Field desorption microscopy of the 〈111〉 trihedral angle of a reconstructed tungsten tip

The surface structure near the 〈111〉 trihedral angle, which forms in an electric-field-heated tungsten tip, is studied by field electron microscopy, continuous-mode field desorption microscopy, and high-temperature field evaporation microscopy. The shape and structure of the surface depend on the temperature, field, and time. The angle is formed by three {011} planes, with the (111) plane at its vertex being retained in the form of a triangle or a hexagon with randomly arranged atomic clusters. The edges between {011} faces represent long and narrow {112} planes having longitudinal or transverse steps. In the absence of field evaporation, the edges and angle sharpen, becoming monoatomic. Field evaporation from the angle or microprotrusions on the edges extends these edges and causes transverse steps to appear on them. The explanation of the changes in the shape and structure of the surface is based on considering the competition of surface diffusion, crystal growth in an electric field, and field evaporation.     

Atomically sharp 〈111〉 trihedral angle of a tungsten tip

The processes occuring in the course of heating of a tungsten tip in an electric field and resulting in the formation of the 〈111〉 trihedral angle at the intersection of three {011} closest packed planes in the crystal lattice of tungsten are investigated using field-emission microscopy, continuous-mode field-desorption microscopy, and high-temperature field evaporation microscopy. It is demonstrated that atomically sharp angles can be formed at temperatures above 2200 K in the absence of field evaporation. An atom forming the apex of the trihedral angle lies in the triangle of atoms arranged in the (111) plane. In the triangle, each atom is located at the intersection of the 〈111〉 close-packed atomic rows, which are the boundaries of the {011} planes forming the trihedral angle and the {112} planes forming the angle edges two rows in width.     

STM study of SiC crystallite growth on vicinal Si(111) surfaces

The initial stage of epitaxial growth of cubic β-SiC on vicinal Si(111) misoriented towards the [112̄] direction is studied by scanning tunneling microscopy in ultra-high vacuum. The clean Si(111) surface contains terraces separated by groups of atomic steps. The separation between the atomic steps within a group is observed to be approximately equal to the length of the long axis of the Si(111)7 × 7 unit cell. We postulate that the SiC forms three-sided pyramids with surfaces of (110) orientation. The pyramids are located mostly at the step edges and are sharper than the end of the tip. This results in a series of identically shaped tip images located at the step edges, which display the structure of the tip.     

在双热舟化学气相沉积系统中通过掺In技术生长GaN纳米线和纳米锥

在现有的一台蒸发镀膜机基础上，设计加工了一个双热舟化学气相沉积系统.该系统具有真空度高、升温速度快、源和衬底温度可分别控制等优点，有利于化合物半导体纳米材料的生长.利用该生长系统，通过在生长过程中掺入等电子杂质In作为表面活性剂，分别在Si衬底和3C-SiC/Si衬底上生长出高质量的具有纤锌矿结构的单晶GaN纳米线和纳米尖三棱锥.所得产物通过场发射扫描电子显微镜、高分辨透射电子显微镜、能量色散x射线谱仪、x射线衍射仪，和荧光谱仪进行表征.这里所用的生长方法新颖，生长出的GaN纳米尖三棱锥在场发射和激光方面有潜在的应用价值. 关键词： GaN 纳米结构 透射电子显微镜 光致荧光谱     

3D atom probe assisted by femtosecond laser pulses

The 3DAP allows to image a material in 3D on a nearly atomic scale. It is based on the field evaporation occurring at the surface of a biased tip like shape specimen with an end radius of 50 nm. Surface atoms are removed one by one from the tip by means of fs laser pulses so that the physical process involved in this laser enhanced field evaporation might correspond to the very early stages of the ablation process. This technique makes possible to distinguish between different regimes of material removal such as thermal evaporation or in the case of metals or semiconductors an evaporation assisted by the rectification of the optical field at the surface. In this paper the principle of the 3DAP is presented and the underlying physics involved in the field evaporation assisted by femtosecond laser pulses is discussed.     

