Nanoscale Tip Research Articles

Vibration Induced Reciprocating Sliding Contacts between Nanoscale Multi-Asperity Tips and a Textured Surface

Microgravity and vacuum are two main environments in outer space. In the microgravity environment, vibration is a typical phenomenon, and it will induce a reciprocating sliding contact between a journal and a bearing in a clearance joint. In vacuum environment, the adhesion effects are severe, and the friction forces are much higher than the ground environment. Nanoscale textures can reduce the contact area and trap the wear particles, which are beneficial to the friction reduction. Most of the current studies focus on the single-pass sliding contact. Actually, considering the roughness of the contact surfaces, a multi-asperity tip should be more reasonable. In this paper, vibration induced reciprocating sliding contacts between nanoscale multi-asperity tips and a textured surface are investigated using a multiscale method, and the material is FCC copper. Six rigid tips are modelled with different cylindrical asperities and slid on the textured surface. Corresponding to the tips, the average friction forces are compared, and the effects of the tip radii are analyzed. The total average friction forces of the textured surface are compared with the case of a smooth surface, and the mechanism of the friction reduction is discussed. The results showed that the total average friction forces decrease as the increase of the asperity radii, and the textured surface can reduce friction forces effectively compared with the smooth surface. This work could contribute to reducing friction by designing the tip and the textured surface in vibration induced reciprocating sliding contacts under microgravity, which will be beneficial to prolong the life of components on the spacecraft.

Intracellular Photothermal Delivery for Suspension Cells Using Sharp Nanoscale Tips in Microwells.

Efficient intracellular delivery of biomolecules into cells that grow in suspension is of great interest for biomedical research, such as for applications in cancer immunotherapy. Although tremendous effort has been expended, it remains challenging for existing transfer platforms to deliver materials efficiently into suspension cells. Here, we demonstrate a high-efficiency photothermal delivery approach for suspension cells using sharp nanoscale metal-coated tips positioned at the edge of microwells, which provide controllable membrane disruption for each cell in an array. Self-aligned microfabrication generates a uniform microwell array with three-dimensional nanoscale metallic sharp tip structures. Suspension cells self-position by gravity within each microwell in direct contact with eight sharp tips, where laser-induced cavitation bubbles generate transient pores in the cell membrane to facilitate intracellular delivery of extracellular cargo. A range of cargo sizes were tested on this platform using Ramos suspension B cells with an efficiency of >84% for Calcein green (0.6 kDa) and >45% for FITC-dextran (2000 kDa), with retained viability of >96% and a throughput of >100 000 cells delivered per minute. The bacterial enzyme β-lactamase (29 kDa) was delivered into Ramos B cells and retained its biological activity, whereas a green fluorescence protein expression plasmid was delivered into Ramos B cells with a transfection efficiency of >58%, and a viability of >89% achieved.

Improving accuracy of nanothermal measurements via spatially distributed scanning thermal microscope probes

Advances in material design and device miniaturization lead to physical properties that may significantly differ from the bulk ones. In particular, thermal transport is strongly affected when the device dimensions approach the mean free path of heat carriers. Scanning Thermal Microscopy (SThM) is arguably the best approach for probing nanoscale thermal properties with few tens of nm lateral resolution. Typical SThM probes based on microfabricated Pd resistive probes (PdRP) using a spatially distributed heater and a nanoscale tip in contact with the sample provide high sensitivity and operation in ambient, vacuum, and liquid environments. Although some aspects of the response of this sensor have been studied, both for static and dynamic measurements, here we build an analytical model of the PdRP sensor taking into account finite dimensions of the heater that improves the precision and stability of the quantitative measurements. In particular, we analyse the probe response for heat flowing through a tip to the sample and due to probe self-heating and theoretically and experimentally demonstrate that they can differ by more than 50%, hence introducing significant correction in the SThM measurements. Furthermore, we analyzed the effect of environmental parameters such as sample and microscope stage temperatures and laser illumination, which allowed reducing the experimental scatter by a factor of 10. Finally, varying these parameters, we measured absolute values of heat resistances and compared these to the model for both ambient and vacuum SThM operations, providing a comprehensive pathway improving the precision of the nanothermal measurements in SThM.

