Tip Apex Radius Research Articles

The art of electrochemical etching for preparing tungsten probes with controllable tip profile and characteristic parameters

Using custom made experimental apparatus, the art of electrochemical etching was systematically studied for fabricating micro/nano tungsten probes with controllable tip profiles of exponential, conical, multidiameter, and calabashlike shapes. The characteristic parameters of probe including length, aspect ratio, and tip apex radius could also be well defined. By combining of static and dynamic etching, the conical-shape probe with length up to several millimeters, controllable tip apex radius, and cone angle could be fabricated. In addition, by continuously lifting the tungsten wire up during the electrochemical etching with different speeds and distances, the multidiameter shape probe could be fabricated. Finally by controlling the anodic flow, the multiple "neck-in" could be realized creating a calabashlike probe. The aspect ratio of probes depends on (i) the effective contact time between the surrounding electrolyte and the wire, (ii) the neck-in position of immersed tungsten wire. Under the optimized etching parameters, tungsten probes with a controllable aspect ratio from 20:1 to 450:1, apex radius less than 20 nm, and cone angle smaller than 3° could be achieved. The technique is well suited for the tungsten probe fabrication with a stabilized stylus contour, ultra-sharp apex radius, and high production reproducibility. The art for preparing microprobes will facilitate the application of such microprobes in diverse fields such as dip-pen nanolithography, scanning probe microscopy, micromachining, and biological cellular studies.

Confinement of the field electron emission to atomic sites on ultra sharp tips

The spatially controlled field assisted etching method for sharpening metallic tips, in a field ion microscope (FIM), is used to study the evolution of the field emission when the tip apex radius is decreased below 1 nm. Unlike the conventional image formation in a field emission microscope (FEM), we demonstrate that at this scale the field emission is rather confined to atomic sites. A single atom apex fabricated at the end of such tips exhibits an outstanding brightness compared to other atomic tips. The measurements have been repeated for two double atom tips, with different atom-atom separations, and images of atomic field emission localization have also been obtained. We have found that the field emission intensity alternates between adjacent atoms when the applied voltage is gradually increased beyond a threshold value.

Sharpening Conical Tungsten Nanotips in KOH Solution under Laser Irradiation

Laser-enhanced chemical etching with a 532 nm neodymium-doped yttrium aluminum garnet pulsed laser in KOH solution was used for sharpening conical tungsten nanotips which were prefabricated with a conventional electrochemical etch. The laser irradiation was incident axially on the tip apex. Due to different light absorption mechanisms by objectives in different sizes, the laser irradiation generates a controllable temperature gradient profile from the tip apex to the tip base. In the KOH solution, the etching rate varies with the temperature profile along the tip axis. The tip apex radius was greatly reduced, with the shape and aspect ratio in good control.

Simulation of optical near and far fields of dielectric apertureless scanning probes

We study apertureless field enhancing optical probes beyond thespherical approximation in a smooth transition towards up to3 µm long conical silicon tips. Such tips are used in apertureless scanning near fieldoptical microscopy, which holds the promise of sub 10 nm lateral resolution. A fullythree-dimensional numerical solution to the Maxwell equations is obtained with themultiple multipole method giving simultaneously both near fields and scattered far fields.The significance of focused beam excitation for work with long tips is illustrated and therelative influence of relevant length scales such as tip length, excitation wavelength, andbeam waist radius is discussed. In the limit of vanishing tip apex radius, the near fieldgrows without bounds, whereas the far field remains finite. We verify that forsmall apex radii the near field confinement at the tip apex, which is related tothe achievable lateral resolution, scales with the inverse of the radius. We find,however, that long tips exhibit a markedly lower confinement than spherical or veryshort tips. Relevant for experimental studies, we demonstrate how scanning theexcitation field with long conical tips can be a useful technique for mapping the focalvolume. We show that the normalized near field at the tip apex is robustly tolerantagainst small misalignments or misorientations of illumination focus and tip apex.

Sensitivity and resolution in noncontact electrostatic force microscopy in the case of a constant potential

The basic dynamic equations of the electrostatic force microscope are derived, in the case of a constant potential applied to a spherical tip above a conducting plane sample. The microscope tip moves in the electrostatic field, providing a force supposed to be inversely proportional to the distance. The linear and nonlinear approximations are treated to obtain the amplitude and phase of the vibration, at a fixed frequency. Optimal experiment conditions, i.e., distance, frequency, and voltage, which give the best sensitivity and lateral resolution, are discussed. The stability of the oscillations during the scans is dependent on the experiment conditions. In addition to the tapping regime, we show that two stable noncontact regimes may exist in some conditions. Only one stable noncontact oscillation occurs at a working frequency equal or superior to the natural frequency of the system. When a distance equal to the free amplitude is selected, the phase shift due to the potential is linear in function of the applied potential (instead of quadratic as observed at larger distances) and a good sensitivity to the potential of the sample is obtained. The theoretical lateral resolution is derived in the linear and nonlinear approximations, and is shown to depend strongly on the mean tip-sample distance. In the examples shown, the best resolution values are reached at a very close distance, with values much lower than the tip apex radius.

