Electronic density oscillations in gold atomic chains assembled atom by atom

Assembling fullerene into nanostructures over micrometer scale with atomic precision

Assembling large organic molecules into predesigned structures for nanoscale devices is a long-standing challenge. Here, we present the atom-scale precise repositions of individual fullerene molecules and molecule transportation over the micrometer scale on a Si(111) surface via reproducible and reversible vertical manipulation by a scanning tunneling microscopy tip. A two-rod abacus consisting of ten fullerene molecules was used to perform arithmetic operations with double digits. This opens the door for the use of larger organic molecules displaying intrinsic characteristics as complex molecular devices with novel functions.

Theoretical Insights into 1D Transition-Metal Nanoalloys Grown on the NiAl(110) Surface

Metallic nanoalloys are essential because of the synergistic effects rather than the merely additive effects of the metal components. Nanoscience is currently able to produce one-atom-thick linear atomic chains (LACs), and the NiAl(110) surface is a well-tested template used to build them. We report the first study based on ab initio density functional theory methods of one-dimensional transition-metal (TM) nanoalloys (i.e., LACs) grown on the NiAl(110) surface. This is a comprehensive and detailed computational study of the effect of alloying groups 10 and 11 metals (Pd, Pt, Cu, Ag, and Au) in LACs supported on the NiAl(110) surfaces to elucidate the structural, energetic, and electronic properties. From the TM series studied here, Pt appears to be an energy-stabilization species; meanwhile, Ag has a contrasting behavior. The work function changes because the alloying in LACs was satisfactorily explained from the explicit surface dipole moment calculations using an ab initio calculation-based approach, which captured the electron density redistribution upon building the LAC.

Plasmon-induced nonlinear response of silver atomic chains

Nonlinear response of a linear silver atomic chain upon ultrafast laser excitation has been studied in real time using the time-dependent density functional theory. We observe the presence of nonlinear responses up to the fifth order in tunneling current, which is ascribed to the excitation of high-energy electrons generated by Landau damping of plasmons. The nonlinear effect is enhanced after adsorption of polar molecules such as water due to the enhanced damping rates during plasmon decay. Increasing the length of atomic chains also increases the nonlinear response, favoring higher-order plasmon excitation. These findings offer new insights towards a complete understanding and ultimate control of plasmon-induced nonlinear phenomena to atomic precision.

Tunneling-Electron-Induced Light Emission from Single Gold Nanoclusters

The coupling of tunneling electrons with the tip-nanocluster-substrate junction plasmon was investigated by monitoring light emission in a scanning tunneling microscope (STM). Gold atoms were evaporated onto the ∼5 Å thick Al2O3 thin film grown on the NiAl (110) surface where they formed nanoclusters 3-7 nm wide. Scanning tunneling spectroscopy (STS) of these nanoclusters revealed quantum-confined electronic states. Spatially resolved photon imaging showed localized emission hot spots. Size dependent study and light emission from nanocluster dimers further support the viewpoint that coupling of tunneling electrons to the junction plasmon is the main radiative mechanism. These results showed the potential of the STM to reveal the electronic and optical properties of nanoscale metallic systems in the confined geometry of the tunnel junction.

Confinement properties of 2D porous molecular networks on metal surfaces

Quantum effects that arise from confinement of electronic states have been extensively studied for the surface states of noble metals. Utilizing small artificial structures for confinement allows tailoring of the surface properties and offers unique opportunities for applications. So far, examples of surface state confinement include thin films, artificial nanoscale structures, vacancy and adatom islands, self-assembled 1D chains, vicinal surfaces, quantum dots and quantum corrals. In this review we summarize recent achievements in changing the electronic structure of surfaces by adsorption of nanoporous networks whose design principles are based on the concepts of supramolecular chemistry. Already in 1993, it was shown that quantum corrals made from Fe atoms on a Cu(1 1 1) surface using single atom manipulation with a scanning tunnelling microscope confine the Shockley surface state. However, since the atom manipulation technique for the construction of corral structures is a relatively time consuming process, the fabrication of periodic two-dimensional (2D) corral structures is practically impossible. On the other side, by using molecular self-assembly extended 2D porous structures can be achieved in a parallel process, i.e. all pores are formed at the same time. The molecular building blocks are usually held together by non-covalent interactions like hydrogen bonding, metal coordination or dipolar coupling. Due to the reversibility of the bond formation defect-free and long-range ordered networks can be achieved. However, recently also examples of porous networks formed by covalent coupling on the surface have been reported. By the choice of the molecular building blocks, the dimensions of the network (pore size and pore to pore distance) can be controlled. In this way, the confinement properties of the individual pores can be tuned. In addition, the effect of the confined state on the hosting properties of the pores will be discussed in this review article.