High‐Definition X‐Ray Imaging of Small Gecko Skin Surface Protuberances for Digitization and 3D Printing

AbstractA detailed analysis of zoological and botanical engineering structures reveals remarkable ideas for developing new healthcare technologies. One eclectic example is the surface microprotrusions of Gecko lizard skins, featuring nanoscale tips that engage in bacterial repulsion and killing. However, building biomimetic synthetic nanostructures in long range for technological use is hampered by the lack of concentrated digital information for 3D nano‐fabrication. For instance, standard micro‐computed tomography (µCT), confocal laser scanning microscopy (CLSM) and atomic force microscopy (AFM) imaging failed to replicate gecko spinules at the nanoscale. The authors now show that the latest generation of high‐power X‐ray microscopy (Zeiss X‐Radia 810) builds high contrast images of the gecko skin surface, and following 3D digital conversion the individual spinules, in the large array, are perfectly delineated at 150 nm and clearly featured the 50 nm nanotips crucial for bacterial cell rupturing. Consequently, the authors generate stenographic (STL.) digital file formats loaded with the instructions for 3D printing and the design blueprints for soft lithography replication. A further reconstruction of the digital data is undertaken into a 3D solid object using MeshLabs, yielding biologically realistic, virtual‐world spinule arrays. In this form, the authors are able to edit the spinules for optimal killing performance against bacterial cells.

Optical shaping of a nano-scale tip by femtosecond laser assisted field evaporation

We have investigated the morphology of a nanotip under femtosecond laser pulse illumination and a high electric field. We show that both the symmetry and the local radius of the tip change with the direction of laser polarization as against the tip axis. The experiments were performed on the very same GaN nanotip by laser-assisted atom probe tomography and electron tomography. This allowed an accurate assessment of the tip features by following the order of evaporation of single atoms from the surface. A change of atom emission sites was observed when a change of the angle between the tip axis and the linearly polarized electric field of the laser was imposed. This enables an optical control of field-evaporation sites. A close optical control of the tip morphology on a scale below 10 nm is thus achievable. Calculations of the field at nanotip apex and absorption maps support the experimental observations. Based on the present study, methods can be developed for reshaping nanotips at the nanometer level. This finding opens perspectives for numerous applications, making use of nanotips as probes or field emitters, and for plasmonic devices.

Laser-assisted atom probe tomography of semiconductors: The impact of the focused-ion beam specimen preparation

This paper demonstrates the increased light absorption efficiency of semiconducting atom probe tips resulting from focused-ion-beam (FIB) preparation. We use transmission electron microscopy to show that semiconducting tips prepared with FIB are surrounded with an amorphized shell. Photomodulated optical reflectance measurements then provide evidence that FIB-induced damage leads to an increase in both sub- and supra-bandgap light absorption efficiency. Using laser-assisted atom probe tomography (La-APT) measurements, we finally show that, for a nanoscale tip geometry, the laser-induced heating of a tip during La-APT is enhanced by the FIB preparation. We conclude that, upon supra-bandgap illumination, the presence of a FIB-amorphized surface dramatically increases the light-induced heat generation inside semiconducting tips during La-APT. Furthermore, we also deduce that, in the intriguing case of sub-bandgap illumination, the amorphization plays a crucial role in the unexpected light absorption.

