Nanoscale Tip Research Articles

Advances in Manufacturing of Molded Tips for Scanning Probe Microscopy

A common method for producing sharp tips used in scanning probe microscopy (SPM) and other applications involving nanoscale tips is to deposit thin-film materials, such as metals, silicon nitride, or diamond-based films, into four-faceted pyramidal molds that are formed by anisotropic etching into a (100) silicon substrate. This well-established method is capable of producing tips with radii as small as a few nanometers. However, the shape of the tip apex is difficult to control with this method, and wedge-shaped tips that are elongated in one dimension are often obtained. This limitation arises due to the practical difficulty of having four planes intersecting at a single point. Here, a new method for producing three-sided molds for SPM tips is demonstrated through the use of etching in (311) silicon wafers. It is shown that silicon nitride and ultrananocrystalline diamond tips fabricated with this new method are wedge free and sharp (<; 10 nm radius), thereby restoring tip molding as a well-controlled manufacturing process for producing ultrasharp SPM tips.

Suppressing nano-scale stick-slip motion by feedback

When a micro cantilever with a nano-scale tip is manipulated on a substrate with atomic-scale roughness, the periodic lateral frictional force and stochastic fluctuations may induce stick-slip motion of the cantilever tip, which greatly decreases the precision of the nano manipulation. This unwanted motion cannot be reduced by open-loop control especially when there exist parameter uncertainties in the system model, and thus needs to introduce feedback control. However, real-time feedback cannot be realized by the existing virtual reality virtual feedback techniques based on the position sensing capacity of the atomic force microscopy (AFM). To solve this problem, we propose a new method to design real-time feedback control based on the force sensing approach to compensate for the disturbances and thus reduce the stick-slip motion of the cantilever tip. Theoretical analysis and numerical simulations show that the controlled motion of the cantilever tip tracks the desired trajectory with much higher precision. Further investigation shows that our proposal is robust under various parameter uncertainties. Our study opens up new perspectives of real-time nano manipulation.

Wear‐Resistant Nanoscale Silicon Carbide Tips for Scanning Probe Applications

AbstractThe search for hard materials to extend the working life of sharp tools is an age‐old problem. In recent history, sharp tools must also often withstand high temperatures and harsh chemical environments. Nanotechnology extends this quest to tools such as scanning probe tips that must be sharp on the nanoscale, but still very physically robust. Unfortunately, this combination is inherently contradictory, as mechanically strong, chemically inert materials tend to be difficult to fabricate with nanoscale fidelity. Here a novel process is described, whereby the surfaces of pre‐existing, nanoscale Si tips are exposed to carbon ions and then annealed, to form a strong silicon carbide (SiC) layer. The nanoscale sharpness is largely preserved and the tips exhibit a wear resistance that is orders of magnitude greater than that of conventional silicon tips and at least 100‐fold higher than that of monolithic, SiO‐doped diamond‐like‐carbon (DLC) tips. The wear is well‐described by an atom‐by‐atom wear model, from which kinetic parameters are extracted that enable the prediction of the long‐time scale reliability of the tips.

Analytical Model for Plowing Friction at Nanoscale

Existing analytical models for elastic–plastic friction, which have been developed for microscale single-asperity contacts, do not apply when the contact dimensions are reduced to the nanometer regime. We demonstrate that although the microscale models correctly describe elastic recovery behind a sliding nanoscale tip, they fail to predict the corresponding coefficient of friction. We show that the breakdown of microscale models can be attributed to the large contribution to friction from pileup. We propose an analytical model for plowing friction for single-asperity contacts, which includes the effects of both elastic recovery and pileup. Functional dependence of the average pileup height on elastic properties of the worn material and on the depth of cut is also proposed. Applicability of the new model is demonstrated in the examples of SiC and Cu.