Interactions of an atomic force microscope tip with a reversed ferroelectric domain

A theory of interactions between an atomic force microscope (AFM) tip and a ferroelectric domain that forms under its electric field is proposed. We show that even for low potentials these interactions are dominant compared to the van der Waals forces and the electrostatic forces, within a certain distance interval. This interval expands, almost linearly, with the applied voltage. Dependence of the critical voltage required for domain formation as a function of the tip-ferroelectric surface distance is defined. This interaction force drops abruptly with removal of the tip from the ferroelectric surface, increases with the applied voltage and the tip apex radius, and depends logarithmically weak on the tip cone length.

A novel method for making high aspect ratio solid diamond tips

A novel method for making high aspect ratio solid diamond tips is presented. The new method is based on focused ion beam (FIB) milling of high aspect ratio inverted conical-shape holes in silicon, then the holes are filled with diamond by hot-filament chemical vapor deposition (HFCVD) method, the solid diamond tips are formed after removal of silicon by wet chemical etching. Inverted conical shape holes of high aspect ratio with controllable depth and curvature has been achieved by optimising the beam current, the diameter of the milled holes, the milling time and the scan scheme. SEM inspection has revealed that the diamond tip has up to 7.5:1 aspect ratio, and tip apex radius can be as small as sub-50 nm. The micro-Raman (mRm) spectrum from a diamond tip shows dominant diamond line at 1332 cm −1 and a slight peak at 1500 cm −1 from the sp 2 bonded carbon. The high aspect ratio solid diamond tips have potential applications in indenters and electron emitters due to the excellent mechanical and negative electron affinity (NEA) properties of diamond.

The field emission properties of high aspect ratio diamond nanocone arrays fabricated by focused ion beam milling

High aspect ratio diamond nanocone arrays are formed on freestanding diamond film by means of focused ion beam (FIB) milling technology and hot-filament chemical vapor deposition (HFCVD) method. The structure and phase purity of an individual diamond nanocone are characterized by scanning electron microscopy (SEM) and micro-Raman spectroscopy. The result indicatesthat the diamond cones with high aspect ratio and small tip apex radius can be obtained by optimizing the parameters ofFIB milling and diamond growth. The diamond nanocone arrays were also used to study the electron field emission propertiesand electric field shielding effect, finding high emission current density, low threshold and weak shielding effect, all attributable to the high field enhancement factor and suitable cone density of the diamond nanocone emitter.

A systematic study of dry etch process for profile control of silicon tips

Control of silicon tip profile is desirable in many applications. Although there are many parameters that have influence on the final tip profile, the study reported in this paper is concentrated on the effect of adding CHF 3, O 2 and Ar gases to the normal SF 6 reactive etching process. With fixed R.F. power, total pressure and SF 6 flow rate, addition of these gases has been found to have significant influence on tip profile, including tip size, height and apex radius. A particular shape of silicon tips can be achieved by fine tuning the flow rate of these added gases in combination with the tuning of other process parameters. Comparing with other processes, our technique is more simplified and is capable of manufacturing sharp tips with a larger variety of shapes.

Scanning tunneling spectroscopy on organic semiconductors: Experiment and model

Scanning-tunneling spectroscopy experiments performed on conjugated polymer films are compared with three-dimensional numerical model calculations for charge injection and transport. It is found that if a sufficiently sharp tip is used, the field enhancement near the tip apex leads to a significant increase in the injected current, which can amount to more than an order of magnitude and can even change the polarity of the predominant charge carrier. We show that when charge injection from the tip into the organic material predominates, it is possible to probe the electronic properties of the interface between the organic material and a metallic electrode directly by means of tip height versus bias voltage measurements. Thus, one can determine the alignment of the molecular orbital energy levels at the buried interface, as well as the single-particle band gap of the organic material. By comparing the single-particle energy gap and the optical absorption threshold, it is possible to obtain an estimate of the exciton binding energy. In addition, our calculations show that by using a one-dimensional model, reasonable parameters can only be extracted from $z\text{\ensuremath{-}}V$ and $I\text{\ensuremath{-}}V$ curves if the tip apex radius is much larger than the tip height. In all other cases, the full three-dimensional problem needs to be considered.

Preparation of gold tips suitable for tip-enhanced Raman spectroscopy and light emission by electrochemical etching

We describe a method of preparing gold scanning tunneling microscopy (STM) tips by direct current electrochemical etching in concentrated HCl and ethanol solution. Gold tips with tip apex radius lower than 30 nm can be reproducibly prepared by this method. The influence of the solution composition, etching voltage on the surface structure, and sharpness has been investigated. These tips can be efficiently used for STM imaging, tip-enhanced Raman spectroscopy, and light emission investigations on the same sample.