Emission properties of undoped and boron-doped nanocrystalline diamond films coated silicon carbide field emitter arrays

A new process for the fabrication of two-tier silicon carbide field emission array (FEA) of nanoscale tips coated with thin nanocrystalline undoped and highly boron-doped diamond films has been developed to improve the electron emission characteristics of the pure silicon carbide tips. The effects of boron-doped film on field emission properties have been studied in comparison with undoped ones, using a diode configuration. The FEA covered with highly boron-doped film demonstrated the lower turn on electric field and higher emission current due to lower work function, 1.5 times lower than for undoped one. Emission current at various values of the electrode gap from 10 to 500 μm has been studied. It was shown that field enhancement factor of two-tier FEA decreases at small interelectode gap (<100 μm) due to perturbations of electric field and mutual screening of microtips, which leads to significant increase in an applied electric field required for maintaining the preset emission current. Both fabricated FEAs demonstrated high current stability with fluctuations not exceeding 5% at relatively low vacuum (10−5–10−6 Torr).

Wavelength-Dependent Optical Force Imaging of Bimetallic Al-Au Heterodimers.

Many important applications of nanometer-scale metallic complexes arise from the light-induced, near-field interactions between their component structures. Here we examine the near-field interactions in bimetallic Al-Au plasmonic nanodisk heterodimers, where the coupling between the primitive plasmons of nanostructures composed of two different metals is studied. Understanding the correlations between nanoparticle morphology and near-field optical properties, particularly for nanostructures composed of two different metals, requires spectrally resolved near-field spatial information. An ideal tool for such investigations is the recently developed photoinduced force microscopy, where the electromagnetic forces between an optically excited plasmonic nanostructure and an adjacent scanning nanoscale tip are measured. Using this approach, we visualize the wavelength-dependent near-field interactions in these bimetallic heterodimers. This system provides a prime example of the diabatic, antenna-reactor picture of plasmon coupling where for a given wavelength the more resonant primitive "driving" plasmon induces a response, the "forced" plasmon, in the off-resonant component. We critically examine spectrally resolved tip-nanostructure forces, comparing experiment with theory, for tips and nanoscale structures of realistic dimensions relative to frequently used approximations for tip geometries. The contrasting effects of dielectric versus metallic tips on acquired spectral force profiles are also examined.

Improved switching reliability achieved in HfOx based RRAM with mountain-like surface-graphited carbon layer

In this work, we demonstrated an effective method to improve the switching reliability of HfOx based RRAM device by inserting mountain-like surface-graphited carbon (MSGC) layer. The MSGC layer was fabricated through thermal annealing of amorphous carbon (a-C) film with high sp2 proportion (49.7%) under 500 °C on Pt substrate, whose characteristics were validated by XPS and Raman spectrums. The local electric-field (LEF) was enhanced around the nanoscale tips of MSGC layer due to large surface curvature, which leads to simplified CFs and localization of resistive switching region. It takes responsibility to the reduction of high/low resistance states (HRS/LRS) fluctuation from 173.8%/64.9% to 23.6%/6.5%, respectively. In addition, the resulting RRAM devices exhibited fast switching speed (<65 ns), good retention (>104 s at 85 °C) and low cycling degradation. This method could be promising to develop reliable and repeatable high-performance RRAM for practical applications.

Molecular features of hydration layers probed by atomic force microscopy.

Structurally-ordered layers of water are universally formed on a solid surface in aqueous solution or under ambient conditions. Although such hydration layers are commonly probed via atomic force microscopy (AFM), the current understanding on how the hydration layers manifest themselves in an AFM experiment is far from complete. By using molecular dynamics simulation, we investigate the hydration layers on a hydrophilic or hydrophobic surface probed by a nanoscale tip. We study the density and molecular orientation of water, the free energy, and the force on the tip by varying the tip-surface distance. The force-distance curve oscillates due to the transition between the mono-, bi-, and tri-layers of water confined between the tip and the surface. If both the tip and the surface are hydrophobic, water confined between the tip and the surface evaporates due to the dewetting transition, giving a hydrophobic force without oscillation. The periodicity of oscillation in the force differs from the structural periodicity of water. With a close proximity of the tip, the molecular dipoles align parallel to the surface, regardless of whether the tip and the surface are hydrophilic or hydrophobic.

