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Visualizing Non-abrupt Transition of Quantum Well States at Stepped Silver Surfaces.

Saha SK, Manna S, Stepanyuk VS, Kirschner J - Sci Rep (2015)

Bottom Line: This study reveals a clear spatially dependent, nearly continuous trend in the energetic shifts of quantum well (QW) states of thin Ag(111) film grown on Cu(111) substrate, showing the strongest change near the step edge.A large energetic shift equaling up to ~200 meV with a lateral extension of the QW states of the order of ~20 Å is found, even though the step-edge is atomically sharp as evidenced by a line scan.The observed lateral extension and the nearly smooth transition of QW states are understood within the context of step-induced charge oscillation, and Smoluchowski-type charge spreading and smoothing.

View Article: PubMed Central - PubMed

Affiliation: Max-Planck-Institut für Mikrostrukturphysik, 06120 Halle, Germany.

ABSTRACT
We use scanning tunneling spectroscopy (STS) experiments and first-principles density functional theory (DFT) calculations to address a fundamental question of how quantum well (QW) states for electrons in a metal evolve spatially in the lateral direction when there is a surface step that changes the vertical confinement thickness. This study reveals a clear spatially dependent, nearly continuous trend in the energetic shifts of quantum well (QW) states of thin Ag(111) film grown on Cu(111) substrate, showing the strongest change near the step edge. A large energetic shift equaling up to ~200 meV with a lateral extension of the QW states of the order of ~20 Å is found, even though the step-edge is atomically sharp as evidenced by a line scan. The observed lateral extension and the nearly smooth transition of QW states are understood within the context of step-induced charge oscillation, and Smoluchowski-type charge spreading and smoothing.

No MeSH data available.


Orbital selective k-resolved electronic density of states EDOS of bulk Ag along with its band structure.The  band mostly has sp-orbital character responsible for our observed QW states. The color scale yellow (light) and red (dark) correspond low and high EDOS respectively.
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f1: Orbital selective k-resolved electronic density of states EDOS of bulk Ag along with its band structure.The band mostly has sp-orbital character responsible for our observed QW states. The color scale yellow (light) and red (dark) correspond low and high EDOS respectively.

Mentions: Ag is a monovalent noble metal with fully filled 4d shell crystallizing in the fcc structure. For Ag, the bottom of the s-type valence band is situated below the binding energy of −7 eV and the top of the s-band above the Fermi energy (EF), leaving the energy range of −2.3 to −0.3 eV (where the QW states are observed in our experiment) for the bands with mostly p-type character (see Fig. 1). In this energy window, the band structures along the Γ-L(111) direction mainly consist of two bands : a highly dispersive parabolic band originating mostly from p-states and a relatively flat band derived from d-states. Within the d-band region, two sharp peaks are located around binding energies of −3.8 and −5 eV. The band touches EF at the L point on the boundary of the first Brillouin zone, and decreases monotonically in energy towards the Γ point. There is an energy gap of about 4 eV at the L point above EF. When the Ag film becomes sufficiently thin along the growth axis direction (say, z) to realize the quantum confinement of Ag electrons in the film, the bands originating from sp-states are expected to form quantized states in the energy range of −2.3 to −0.3 eV. Indeed, for Ag(111) film, the QW states have been observed by photoemission over the widest range of thickness and binding energy for any overlayer/substrate combination hitherto studied15. For this reason and because of the metallurgical simplicity (Ag and Cu are completely immiscible at room temperature)16, we choose here the Ag/Cu(111) system for studying the interaction of QW states with the surface steps. An efficient experimental tool for studying the interaction of these QW states with surface steps is STM which probes the weak evanescent tails of QW electrons outside the metal in the vaccum region without destroying the interference pattern17.


Visualizing Non-abrupt Transition of Quantum Well States at Stepped Silver Surfaces.

Saha SK, Manna S, Stepanyuk VS, Kirschner J - Sci Rep (2015)

Orbital selective k-resolved electronic density of states EDOS of bulk Ag along with its band structure.The  band mostly has sp-orbital character responsible for our observed QW states. The color scale yellow (light) and red (dark) correspond low and high EDOS respectively.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4525284&req=5

f1: Orbital selective k-resolved electronic density of states EDOS of bulk Ag along with its band structure.The band mostly has sp-orbital character responsible for our observed QW states. The color scale yellow (light) and red (dark) correspond low and high EDOS respectively.
Mentions: Ag is a monovalent noble metal with fully filled 4d shell crystallizing in the fcc structure. For Ag, the bottom of the s-type valence band is situated below the binding energy of −7 eV and the top of the s-band above the Fermi energy (EF), leaving the energy range of −2.3 to −0.3 eV (where the QW states are observed in our experiment) for the bands with mostly p-type character (see Fig. 1). In this energy window, the band structures along the Γ-L(111) direction mainly consist of two bands : a highly dispersive parabolic band originating mostly from p-states and a relatively flat band derived from d-states. Within the d-band region, two sharp peaks are located around binding energies of −3.8 and −5 eV. The band touches EF at the L point on the boundary of the first Brillouin zone, and decreases monotonically in energy towards the Γ point. There is an energy gap of about 4 eV at the L point above EF. When the Ag film becomes sufficiently thin along the growth axis direction (say, z) to realize the quantum confinement of Ag electrons in the film, the bands originating from sp-states are expected to form quantized states in the energy range of −2.3 to −0.3 eV. Indeed, for Ag(111) film, the QW states have been observed by photoemission over the widest range of thickness and binding energy for any overlayer/substrate combination hitherto studied15. For this reason and because of the metallurgical simplicity (Ag and Cu are completely immiscible at room temperature)16, we choose here the Ag/Cu(111) system for studying the interaction of QW states with the surface steps. An efficient experimental tool for studying the interaction of these QW states with surface steps is STM which probes the weak evanescent tails of QW electrons outside the metal in the vaccum region without destroying the interference pattern17.

Bottom Line: This study reveals a clear spatially dependent, nearly continuous trend in the energetic shifts of quantum well (QW) states of thin Ag(111) film grown on Cu(111) substrate, showing the strongest change near the step edge.A large energetic shift equaling up to ~200 meV with a lateral extension of the QW states of the order of ~20 Å is found, even though the step-edge is atomically sharp as evidenced by a line scan.The observed lateral extension and the nearly smooth transition of QW states are understood within the context of step-induced charge oscillation, and Smoluchowski-type charge spreading and smoothing.

View Article: PubMed Central - PubMed

Affiliation: Max-Planck-Institut für Mikrostrukturphysik, 06120 Halle, Germany.

ABSTRACT
We use scanning tunneling spectroscopy (STS) experiments and first-principles density functional theory (DFT) calculations to address a fundamental question of how quantum well (QW) states for electrons in a metal evolve spatially in the lateral direction when there is a surface step that changes the vertical confinement thickness. This study reveals a clear spatially dependent, nearly continuous trend in the energetic shifts of quantum well (QW) states of thin Ag(111) film grown on Cu(111) substrate, showing the strongest change near the step edge. A large energetic shift equaling up to ~200 meV with a lateral extension of the QW states of the order of ~20 Å is found, even though the step-edge is atomically sharp as evidenced by a line scan. The observed lateral extension and the nearly smooth transition of QW states are understood within the context of step-induced charge oscillation, and Smoluchowski-type charge spreading and smoothing.

No MeSH data available.