the emission along with tunneling spectroscopy are used to show that the emission is comprised of two components. A sharp spectral feature is found to be due to interband transitions between quantum well states of the Na layer. The other component is similar to plasmon-mediated emission observed previously from noble metal surfaces. The results suggest that the light emission is a unique local probe of the electronic structure of this quantum well system.

[BibTeX]
@article{GHO_IJNS02,
  author = {G. Hoffmann and R. Berndt},
  title = {Quantum Well Luminescence from Metallic Monolayers},
  journal = {Int. J. Nanosci.},
  volume = {1},
  pages = {53--62},	
  year = {2002},
  url = {http://ejournals.wspc.com.sg/175/01/0101/S0219581X02000048.html},
  doi = {http://doi.org/10.1142/S0219581X02000048}
}
 
 
Color Imaging with a Low Temperature Scanning Tunneling Microscope
G. Hoffmann, J. Kröger, R. Berndt, Rev. Sci. Instrum. 73, 305 (2002)

Abstract:
We report on an improved optical design for detecting light emitted from a scanning tunneling microscope (STM). The achieved detection efficiency enables acquisition of statistically significant spectra from noble metal surfaces in milliseconds. Thus new modes of measurement become available which encompass spectroscopic imaging -acquisition of fluores- cence spectra at each point of a STM image- and excitation spectroscopy -acquisition of fluorescence spectra while varying the tip-sample bias-. Spectroscopic imaging is used to observe gradual changes of the emission spectra as the STM tip approaches a monoatomic step of Ag(111) on a nanometer scale. Excitation spectroscopy with high resolution in both wavelength and bias voltage is demonstrated for a Ag(111) surface.

[BibTeX]
@article{GHO_RSI02,
  author = {G. Hoffmann and J. Kröger and R. Berndt},
  title = {Color Imaging with a Low Temperature Scanning Tunneling Microscope},
  journal = {Rev. Sci. Instrum.},
  volume = {73},
  pages = {305--309},	
  year = {2002},
  url = {http://dx.doi.org/10.1063/1.1433946},
  doi = {http://doi.org/10.1063/1.1433946}
}
 
 
La manipulation des ondes électroniques
R. Berndt, Pour la Science 290, 128 (2001)
[BibTeX]
@article{RBE_PLS01,
  author = {R. Berndt},
  title = {La manipulation des ondes électroniques},
  journal = {Pour la Science},
  volume = {290},
  pages = {128--131},	
  year = {2001}
}
 
 
Scanning tunnelling spectroscopy of electron resonators
J. Kliewer, R. Berndt, S. Crampin, New J. Phys. 3, 22.1 (2001)

Abstract:
The electronic structure of artificial Mn atom arrays on Ag(111) is characterised in detail with scanning tunnelling spectroscopy and spectroscopic imaging at low temperature. We demonstrate the degree to which variations in geometry may be used to control spatial and spectral distributions of surface state electrons con.ned within the arrays, how these are influenced by atoms placed within the structure and how the ability to induce spectral features at specific energies may be exploited through lineshape analyses to deduce quasiparticle lifetimes near the Fermi level. Through extensive comparison of dI/dV maps and spectra we demonstrate the utility of a model based upon twodimensional s-wave scatterers for describing and predicting the characteristics of specific resonators.

[BibTeX]
@article{JKL_NJP01,
  author = {J. Kliewer and R. Berndt and S. Crampin},
  title = {Scanning tunnelling spectroscopy of electron resonators},
  journal = {New J. Phys.},
  volume = {3},
  pages = {22.1--22.11},	
  year = {2001},
  url = {http://stacks.iop.org/NJP/3/22},
  doi = {http://doi.org/10.1088/1367-2630/3/1/322}
}
 
 
Sharper images: Focussing soft X-rays with photon sieves
L. Kipp, M. Skibowski, R. Johnson, R. Berndt, R. Adelung, S. Harm, R. Seemann, Nature 414, 184 (2001)

Abstract:
Fresnel zone plates consisting of alternating transmissive and opaque circular rings can be used to focus X-rays. The spatial resolution that can be achieved with these devices is of the order of the width of the outermost zone and is therefore limited by the smallest structure (20-40 nm) that can be fabricated by lithography today. Here we show that a large number of pinholes distributed appropriately over the Fresnel zones make it possible to focus soft X-rays to spot sizes smaller than the diameter of the smallest pinhole. In addition, higher orders of diffraction and secondary maxima can be suppressed by several orders of magnitude. In combination with the next generation of synchrotron light sources (free-electron lasers) these photon sieves offer new opportunities for high-resolution X-ray microscopy and spectroscopy in physical and life sciences.