Multi-excitonic emission from Stranski-Krastanov GaN/AlN quantum dots inside a nanoscale tip

Single-dot time-resolved micro-photoluminescence spectroscopy and correlated electron tomography (ET) have been performed on self-assembled GaN/AlN quantum dots isolated within a field-emission nanoscale tip by focused ion beam (FIB). Despite the effect of the FIB, the system conserves the capability of emitting light through multi-excitonic complexes. The optical spectroscopy data have then been correlated with the electronic structure and lifetime parameters that could be extracted using the structural parameters obtained by ET via a 6 band k.p model. A biexciton-exciton cascade could be identified and thoroughly analysed. The biexciton-exciton states exhibit a non-negligible polarization component along the [0001] polar crystal axis, indicating a significant valence band mixing, while the relationship between exciton energy and biexciton binding energy is consistent with a hybrid character of the biexciton.

Direct-current triboelectricity generation by a sliding Schottky nanocontact on MoS2 multilayers.

The direct conversion of mechanical energy into electricity by nanomaterial-based devices offers potential for green energy harvesting 1-3 . A conventional triboelectric nanogenerator converts frictional energy into electricity by producing alternating current (a.c.) triboelectricity. However, this approach is limited by low current density and the need for rectification 2 . Here, we show that continuous direct-current (d.c.) with a maximum density of 106 A m-2 can be directly generated by a sliding Schottky nanocontact without the application of an external voltage. We demonstrate this by sliding a conductive-atomic force microscope tip on a thin film of molybdenum disulfide (MoS2). Finite element simulation reveals that the anomalously high current density can be attributed to the non-equilibrium carrier transport phenomenon enhanced by the strong local electrical field (105-106 V m-2) at the conductive nanoscale tip 4 . We hypothesize that the charge transport may be induced by electronic excitation under friction, and the nanoscale current-voltage spectra analysis indicates that the rectifying Schottky barrier at the tip-sample interface plays a critical role in efficient d.c. energy harvesting. This concept is scalable when combined with microfabricated or contact surface modified electrodes, which makes it promising for efficient d.c. triboelectricity generation.

A customizable class of colloidal-quantum-dot spasers and plasmonic amplifiers.

Colloidal quantum dots are robust, efficient, and tunable emitters now used in lighting, displays, and lasers. Consequently, when the spaser-a laser-like source of high-intensity, narrow-band surface plasmons-was first proposed, quantum dots were specified as the ideal plasmonic gain medium for overcoming the significant intrinsic losses of plasmons. Many subsequent spasers, however, have required a single material to simultaneously provide gain and define the plasmonic cavity, a design unable to accommodate quantum dots and other colloidal nanomaterials. In addition, these and other designs have been ill suited for integration with other elements in a larger plasmonic circuit, limiting their use. We develop a more open architecture that decouples the gain medium from the cavity, leading to a versatile class of quantum dot-based spasers that allow controlled generation, extraction, and manipulation of plasmons. We first create aberration-corrected plasmonic cavities with high quality factors at desired locations on an ultrasmooth silver substrate. We then incorporate quantum dots into these cavities via electrohydrodynamic printing or drop-casting. Photoexcitation under ambient conditions generates monochromatic plasmons (0.65-nm linewidth at 630 nm, Q ~ 1000) above threshold. This signal is extracted, directed through an integrated amplifier, and focused at a nearby nanoscale tip, generating intense electromagnetic fields. More generally, our device platform can be straightforwardly deployed at different wavelengths, size scales, and geometries on large-area plasmonic chips for fundamental studies and applications.