High‐Voltage Nano‐oxidation in Deionized Water and Atmospheric Environments by Atomic Force Microscopy

This study used atomic force microscopy (AFM), metallic probes with a nanoscale tip, and high-voltage generators to investigate the feasibility of high-voltage nano-oxidation processing in deionized water (DI water) and atmospheric environments. Researchers used a combination of wire-cutting and electrochemical etching to transform a 20-μm-thick stainless steel sheet into a conductive metallic AFM probe with a tip radius of 60 nm, capable of withstanding high voltages. The combination of AFM, high-voltage generators, and nanoscale metallic probes enabled nano-oxidation processing at 200 V in DI water environments, producing oxides up to 66.6 nm in height and 467.03 nm in width. Oxides produced through high-voltage nano-oxidation in atmospheric environments were 117.29 nm in height and 551.28 nm in width, considerably exceeding the dimensions of those produced in DI water. An increase in the applied bias voltage led to an apparent logarithmic increase in the height of the oxide dots in the range of 200-400 V. The performance of the proposed high-voltage nano-oxidation technique was relatively high with seamless integration between the AFM machine and the metallic probe fabricated in this study.

Infrared Nanoscopy of Dirac Plasmons at the Graphene–SiO2 Interface

We report on infrared (IR) nanoscopy of 2D plasmon excitations of Dirac fermions in graphene. This is achieved by confining mid-IR radiation at the apex of a nanoscale tip: an approach yielding 2 orders of magnitude increase in the value of in-plane component of incident wavevector q compared to free space propagation. At these high wavevectors, the Dirac plasmon is found to dramatically enhance the near-field interaction with mid-IR surface phonons of SiO2 substrate. Our data augmented by detailed modeling establish graphene as a new medium supporting plasmonic effects that can be controlled by gate voltage.

Atomic Friction Modulation on the Reconstructed Au(111) Surface

Friction between a nanoscale tip and a reconstructed Au(111) surface is investigated both by atomic force microscopy (AFM) and molecular statics calculations. Lateral force AFM images exhibit atomic lattice stick–slip behavior with a superstructure corresponding to the herringbone reconstruction pattern. However, the superstructure contrast is not primarily due to variations in the local frictional dissipation (which corresponds to the local width of the friction loop). Rather, the contrast occurs primarily because the local centerline position of the friction loop is periodically shifted from its usual value of zero. Qualitatively, similar behavior is reproduced in atomistic simulations of an AFM tip sliding on the reconstructed Au(111) substrate. In both simulations and experiments, this centerline modulation effect is not observed on unreconstructed surfaces. Similarly, using a topographically flat surface as a hypothetical control system, the simulations show that the centerline modulation is not caused by variations in the reconstructed surface’s topography. Rather, we attribute it to the long-range variation of the local average value of the tip-sample interaction potential that arises from the surface reconstruction. In other words, surface atoms located at unfavorable sites, i.e., in the transition between face-centered-cubic (FCC) and hexagonal-close-packed (HCP) regions, have a higher surface free energy. This leads to a varying conservative force which locally shifts the centerline position of the friction force. This demonstrates that stick–slip behavior in AFM can serve as a rather sensitive probe of the local energetics of surface atoms, with an attainable lateral spatial resolution of a few nanometers.

Effects of a silicon probe on gold nanoparticles on glass under evanescent illumination

We have numerically investigated the influence of a nanoscale silicon tip in proximity to an illuminated gold nanoparticle. We describe how the position of the high-permittivity tip and the size of the nanoparticle impact the absorption, peak electric field and surface plasmon resonance wavelength under different illumination conditions. We detail the finite element method (FEM) approach we have used, whereby we specify a volume excitation field analytically and calculate the difference between this source field and the total field (i.e., scattered-field formulation). We show that a nanoscale tip can locally enhance the absorption of the particle as well as the peak electric field at length scales far smaller than the wavelength of the incident light.

