Photon emission at molecular resolution induced by a scanning tunneling microscope

The tip-surface region of a scanning tunneling microscope (STM) emits light when the energy of the tunneling electrons is sufficient to excite luminescent processes. These processes provide access to dynamic aspects of the local electronic structure that are not directly amenable to conventional STM experiments. From monolayer films of carbon-60 fullerenes on gold(110) surfaces, intense emission is observed when the STM tip is placed above an individual molecule. The diameter of this emission spot associated with carbon-60 is approximately 4 angstroms. These results demonstrate the highest spatial resolution of light emission to date with a scanning probe technique.     

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.     

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.     

Current-induced hydrogen tautomerization and conductance switching of naphthalocyanine molecules

The bistability in the position of the two hydrogen atoms in the inner cavity of single free-base naphthalocyanine molecules constitutes a two-level system that was manipulated and probed by low-temperature scanning tunneling microscopy. When adsorbed on an ultrathin insulating film, the molecules can be switched in a controlled fashion between the two states by excitation induced by the inelastic tunneling current. The tautomerization reaction can be probed by resonant tunneling through the molecule and is expressed as considerable changes in the conductivity of the molecule. We also demonstrated a coupling of the switching process so that the charge injection in one molecule induced tautomerization in an adjacent molecule.     

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.     

Scanning tunneling microscopy of recA-DNA complexes coated with a conducting film

A link between scanning tunneling microscopy (STM) and conventional transmission electron microscopy has been established for biological material by applying STM on freeze-dried recA-DNA complexes coated with a conducting film. The topography of the complexes observed by means of STM revealed a right-handed single helix composed of about six recA monomers per helical turn.     

Chemistry. How to assemble a molecular junction

Junctions between metals and molecules play an important role in molecular electronics. Advances in this field are hampered by the lack of understanding of the electronic structure of organic-metal interfaces. In his Perspective, Kummel highlights the report by Nazin et al. (3), who have used scanning tunneling microscopy (STM) to assemble a metal-molecule-metal junction. Subsequently, they employed the STM tip to probe the atomic structure and local electronic properties of the metal-molecule interface in unprecedented detail. They find evidence for strong coupling between the molecular and metal states. Such coupling affects the conductivity of metal-molecule-metal junctions.     

Single-bond formation and characterization with a scanning tunneling microscope

A scanning tunneling microscope (STM) was used to manipulate the bonding of a carbon monoxide (CO) molecule and to analyze the structure and vibrational properties of individual products. Individual iron (Fe) atoms were evaporated and coadsorbed with CO molecules on a silver (110) surface at 13 kelvin. A CO molecule was transferred from the surface to the STM tip and bonded with an Fe atom to form Fe(CO). A second CO molecule was similarly transferred and bonded with Fe(CO) to form Fe(CO)(2). Controlled bond formation and characterization at the single-bond level probe chemistry at the spatial limit.     

Coupling and entangling of quantum states in quantum dot molecules

We demonstrate coupling and entangling of quantum states in a pair of vertically aligned, self-assembled quantum dots by studying the emission of an interacting electron-hole pair (exciton) in a single dot molecule as a function of the separation between the dots. An interaction-induced energy splitting of the exciton is observed that exceeds 30 millielectron volts for a dot layer separation of 4 nanometers. The results are interpreted by mapping the tunneling of a particle in a double dot to the problem of a single spin. The electron-hole complex is shown to be equivalent to entangled states of two interacting spins.     

Molecular structure of DNA by scanning tunneling microscopy

Uncoated DNA molecules marked with an activated tris(l-aziridinyl) phosphine oxide (TAPO) solution were deposited on gold substrates and imaged in air with the use of a high-resolution scanning tunneling microscope (STM). Constant-current and gap-modulated STM images show clear evidence of the helicity of the DNA structure: pitch periodicity ranges from 25 to 35 angstroms, whereas the average diameter is 20 angstroms. Molecular structure within a single helix turn was also observed.     

"Molecule corrals" for studies of monolayer organic films

Scanning tunneling microscopy (STM) studies have demonstrated that monolayer-deep, flat-bottomed, circular etch pits can be grown on highly ordered pyrolytic graphite by high-temperature etching in the presence of oxygen. In this work, these graphite etch pits are used as "molecule corrals" to isolate ensembles of molecules for study by STM. The nucleation of self-assembled molecular films in the corrals took place by nucleation events separate from those leading to self-assembly on the surrounding terrace and allowed the measurement of the nucleation rate constant in the corrals. The dependence of the nucleation rate for self-assembly on pit size shows that nucleation occurs at open terrace sites and that step edges (that is, the corral's perimeter) and confinement inhibit film growth.     

Creating favorable geometries for directing organic photoreactions in alkanethiolate monolayers

The products of photoreactions of conjugated organic molecules may be allowed by selection rules but not observed in solution reactions because of unfavorable reaction geometries. We have used defect sites in self-assembled alkanethiolate monolayers on gold surfaces to direct geometrically unfavorable photochemical reactions between individual organic molecules. High conductivity and stochastic switching of anthracene-terminated phenylethynylthiolates within alkanethiolate monolayers, as well as in situ photochemical transformations, have been observed and distinguished with the scanning tunneling microscope (STM). Ultraviolet light absorbed during imaging increases the apparent heights of excited molecules in STM images, a direct manifestation of probing electronically excited states.     

