Probing single molecule dynamics

The room temperature dynamics of single sulforhodamine 101 molecules dispersed on a glass surface are investigated on two different time scales with near-field optics. On the 10(-2)- to 10(2)-second time scale, intensity fluctuations in the emission from single molecules are examined with polarization measurements, providing insight into their spectroscopic properties. On the nanosecond time scale, the fluorescence lifetimes of single molecules are measured, and their excited-state energy transfer to the aluminum coating of the near-field probe is characterized. A movie of the time-resolved emission demonstrates the feasibility of fluorescence lifetime imaging with single molecule sensitivity, picosecond temporal resolution, and a spatial resolving power beyond the diffraction limit.     

Ballistic electron microscopy of individual molecules

We analyzed the transport of ballistic electrons through organic molecules on uniformly flat surfaces of bismuth grown on silicon. For the fullerene C60 and for a planar organic molecule (3,4,9,10-perylene-tetracarboxylic acid dianhydride), the signals revealed characteristic submolecular patterns that indicated where ballistic transport was enhanced or attenuated. The transport was associated to specific electronic molecular states. At electron energies of a few electron volts, this "scanning near-field electron transmission microscopy" method could be applied to various adsorbates or thin layers.     

Complement fixation on cell surfaces by 19S and 7S antibodies

The mechanism of complement fixation on cell surfaces by whole antiserums, and by 19S and 7S fractions has been studied with a new comple-ment-fixation test. This test is based on the fixation and transfer of the activated first component of complement (C1a). We have concluded that a single molecule of 19S antibody in combination with antigen at the cell surface is sufficient to bind one molecule of C1a. For 7S antibodies at least two molecules in close proximity at the cell surface are required to fix one molecule of C1a.     

Near-field optics: microscopy, spectroscopy, and surface modification beyond the diffraction limit

The near-field optical interaction between a sharp probe and a sample of interest can be exploited to image, spectroscopically probe, or modify surfaces at a resolution (down to approximately 12 nm) inaccessible by traditional far-field techniques. Many of the attractive features of conventional optics are retained, including noninvasiveness, reliability, and low cost. In addition, most optical contrast mechanisms can be extended to the near-field regime, resulting in a technique of considerable versatility. This versatility is demonstrated by several examples, such as the imaging of nanometric-scale features in mammalian tissue sections and the creation of ultrasmall, magneto-optic domains having implications for highdensity data storage. Although the technique may find uses in many diverse fields, two of the most exciting possibilities are localized optical spectroscopy of semiconductors and the fluorescence imaging of living cells.     

Chemistry. The motions of an enzyme soloist

Dynamics of proteins are crucial to their function. In his Perspective, Orrit stresses the advantages of studying these dynamics with single-molecule methods--which require no synchronization--rather than with conventional ensemble measurements. He highlights the report by Yang et al., who follow the fluorescence of a single enzyme molecule. Electron transfer from the fluorophore to a quencher induces fluctuations of the fluorescence lifetime along with the fluorophore-quencher distance. The wide range of characteristic times of those fluctuations reveals the complexity of the protein's potential energy landscape. As a new molecular ruler, electron transfer complements other single-molecule methods such as energy transfer (FRET) for distances shorter than a few nanometers.     

Inducing and viewing the rotational motion of a single molecule

Tunneling electrons from the tip of a scanning tunneling microscope were used to induce and monitor the reversible rotation of single molecules of molecular oxygen among three equivalent orientations on the platinum(111) surface. Detailed studies of the rotation rates indicate a crossover from a single-electron process to a multielectron process below a threshold tunneling voltage. Values for the energy barrier to rotation and the vibrational relaxation rate of the molecule were obtained by comparing the experimental data with a theoretical model. The ability to induce the controlled motion of single molecules enhances our understanding of basic chemical processes on surfaces and may lead to useful single-molecule devices.     