Approaching the quantum limit for plasmonics: linear atomic chains

Optical excitations in atomic-scale materials can be strongly mixed, with contributions from both single-particle transitions and collective response. This complicates the quantum description of these excitations, because there is no clear way to define their quantization. To develop a quantum theory for these optical excitations, they must first be characterized so that single-particle-like and collective excitations can be identified. Linear atomic chains, such as atom chains on surfaces, linear arrays of dopant atoms in semiconductors, or linear molecules, provide ideal testbeds for studying collective excitations in small atomic-scale systems. We use exact diagonalization to study the many-body excitations of finite (10 to 25) linear atomic chains described by a simplified model Hamiltonian. Exact diagonalization results can be very different from the density functional theory (DFT) results usually obtained. Highly correlated, multiexcitonic states, strongly dependent on the electron-electron interaction strength, dominate the exact spectral and optical response but are not present in DFT excitation spectra. The ubiquitous presence of excitonic many-body states in the spectra makes it hard to identify plasmonic excitations. A combination of criteria involving a many-body state's transfer dipole moment, balance, transfer charge, dynamical response, and induced-charge distribution do strongly suggest which many-body states should be considered as plasmonic. This analysis can be used to reveal the few plasmonic many-body states hidden in the dense spectrum of low-energy single-particle-like states and many higher-energy excitonic-like states. These excitonic states are the predominant excitation because of the many possible ways to develop local correlations.

Adsorption of Pd, Pt, Cu, Ag, and Au monomers on NiAl(110) surface: a comparative study from DFT calculations

First principles calculations based on periodic density functional theory (DFT) have been used to investigate the structural, energetic and electronic properties of different transition metal atoms (Pd, Pt, Cu, Ag, and Au) on the NiAl(110) surface at low coverages (0.08 and 0.25 monolayer). All adatoms prefer to adsorb on 4-fold coordinated sites interacting with two Al and two Ni atoms and forming polar and covalent bonds, respectively. The calculated negative work function changes are explained by the effect of positive surface image created after adsorption, which induces the polarization of the negatively charged adsorbates. Consequently, for metals with similar electronegativity as Ni (Ag and Cu), this polarization effect becomes more significant and leads to larger negative work function changes, but the charge transferred is small.

Atom-by-atom assembly

Atomic manipulation using a scanning tunneling microscope (STM) tip enables the construction of quantum structures on an atom-by-atom basis, as well as the investigation of the electronic and dynamical properties of individual atoms on a one-atom-at-a-time basis. An STM is not only an instrument that is used to 'see' individual atoms by means of imaging, but is also a tool that is used to 'touch' and 'take' the atoms, or to 'hear' their movements. Therefore, the STM can be considered as the 'eyes', 'hands' and 'ears' of the scientists, connecting our macroscopic world to the exciting atomic world. In this article, various STM atom manipulation schemes and their example applications are described. The future directions of atomic level assembly on surfaces using scanning probe tips are also discussed.

Fuel mediated solution combustion synthesis of ZnO supported gold clusters and nanoparticles and their catalytic activity and in vitro cytotoxicity

Well characterized atomic gold clusters and nanoparticles on the surface of ZnO are found to be effective against cancer cells.

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.

Three-dimensional scanning force/tunneling spectroscopy at room temperature

We simultaneously measured the force and tunneling current in three-dimensional (3D) space on the Si(111)-(7 × 7) surface using scanning force/tunneling microscopy at room temperature. The observables, the frequency shift and the time-averaged tunneling current were converted to the physical quantities of interest, i.e. the interaction force and the instantaneous tunneling current. Using the same tip, the local density of states (LDOS) was mapped on the same surface area at constant height by measuring the time-averaged tunneling current as a function of the bias voltage at every lateral position. LDOS images at negative sample voltages indicate that the tip apex is covered with Si atoms, which is consistent with the Si-Si covalent bonding mechanism for AFM imaging. A measurement technique for 3D force/current mapping and LDOS imaging on the equivalent surface area using the same tip was thus demonstrated.

Extraordinary electron propagation length in a metallic double chain supported on a metal surface

The present theoretical study shows that a double chain of Cu metal atoms adsorbed on a Cu(111) metal surface can guide an excited electron for distances exceeding 10 nm. The nanostructure appears to be quasi-decoupled from the substrate and thus to act as a nanowire. The origin of the above phenomenon is the interference between the decay of the quasistationary 1D sp-band states localized on each chain. This allows to approach the situation of the formation of bound states in the continuum first discussed in quantum systems by von Neumann and Wigner.

