Atomic-scale coupling of photons to single-molecule junctions

Spatial resolution at the atomic scale has been achieved in the coupling of light to single molecules adsorbed on a surface. Electron transfer to a single molecule induced by green to near-infrared light in the junction of a scanning tunneling microscope (STM) exhibited spatially varying probability that is confined within the molecule. The mechanism involves photo-induced resonant tunneling in which a photoexcited electron in the STM tip is transferred to the molecule. The coupling of photons to the tunneling process provides a pathway to explore molecular dynamics with the combined capabilities of lasers and the STM.     

Single-molecule vibrational spectroscopy and microscopy

Vibrational spectra for a single molecule adsorbed on a solid surface have been obtained with a scanning tunneling microscope (STM). Inelastic electron tunneling spectra for an isolated acetylene (C2H2) molecule adsorbed on the copper (100) surface showed an increase in the tunneling conductance at 358 millivolts, resulting from excitation of the C-H stretch mode. An isotopic shift to 266 millivolts was observed for deuterated acetylene (C2D2). Vibrational microscopy from spatial imaging of the inelastic tunneling channels yielded additional data to further distinguish and characterize the two isotopes. Single-molecule vibrational analysis should lead to better understanding and control of surface chemistry at the atomic level.     

Carbynes in meteorites: detection, low-temperature origin, and implications for interstellar molecules

Carbon from the Allende meteorite is not graphite but carbyne (triply bonded elemental carbon), inasmuch as on heating to 250 degrees to 330 degrees C it releases mainly triply bonded fragments: -(C identical withC)(n),- with n = 1 to 5, and -(C identical withC)(n)-CN, with n = 1 to 3. Although carbynes have been known to form only by condensation of carbon vapor above 2600 K or by explosive shock of > 600 kilobars, it is found that they also form metastably by the reaction 2CO --> CO(2) + C (solid) at 300 degrees to 400 degrees C in the presence of a chromite catalyst. Such low-temperature formation by surface catalysis may be the dominant source of carbynes on the earth and in meteorites, and a major source of interstellar carbynes and cyanopolyacetylenes.     

Dissociation of individual molecules with electrons from the tip of a scanning tunneling microscope

The scanning tunneling microscope (STM) can be used to select a particular adsorbed molecule, probe its electronic structure, dissociate the molecule by using electrons from the STM tip, and then examine the dissociation products. These capabilities are demonstrated for decaborane(14) (B(10)H(14)) molecules adsorbed on a silicon(111)-(7 x 7) surface. In addition to basic studies, such selective dissociation processes can be used in a variety of applications to control surface chemistry on the molecular scale.     

Infrared Reflection-Absorption Spectroscopy and STM Studies of Model Silica-Supported Copper Catalysts

The structure of model silica-supported copper catalysts has been investigated with scanning tunneling microscopy (STM) and infrared reflection-absorption spectroscopy (IRAS). The IRAS studies of CO on the model silica-supported copper catalysts indicate that there are several types of copper clusters with surface structures similar to (111), (110), and other high-index planes of single-crystal copper. The STM studies show several types of copper clusters on silica and reveal images of metal clusters on an amorphous oxide support with atomic resolution.     

Vibrationally resolved fluorescence excited with submolecular precision

Tunneling electrons from a scanning tunneling microscope (STM) were used to excite photon emission from individual porphyrin molecules adsorbed on an ultrathin alumina film grown on a NiAl(110) surface. Vibrational features were observed in the light-emission spectra that depended sensitively on the different molecular conformations and corresponding electronic states obtained by scanning tunneling spectroscopy. The high spatial resolution of the STM enabled the demonstration of variations in light-emission spectra from different parts of the molecule. These experiments realize the feasibility of fluorescence spectroscopy with the STM and enable the integration of optical spectroscopy with a nanoprobe for the investigation of single molecules.     

Confinement of electrons to quantum corrals on a metal surface

A method for confining electrons to artificial structures at the nanometer lengthscale is presented. Surface state electrons on a copper(111) surface were confined to closed structures (corrals) defined by barriers built from iron adatoms. The barriers were assembled by individually positioning iron adatoms with the tip of a 4-kelvin scanning tunneling microscope (STM). A circular corral of radius 71.3 A was constructed in this way out of 48 iron adatoms. Tunneling spectroscopy performed inside of the corral revealed a series of discrete resonances, providing evidence for size quantization. STM images show that the corral's interior local density of states is dominated by the eigenstate density expected for an electron trapped in a round two-dimensional box.     