Atmospheric Lifetime of CHF2Br, a Proposed Substitute for Halons

The rate coefficients, k(1), for the reaction of OH with CHF(2)Br have been measured using pulsed photolysis and discharge flow techniques at temperatures (T) between 233 and 432 K to be k(1), = (7.4 +/- 1.6) x 10(-13) exp[-(1300 +/- 100)/T] cubic centimeters per molecule per second. The ultraviolet absorption cross sections, sigma, of this molecule between 190 and 280 nanometers were measured at 296 K. The k(1), and sigma values were used in a one-dimensional model to obtain an atmospheric lifetime of approximately 7 years for CHF(2)Br. This lifetime is shorter by approximately factors of 10 and 2 than those for CF(3)Br and CF(2)ClBr, respectively. The ozone depletion potentials of the three compounds will reflect these lifetimes.     

Self-assembled hexa-peri-hexabenzocoronene graphitic nanotube

An amphiphilic hexa-peri-hexabenzocoronene self-assembles to form a pi-electronic, discrete nanotubular object. The object is characterized by an aspect ratio greater than 1000 and has a uniform, 14-nanometer-wide, open-ended hollow space, which is an order of magnitude larger than those of carbon nanotubes. The wall is 3 nanometers thick and consists of helical arrays of the pi-stacked graphene molecule, whose exterior and interior surfaces are covered by hydrophilic triethylene glycol chains. The graphitic nanotube is redox active, and a single piece of the nanotube across 180-nanometer-gap electrodes shows, upon oxidation, an electrical resistance of 2.5 megohms at 285 kelvin [corrected]. This family of molecularly engineered graphite with a one-dimensional tubular shape and a chemically accessible surface constitutes an important step toward molecular electronics.     

Experimental detection of tetranitrogen

Tetranitrogen (N4), which has been the subject of great theoretical interest, has been prepared from the N4+ cation and positively detected as a gaseous metastable molecule with a lifetime exceeding 1 microsecond in experiments based on neutralization-reionization mass spectrometry. An examination of the geometry of N4+ and the fragmentation pattern of the (14)N2(15)N2 neutral molecule has revealed that the latter is characterized by an open-chain geometry with two distinct, closely bound N2 units joined by a longer weaker bond.     

Near-field microscopy through a SiC superlens

The wave nature of light limits the spatial resolution in classical microscopy to about half of the illumination wavelength. Recently, a new approach capable of achieving subwavelength spatial resolution, called superlensing, was invented, challenging the already established method of scanning near-field optical microscopy (SNOM). We combine the advantages of both techniques and demonstrate a novel imaging system where the objects no longer need to be in close proximity to a near-field probe, allowing for optical near-field microscopy of subsurface objects at sub-wavelength-scale lateral resolution.     

The microwave drill

We present a drilling method that is based on the phenomenon of local hot spot generation by near-field microwave radiation. The microwave drill is implemented by a coaxial near-field radiator fed by a conventional microwave source. The near-field radiator induces the microwave energy into a small volume in the drilled material under its surface, and a hot spot evolves in a rapid thermal-runaway process. The center electrode of the coaxial radiator itself is then inserted into the softened material to form the hole. The method is applicable for drilling a variety of nonconductive materials. It does not require fast rotating parts, and its operation makes no dust or noise.     

Breaking the diffraction barrier: optical microscopy on a nanometric scale

In near-field scanning optical microscopy, a light source or detector with dimensions less than the wavelength (lambda) is placed in close proximity (lambda/50) to a sample to generate images with resolution better than the diffraction limit. A near-field probe has been developed that yields a resolution of approximately 12 nm ( approximately lambda/43) and signals approximately 10(4)- to 10(6)-fold larger than those reported previously. In addition, image contrast is demonstrated to be highly polarization dependent. With these probes, near-field microscopy appears poised to fulfill its promise by combining the power of optical characterization methods with nanometric spatial resolution.     

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.     

