Light-matter interaction at the atomic scale rules fundamental phenomena such as photoemission and lasing, while enabling basic everyday technologies, including photovoltaics and optical communications. In this context, plasmons -the collective electron oscillations in conducting materials-are important because they allow manipulating optical fields at the nanoscale. The advent of graphene and other two-dimensional crystals has pushed plasmons down to genuinely atomic dimensions, displaying appealing properties such as a large electrical tunability. However, plasmons in these materials are either too broad or lying at low frequencies, well below the technologically relevant nearinfrared regime. Here we demonstrate sharp near-infrared plasmons in lithographically-patterned wafer-scale atomically-thin silver crystalline films. Our measured optical spectra reveal narrow plasmons (quality factor ∼ 4), further supported by a low sheet resistance comparable to bulk metal in few-atomic-layer silver films down to seven Ag(111) monolayers. Good crystal quality and plasmon narrowness are obtained despite the addition of a thin passivating dielectric, which renders our samples resilient to ambient conditions. The observation of spectrally sharp and strongly confined plasmons in atomically thin silver holds great potential for electro-optical modulation and optical sensing applications. * These two authors contributed equally to the work. † Electronic address: enrique.ortega@ehu.es ‡ Electronic address: javier.garciadeabajo@icfo.es arXiv:1901.07739v2 [cond-mat.mes-hall]

Quantum dots are known to confine electrons within their structure. Whenever they periodically aggregate into arrays and cooperative interactions arise, novel quantum properties suitable for technological applications show up. Control over the potential barriers existing between neighboring quantum dots is therefore essential to alter their mutual crosstalk. Here we show that precise engineering of the barrier width can be experimentally achieved on surfaces by a single atom substitution in a haloaromatic compound, which in turn tunes the confinement properties through the degree of quantum dot intercoupling. We achieved this by generating self-assembled molecular nanoporous networks that confine the two-dimensional electron gas present at the surface. Indeed, these extended arrays form up on bulk surface and thin silver films alike, maintaining their overall interdot coupling. These findings pave the way to reach full control over two-dimensional electron gases by means of self-assembled molecular networks.

Rubidium adsorption on the surface of the topological insulator Bi(2)Se(3) is found to induce a strong downward band bending, leading to the appearance of a quantum-confined two-dimensional electron gas state (2DEG) in the conduction band. The 2DEG shows a strong Rashba-type spin-orbit splitting, and it has previously been pointed out that this has relevance to nanoscale spintronics devices. The adsorption of Rb atoms, on the other hand, renders the surface very reactive, and exposure to oxygen leads to a rapid degrading of the 2DEG. We show that intercalating the Rb atoms, presumably into the van der Waals gaps in the quintuple layer structure of Bi(2)Se(3), drastically reduces the surface reactivity while not affecting the promising electronic structure. The intercalation process is observed above room temperature and accelerated with increasing initial Rb coverage, an effect that is ascribed to the Coulomb interaction between the charged Rb ions. Coulomb repulsion is also thought to be responsible for a uniform distribution of Rb on the surface.

Atomic staircases in noble metal surfaces are model one-dimensional superlattices, where free-electron-like surface states transform into superlattice bands with sizeable quantum size shifts and gaps. At critical step spacings d = n × (λF /2), such superlattice gaps lie at the Fermi energy, affecting the electronic energy, and hence the structural stability of the step lattice, which is held by weak elastic interactions. We use Cu, Ag, and Au curved crystals to smoothly tune the superlattice constant d in Angle Resolved Photoemission (ARPES) and Scanning Tunneling Microscopy (STM) experiments. With ARPES we accurately quantify terrace size effects and determine the superlattice potential, which increases from Ag to Cu and to Au. With STM we analyze the d-dependent terrace width distribution for Cu and Ag, and observe non-linear variations in Cu. On the grounds of simple electronic and elastic models, we conclude that terrace width distribution instabilities and electronic energy variations at d = n×(λF /2) have the same order of magnitude for Cu. In contrast, the weak superlattice potential in Ag, i.e., its smoother band structure modulation is not sufficient to alter the step lattice.