基于ARM的温室大棚CO_2监控系统的设计与实现

CO2浓度的监控在以温室大棚为代表的现代农业设施中发挥着巨大的作用。本文所述系统是基于ARM的控制芯片,根据植物生长规律,在上午9时至11时监控CO2浓度,使温室中的CO2浓度始终维持在400ppm到550ppm之间。系统将CO2浓度变送器采集到的数据传送至STM32,STM32经过分析处理数据来控制后端执行模块开断CO2气瓶,以使温室内CO2维持在适宜的浓度,这样既节约了资源又能满足作物的生理需求。     

Atomic resolution imaging of adsorbates on metal surfaces in air: iodine adsorption on pt(111)

The adsorption of iodine on platinum single crystals was studied with the scanning tunneling microscope (STM) to define the limits of resolution that can be obtained while imaging in air and to set a target resolution for STM imaging of metal surfaces immersed in an electrochemical cell. Two iodine adlattice unit cells of slightly different iodine packing density were clearly imaged: ( radical7 x radical7) R19.1 degrees -I, surface coverage ?(I) = 3/7; and (3 x 3)-I, ?(I) = 4/9. The three iodine atoms in the ( radical7 x radical7) unit cell form a regular hexagonal lattice interatomic distance d(I) = 0.424 nanometer, with two atoms adsorbed in threefold hollow sites and one atom adsorbed at an atop site. The (3 x 3) unit cell showed two different packing arrangements of the four iodine atoms exit. In one of the (3 x 3) structures, the iodine atoms pack to form a hexagonal lattice, d(I) = 0.417 nanometer, with three of the iodine atoms at twofold adsorption sites and one atom at an atop site. Another packing arrangement of iodine into the (3 x 3) unit cell was imaged in which the iodine atoms are not arranged symmetrically.     

Crystal structure of a silyl cation with no coordination to anion and distant coordination to solvent

The crystal structure of a stable silyl cation, triethylsilylium, in the form of its tetrakis (pentafluorophenyl)borate salt [Et(3)Si(+) (C(6)F(5))(4)B(-)] (Et, ethyl) shows no coordination between cation and anion. The closest silicon-fluorine distance is greater than 4 angstroms. A toluene solvent molecule is close enough to cause some deviations from planarity at the silicon, but the silicon-toluene distance is well beyond the sum of the silicon and carbon covalent radii. The toluene molecule is essentially planar and undistorted, as expected if little or no positive charge has been transferred from silicon to toluene.     

Determining transition-state geometries in liquids using 2D-IR

Many properties of chemical reactions are determined by the transition state connecting reactant and product, yet it is difficult to directly obtain any information about these short-lived structures in liquids. We show that two-dimensional infrared (2D-IR) spectroscopy can provide direct information about transition states by tracking the transformation of vibrational modes as a molecule crossed a transition state. We successfully monitored a simple chemical reaction, the fluxional rearrangement of Fe(CO)5, in which the exchange of axial and equatorial CO ligands causes an exchange of vibrational energy between the normal modes of the molecule. This energy transfer provides direct evidence regarding the time scale, transition state, and mechanism of the reaction.     

A molecule carrier

We found that anthraquinone diffuses along a straight line across a flat, highly symmetric Cu111 surface. It can also reversibly attach one or two CO2 molecules as "cargo" and act as a "molecule carrier," thereby transforming the diffusive behavior of the CO2 molecules from isotropic to linear. Density functional theory calculations indicated a substrate-mediated attraction of approximately 0.12 electron volt (eV). Scanning tunneling microscopy revealed individual steps of the molecular complex on its diffusion pathway, with increases of approximately 0.03 and approximately 0.02 eV in the diffusion barrier upon attachment of the first and second CO2 molecule, respectively.     

Rectification of STM Current to Graphite Covered with Phthalocyanine Molecules

The scanning tunneling microscope (STM) can be used to measure current-voltage characteristics on an atomic scale. The attachment of copper phthalocyanine molecules, in contrast to a variety of other molecules, to graphite changes the electrical characteristics of the STM from relatively symmetric to highly asymmetric or rectifying. Evidence is presented to show that the asymmetry arises because of the electronic energy levels of the copper phthalocyanine. The organic molecules were bonded to the graphite by an acid-base reaction that may have wide applicability.     