Breaking the Mucus Barrier Unveils Cancer Cell Secrets

Measuring the mechanical strength of mucus layers surrounding cancer cells offers clues to better ways to treat cancer, and also suggests why some cancer cells are more resistant to drugs than others, according to a report in the January 2010 online issue of the journal Applied Physics Letters (available: http://link.aip.org/link/applab/v97/i26/p263703/s1). Researchers led by Kai-tak Wan, associate professor of engineering at Northeastern University, Boston, Massachusetts, say that uptake of cytotoxic drugs by typical tumor cells is limited by the dense dendritic network of oligosaccharide mucin chains that forms a mechanical barrier. “Mucins are high molecular weight glycoproteins having oligosaccharides attached to a protein backbone core by O-glycosidic linkages, and are approximately 50%–80% carbohydrates in terms of total molecular mass,” the authors state. “The long chain molecules are naturally produced in a wide range of host tissues including the gastrointestinal tract, lungs, kidneys, ovaries, breast, and pancreas.” This tangled brush forms a physical barrier and, when over-expressed, can prevent drugs from reaching the underlying cancer cells. Over-expressed mucus also makes it easier for cancer cells to break away from surrounding cells and metastasize. According to Wan, healthy tissues naturally secrete mucus to protect against infection. Cancer cells, however, produce far more mucus than healthy cells. Study co-author Robert B. Campbell, an associate professor of pharmaceutical sciences at Massachusetts College of Pharmacy and Health Sciences, Worcester, Massachusetts, has been investigating the use of chemical agents that limit the formation of this tangled mucus barrier to allow chemotherapeutic agents to permeate the barrier. To determine how well those agents work, Wan used the nanoscale tip of an atomic force microscope to push against the mucus barrier. The less resistance it encountered, the less tangled the barrier. Six types of mucinous human in vitro adenocarcinoma models were investigated: pancreatic Capan-1; breast ZR-75-1; colorectal Colo-205; lung Chago-K-1; ovarian (wide type [WT]) SK-OV3 WT; and ovarian (multidrug resistant [MDR]) SK-OV3 MDR. A nonmucous cancer cell type derived from the brain, U87-MG, served as a negative control for the study. Benzyl-α-GalNac (analog of N-acetylgalactosamine) was used to inhibit type-O-glycosylation and to reduce the carbohydrate chains associated with the protein backbone of mucin. The chemotherapeutic agent significantly reduced the energy needed to pierce the mucus barrier in lung, breast, colorectal, pancreatic, and wild type (natural) ovarian cancer cells. However, the treatment barely registered any change in MDR ovarian cancer cells. It currently remains unclear how those cells resist drugs that ordinarily eliminate wild-type ovarian cancer, but Wan's research points to an important difference: the mucus layer formed by the 2 types of cells reacts differently to the same chemical treatment. “Although the mechanical barrier alone is certainly not the only mechanism that hinders drug transport, it at least contributes quite significantly to ineffective cytotoxic drug therapy,” the authors state. “The studies reported herein offer additional support for the development of clinical and pharmaceutical approaches to combat mucin overexpression in tumors during cancer chemotherapy.”

Finite Element Modeling of the Nano-scale Adhesive Contact and the Geometry-based Pull-off Force

Nano-scale adhesive contact mediated by intermolecular van der Waals forces has become a typical fundamental problem in many areas. Interpretation and control of the strength and efficiency of the nano-scale adhesive contacts require a proper modeling considering the actual interfacial forces, the varying contact area, and clearance. In this article, the finite-element (FE) method is developed to model the nano-scale adhesive contact of elastic bodies with an adhesive pressure derived from the interatomic interaction Lennard-Jones potential, which permits numerical solutions for a variety of interface geometries. Compared with the analytical results from conventional Hertz, JKR, and DMT models, the validity of the FE model is verified. For nano-scale contact, the assumption of equivalent radius adopted in the Hertz model is initially investigated and proved to be improper for nano-scale adhesive contact due to the distribution variations of interfacial force caused by local contact geometry. Then adhesive contact behaviors of four typical nano-scale contact geometries inspired by tip shapes of bio-adhesive pads are investigated in detail, which are flat punch tip, sphere tip, mushroom tip, and empty cup tip. The simulation results indicate that the nano-scale tip geometry plays a dominant role on the pull-off strength. Within the investigated geometries, cup tip results in a highest adhesion efficiency followed by flat punch tip, sphere tip, and mushroom tip, respectively, which are highly geometry dependent and verified by former experimental results. The dominant effect is found coming from the contact area ratio of the adhesive area to the sticking area or the whole contact area. The FE modeling can serve a useful purpose in revealing the nano-scale geometry-based adhesion contact for surface topography design in MEMS to avoid stiction failure and for the artificial sticky feet in bionics to increase adhesion strength.