[BibTeX]
@article{LKI_NAT01,
  author = {L. Kipp and M. Skibowski and R.L. Johnson and R. Berndt and R. Adelung and S. Harm and R. Seemann},
  title = {Sharper images: Focussing soft X-rays with photon sieves},
  journal = {Nature},
  volume = {414},
  pages = {184--188},	
  year = {2001},
  url = {http://www.nature.com/nature/journal/v414/n6860/full/414184a0.html},
  doi = {http://doi.org/10.1038/35102526}
}
 
 
Scanning Tunneling Spectroscopy of Na on Cu(111)
J. Kliewer, R. Berndt, Phys. Rev. B 65, 035412-1--035412-6 (2001)

Abstract:
Monolayer films of Na evaporated onto Cu(111) under UHV conditions are investigated with scanning tunneling microscopy and spectroscopy (STS) at low temperature. The STS data clearly reveal a two-dimensional overlayer state of Na on Cu(111). From STS and spectroscopic imaging, the binding energies and effective masses of this state are determined for various coverages. The state is demonstrated to be affected by spatial inhomogeneities such as Cu steps or local coverage variations. Drastic bias-dependent variations of apparent step heights and tunneling barrier heights are reported and interpreted in terms of the large contributions of the overlayer state to the tunneling current.

[BibTeX]
@article{JKL_PRB01,
  author = {J. Kliewer and R. Berndt},
  title = {Scanning Tunneling Spectroscopy of Na on Cu(111)},
  journal = {Phys. Rev. B},
  volume = {65},
  pages = {035412-1--035412-6},	
  year = {2001},
  url = {http://link.aps.org/abstract/PRB/v65/p035412},
  doi = {http://doi.org/10.1103/PhysRevB.65.035412}
}
 
 
Luminescence from Metallic Quantum Wells in a Scanning Tunneling Microscope
G. Hoffmann, J. Kliewer, R. Berndt, Phys. Rev. Lett. 87, 176803 (2001)

Abstract:
The tunneling current of a scanning tunneling microscope is reported to cause light emission from the metallic quantum well system Na on Cu(111). Optical spectra along with tunneling spectroscopy show that one component of the emission is due to interband transitions between quantum well states of the Na layer. The other component is similar to plasmon-mediated emission observed previously from noble metals. The emission is a unique local probe of the electronic structure of this quantum well system.

[BibTeX]
@article{GHO_PRL01,
  author = {G. Hoffmann and J. Kliewer and R. Berndt},
  title = {Luminescence from Metallic Quantum Wells in a Scanning Tunneling Microscope},
  journal = {Phys. Rev. Lett.},
  volume = {87},
  pages = {176803},	
  year = {2001},
  url = {http://link.aps.org/abstract/PRL/v87/p176803},
  doi = {http://doi.org/10.1103/PhysRevLett.87.176803}
}
 
 
Supramolecular self-assembly and selective step decoration on the Au(111) surface
M. Vladimirova, A.V. M. Stengel, A. Baldereschi, M. Böhringer, K. Morgenstern, R. Berndt, W.-D. Schneider, Europhys. Lett. 56, 254 (2001)

Abstract:
Scanning tunneling microscopy of low coverages of 1-nitronaphtalene (NN) on reconstructed Au(111) reveals that the face-centered-cubic (fcc) reconstruction domains of steps are densely and selectively decorated. Molecular dynamics simulations identify this phenomenon as the self-assembly of hydrogen-like bonded supramolecular chains driven by the step electrostatic potential. First principles calculations predict weak adsorption at domain boundaries, and stable adsorption at the step edge of the fcc domains, explaining the selective decoration.