Manipulation of the Reconstruction of the Au(111) Surface with the STM

Modification of the reconstruction of an Au(111) surface with a scanning tunneling microscope (STM) is demonstrated. This modification is accomplished by transferring a number of surface atoms to the STM tip to generate a surface multivacancy (hole), which modifies the stress distribution at the surface. The structural changes that follow the tip-induced surface perturbation are imaged in a time-resolved manner. The structural modification is the result of both short-range interactions, which lead to local atomic relaxation, and long-range elastic interactions, which produce large-scale rearrangements.     

An atomic seesaw switch formed by tilted asymmetric Sn-Ge dimers on a Ge (001) surface

When tin (Sn) atoms are deposited on a clean germanium (Ge) (001) surface at room temperature, buckled dimers originating from the Sn atoms are formed at the Ge-dimer position. We identified the dimer as a heterogeneous Sn-Ge dimer by reversing its buckling orientation with a scanning tunneling microscope (STM) at 80 kelvin. An atomic seesaw switch was formed for one-dimensional electronic conduction in the Ge dimer-row direction by using the STM to reversibly flip the buckling orientation of the Sn-Ge dimer and to set up standing-wave states.     

A stable high-index surface of silicon: si(5 5 12)

A stable high-index surface of silicon, Si(5 5 12), is described. This surface forms a 2 x 1 reconstruction with one of the largest unit cells ever observed, 7.7 angstroms by 53.5 angstroms. Scanning tunneling microscopy (STM) reveals that the 68 surface atoms per 2 x 1 unit cell are reconstructed only on a local scale. A complete structural model for the surface is proposed, incorporating a variety of features known to exist on other stable silicon surfaces. Simulated STM images based on this model have been computed by first-principles electronic-structure methods and show excellent agreement with experiment.     

Dimer preparation that mimics the transition state for the adsorption of H2 on the Si(100)-2 x 1 surface

A chemically induced dimer configuration was prepared on the silicon (Si) (100) surface and was characterized by scanning tunneling microscopy (STM) and spectroscopy (STS). These prepared dimers, which are essentially untilted and differ both electronically and structurally from the dynamically tilting dimers normally found on this surface, are more reactive than normal dimers. For molecular hydrogen (H2) adsorption, the enhancement is about 10(9) at room temperature. There is no appreciable barrier for the H2 reaction at prepared sites, indicating the prepared configuration closely approximates the actual dimer structure in the transition state. This previously unknown ability to prepare specific surface configurations has important implications for understanding and controlling reaction dynamics on semiconductor surfaces.     

Elementary structural motifs in a random network of cytosine adsorbed on a gold(111) surface

Nonsymmetrical organic molecules adsorbed on solid surfaces may assemble into random networks, thereby providing model systems for organic glasses that can be directly observed by scanning tunneling microscopy (STM). We investigated the structure of a disordered cytosine network on a gold(111) surface created by thermal quenching, to temperatures below 150 K, of the two-dimensional fluid present on the surface at room temperature. Comparison of STM images to density functional theory calculations allowed us to identify three elementary structural motifs (zigzag filaments and five- and six-membered rings) that underlie the whole supramolecular random network. The identification of elementary structural motifs may provide a new framework for understanding medium-range order in amorphous and glassy systems.     

Reversible rotation of antimony dimers on the silicon (001) surface with a scanning tunneling microscope

The scanning tunneling microscope (STM) was used to control the configuration of antimony clusters on the (001) surface of silicon. In particular, the STM tip induced a reversible rotation between two orthogonal orientations of individual antimony dimers on the surface. This simple rotation can be explained by an atomic-scale torque exerted on the antimony dimers by the STM tip. The reversibility of this process could provide a basis for making atomic-scale memory cells.     

Reversible control of hydrogenation of a single molecule

Low-temperature scanning tunneling microscopy was used to selectively break the N-H bond of a methylaminocarbyne (CNHCH3) molecule on a Pt(111) surface at 4.7 kelvin, leaving the C-H bonds intact, to form an adsorbed methylisocyanide molecule (CNCH3). The methylisocyanide product was identified through comparison of its vibrational spectrum with that of directly adsorbed methylisocyanide as measured with inelastic electron tunneling spectroscopy. The CNHCH3 could be regenerated in situ by exposure to hydrogen at room temperature. The combination of tip-induced dehydrogenation with thermodynamically driven hydrogenation allows a completely reversible chemical cycle to be established at the single-molecule level in this system. By tailoring the pulse conditions, irreversible dissociation entailing cleavage of both the C-H and N-H bonds can also be demonstrated.