Scanning tunneling spectroscopy

The scanning tunneling microscope (STM) has revolutionized our ability to explore and manipulate atomic-scale solid surfaces. In addition to its unparalleled spatial power, the STM can study dynamical processes, such as molecular conformational changes, by recording current traces as a function of time. It can also be employed to measure the physical properties of molecules or nanostructures down to the atomic scale. Combining STM imaging with measurement of current-voltage (I-V) characteristics [i.e., scanning tunneling spectroscopy (STS)] at similar resolution makes it possible to obtain a detailed map of the electronic structure of a surface. For many years, STM lacked chemical specificity; however, the recent development of STM-IETS (inelastic electron tunneling spectroscopy) has allowed us to measure the vibrational spectrum of a single molecule. This review introduces and illustrates these recent developments with a few simple scholarly examples.

Nonlocal conductance modulation by molecules: scanning tunneling microscopy of substituted styrene heterostructures on H-terminated Si(100)

One-dimensional organic heterostructures consisting of contiguous lines of CF3- and OCH3-substituted styrene molecules on silicon are studied by scanning tunneling microscopy and ab initio simulation. Dipole fields of OCH3-styrene molecules are found to enhance conduction through molecules near CF3-styrene/OCH_{3}-styrene heterojunctions. Those of CF3-styrene depress transport through the nearby silicon. Thus the choice of substituents and their attachment site on host molecules provide a means of differentially tuning molecule and substrate transport at the molecular scale.

Electron propagation along Cu nanowires supported on a Cu(111) surface

We present a joint experimental-theoretical study of the one-dimensional band of excited electronic states with sp character localized on Cu nanowires supported on a Cu(111) surface. Energy dispersion and lifetime of these states have been obtained, allowing the determination of the mean distance traveled by an excited electron along the nanowire before it escapes into the substrate. We show that a Cu nanowire supported on a Cu(111) surface can guide a one-dimensional electron flux over a short distance and thus can be considered as a possible component for nanoelectronics devices.

One-dimensional 3d electronic bands of monatomic Cu chains

The electronic structure of an array of monatomic Cu chains grown on the Pt(997) surface has been examined by angle-resolved photoemission. The monatomic wires exhibit properties associated with 3d electron confinement in one dimension. Along the wire direction, the 3d bands states display a dispersive character, with periodicity in reciprocal space defined by the wire array geometry. These observations are compared and analyzed with ab initio calculations based on the full-potential linearized augmented plane-wave method.

Size-dependent structure of MoS2 nanocrystals

Molybdenum disulphide nanostructures are of interest for a wide variety of nanotechnological applications ranging from the potential use of inorganic nanotubes in nanoelectronics to the active use of nanoparticles in heterogeneous catalysis. Here, we use atom-resolved scanning tunnelling microscopy to systematically map and classify the atomic-scale structure of triangular MoS2 nanocrystals as a function of size. Instead of a smooth variation as expected from the bulk structure of MoS2, we observe a very strong size dependence for the cluster morphology and electronic structure driven by the tendency to optimize the sulphur excess present at the cluster edges. By analysing of the atomic-scale structure of clusters, we identify the origin of the structural transitions occurring at unique cluster sizes. The novel findings suggest that good size control during the synthesis of MoS2 nanostructures may be used for the production of chemically or optically active MoS2 nanomaterials with superior performance.

Spatial mapping of the electronic states of a one-dimensional system

Using low-temperature scanning tunneling microscopy and spectroscopy, we have recorded spatial maps of confined electronic states in the troughs between self-organized Pt nanowires on Ge(001) that are spaced 2.4 nm apart. Two sub-bands are resolved, which correspond to the lowest energy levels of a quantum mechanical particle in a box. As expected, the spatial dI/dV maps exhibit a maximum and a minimum in the middle of the troughs for the n = 1 and n = 2 states, respectively.

Realization of a Particle-in-a-Box: Electron in an Atomic Pd Chain

Well-defined Pd chains were assembled from single atoms on a NiAl(110) surface with the tip of a scanning tunneling microscope. The electronic properties of the chains were determined by spatially resolved conductance measurements, revealing a series of quantum well states with parabolic dispersion. The particle-in-a-box states in Pd chains show higher onset energy and larger effective mass than those in Au chains investigated before, reflecting the influence of elemental composition on one-dimensional electronic systems. The intrinsic widths and spectral intensities of Pd induced states provide information on lifetime and spatial localization of states in the atomic chain.

