Sharp Metal Tip Research Articles

Field-based ion generation from microscale emitters on natural and artificial objects for atmospheric pressure mass spectrometry

Field-based ion generation is described for ambient mass spectrometry. The technique allows the analysis of endogenously expressed chemicals and exogenously applied compounds directly from the cuticle of live insects in real time. Cuticular hairs serve as electric field-enhancing structures and play a key role in ion generation. Artificial emitters such as graphite whiskers or sharp metal tips replicate this effect.

FABRICATION OF STM TUNGSTEN NANOTIP BY ELECTROCHEMICAL ETCHING METHOD

With developments in nanoscience and nanotechnology, Scanning Tunneling Microscope (STM) has found a wide application in imaging the atoms, molecules, and nanostructures. This microscope uses an ultra sharp metallic tip for scanning sample surface to produce surface topographic image with atomic resolution. Reliability and resolution of STM images depend largely on the sharpness of the tip apex and repeatability of images depends on mechanical strength of tip material. During last decades, a variety of techniques and processes have been developed for fabrication of different metallic tips made from tungsten, platinum, platinum–iridium, gold, and silver. Electrochemical etching process is the most popular method for fabrication of nanotips with desired quality, reliability, and reproducibility and tungsten is normally the first choice for fabrication of STM tips as it has a high mechanical strength as well as a good electrical conductivity. Fabrication of STM tungsten tip by using electrochemical etching method and tip characterization has been the subject of several researches. Nevertheless, to our knowledge, effect of voltage type (AC/DC), time delay in turning off the voltage, tungsten wire diameter, environmental vibrations, electrolyte type, cathode material, and perpendicularity of tungsten wire (toward electrolyte surface) on the tip sharpness have not been studied so far. In this paper, effects of these parameters on the tip shape and sharpness are investigated. A proper set-up for STM tungsten nanotip fabrication by using electrochemical etching method is presented.

Optical Properties of Metal Tips for Tip-Enhanced Spectroscopies

We develop a full 3D approach to simulate the optical properties of sharp metal tips that could be quantitatively compared with experiments and provide accurate predictions. Similar to metallic wires, elongated metal tips support a series of extended mode resonances whose properties are largely determined by the tip geometry and are essentially independent of the tip material. For sufficiently long tips, these resonances are energetically well separated from a broader, high-energy feature, whose position and line shape are independent of the tip length but vary strongly with the tip material. The latter is a localized plasmon mode characterized by the hemisphere terminating the tip.

Dual mode near-field scanning optical microscopy for near-field imaging of surface plasmon polariton

In this paper, we theoretically and experimentally demonstrate that metal coated apertured probes are efficient near-field probes on surfaces with high reflectivity for the scattering as well as for the collection mode near-field scanning optical microscopy (NSOM). We show that a blunt apertured metal coated tip is very effective in suppressing image dipoles which affect strongly the signals scattered from frequently used sharp metal tips or gold nanoparticle attached probes. By using a simultaneous collection and scattering mode (dual mode) NSOM we measure the near-field images of surface plasmon polariton (SPP) launched from a slit. The collection mode measures propagating SPP along lateral distance in a long scan range with high signal-to-noise ratio, and the scattering mode measures the polarization resolved near-field of SPP. Comparisons of the measured data obtained in the dual mode enable to easily characterize SPP and to separate the measured near-field into the propagating SPP and the directly transmitted light.

Direct growth of carbon nanotubes atom by atom during field emission

AbstractWe have designed a field emission microscope (FEM) coupled to a chemical vapor deposition (CVD) reactor in order to observe directly the growths of individual carbon nanotubes (CNTs) from the nucleation stage. Catalyst metals are first deposited in situ on a sharp metallic tip during direct FEM imaging and formed into nanoparticles by dewetting. CNTs are then grown directly on these nanoparticles by CVD in acetylene or other hydrocarbon gases at appropriate temperatures (600-900°C). The FEM patterns are formed by electrons emitted from individual CNT caps. The videos are analyzed to extract the growth rates and models. In situ field emission I/V measurements are also performed. The most interesting new discovery is that the CNTs often rotate axially during growth, thus strongly supporting a recently proposed model of ‘screw-dislocation-like’ (SDL) mechanism. The event is not rare as four rotating CNT growths versus six non-rotating growths were observed. In one case the CNT rotated quite uniformly ∼180 times during its 11 min growth. This observation should aid researchers to better understand and control the growth of SWNTs.