The properties of the nonequilibrium LaB6 surface resulting from field evaporation

The nonequilibrium surface of single-crystal lanthanum hexaboride needles and its modifications are studied with a time-of-flight atomic probe. The surface is obtained by room-temperature field evaporation. The mass spectra of field evaporation shed light on the surface composition at the needle tip immediately after tip etching, corrosion in residual gases, intense cleaning by field evaporation, and the relaxation of the nonequilibrium surface by heating to 1250 K. Conditions for the breakdown of an oxide film on the tip surface and for obtaining the mass spectra of field evaporation for stoichiometric or lanthanum-enriched pure LaB₆ single crystals are discussed.     

Field-induced evaporation of carbon nanotubes

We report here an experimental observation of field emission from arrays of multiwall carbon nanotubes. Current densities in the range 10–30 mA/cm² with excellent long-term stability were recorded. A detailed study of the destruction of nanotubes at extreme operation conditions is performed. We established that field evaporation of nanotubes accompanies field emission from a cold cathode at electric fields higher than 2 V/?. Electron microscopy of the evaporation products reveals irregularly shaped carbon nanoparticles with a hollow core. The diameter of the particles is ∼20 nm. A mechanism of the process is proposed and discussed. Received: 6 October 2000 / Accepted: 28 April 2001 / Published online: 27 June 2001     

Change in the emitter surface during high-current-density field emission from the 〈111〉 trihedral angle of a reconstructed tungsten tip

Changes in the shape and emission characteristics of the atomically sharp trihedral 〈111〉 angle of a tungsten tip reconstructed in an electric field are studied by continuous-mode field desorption microscopy during high-current-density field emission. The main changes in the tip shape and the slope of the Fowler-Nordheim characteristic occur at an emission current of 1–5 μA. At a current of 50–100 μA taken from the angle, the tip shape and emission characteristics stabilize and remain unchanged in the range 0–150 μA. The new tip shape is characterized by the widening of the angle edges; the appearance of {112} and {001} plane steps in them; a decrease in the sizes of the {011} planes forming the angle faces; and the appearance of steplike transition regions between {011}, {001}, and {112} faces. These changes in the tip shape are related to the fact that the emission field is weaker than the electric field used for preliminary tip reconstruction, the weakening of the field by the space charge of emitted electrons, and a nonuniform temperature distribution in the tip.     

The influence of the oxygen exposure on the thermal faceting of W[1 1 1] tip

The oxygen induced faceting of the macroscopic W[1 1 1] tip has been studied for oxygen exposures in the range 0.5-31 L and annealing temperatures 800-1800 K using the field ion microscopy (FIM) technique. After annealing at temperatures lower than 800 K, higher than 1850 K or for exposures lower than 0.5 L faceting was not observed. For exposures 0.5-1.9 L and annealing temperatures 800-1600 K well developed {1 1 2} facets with sharp edges formed. For exposures higher than 2.0 L edges of the {1 1 2} facets were broadening and disappearing, what has been attributed to the formation of three-dimensional tungsten oxides. The oxides could be easily removed by annealing the tip at 1700 K, what leads to formation of sharp facet edges. On the basis of these results a modified procedure of the ultrasharp tip fabrication has been proposed.     

碳纳米管薄膜场蒸发效应

在超高真空系统中对基于丝网印刷方法制备的碳纳米管薄膜的场蒸发效应进行实验研究. 实验发现, 碳纳米管薄膜样品存在场蒸发现象, 蒸发阈值场在10.0-12.6 V/nm之间, 蒸发离子流可以达到百皮安量级; 扫描电子显微镜分析和场致电子发射测量结果表明, 场蒸发会使碳纳米管分布变得更加不均匀, 会导致薄膜的场致电子发射开启电压上升(240→300V)、场增强因子下降(8300→4200)、蒸发阈值场上升(10→12.6V/nm), 同时使得薄膜场致电子发射的可重复性明显变好. 场蒸发也是薄膜自身电场一致性修复的表现, 这种修复并非表现在形貌上, 而是不同区域场增强因子之间的差距会越来越小, 这样薄膜场致电子发射的可重复性和稳定性自然会得到改善.     