Direct deposition of micro- and nanoscale hydrogels using Dip Pen Nanolithography (DPN)

Dip Pen Nanolithography® (DPN™) is the controlled delivery of material from nanoscale tips to a surface. Its advantages over other lithography techniques include low operating costs, high resolution, nanoscale registry and multiplexed deposition capabilities. With NanoInk's latest nanolithography tools and new biocompatible patterning protocols, any scientist can easily and rapidly create arbitrary hydrogel patterns with subcellular feature sizes to be used for cell adhesion, cell migration and drug delivery studies.

Nanoscale sharpening tips of vapor–liquid–solid grown silicon microwire arrays

We developed out-of-plane, high aspect ratio, nanoscale tip silicon microwire arrays forapplication to penetrating, multisite, nanoscale biological sensors. Silicon microwire arraysselectively grown by gold-catalyzed vapor–liquid–solid growth of silicon can be formed tocreate sharpened nanotips with a tip diameter of less than 100 nm by utilizing batch-processedsilicon chemical etching for only 1–3 min. The tip angles achieved ranged from11° to38°. The nanotip silicon microwires can perform gelatin penetration without wire breakdown,indicating their potential penetrating capability for measurements inside biological tissues.

Field emission from a large area of vertically-aligned carbon nanofibers with nanoscale tips and controlled spatial geometry

A cost-effective method was used to fabricate large area (e.g. 2.5×2.5cm2 for a pitch of 0.5μm) nanometer scale pyramid arrays with precise and controllable location, spacing and size by interference lithography. Using this method a large-area of vertically-aligned carbon nanofibers (VACNFs) with different interfiber-distance-to-fiber-height ratios and fiber aspect ratios were synthesised. Very sharp tipped VACNFs were obtained and their field emission properties were measured and are compared to results in the literature. We find that the field emission performance of VACNF arrays is optimized when the interfiber-distance-to-fiber-height ratio is equal to 1, in line with some experimental findings but different from some predictions.

Synthesis of Nanoscale Tips Using Femtosecond Laser Radiation under Ambient Condition

We report a unique growth of platelet-shaped nanoscale tips of transparent dielectric using femtosecond laser radiation at MHz pulse repetition rate with nitrogen background gas flow under ambient condition. The tips grew with sharp nanoscale apex while their base and lengths are of the order of few hundred nanometers. In the absence of nitrogen, the irradiation leads to nanofibrous structure formation. The collision between the nitrogen gas atoms and the vapor species slows down plume expansion and lead to an increase of nanoparticles size. This prevents the fibrous structure formation and provides appropriate condition for nanoscale tips growth.

Single living cell detection of telomerase over-expression for cancer detection by an optical fiber nanobiosensor

An optical fiber nanobiosensor was constructed to successfully detect a general cancer biomarker, telomerase at single cell level with its nanoscale tip. The nanotip immobilized with a specific antibody was inserted into a MCF-7 breast cancer cell nucleus to capture telomerases directly, after which an in vitro enzymatic sandwich immunoassay was performed to achieve sensitive single living cell detection. The nanotip inserted into MCF-7 cell nucleus provides significantly higher average ( F − F 0)/ F 0 ratio than that of human mesenchymal stem cell (hMSC) nucleus, demonstrating the successful detection of the telomerase over-expression in cancer cells as compared to normal cells. The detection in the cytoplasm shows much smaller average ratio than that in the nucleus of MCF-7 cells while clearly verifies the nuclear localization of telomerase. The successful detection of telomerase over-expression in a single living cell for the first time may provide a potential method for cancer detection, and also demonstrate a universal approach that can be used to detect other low expression proteins in a single living cell.

Fabrication of ball-shaped atomic force microscope tips by ion-beam-induced deposition of platinum on multiwall carbon nanotubes

Ball-shaped atomic force microscope (AFM) tips (ball tips) are useful in AFM metrology, particularly in critical dimension AFM metrology and in micro-tribology. However, a systematic fabrication method for nano-scale ball tips has not been reported. We report that nano-scale ball tips can be fabricated by ion-beam-induced deposition (IBID) of Pt at the free end of multiwall carbon nanotubes that are attached to AFM tips. Scanning electron microscopy and transmission electron microscopy analyses were done on the Pt ball tips produced by IBID in this manner, using ranges of Ga ion beam conditions. The Pt ball tips produced consisted of aggregated Pt nano-particles and were found to be strong enough for AFM imaging.