(Invited) Photo-Induced Force Mapping of Plasmonic Nanostructures

Plasmonic nanoparticles serve as efficient optical nanoantennae that interact with light with an effective cross section exceeding their geometric size. We have recently exploited this antenna effect to engineer photoelectrodes for solar water splitting, optimizing for different plasmonic decay mechanisms such as near-field energy transfer to the surrounding semiconductor [1], plasmon-induced hot carrier generation and utilization [2], and broadband extreme light absorption within monolayer MoS2 [3]. However, often more than one plasmonic decay mechanism is at work, which makes it challenging to separate out the relative contributions and optimize them efficiently for solar water splitting. Novel characterization tools to study the heterogeneity of plasmonic photocatalysts both in terms of their morphology and optical near-field enhancements in-situ are therefore of widespread interest. Here we introduce a novel technique called photo-induced force microscopy that allows for simultaneous imaging of topographical features and spectroscopic characterization of near-field distributions with high spectral selectivity and temporal resolution. In this technique, a nanoscale tip is brought in the vicinity of the sample, which is optically excited. The photo-induced gradient forces between the tip and the sample can be detected with nanometer-scale spatial resolution, along with topographical information in atomic force microscopy tapping mode. I will show examples of our combined experimental-theoretical study [4] where we map force intensity enhancements in nanostructures and compare them to simulated force intensity enhancements in which a realistic tip-sample geometry is modeled using Maxwell-Stress tensor methods. Near-field inhomogenities with a spatial resolution less than 10 nm are easily resolved. As such, photo-induced force microscopy will be a welcome addition to the toolbox of nanoscale characterization methods, and will find additional useful applications for the characterization of precisely manufactured nanostructures and surface-enhanced Raman scattering (SERS) substrates. Time permitting, I will discuss charge carrier dynamics in hybrid nanoparticles composed of plasmonic / two-dimensional materials, and applications of photo-induced force microscopy to these study photocatalytic processes.[1] Nano Letters 2011, 11, p 3440[2] Nano Letters 2015, 15 (9), p 6155; Chemistry of Materials 2016, 28 (13), p 4546[3] ACS Photonics 2016, 3 (5), p 853[4] Nano Letters 2016, Articles ASAP (As Soon As Publishable)

Adhesion Mechanics between Nanoscale Silicon Oxide Tips and Few-Layer Graphene

Finite element method (FEM) simulations of the adhesive contact between a nanoscale tip and a silicon oxide substrate covered with graphene were performed, modelling experimental atomic force microscopy pull-off measurements. Simulations showed a slight increase in the pull-off force as layer number increased. This small enhancement was within reported experimental error, agreeing with the experimental findings of layer-independent adhesion forces. Pull-off forces did not vary with the elastic strain in the system for a given number of layers, but were influenced by the greater adhesive stresses for tip–graphene interaction compared with tip–substrate interactions. FEM simulations were also performed on suspended graphene and showed that the adhesive forces increased slightly beyond one layer of graphene, but then varied little from two to four layers of graphene. The results indicate that while there is some local delamination of the graphene sheets from the substrate, the adhesive stresses between the graphene layers in multilayer graphene effectively prevent out-of-plane mechanical deformation of the graphene layers that could result from tip–graphene interactions. Thus, the increased pull-off forces observed beyond one monolayer results from a change in the amount of material between the tip and substrate, or in this case the number of graphene layers, thus increasing the van der Waals force between tip and graphene.

Effect of anodization time on the growth of twinned pyramid crystals of bismite from polyhedral bismuth particle by facile electrolysis-based oxidation

ABSTRACTNanostructured bismite films were fabricated by both thermal oxidation and electrochemical anodization of Bi film in NaOH aqueous electrolyte. Pure phase of α-bismite was obtained by the thermal oxidation technique. Pure-phase α and β bismites were successfully prepared by varying the anodization time. Truncated polyhedral aggregates of bismuth particles were converted to twinned pyramids particles having nanoscale tips with increase in anodization time. The crystallite size increased with anodization time from 43 nm to 79 nm. The band gap value lowered from 2.835(2) eV to 2.422(1) eV with anodization time and this is in agreement with the increase in crystallite size obtained from XRD. The shift in the absorption edge to higher wavelength with anodization time was attributed to the smoothening of surface and grain growth.

SERS Taper-Fiber Nanoprobe Modified by Gold Nanoparticles Wrapped with Ultrathin Alumina Film by Atomic Layer Deposition.