Quantum confinement between self-organized Pt nanowires on Ge(001)

The existence of one-dimensional (1D) electronic states between self-organized Pt nanowires spaced 1.6 or 2.4 nm apart on a Ge(001) surface is revealed by low-temperature scanning tunneling microscopy. These perfectly straight Pt nanowires act as barriers for a surface state (located just below the Fermi level) of the underlying terrace. The energy positions of the 1D electronic states are in good agreement with the energy levels of a quantum particle in a well. Spatial maps of the differential conductivity of the 1D electronic states conclusively reveal that these states are exclusively present in the troughs between the Pt nanowires.

End states in one-dimensional atom chains

End states--the zero-dimensional analogs of the two-dimensional states that occur at a crystal surface--were observed at the ends of one-dimensional atom chains that were self-assembled by depositing gold on the vicinal Si(553) surface. Scanning tunneling spectroscopy measurements of the differential conductance along the chains revealed quantized states in isolated segments with differentiated states forming over end atoms. A comparison to a tight-binding model demonstrated how the formation of electronic end states transforms the density of states and the energy levels within the chains.

Electronic properties of artificial Au chains with individual Pd impurities

Artificial Au atomic chains with individual Pd impurities were assembled from single metal atoms with a scanning tunneling microscope on a NiAl(110) surface. Scanning tunneling spectroscopy (STS) revealed an electronic resonance 2.15 eV above the Fermi energy localized within 4 A of single Pd atom impurities and two electronic resonances 2.25 eV and 2.95 eV above the Fermi energy localized within 8 A of Pd dimer impurities. The emergence of these localized resonances was studied by STS at each stage of the atom-by-atom assembly. Additionally, conductance images of the chains revealed delocalized electronic density oscillations in the pure Au segments of the chains.

Theoretical study of encapsulated alkali metal atoms in nanoporous channels of ITQ-4 zeolite: one-dimensional metals and inorganic electrides

Electronic structure calculations within density-functional theory have been carried out in a class of novel inorganic electrides M-ITQ-4 zeolite (M = Na, K, Rb, Cs) to understand the competing effects of guest-guest (M-M) and guest-host (M-ITQ-4) interactions. We find that Na forms a nearly perfect 1D metal undergoing Peierls distortion whereas Cs couples rather strongly to the host accompanied by a large charge transfer. In addition to the guest-host high energy charge transfer excitations we find a new far infrared excitation peak in Na at approximately 0.35 eV that can be ascribed to the Peierls distortion.

Electronic states of linear Au clusters supported on metal surfaces: why are they like those of a particle in a box?

Scanning tunneling spectroscopy and microscopy show that the empty states of linear Au clusters supported on a metal surface behave as if they are the states of an electron in an empty one-dimensional box. We show here that certain difficulties of this description are removed by a particle-in-a-cylinder model. This interpretation is supported by density functional calculations.

Quantum confinement in monatomic Cu chains on Cu(111)

The existence of one-dimensional (1D) electronic states in Cu/Cu(111) chains assembled by atomic manipulation is revealed by low-temperature scanning tunneling spectroscopy and density functional theory (DFT) calculations. Our experimental analysis of the chain-localized electron dynamics shows that the dispersion is fully described within a 1D tight-binding approach. DFT calculations confirm the confinement of unoccupied states to the chain in the relevant energy range, along with a significant extension of these states into the vacuum region.

Visualization and spectroscopy of a metal-molecule-metal bridge

Artificial nanostructures, each composed of a copper(II) phthalocyanine (CuPc) molecule bonded to two gold atomic chains with a controlled gap, were assembled on a NiAl(110) surface by manipulation of individual gold atoms and CuPc molecules with a scanning tunneling microscope. The electronic densities of states of these hybrid structures were measured by spatially resolved electronic spectroscopy and systematically tuned by varying the number of gold atoms in the chains one by one. The present approach provides structural images and electronic characterization of the metal-molecule-metal junction, thereby elucidating the nature of the contacts between the molecule and metal in this junction.

Localized molecular constraint on electron delocalization in a metallic chain

An artificial quantum structure consisting of a single CO molecule adsorbed on a Au chain was assembled by manipulating single Au atoms on NiAl(110) at 12 K with a scanning tunneling microscope (STM). The CO disrupts the delocalization of electron density waves in the chain, as it suppresses the coupling between neighboring chain atoms. The possibility to specify the CO position on the chain allows controlled modification of the electronic properties in a quantum system. Inelastic electron tunneling spectroscopy with the STM provides vibrational characterization of the adsorbed CO.