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Designing molecular organic semiconductors with distinct frontier orbitals is key for the development of devices with desirable properties. Generating defined organic nanostructures with atomic precision can be accomplished by on-surface synthesis. We use this "dry" chemistry to introduce topological variations in a conjugated poly(para-phenylene) chain in the form of meta-junctions. As evidenced by STM and LEED, we produce a macroscopically ordered, monolayer thin zigzag chain film on a vicinal silver crystal. These cross-conjugated nanostructures are expected to display altered electronic properties, which are now unraveled by highly complementary experimental techniques (ARPES and STS) and theoretical calculations (DFT and EPWE). We find that meta-junctions dominate the weakly dispersive band structure, while the band gap is tunable by altering the linear segment's length. These periodic topology effects induce significant loss of the electronic coupling between neighboring linear segments leading to partial electron confinement in the form of weakly coupled quantum dots. Such periodic quantum interference effects determine the overall semiconducting character and functionality of the chains. TOC

Quantum dot arrays in the form of molecular nanoporous networks are renown for modifying the electronic surface properties through quantum confinement. Here we show that, compared to the pristine surface state, the fundamental energy of the confined states can exhibit downward shifts accompanied by a lowering of the effective masses simultaneous to the appearance of tiny gaps at the Brillouin zone boundaries. We observed these effects by angle resolved photoemission for two self-assembled homothetic (scalable) Co-coordinated metalorganic networks. Complementary scanning tunneling spectroscopy measurements confirmed these findings. Electron plane wave expansion simulations and density functional theory calculations provide insight into the nature of this phenomenon, which we assign to metal-organic overlayer-substrate interactions in the form of adatom-substrate hybridization. The absence to date of the experimental band structure resulting from single adatom metal-coordinated nanoporous networks has precluded the observation of the significant surface state renormalization reported here, which we infer are general of low interacting and well-defined adatom arrays. * Corresponding AuthorsOver the last decades, the concepts of supramolecular chemistry have been successfully transferred to the construction of two-dimensional (2D) self-assembled molecular arrangements on metallic surfaces [1][2][3][4]. By selecting the proper tectons (molecular constituents and, if required, metal linkers) and depositing them onto selected substrates, longrange ordered, regular and robust nanoporous networks have been achieved, ranging from hydrogen- [5] or halogen-bonded [6], to metal-organic structures [7,8]. Such regular structures stand out as ideal templates for nanopatterning organic and inorganic adsorbates by selective adsorption [9][10][11][12][13][14]. Nanoporous networks, also referred to as quantum dot (QD) arrays since they can confine surface state (SS) electrons, provide a vast playground for studying and engineering the electronic properties of new and exotic 2D materials. Indeed, metal-organic networks are known to show novel magnetic properties [15,16], catalytic effects [17], oxidation states [18], exotic tesellation [19][20][21] and bear the prospect of exhibiting topological electronic bands [22,23].The dominant electronic signature around the Fermi level due to the presence of nanoporous networks comes from the substrate's surface state electrons, which scatter at the molecular array and become confined within individual nanopores [24,25]. The tunability of the confined electronic state has so far been achieved by varying the pore dimensions, i.e. the QD size [25,26]. However, since the confining potential barriers are not infinite, these QDs are not independent but coupled, as has been shown by angle resolved photoemission (ARPES) through the existence of new dispersive electronic bands [24], as well as by Fourier-transform scanning tunneling spectroscopy (FT-STS) data [27]. These QD array bands can be modified...

We investigate the scattering of electrons belonging to Shockley states of (111)-oriented noble metal surfaces using angle resolved photoemission (ARPES) and scanning tunneling microscopy (STM). Both ARPES and STM indicate that monatomic steps on a noble metal surface may act either as strongly repulsive or highly transmissive barriers for surface electrons, depending on the coherence of the step lattice, and irrespectively of the average step spacing. By measuring curved crystal surfaces with terrace length ranging from 30 to 180Å, we show that vicinal surfaces of Au and Ag with periodic step arrays exhibit a remarkable wave function coherence beyond 100Å step spacings, well beyond the Fermi wavelength limit and independently of the projection of the bulk band gap on the vicinal plane. In contrast, the analysis of transmission resonances investigated by STM shows that a pair of isolated parallel steps defining a 58Å wide terrace confines and decouples the surface state of the small terrace from that of the (111) surface. We conclude that the formation of laterally confined quantum well states in vicinal surfaces as opposed to propagating superlattice states depends on the loss of coherence driven by imperfection in the superlattice order.