Imaging individual chains and aggregates on conjugated polymer films

AbstractWe demonstrate a novel method to visualize individual molecular chains in the surface layer of organic semiconducting polymer films of arbitrary thickness. The method is based on scanning force microscopy and employs a combination of a sensitive phase detection system and metal coated tips with a low (∼1 N/m) spring constant and a relatively high Q factor (∼200). The molecularly resolved morphology is observed in phase images that are taken simultaneously with the topography. Surprisingly, we found that, when the tip apex radius exceeds the intermolecular spacing in the surface layer, surface aggregates –when present- are visualized. In both cases, the phase contrast is shown to result from van der Waals interaction between the conjugated backbone of the polymer chains and the metallic tip, and can quantitatively be described by a simple harmonic oscillator model. We have employed this method to study the morphology of poly(p-phenylene vinylene) (PPV) derivatives with different substitutions.

Finite element simulations of the resolution in electrostatic force microscopy

In this paper, we present simulation results for the electrostatic force between two conducting parts placed at different voltages: an atomic force microscope (AFM) sensor and a metallic sample. The sensor is composed of a cantilever supporting a conical tip terminated by a spherical apex. The simulations are based on the finite element method. For tip–sample distances (5–50 nm) and for an electrically homogeneous plane, the electrostatic force can be compared to the results obtained with the equivalent charge model and experiment. By scanning a plane with a potential step, the variation of the electrostatic force near the discontinuity gives the spatial resolution in electrostatic force microscopy (EFM). We establish then the relationships between the resolution, tip– sample distance, and tip apex radius. The electrostatic force microscope results from one of many specializations of tip sensor in near-field microscopy [1, 2]. More precisely, this type of microscope is realized by applying a voltage on a conducting AFM tip. It is a good tool for imaging samples that present a gradient of electrical properties [3–5]. Variations of flexion of the cantilever holding the tip during a scan allow us to construct an electrical image [6] on inhomogeneous materials as well as on nanostructures (superlattices, nanoelectronics, etc.) [7–9]. In the simple case where the tip is in front of a conductive plane sample, we can deduce the force applied on the sensor by means of analytical expressions [10–12] or an equivalent charge model [13]. As soon as the geometry of the sample becomes complex (integrated circuits, dielectrics), the theoretical behavior of the system can be obtained by numerical methods such as the surface charge method [14], finite difference method [15], or finite element method [16]. To determine the properties of the electrostatic force microscope in front of a sample with areas at different potentials, we propose to use the finite element method. In Sect. 1 we verify the results obtained by this numerical method in the simple case of a tip in front of a plane sample at constant potential [13]. In Sect. 2 we consider the response of the microscope near a potential step [17]. For this, we study the 3-dimensional tip–object system and determine the force applied on the tip by the finite element method and then we deduce the resolution for a potential step. 1 Mathematical model 1.1 The electrostatic problem The problem consists in determining the interaction between an AFM sensor (tip + cantilever) and an infinite plane (both conducting). If the tip is long enough or the distance d between the tip and the sample is small, we can neglect the effect of the cantilever [18]. Then, the study is reduced to the calculation of the force exerted on a conical tip in front of a metallic plane. We treat the problem in 3-dimensional space because heterogeneities, as introduced in Sect. 2, cause the revolution symmetry to disappear. First, we must obtain the potential distribution in the space between the tip and the plane. We solved the Laplace equation in a domain Ω bounded by Γ . Γ is composed of three parts Γ0, Γ1, and Γ2, which are defined given potentials and electrical fields (see Fig. 1). The problem is written as follows: ∆v= 0 in Ω (1) v= v0 on Γ0 (2) v= v1 on Γ1, for a simple conducting plane (3)

Reliable tip preparation for high-resolution scanning tunneling microscopy

We describe a method to prepare electron emitters (tips) designed to achieve routine atomic resolution in a scanning tunneling microscope. Highly reliable results were obtained with repeated sputter-anneal processes in ultrahigh vacuum using polycrystalline tungsten wires as the base material. A tip apex radius of less than 5 nm and an atomic resolution of III–V (110) semiconductor surfaces are routinely obtained. The analysis involves scanning electron microscopy, transmission electron microscopy, scanning tunneling microscopy, and current–voltage characteristics in the field emission regime.

Tip shape evolution: Capillarity-induced matter transport by surface diffusion. I

From basic equations describing physical phenomena occuring at different temperatures, we have calculated numerically the matter transport by surface diffusion alone or by simultaneous action of surface diffusion and evaporation. Tip morphological evolution is then obtained by computer simulation for different cone angles and for different temperatures. Theoretical results show a critical cone angle. For tips with cone angles below this critical value a solid drop formation and detachment is observed at the tip end. For angles greater than this critical value two cases must be considered. First, for matter transport by surface diffusion only, steady-state profile evolution is predicted; such an evolution follows the Herring scaling law t 1 4 . Second, in the case of simultaneous action of surface diffusion and evaporation and for these high angles, calculations show tip evolution toward pseudo-stationary profiles; these profiles are characteristic of each cone angle, but their geometrical dimensions are function of the heating conditions (i.e. surface diffusion and evaporation values). Equations for the variations of the tip apex radius as function of surface diffusion coefficient and evaporation rate are deduced for these two cases, as well as variations of geometrical constants as a function of tip half-cone angles.