[BibTeX]
@article{MVL_EPL01,
  author = {M. Vladimirova and M. Stengel,A.De Vita and A. Baldereschi and M. Böhringer and K. Morgenstern and R. Berndt and W.-D. Schneider},
  title = {Supramolecular self-assembly and selective step decoration on the Au(111) surface},
  journal = {Europhys. Lett.},
  volume = {56},
  pages = {254--260},	
  year = {2001},
  url = {http://stacks.iop.org/EPL/56/254},
  doi = {http://doi.org/10.1209/epl/i2001-00514-3}
}
 
 
Low Temperature Scanning Tunnelling Microscopy of Na on Cu(111)
J. Kliewer, R. Berndt, Surf. Sci. 477, 250 (2001)

Abstract:
Room temperature grown Na Films on Cu(111) were investigated with low temperature scanning tunnelling microscopy at coverages Θ = 0.3 to 1.1 monolayers. A p(3 × 3) reconstruction with 4 atoms per unit cell at full monolayer coverage is observed in accord with previous low energy electron diffraction data. For coverages below Θ = 1 ML, a structure model is proposed. We find that an analysis of the electronic structure of the Na overlayer state provides complementary information for the characterization of the films. For the 2nd ML Na film, island growth is observed. From an analysis of the STM data of the 2nd ML Na film, a structure model is developed.

[BibTeX]
@article{JKL_SSI01,
  author = {J. Kliewer and R. Berndt},
  title = {Low Temperature Scanning Tunnelling Microscopy of Na on Cu(111)},
  journal = {Surf. Sci.},
  volume = {477},
  pages = {250--258},	
  year = {2001},
  url = {http://dx.doi.org/10.1016/S0039-6028(01)00891-3},
  doi = {http://doi.org/10.1016/S0039-6028(01)00891-3}
}
 
 
Controlling Adsorbate Electronic Structure
J. Kliewer, R. Berndt, S. Crampin, Adv. Funct. Mater. 11, 186 (2001)

Abstract:
We have used nanocavities constructed from Mn adatoms on Ag(111) to achieve a controlled modification of the electronic structure of the Ag surface. Local tunnelling spectroscopy reveals an intriguing modification of the electronic structure of a single Mn adsorbate put into these cavities. Model calculations show that the changes result from the coupling between the Mn s level and surface electronic states of the substrate, which are strongly influenced by the size and geometry of the nanocavity. The dimension and geometry of the adatom array thus provide a degree of control over the induced changes.

[BibTeX]
@article{JKL_AFMA01,
  author = {J. Kliewer and R. Berndt and S. Crampin},
  title = {Controlling Adsorbate Electronic Structure},
  journal = {Adv. Funct. Mater.},
  volume = {11},
  pages = {186--187},	
  year = {2001},
  url = {http://www3.interscience.wiley.com/cgi-bin/abstract/82003996/ABSTRACT},
  doi = {http://doi.org/10.1002/1616-3028(200106)11:3%3C186::AID-ADFM186%3E3.0.CO;2-D}
}
 
 
Scanning tunnelling spectroscopy of a single Kondo impurity
R. Berndt, W.-D. Schneider, in Kondo Effect and Dephasing in Low-Dimensional Metallic Systems, V. Chandrasekhar, C. van Haesendonck and A. Zawadowski (Hrsg.) NATO Science Series II on Mathematics, Physics and Chemistry 50, 87-95 (Kluwer, Dordrecht, 2001)

Abstract:
We have used scanning tunnelling microscopy to study tunnelling spectra of individual Ce atoms on Ag(111) surfaces. These experiments led to the first observation of the Kondo resonance from a single impurity atom. The Kondo resonance is reflected by an unexpected Fano lineshape.

[BibTeX]
@inbook{RBE_NSS01,
  author = {R. Berndt and W.-D. Schneider},
  chapter = {Scanning tunnelling spectroscopy of a single Kondo impurity},
  title = {Kondo Effect and Dephasing in Low-Dimensional Metallic Systems},
  editor = {V. Chandrasekhar and Ch.  van Haesendonck and A. Zawadowski},
  series = {NATO Science Series II on Mathematics, Physics and Chemistry},
  volume = {50},
  pages = {87--95},
  publisher = {Kluwer, Dordrecht},	
  year = {2001},
  isbn = {978-1-4020-0401-8}
}
 
 
Electromagnetic coupling at an atomic scale
G. Hoffmann, R. Berndt, Appl. Phys. A 72, S173 (2001)

Abstract:
We report the first spectrally resolved measurements of scanning tunnelling microscope induced fluorescence from monoatomic steps. As a function of lateral distance from the step edge on a sub nanometre scale increasing red (blue) shifts of the emission are observed as the tip approaches the lower (upper) step edge.

