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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.


(a) Lower Panel : A linescan taken across the step indicated by dashed black line. Upper Panel: Calculated (DFT) height profile across the step for this linescan. (b) Calculated in-plane (along x-direction) bond-lengths (Δxi+1,I = xi+1 − xi, where i stands for atom index) across the step for this linescan.
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f3: (a) Lower Panel : A linescan taken across the step indicated by dashed black line. Upper Panel: Calculated (DFT) height profile across the step for this linescan. (b) Calculated in-plane (along x-direction) bond-lengths (Δxi+1,I = xi+1 − xi, where i stands for atom index) across the step for this linescan.

Mentions: The Ag(111) slab is represented by a supercell built from fcc Ag by stacking a variable number of fcc unit cells along the 111 direction and a Cu(111) substrate is modeled by continuing the fcc Ag lattice by Cu atoms [see lower panel of Fig. 3(a) and Fig. 4]. In the xy plane, which is the interface plane, the Cu lattice constant is adapted to the Ag value in order to avoid a lattice mismatch. The stepped Ag surface on Cu(111) substrate [for instance, Ag(4.5 ML)/Cu(111) system] is constructed by using a half-layer model where half of the atoms of one surface layer of a silver 111 oriented slab are removed. This leads to an Ag(111) overlayer slab with stepped top and flat bottom. In this structure, terraces form an infinite (along y-direction) stripe of Ag atoms, which is 15 atomic rows wide in the upper surface layer and 15 for the lower surface layer. This striped surface are five layers thick in cross-section through the upper terrace and four layers thick through the lower terraces (trenches) in between. Except the substrate (bottommost) layer, the rest of the whole system is relaxed so as to minimize the forces acting on the atoms using a conjugate-gradient algorithm. It is easy to conceive that the atoms near a surface step will suffer displacement from their (regular) flat (2D) surface lattice sites. Unlike flat surfaces, stepped surfaces relax in both z and x directions, since the existence of steps at the surface leads to broken symmetry in both of these directions. While relaxations along the z-direction yield modified interlayer separations, those along the x-direction provide new registries of atoms, as compared to those in the bulk. In contrast to bulk atoms the forces acting on surface atoms near step are unbalanced leading to resultant forces and hence, to a geometrical relaxation (in addition to the relaxation of the surface atom layer as a whole). This relaxation can further unbalance the forces acting on the next-nearest neighbors and so forth, giving rise to an elastic strain emanating from the step site.


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

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

(a) Lower Panel : A linescan taken across the step indicated by dashed black line. Upper Panel: Calculated (DFT) height profile across the step for this linescan. (b) Calculated in-plane (along x-direction) bond-lengths (Δxi+1,I = xi+1 − xi, where i stands for atom index) across the step for this linescan.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f3: (a) Lower Panel : A linescan taken across the step indicated by dashed black line. Upper Panel: Calculated (DFT) height profile across the step for this linescan. (b) Calculated in-plane (along x-direction) bond-lengths (Δxi+1,I = xi+1 − xi, where i stands for atom index) across the step for this linescan.
Mentions: The Ag(111) slab is represented by a supercell built from fcc Ag by stacking a variable number of fcc unit cells along the 111 direction and a Cu(111) substrate is modeled by continuing the fcc Ag lattice by Cu atoms [see lower panel of Fig. 3(a) and Fig. 4]. In the xy plane, which is the interface plane, the Cu lattice constant is adapted to the Ag value in order to avoid a lattice mismatch. The stepped Ag surface on Cu(111) substrate [for instance, Ag(4.5 ML)/Cu(111) system] is constructed by using a half-layer model where half of the atoms of one surface layer of a silver 111 oriented slab are removed. This leads to an Ag(111) overlayer slab with stepped top and flat bottom. In this structure, terraces form an infinite (along y-direction) stripe of Ag atoms, which is 15 atomic rows wide in the upper surface layer and 15 for the lower surface layer. This striped surface are five layers thick in cross-section through the upper terrace and four layers thick through the lower terraces (trenches) in between. Except the substrate (bottommost) layer, the rest of the whole system is relaxed so as to minimize the forces acting on the atoms using a conjugate-gradient algorithm. It is easy to conceive that the atoms near a surface step will suffer displacement from their (regular) flat (2D) surface lattice sites. Unlike flat surfaces, stepped surfaces relax in both z and x directions, since the existence of steps at the surface leads to broken symmetry in both of these directions. While relaxations along the z-direction yield modified interlayer separations, those along the x-direction provide new registries of atoms, as compared to those in the bulk. In contrast to bulk atoms the forces acting on surface atoms near step are unbalanced leading to resultant forces and hence, to a geometrical relaxation (in addition to the relaxation of the surface atom layer as a whole). This relaxation can further unbalance the forces acting on the next-nearest neighbors and so forth, giving rise to an elastic strain emanating from the step site.

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.