Negative differential resistance on the atomic scale: implications for atomic scale devices

Negative differential resistance (NDR) is the essential property that allows fast switching in certain types of electronic devices. With scanning tunneling microscopy (STM) and scanning tunneling spectroscopy, it is shown that the current-voltage characteristics of a diode configuration consisting of an STM tip over specific sites of a boron-exposed silicon(111) surface exhibit NDR. These NDR-active sites are of atomic dimensions ( approximately 1 nanometer). NDR in this case is the result of tunneling through localized, atomic-like states. Thus, desirable device characteristics can be obtained even on the atomic scale.     

Identification of the products from the reaction of chlorine with the silicon(111)-(7x7) surface

The various products from the reaction of chlorine (Cl) with the adatom layer of the Si(111)-(7x7) surface have been identified with scanning tunneling microscopy (STM). Initially, a single Cl atom reacts with the adatom dangling bond. At higher surface coverage, additional Cl atoms insert themselves into the Si-Si backbonds between the adatom and rest-atom layers, producing adatoms that have reacted with two or three Cl atoms. These products are characterized by different registries with respect to the underlying rest layer and appear in STM images as adatoms of different sizes, consistent with the breaking of Si-Si backbonds and the formation ofnew Si-Cl bonds.     

Manipulating Chlorine Atom Bonding on the Si(100)-(2 x 1) Surface with the STM

Chlorine atoms strongly chemisorbed at dangling bond sites on the Si(100)-(2 x 1) surface are observed by scanning tunneling microscopy (STM) to hop between adjacent sites. The origin of this behavior is suggested to be an interaction between the field of the probe tip and the dipole moment of the silicon-chlorine bond. Chlorine atom migration is shown to be facilitated by the presence of a metastable chlorine bridge-bonded minimum. The STM probe was used to excite single chlorine atoms into this bridging configuration, resulting in a local population inversion. Selective application of voltage pulses between the probe tip and the surface rearranged the local bonding and induced transformations between different types of chlorine sites. In this manner, adsorbed species can be dissected and their composition and structure directly probed.     

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.     

利用下端腐蚀法制备纳米级STM探针

在深入分析电化学腐蚀原理的基础上,发现了一种提高针尖的尖锐程度的新方法,即:利用下端腐蚀方法得到了比传统的上端腐蚀方法更尖锐的针尖.通过对腐蚀电压、腐蚀溶液浓度、切断时间的研究和分析,总结出下端腐蚀法制备纳米级STM探针的最佳综合条件;并通过对前人方案的修改和完善,研制了一套自动控制切断电路装置,该电路装置可以任意设置切断条件,以此得到不同粗细的针尖;最后将据此制作的纳米级针尖成功地应用于Unisoku-STM仪器的扫描,得到了清晰、稳定的原子级分辨Bi(0001)图像.     

Near--Atomic Resolution Imaging of Ferroelectric Liquid Crystal Molecules on Graphite by STM

Near-atomic resolution images of a two-dimensional heteroepitaxial crystal composed of the relatively "functionally rich" chiral liquid crystal mesogen MDW 74 on graphite have been obtained by scanning tunneling microscopy (STM). This work is aimed at developing an improved understanding of the commercially crucial phenomenon of liquid crystal alignment by studying well-characterized surfaces. Herein is reported molecular-level characterization of the surface underlying a ferroelectric liquid crystal in situ, a requisite starting point for understanding the liquid crystal-solid interface at the molecular level. The results are also important in the context of developing a model for the molecular. origins of the contrast observed in STM images of organic monolayers on conductor surfaces. The data and analysis provide strong evidence that neither frontier orbital alone (highest occupied or lowest unoccupied molecular orbital) is sufficient to describe the observed tunneling efficiency.     

Electromechanical properties of graphene drumheads

We determined the electromechanical properties of a suspended graphene layer by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) measurements, as well as computational simulations of the graphene-membrane mechanics and morphology. A graphene membrane was continuously deformed by controlling the competing interactions with a STM probe tip and the electric field from a back-gate electrode. The probe tip-induced deformation created a localized strain field in the graphene lattice. STS measurements on the deformed suspended graphene display an electronic spectrum completely different from that of graphene supported by a substrate. The spectrum indicates the formation of a spatially confined quantum dot, in agreement with recent predictions of confinement by strain-induced pseudomagnetic fields.     

Picometer-scale electronic control of molecular dynamics inside a single molecule

Tunneling electrons from a low-temperature (5 kelvin) scanning tunneling microscope were used to control, through resonant electronic excitation, the molecular dynamics of an individual biphenyl molecule adsorbed on a silicon(100) surface. Different reversible molecular movements were selectively activated by tuning the electron energy and by selecting precise locations for the excitation inside the molecule. Both the spatial selectivity and energy dependence of the electronic control are supported by spectroscopic measurements with the scanning tunneling microscope. These experiments demonstrate the feasibility of controlling the molecular dynamics of a single molecule through the localization of the electronic excitation inside the molecule.