[BibTeX]
@article{GHO_APA01,
  author = {G. Hoffmann and R. Berndt},
  title = {Electromagnetic coupling at an atomic scale},
  journal = {Appl. Phys. A},
  volume = {72},
  pages = {S173--S175},	
  year = {2001},
  url = {http://www.springerlink.com/openurl.asp?genre=article&id=doi:10.1007/s003390100671},
  doi = {http://doi.org/10.1007/s003390100671}
}
 
 
Local spectroscopy of two-dimensional states in Na films on Cu(111)
J. Kliewer, R. Berndt, Appl. Phys. A 72, S155 (2001)

Abstract:
Alkali overlayers on noble metal surfaces exhibit two-dimensional electronic states. We have investigated the electronic structure of Na on Cu(111) at T=5 K with scanning tunneling spectroscopy. In agreement with photoelectron spectroscopy we find two electronic bands close to the Fermi energy which correspond to quantum well states (QWS). In second monolayer islands confinement of the QWS is observed.

[BibTeX]
@article{JKL_APA01_2,
  author = {J. Kliewer and R. Berndt},
  title = {Local spectroscopy of two-dimensional states in Na films on Cu(111)},
  journal = {Appl. Phys. A},
  volume = {72},
  pages = {S155--S157},	
  year = {2001},
  url = {http://www.springerlink.com/openurl.asp?genre=article&id=doi:10.1007/s003390100674},
  doi = {http://doi.org/10.1007/s003390100674}
}
 
 
Electronic structure of an adsorbate in a resonator
J. Kliewer, R. Berndt, S. Crampin, Appl. Phys. A 72, S147 (2001)

Abstract:
We report the controlled modification of the electronic structure on a single Mn adsorbate. This modification was achieved by placing the Mn atom within geometrical arrays of adatoms on Ag(111), and observed with local tunnelling spectroscopy. The changes result from the coupling between the Mn s level and surface electronic states of the substrate, which are strongly influenced by the size and geometry of the nanocavity. The dimension and geometry of the adatom array thus provide a degree of control over the induced changes.

[BibTeX]
@article{JKL_APA01,
  author = {J. Kliewer and R. Berndt and S. Crampin},
  title = {Electronic structure of an adsorbate in a resonator},
  journal = {Appl. Phys. A},
  volume = {72},
  pages = {S147--S148},	
  year = {2001},
  url = {http://www.springerlink.com/openurl.asp?genre=article&id=doi:10.1007/s003390100672},
  doi = {http://doi.org/10.1007/s003390100672}
}
 
 
Influence of the tip geometry in light emission from the scanning tunneling microscope
G. Hoffmann, J. Aizpurua, S. Apell, R. Berndt, Surf. Sci. 482, 1159 (2001)

Abstract:
In the scanning tunnelling microscopy of metal surfaces, electrons tunnelling inelastically emit light via the decay of coupled plasmons of the tip and the sample. We present an experimental study of the influence of the tip shape in light emission for a double tip system which is specially adequate to isolate the influence of the tip shape from other features in fluorescence spectra. Model calculations for various tip shapes show qualitative agreement with the experimental features, emphasizing the importance of tip shape in light emission.

[BibTeX]
@article{GHO_SSI01,
  author = {G. Hoffmann and J. Aizpurua and S.P. Apell and R. Berndt},
  title = {Influence of the tip geometry in light emission from the scanning tunneling microscope},
  journal = {Surf. Sci.},
  volume = {482},
  pages = {1159--1162},	
  year = {2001},
  url = {http://dx.doi.org/10.1016/S0039-6028(00)01079-7},
  doi = {http://doi.org/10.1016/S0039-6028(00)01079-7}
}