Atomic resolution of the silicon (111)-(7x7) surface by atomic force microscopy

Achieving high resolution under ultrahigh-vacuum conditions with the force microscope can be difficult for reactive surfaces, where the interaction forces between the tip and the samples can be relatively large. A force detection scheme that makes use of a modified cantilever beam and senses the force gradient through frequency modulation is described. The reconstructed silicon (111)-(7x7) surface was imaged in a noncontact mode by force microscopy with atomic resolution (6 angstroms lateral, 0.1 angstrom vertical).     

The force needed to move an atom on a surface

Manipulation of individual atoms and molecules by scanning probe microscopy offers the ability of controlled assembly at the single-atom scale. However, the driving forces behind atomic manipulation have not yet been measured. We used an atomic force microscope to measure the vertical and lateral forces exerted on individual adsorbed atoms or molecules by the probe tip. We found that the force that it takes to move an atom depends strongly on the adsorbate and the surface. Our results indicate that for moving metal atoms on metal surfaces, the lateral force component plays the dominant role. Furthermore, measuring spatial maps of the forces during manipulation yielded the full potential energy landscape of the tip-sample interaction.     

Quantitative measurement of short-range chemical bonding forces

We report direct force measurements of the formation of a chemical bond. The experiments were performed using a low-temperature atomic force microscope, a silicon tip, and a silicon (111) 7x7 surface. The measured site-dependent attractive short-range force, which attains a maximum value of 2.1 nanonewtons, is in good agreement with first-principles calculations of an incipient covalent bond in an analogous model system. The resolution was sufficient to distinguish differences in the interaction potential between inequivalent adatoms, demonstrating the ability of atomic force microscopy to provide quantitative, atomic-scale information on surface chemical reactivity.     

Complex patterning by vertical interchange atom manipulation using atomic force microscopy

The ability to incorporate individual atoms in a surface following predetermined arrangements may bring future atom-based technological enterprises closer to reality. Here, we report the assembling of complex atomic patterns at room temperature by the vertical interchange of atoms between the tip apex of an atomic force microscope and a semiconductor surface. At variance with previous methods, these manipulations were produced by exploring the repulsive part of the short-range chemical interaction between the closest tip-surface atoms. By using first-principles calculations, we clarified the basic mechanisms behind the vertical interchange of atoms, characterizing the key atomistic processes involved and estimating the magnitude of the energy barriers between the relevant atomic configurations that leads to these manipulations.     

Surface order and stability of langmuir-blodgett films

Angstrom-resolution atomic force microscope images of Langmuir-Blodgett monolayers and multilayers of cadmium arachidate in air and under water show a dramatic change from a disordered arrangement to a crystalline lattice by the addition or removal of a single layer of molecules. The disordered surface is less stable than the ordered one to mechanical stresses such as atomic force microscopy tip forces or at the air-water contact line during contact angle measurements. The difference in the degree of order in the alkyl chains is attributed to the strong attractive interaction between headgroups in the presence of the divalent cation.     

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.     

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.     

Revealing the angular symmetry of chemical bonds by atomic force microscopy

We have measured the angular dependence of chemical bonding forces between a carbon monoxide molecule that is adsorbed to a copper surface and the terminal atom of the metallic tip of a combined scanning tunneling microscope and atomic force microscope. We provide tomographic maps of force and current as a function of distance that revealed the emergence of strongly directional chemical bonds as tip and sample approach. The force maps show pronounced single, dual, or triple minima depending on the orientation of the tip atom, whereas tunneling current maps showed a single minimum for all three tip conditions. We introduce an angular dependent model for the bonding energy that maps the observed experimental data for all observed orientations and distances.     

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.     

Scanned probe imaging of single-electron charge states in nanotube quantum dots

An atomic force microscope was used to study single-electron motion in nanotube quantum dots. By applying a voltage to the microscope tip, the number of electrons occupying the quantum dot could be changed, causing Coulomb oscillations in the nanotube conductance. Spatial maps of these oscillations were used to locate individual dots and to study the electrostatic coupling between the dot and the tip. The electrostatic forces associated with single electrons hopping on and off the quantum dot were also measured. These forces changed the amplitude, frequency, and quality factor of the cantilever oscillation, demonstrating how single-electron motion can interact with a mechanical oscillator.     

Energy dependence of abstractive versus dissociative chemisorption of fluorine molecules on the silicon (111)-(7x7) surface

Scanning tunneling microscopy and monoenergetic molecular beams have been used to obtain real-space atomic images of the competition between abstractive and dissociative chemisorption. The size distribution of Si-F adsorbates on the Si(111)-(7x7) surface was examined as a function of the incident translational energy of the F(2) molecules. For F(2) molecules with 0.03 electron volt of incident energy, the dominant adsorbate sites were isolated Si-F species. As an F(2) molecule with low translational energy collides with the surface, abstraction occurs and only one of the F atoms chemisorbs; the other is ejected into the gas phase. For F(2) molecules with 0.27 electron volt of incident energy, many adjacent Si-F adsorbates (dimer sites) were observed because F(2) molecules with high translational energy collide with the surface and chemisorb dissociatively so that both F atoms react to form adjacent Si-F adsorbates. For halogens with very high incident energy (0.5-electron volt Br(2)), dissociative chemisorption is the dominant adsorption mechanism and dimer sites account for nearly all adsorbates.     

Force microscopy with light-atom probes

The charge distribution in atoms with closed electron shells is spherically symmetric, whereas atoms with partially filled shells can form covalent bonds with pointed lobes of increased charge density. Covalent bonding in the bulk can also affect surface atoms, leading to four tiny humps spaced by less than 100 picometers in the charge density of adatoms on a (001) tungsten surface. We imaged these charge distributions by means of atomic force microscopy with the use of a light-atom probe (a graphite atom), which directly measured high-order force derivatives of its interaction with a tungsten tip. This process revealed features with a lateral distance of only 77 picometers.     

