Surface-state depopulation on small Ag(111) terraces

Synthesis of Quantum-Confined Borophene Nanoribbons

Borophene nanoribbons (BNRs) are one-dimensional strips of atomically thin boron expected to exhibit quantum-confined electronic properties that are not present in extended two-dimensional borophene. While the parent material borophene has been experimentally shown to possess anisotropic metallicity and diverse polymorphic structures, the atomically precise synthesis of nanometer-wide BNRs has not yet been achieved. Here, we demonstrate the synthesis of multiple BNR polymorphs with well-defined edge configurations within the nanometer-scale terraces of vicinal Ag(977). Through atomic-scale imaging, spectroscopy, and first-principles calculations, the synthesized BNR polymorphs are characterized and found to possess distinct edge structures and electronic properties. For single-phase BNRs, v1/6-BNRs and v1/5-BNRs adopt reconstructed armchair edges and sawtooth edges, respectively. In addition, the electronic properties of single-phase v1/6-BNRs and v1/5-BNRs are dominated by Friedel oscillations and striped moiré patterns, respectively. On the other hand, mixed-phase BNRs possess quantum-confined states with increasing nodes in the electronic density of states at elevated biases. Overall, the high degree of polymorphism and diverse edge topologies in borophene nanoribbons provide a rich quantum platform for studying one-dimensional electronic states.

Silicene growth on Ag(110) and Ag(111) substrates reconsidered in light of Si-Ag reactivity

Silicene, the 2D silicon allotrope analogue of graphene, was theoretically predicted in 1994 as a metastable buckled honeycomb silicon monolayer. Similarly to its carbon counterpart it was predicted to present an electronic structure hosting Dirac cones. In the last decade a great deal of work has been done to synthesize silicene and exploit its properties. In this paper we will review our research group activity in the field, dealing in particular with silicon-substrate interaction upon silicon deposition, and discuss the still debated silicene formation starting from the chemistry of silicon unsaturated compounds.

Controlled interaction of surface quantum-well electronic states

We report on the construction of well-defined surface quantum well arrangements by combining self-assembly protocols and molecular manipulation procedures. After the controlled removal of individual porphyrin molecules from dense-packed arrays on Ag(111), the surface state electrons are confined at the bare silver patches. These act as quantum wells that show well-defined unoccupied bound surface states. Scanning tunneling spectroscopy and complementary boundary element method calculations are performed to characterize the interaction between the bound states of adjacent quantum wells and reveal a hybridization of wave functions resulting in bonding and antibonding states. The interwell coupling can be tuned by the deliberate choice of the molecules acting as potential barriers. The fabrication method is shown to be ideally suited to engineer specific configurations as one-dimensional chains or two-dimensional artificial molecules.

Assessment of scanning tunneling spectroscopy modes inspecting electron confinement in surface-confined supramolecular networks

Scanning tunneling spectroscopy (STS) enables the local, energy-resolved investigation of a samples surface density of states (DOS) by measuring the differential conductance (dI/dV) being approximately proportional to the DOS. It is popular to examine the electronic structure of elementary samples by acquiring dI/dV maps under constant current conditions. Here we demonstrate the intricacy of STS mapping of samples exhibiting a strong corrugation originating from electronic density and local work function changes. The confinement of the Ag(111) surface state by a porous organic network is studied with maps obtained under constant-current (CC) as well as open-feedback-loop (OFL) conditions. We show how the CC maps deviate markedly from the physically more meaningful OFL maps. By applying a renormalization procedure to the OFL data we can mimic the spurious effects of the CC mode and thereby rationalize the physical effects evoking the artefacts in the CC maps.

Enhanced charge transfer in a monolayer of the organic charge transfer complex TTF-TNAP on Au(111)

Electronic doping is a key concept for tuning the properties of organic materials. In bulk structures, the charge transfer between donor and acceptor is mainly given by the respective ionization potential and electron affinity. In contrast, monolayers of charge transfer complexes in contact with a metal are affected by an intriguing interplay of hybridization and screening at the metallic interface, determining the resulting charge state. Using scanning tunneling microscopy and spectroscopy, we characterize the electronic properties of the organic acceptor molecule 11,11,12,12-tetracyanonaptho-2,6-quinodimethane (TNAP) adsorbed on a Au(111) surface. The ordered islands remain in a weakly physisorbed state with no charge transfer interaction with the substrate. When the electron donor tetrathiafulvalene (TTF) is added, ordered arrays of alternating TNAP and TTF rows are assembled. In these structures, we find the lowest unoccupied molecular orbital (LUMO) of the free TNAP molecule shifted well below the Fermi level of the substrate. The TNAP is thus charged with more than one electron.