The influence of tip–sample interaction on step fluctuations on Ag(111)

The fluctuations of the position of monatomic steps on Ag(111) are investigated by scanning tunneling microscopy (STM). We analyze the influence of tip–sample interaction by varying the gap impedance over more than two orders of magnitude. For tunneling tips providing a weak tip–sample interaction, we show that the step position autocorrelation function remains essentially unaltered. In this unperturbed case, the kinetics of step fluctuations are found to be dominated by one-dimensional mass transport. For larger variations of the tip–sample distance or for less favorable tip configurations, we observe a tip-induced increase of the step fluctuations. Our measurements suggest that this effect is caused rather by short-range forces than by the electric field in the tunneling gap.     

Calculation of the temperature and thermal expansion of a STM tip heated by a short laser pulse

A mathematical model for the calculation of the temperature field in a scanning tunneling microscope (STM) tip under laser illumination is developed. The duration of the laser pulse is a few nanoseconds or shorter. A Gaussian distribution of the laser light intensity in time and space is assumed. Two different mechanisms of tip heating are taken into account: 1. due to an enhanced electric field on the tip; 2. due to heating of the side surface of the tip by the focused spot of laser light. An average tip temperature is calculated using the heat conductivity equation. The enhanced electric field on the tip is calculated by the method of boundary integral equations. Received: 20 August 2002 / Revised version: 4 December 2002 / Published online: 19 March 2003 RID="*" ID="*"Corresponding author. Fax: +49-2551/962-490, E-mail: sklein@fh-muenster.de     

Atomic structures on faceted W(111) surfaces induced by ultrathin films of Pd

The faceting of Pd/W(111) surfaces has been studied using a Scanning tunneling microscope (STM). Three-sided pyramidal facets having {211} faces with dimensions ranging from 3 to 15 nm can be induced by ultrathin Pd films (≥ 1 monolayer), upon annealing to 700 K or higher. From atomic-resolution STM-images of these surfaces, we obtain direct confirmation of the {211} structure on individual facets of the 3-sided pyramids. In addition, the atomic structure of the facet edges indicates that edge energy may play a role in faceting. When the as-deposited coverage of Pd is greater than the critical value ( 1 monolayer) for inducing faceting, the extra Pd atoms diffuse to form 3-dimensional clusters, some with discernible crystalline structures, upon annealing.     

The effect of an externally applied electric field on the reconstruction of Pt(110)

The influence of an externally applied electric field on the reconstruction of the Pt(110) surface has been examined by field ion microscopy. Electric field strengths up to and including 16 V/nm have no apparent effect on the thermally induced reconstruction of Pt clusters at 305 K. At field strengths of 21 V/nm and above, the thermal reconstruction process at 305 K is enhanced by “high-temperature” field evaporation.     

A determination of the rate and heat of evaporation by measuring the radius variations of a metal tip

A new material loss technique for the experimental determination of the free evaporation rate of a metal is presented. This technique is based on the study of tip profile changes. The tip radius variation is a consequence of the simultaneous action of free evaporation and surface self-diffusion. The rate of free evaporation is related to the tip radius evolution, so the vapor pressure and heat of evaporation can be determined by measuring in situ the radius changes with time. The method permits an easy control of the cleanliness of the surface by field electron microscopy, and should enable one to study the influence of adsorption on evaporation. The method is tested with molybdenum tips. The variation of the radius of the tips, heated in ultra-high vacuum, is determined by field electron and scanning microscope measurements. Vapor pressures and heat of evaporation values obtained are in agreement with values obtained by other techniques.     