Modeling and Validation of the van der Waals Force During the Adhesion of Nanoscale Objects to Rough Surfaces: A Detailed Description

The interactions between nanoparticles and rough surfaces are of great scientific and engineering importance and have numerous applications in surface science and biotechnology. Surface geometry and roughness play crucial roles in observed particle adhesion forces. We previously developed a model and simulation approach to describe adhesion between microscale bodies. This work provides detailed descriptions of the modeling framework, with associated experimental validation, applied to nanoscale systems. The physical systems of interest include nanoscale silicon nitride adhering to different surfaces in both dry and aqueous environments. To perform the modeling work, precise descriptions of the geometry of the particle and the roughness of the particle and substrate were generated. By superimposing the roughness and geometry models for the particle and the substrate, it was possible to precisely describe the spatial configurations of the adhering surfaces. The interacting surfaces were then discretized, and the adhesion force between the two surfaces was calculated by using Hamaker's additive approach, based on van der Waals interactions. In the experimental work, an atomic force microscope (AFM) was used to measure the adhesion force (pull-off force) between nanoscale silicon nitride cantilever tips and a range of substrates in different environments. The measured and predicted force distributions were compared, and good agreement was observed between theory and experiment.

Encapsulated tips for reliable nanoscale conduction in scanning probe technologies

Nanoscale tip apexes of conducting cantilever probes are important enablers for severalconducting probe technologies that require reliable long-term operation, while preservingthe nanoscale integrity of the tip apex. In this paper, the concept of an encapsulated tipwith a nanoscale conducting core is presented. A method to fabricate such tips onconducting silicon microcantilevers is described. Long-term conduction and wear reliabilityof these nanoscale tips are evaluated systematically, and their ability to operate for slidingdistances greater than 2 m in conduction and 11 m in wear on amorphous carbon isdemonstrated. These results are expected to have an impact on the future of conductingprobe-based technologies such as probe-based nanometrology, data storage andnanolithography.

Simulation of Atomic-Scale Wear of Graphite - Nanotip Induced Graphene Formation

The atomic-scale wear, the formation process of the graphene during the lateral line scan process of the nanoscale tip on the multi-layered graphene substrate is studied by using molecular relaxation method. The nanotip is scanned in line forward (along [1 2 -3 0] direction) and backward under the constant-height mode. Analysis of the effect of the tip height on the relative motion of the nanotip, the 1st graphene layer, and the 2nd graphene layer, reveals the transition from the nanotip state to the graphene tip state. During the nanotip state the mean lateral force rapidly increases as the mean loading force increases. Here the friction between the tip and the surface occurs. However, during the graphene tip state, takes nearly the constant value independent of . Here the internal friction among the 1st, the 2nd and the 3rd graphene layers occurs. The marked scan directional dependence and the increase of near the graphene edge appears. The irreversible shift of the graphene layer after all the scan processes can explain the mechanism of the elementary process of the atomic-scale wear. [DOI: 10.1380/ejssnt.2009.173]

Fabrication of Surface Plasmon Resonators by Nanoskiving Single-Crystalline Gold Microplates

This paper demonstrates the sectioning of chemically synthesized, single-crystalline microplates of gold with an ultramicrotome (nanoskiving) to produce single-crystalline nanowires; these nanowires act as low-loss surface plasmon resonators. This method produces collinearly aligned nanostructures with small, regular changes in dimension with each consecutive cross-section: a single microplate thus can produce a number of "quasi-copies" (delicately modulated variations) of a nanowire. The diamond knife cuts cleanly through microplates 35 microm in diameter and 100 nm thick without bending the resulting nanowire and cuts through the sharp edges of a crystal without deformation to generate nanoscale tips. This paper compares the influence of sharp tips and blunt tips on the resonator modes in these nanowires.