Electron states confined within nano-steps on metal surfaces

To elucidate electron states confined within steps on metal surfaces, we demonstrated a new point of view that a linear step on a noble metal surface can be treated as a dipole potential composed of delta functions. For an electron confined by two pairs of dipole potentials, we derived quasi-stationary eigenstates whose eigenenergies are complex numbers which lead to the lifetime of the electron. To derive the local density of states (LDOS), scanning tunneling microscopy (STM) images and scanning tunneling spectra (STS) for stepped surfaces, we incorporated the lifetime effect on them and clarified the relation between the LDOS and the STM current by applying the expression for the STM current derived by Selloni et al (1985 Phys. Rev. B 31 2602). Although, in previous studies, the Fabry-Pérot interference mechanism has been used to explain electron states confined within two steps, it requires four fitting parameters, in contrast our method requires one fitting parameter which specifies the height of the delta functions. Our results for LDOS images, topographical images and STS are consistent with experimental ones for both the cases where electrons stay on a terrace confined by two steps and on a wide terrace outside the step, which confirms the validity of our model.

Narrow Au(111) terraces decorated by self-organized Co nanowires: a low-temperature STM/STS investigation

Deposition of Co atoms on Au(111) surfaces leads to the formation of self-organized bilayer Co nanowires at the step edges between adjacent narrow Au(111) terraces. Scanning tunneling microscopy and spectroscopy at low temperatures is used to probe the influence of the finite dimensions on the local density of states for both the Co wires and the narrow Au terraces. Confinement of Au surface state electrons to narrow Au terraces causes a shift of the Au surface state onset energy to higher energies. For the Co nanowires discrete energy levels are observed. Moreover, standing wave patterns occur at the surface of both the Au and the Co. The patterns increase in complexity with increasing energy. All Co nanowires formed at the edges of narrow Au terraces reveal a characteristic maximum in the local density of states at a significantly different energy when compared to the Co islands that are formed on large Au terraces. The experimental results can be interpreted in terms of a simple particle-in-a-box model.

Tuning the ferromagnetic coupling of Fe nanodots on Cu(111) via dimensionality variation of the mediating electrons

Using in situ magneto-optical Kerr effect measurements and phenomenological modeling, we study the tunability in both the magnetization anisotropy and magnetic coupling of Fe nanodots on a curved Cu(111) substrate with varying vicinity. We observe that, as the terrace width w decreases, the magnetization anisotropy increases monotonically, faster when w is smaller than the nanodot size d. In contrast, the magnetic coupling strength also increases until w approximately d, after which it decreases steeply. These striking observations can be rationalized by invoking the counterintuitive dimensionality variation of the surface electrons mediating the interdot coupling: the electrons are confined to be one dimensional (1D) when w > or = d, but become quasi-2D when w < d due to enhanced electron spillover across the steps bridged by the nanodots.

Dislocation-induced local modulation of the surface states of Ag(111) thin films on Si(111) 7 x 7 substrates

Local modulation of the Shockley-type surface state was studied around threading dislocations at the surfaces of ultrathin Ag(111) epitaxial films on Si(111) substrates. Scanning tunneling microscope (STM) observations indicated that the wavelength of the surface state electron was shortened around the dislocations in the electron standing wave pattern. Scanning tunneling spectroscopy (STS) revealed that the bottom of the local surface state (E0) shifts downward around the dislocation. The shift in E0 and the lattice displacement Delta u(z) have a linear relation, which indicates that the shift of the surface state is caused by local relaxation of the misfit strain around the dislocation.

Contribution of the surface state to the observation of the surface Kondo resonance

Tunneling spectra obtained on and near Co atoms adsorbed on Ag(111) show at 5 to 6 K a Kondo resonance that appears as a characteristic dip around the Fermi energy. The feature is present up to 1.5 nm around Co atoms adsorbed on terraces with the surface state onset in the occupied region of the density of states. On a narrow terrace, where the surface state onset lies in the unoccupied region of the density of states, it is only present up to 0.5 nm. This difference demonstrates directly the importance of the surface state electrons in the observation of the surface Kondo resonance.

Localization of surface states in disordered step lattices

The character of the surface state wave function on regularly stepped Cu(111) is reinvestigated. It is shown that the qualitative change at terrace lengths around 17 A observed previously by Ortega et al. [Phys. Rev. Lett. 84, 6110 (2000)]] must necessarily be described as a change from a propagating superlattice state to a terrace-confined quasi-one-dimensional state. This reconciles previous, apparently contradictory experimental results and sheds new light on the behavior of nearly free electrons in nanostructures. Possible mechanisms driving the localization are discussed on the basis of the surface state bulk penetration depth, which has been measured in both regimes.

Step-lattice-induced band-gap opening at the fermi level

The interaction of the Shockley surface state with the step lattice of vicinal Cu(111) leads to the formation of an electronic superlattice state. On Cu(443), where the average terrace length forms a "shape resonance" with the Fermi wavelength, we find a step-lattice-induced band-gap opening at the Fermi level. A gap magnitude >200 meV is inferred from high resolution photoemission experiments and line shape analysis. The corresponding energy gain with respect to a gapless case is approximately 11 meV/unit cell, and is a substantial contribution to the stabilization of the step lattice.