A taper-fiber SERS nanoprobe modified by gold nanoparticles (Au-NPs) with ultrathin alumina layers was fabricated and its ability to perform remote Raman detection was demonstrated. The taper-fiber nanoprobe (TFNP) with a nanoscale tip size under 80 nm was made by heated pulling combined with the chemical etching method. The Au-NPs were deposited on the TFNP surface with the electrostatic self-assembly technology, and then the TFNP was wrapped with ultrathin alumina layers by the atomic layer deposition (ALD) technique. The results told us that with the increasing thickness of the alumina film, the Raman signals decreased. With approximately 1 nm alumina film, the remote detection limit for R6G aqueous solution reached 10−6 mol/L.

Wear Resistance Limited by Step Edge Failure: The Rise and Fall of Graphene as an Atomically Thin Lubricating Material.

Owing to its intrinsically lubricious property, graphene has a high potential to be an atomically thin solid lubricant for sliding interfaces. Despite its ultrahigh breaking strength at the nanoscale, graphene often fails to maintain its integrity when subjected to macroscale tribological tests. To reveal the true wear characteristics of graphene, a nanoscale diamond tip was used to scratch monolayer graphene mechanically exfoliated to SiO2 substrates. Our experimental results show that while graphene exhibited extraordinary wear resistance in the interior region, it could be easily damaged at the step edge under a much lower normal load (∼2 orders of magnitude smaller). Similar behavior with substantially reduced wear resistance at the edge was also observed for monatomic graphene layer on graphite surface. Using molecular dynamics simulations, we attributed this markedly weak wear resistance at the step edge to two primary mechanisms, i.e., atom-by-atom adhesive wear and peel induced rupture. Our findings shed light on the paradox that graphene is nanoscopically strong yet macroscopically weak. As step edge is ubiquitous for two-dimensional materials at the macroscale, our study also provides a guiding direction for maximizing the mechanical and tribological performance of these atomically thin materials.

Photoinduced Force Mapping of Plasmonic Nanostructures.

The ability to image the optical near-fields of nanoscale structures, map their morphology, and concurrently obtain spectroscopic information, all with high spatiotemporal resolution, is a highly sought-after technique in nanophotonics. As a step toward this goal, we demonstrate the mapping of electromagnetic forces between a nanoscale tip and an optically excited sample consisting of plasmonic nanostructures with an imaging platform based on atomic force microscopy. We present the first detailed joint experimental-theoretical study of this type of photoinduced force microscopy. We show that the enhancement of near-field optical forces in gold disk dimers and nanorods follows the expected plasmonic field enhancements with strong polarization sensitivity. We then introduce a new way to evaluate optically induced tip-sample forces by simulating realistic geometries of the tip and sample. We decompose the calculated forces into in-plane and out-of-plane components and compare the calculated and measured force enhancements in the fabricated plasmonic structures. Finally, we show the usefulness of photoinduced force mapping for characterizing the heterogeneity of near-field enhancements in precisely e-beam fabricated nominally alike nanostructures - a capability of widespread interest for precise nanomanufacturing, SERS, and photocatalysis applications.

Stabilizing effect of diamond thin film on nanostructured silicon carbide field emission array

Silicon carbide field emission arrays (FEA) are at the forefront of development as new promising electron sources for vacuum microelectronic devices. The authors present a new process for the fabrication of array of nanoscale tips with reduced heterogeneity of their heights. The characterization results show that at strong electric fields the heterogeneity is a key factor causing current instability. Diamond is recognized as the best material to obtain field emission, but fabrication of tip array with a reasonable aspect ratio is a challenge. Therefore, the authors have combined the benefits of both the abovementioned materials and fabricated silicon carbide FEA of tips coated with nanocrystalline undoped diamond thin film. The coating smoothens the virtual emitting surface connecting the tips, and therefore reduces heterogeneity of their heights, thereby improving the current stability. Current fluctuation decreased to 5% in compare with 23% in FEA without coating.