Intersubband electroluminescence from silicon-based quantum cascade structures

The quantum cascade laser, which uses electronic transitions within a single band of a semiconductor, constitutes a possible way to integrate active optical components into silicon-based technology. This concept necessitates a transition with a narrow linewidth and an upper state with a sufficiently long lifetime. We report the observation of intersubband electroluminescence from a p-type silicon/silicon-germanium quantum cascade structure, centered at 130 millielectron volts with a width of 22 millielectron volts, with the expected polarization, and discernible up to 180 kelvin. The nonradiative lifetime is found to depend strongly on the design of the quantum well structure, and is shown to reach values comparable to that of an equivalent GaInAs/AlInAs laser structure.     

Single molecules observed by near-field scanning optical microscopy

Individual carbocyanine dye molecules in a sub-monolayer spread have been imaged with near-field scanning optical microscopy. Molecules can be repeatedly detected and spatially localized (to approximately lambda/50 where lambda is the wavelength of light) with a sensitivity of at least 0.005 molecules/(Hz)(1/2) and the orientation of each molecular dipole can be determined. This information is exploited to map the electric field distribution in the near-field aperture with molecular spatial resolution.     

Near-field coherent spectroscopy and microscopy of a quantum dot system

We combined coherent nonlinear optical spectroscopy with nano-electron volt energy resolution and low-temperature near-field microscopy with subwavelength resolution (相似文献   

Scanning interferometric apertureless microscopy: optical imaging at 10 angstrom resolution

Interferometric near-field optical microscopy achieving a resolution of 10 angstroms is demonstrated. The scattered electric field variation caused by a vibrating probe tip in close proximity to a sample surface is measured by encoding it as a modulation in the optical phase of one arm of an interferometer. Unlike in regular near-field optical microscopes, where the contrast results from a weak source (or aperture) dipole interacting with the polarizability of the sample, the present form of imaging relies on a fundamentally different contrast mechanism: sensing the dipole-dipole coupling of two externally driven dipoles (the tip and sample dipoles) as their spacing is modulated.     

Real-time observation of the vibration of a single adsorbed molecule

The newly developed femtosecond field emission camera was used to observe the time dependence of field emission through a single copper phthalocyanine molecule adsorbed on a tungsten tip. In many of the individual 212-picosecond-long recordings, the field emission was found to oscillate with a frequency between 5 x 10(10) and 20 x 10(10) hertz. The oscillations, which were not observed from a bare tip, are believed to arise from the vibration of a single molecule with respect to the surface. Numerical simulations confirmed the statistical significance of the data.     

Inverse velocity dependence of vibrationally promoted electron emission from a metal surface

All previous experimental and theoretical studies of molecular interactions at metal surfaces show that electronically nonadiabatic influences increase with molecular velocity. We report the observation of a nonadiabatic electronic effect that follows the opposite trend: The probability of electron emission from a low-work function surface--Au(111) capped by half a monolayer of Cs--increases as the velocity of the incident NO molecule decreases during collisions with highly vibrationally excited NO(X(2)pi((1/2)), V = 18; V is the vibrational quantum number of NO), reaching 0.1 at the lowest velocity studied. We show that these results are consistent with a vibrational autodetachment mechanism, whereby electron emission is possible only beyond a certain critical distance from the surface. This outcome implies that important energy-dissipation pathways involving nonadiabatic electronic excitations and, furthermore, not captured by present theoretical methods may influence reaction rates at surfaces.     

Measurement of single-molecule resistance by repeated formation of molecular junctions

The conductance of a single molecule connected to two gold electrodes was determined by repeatedly forming thousands of gold-molecule-gold junctions. Conductance histograms revealed well-defined peaks at integer multiples of a fundamental conductance value, which was used to identify the conductance of a single molecule. The resistances near zero bias were 10.5 +/- 0.5, 51 +/- 5, 630 +/- 50, and 1.3 +/- 0.1 megohms for hexanedithiol, octanedithiol, decanedithiol, and 4,4' bipyridine, respectively. The tunneling decay constant (betaN) for N-alkanedithiols was 1.0 +/- 0.1 per carbon atom and was weakly dependent on the applied bias. The resistance and betaN values are consistent with first-principles calculations.