Faceting of nanoscale fingers on the (1 1 1) surface of gold

Gold fingers, one atomic layer (0.25 nm) high, 4-5 nm wide, and several hundred nm long are formed on the (1 1 1) surface of gold at room temperature by a combination of atomic manipulation and surface self-organisation. Each finger has two parallel edges (type A and type B, respectively) running along its length. The type A step is found to have higher step energy and become nanofaceted when disturbed by either thermal energy or the electric field under the STM tip, leading to the transformation of fingers to “nano-knives”. Our findings reveal the important role of step energy in the process of nanostructure fabrication on surfaces. The gold fingers also provide an ideal system for the investigation of meta-stable nanostructures.     

Local fracture of thin metallic films during electromagnetic loading

The pulsed electrodynamic fracture of thin (10–50 nm) aluminum films deposited onto polymer substrates is experimentally studied. The fracture of the films manifests itself in the form of discontinuity (crack) channel growth across an applied electric field. The roles of a magnetic pressure, thermomechanical stresses, and Joule heating at the tip of a growing crack are analyzed at high current densities (∼10¹⁰−10¹² A/m²). In contrast to the results well known for the electrodynamic fracture of bulk metallic samples, the first two factors in the films are insignificant. The effect of the electric explosion of the crack tip is found to play a key role in the beginning of fracture.     

Electric field induced oscillations in the catalytic water production on rhodium: A theoretical analysis

Methods are developed to describe catalytic reactions on multi-facetted surfaces in high electric fields in the conditions of field emission microscopy. These methods are applied to the hydrogen–oxygen–rhodium system for which a mean-field kinetic model is established. This model is shown to reproduce not only the nonequilibrium regimes of bistability and oscillations and their nanopatterns, but also the temperature-programmed desorption spectra of hydrogen and oxygen on rhodium, as well as the equilibrium phase diagram of oxygen on rhodium with the transition to the trilayer surface oxide ORhO. The dependence of the kinetic constants on the surface orientation of the facets is taken into account by expanding them in kubic harmonics suitable for the fcc rhodium crystal. The electric field modifies the gas pressure as well as the activation energies of the different kinetic processes. The tip of the field emission microscope is shown to behave as a nanoreactor.     

Modification of interface structure and contact resistance between a carbon nanotube and a gold electrode by local melting

The electrical connection between a multiwalled carbon nanotube (MWNT) and a gold electrode on applying an electric current was studied by performing in situ transmission electron microscopy observations while simultaneous measuring the bias voltage and the electric current. The tip of the MWNT was brought into contact with the gold surface. When a current density of ∼10⁸ A/cm² flowed through the contact, the gold surface started to melt along the surface of the MWNT tip due to Joule heating. At about twice the current density, a drastic change was observed in the structure of the gold surface in the contact region. This structural change increased the contact area between the MWNT tip and gold, which reduced the electrical contact resistance.     

A One‐Dimensional Particle‐in‐Cell Model of Plasma Build‐Up in Vacuum Arcs

Understanding the mechanism of plasma build‐up in vacuum arcs is essential in many fields of physics. A one‐dimensional particle‐in‐cell computer simulation model is presented, which models the plasma developing from a field emitter tip under electrical breakdown conditions, taking into account the relevant physical phenomena. As a starting point, only an external electric field and an initial enhancement factor of the tip are assumed. General requirements for plasma formation have been identified and formulated in terms of the initial local field and a critical neutral density. The dependence of plasma build‐up on tip melting current, the evaporation rate of neutrals and external circuit time constant has been investigated for copper and simulations imply that arcing involves melting currents around 0.5–1 A/μm², evaporation of neutrals to electron field emission ratios in the regime 0.01 – 0.05, plasma build‐up timescales in the order of ～ 1 – 10 ns and two different regimes depending on initial conditions, one producing an arc plasma, the other one not. Also the influence of the initial field enhancement factor and the external electric field required for ignition has been explored, and results are consistent with the experimentally measured local field value of ～ 10 GV/m for copper (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)