Fabrication of atomic-scale structures on si(001) surfaces

The scanning tunneling microscope has been used to define regular crystalline structures at room temperature by removing atoms from the silicon (001) surface. A single atomic layer can be removed to define features one atom deep and create trenches with ordered floors. Segments of individual dimer rows can be removed to create structures with atomically straight edges and with lateral features as small as one dimer wide. Conditions under which such removal is possible are defined, and a mechanism is proposed.     

X-ray structure of the major adduct of the anticancer drug cisplatin with DNA: cis-[Pt(NH3)2(d(pGpG))

Crystals of the adduct of the anticancer drug cis-diamminedichloroplatinum(II), cis-DDP, with d(pGpG), its putative target on DNA in the cancer cell, have been obtained and used in an x-ray crystallographic study to elucidate the molecular structure to atomic resolution. Each of the four crystallographically independent cis-[Pt(NH3)2(d(pGpG))] molecules is comprised of a square-planar platinum atom bonded to two ammonia ligands and two N(7) atoms of guanosine nucleosides from the same chain. Base stacking of the two adjacent guanine rings is completely disrupted by coordination to the cis-(Pt(NH3)2)2+ unit. Comparison of the backbone and deoxyribose ring torsion angles with those found by previous (nuclear magnetic resonance spectroscopy) studies of this adduct in solution demonstrates that the solid state geometry is substantially the same as that in solution. The relevance of these results to the molecular mechanism of action of cis-DDP is discussed.     

Field-Induced Nanometer- to Atomic-Scale Manipulation of Silicon Surfaces with the STM

The controlled manipulation of silicon at the nanometer scale will facilitate the fabrication of new types of electronic devices. The scanning tunneling microscope (STM) can be used to manipulate strongly bound silicon atoms or clusters at room temperature. Specifically, by using a combination of electrostatic and chemical forces, surface atoms can be removed and deposited on the STM tip. The tip can then move to a predetermined surface site, and the atom or cluster can be redeposited. The magnitude of such forces and the amount of material removed can be controlled by applying voltage pulses at different tip-surface separations.     

Orientational ordering of polymers by atomic force microscope tip-surface interaction

Results of studies on the interaction between the tip of an atomic force microscope and polystyrene molecules in a film spread on a surface are reported. The tip produces a persistent deformation on the film; some of the polymer molecules are eventually pulled up by the tip. Nanometer-size structures are induced, resulting in a pattern that is periodic and is oriented perpendicular to the scan direction.     

Nanofabrication of Small Copper Clusters on Gold(111) Electrodes by a Scanning Tunneling Microscope

The use of scanning tunneling microscopy in an electrochemical environment as a tool for the nanoscale modification of gold electrodes was demonstrated. Small copper clusters, typically two to four atomic layers in height, were precisely positioned on a gold(111) electrode by a process in which copper was first deposited onto the tip of the scanning tunneling microscope, which then acted as a reservoir from which copper could be transferred to the surface during an appropriate approach of the tip to the surface. Tip approach and position were controlled externally by a microprocessor unit, allowing the fabrication of various patterns, cluster arrays, and "conducting wires" in a very flexible and convenient manner.     

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.     

The controlled evolution of a polymer single crystal

We present a method for controlling the initiation and kinetics of polymer crystal growth using dip-pen nanolithography and an atomic force microscope tip coated with poly-dl-lysine hydrobromide. Triangular prisms of the polymer epitaxially grow on freshly cleaved mica substrates, and their in-plane and out-of-plane growth rates can be controlled by raster scanning the coated tip across the substrate. Atomic force microscope images were concomitantly recorded, providing a set of photographic images of the process as it spans the nanometer- to micrometer-length scales as a function of environmental conditions.     

重铬酸钾对大蒜根尖细胞有丝分裂的影响

[目的]研究不同浓度的重铬酸钾溶液对大蒜根尖细胞有丝分裂的影响,为探讨铬离子对植物体的细胞毒性和遗传毒性提供参考。[方法]用12．5、25．0、50．0、100．0、200．0mg／L重铬酸钾溶液分别处理大蒜根尖24、48、72h,设蒸馏水为对照组;应用常规染色体压片技术,观察铬对大蒜根尖细胞有丝分裂指数和染色体畸变现象的影响;应用Tamhane多重比较进行差异性分析。[结果]当重铬酸钾的浓度为0～25．0mg／L时,无论处理时间长短,大蒜根尖细胞有丝分裂指数均呈上升趋势;当重铬酸钾浓度大于25．0mg／L时,有丝分裂指数则呈下降趋势。Tamhane多重比较表明：用不同浓度的重铬酸钾培养大蒜根尖24h,较高浓度（200．0mg／L）与较低浓度（0、12．5、25．0mg／L）处理组间存在极显著差异（P〈0．01）;培养大蒜根尖48h,较高浓度（200．0mg／L）与较低浓度（12．5、25．0、50．0mg／L）处理组间存在极显著差异（P〈0．01）;培养大蒜根尖72h,则大部分处理组间都存在极显著差异。各处理组均有染色体畸变现象。[结论]重铬酸钾对大蒜根尖细胞的毒害作用与其浓度和作用时间有关。在一定时间内,较低浓度的重铬酸钾促进大蒜根尖细胞的有丝分裂,较高浓度的重铬酸钾则抑制大蒜根尖细胞的有丝分裂,说明铬具有二重性。