Extreme localization of electrons in space and time

Electron emission from sharp metal tips can take place on sub-femtosecond time scales if the emission is driven by few cycle femtosecond laser pulses. Here we outline the experimental prerequisites in detail, discuss emission regimes and relate them to recent experiments in the gas phase (attosecond physics). We present a process that leads to single atom tip emitters that are stable under laser illumination and conclude with a discussion of how to achieve short electron pulses at a target.

Angular dependent luminescence of individual suspended ZnO nanorods

We report angular dependent microphotoluminescence measurements on individual suspended ZnO nanorods attached to sharp metal tips. The luminescence measured along and perpendicular to the axis of the same single nanorod indicates quantitatively that >99% of the near band edge emission can be confined effectively in the nanorods and emitted from the end facets. The radially confined luminescence is dominated by P bands and LO phonon replicas of the free exciton emission and P bands. The measurements along the length of the nanorods show that the confinement is evidently influenced by the surface structure.

Light depolarization induced by sharp metallic tips and effects on Tip-Enhanced Raman Spectroscopy

Tip-Enhanced Raman Spectroscopy is based on the field enhancement and confinement provided by sharp metallic tips, used to locally excite the Raman emission from molecules, crystals and nanostructures. This technique has recently demonstrated capabilities for nanometer scale optical resolution in Raman analysis and has enormous potential applications in nano- and bio-technologies. In this paper we describe the state-of-the-art experimental techniques, focusing the attention on the strategies to reduce the far-field background, on new processes to fabricate sharp metallic tips and on the effect of the depolarization produced by metallic tips on Tip-Enhanced Raman spectra of bulk crystals.

Gate Controlled Negative Differential Resistance and Photoconductivity Enhancement in Carbon Nanotube Addressable Intra-connects

ABSTRACTWe have observed gate-controlled N-shaped negative differential resistance (NDR) and photoconductivity enhancement in carbon nanotube (CNT) based addressable intra-connects. The intra-connects – bridges spanning across planar electrodes – were measured at room temperature. Individual single-walled CNT (SWCNT) channels were grown using chemical vapor deposition (CVD) precisely between very sharp metal tips on the pre-fabricated electrodes without post-processing. The electrodes were made of cobalt. Methane and H2 gas mixture were introduced into quartz tube for an hour at 900 C, with flow rate of 1900 sccm for methane and 20 sccm for H2. We have investigated two different cases: in one case, the source-drain current-voltage, Ids-Vds, characteristics were linear. The other case exhibited nonlinear Ids-Vds characteristics. Raman scattering of the intra-connects indicated that each were made of SWCNT with radial breathing mode (RBM) at 191.9 cm-1 and 176.2 cm-1, respectively. Current-voltage Ids-Vds characteristics were measured for various Vgs from -10 V to +10 V. Negative differential resistance (NDR) was found in the Ids-Vgs curves for gate bias in the region of -3>Vgs>-6 V. The NDR peak was shifted to the negative side as the source-drain voltage was increased from Vds=0 to 0.75 V. Otherwise, the intra-connects exhibited characteristics of an ordinary p-type channel. The experiments were repeated under white light illumination. The light increases the carrier density in the channel but not in the metal electrodes and allowed us to study the effective doping of the channel without affecting the work function of the SWCNT/metal contact. The overall channel conductance increased under the light irradiation. Under illumination, the devices became more stable, as well. In summary, we have investigated contact properties between a SWCNT intra-connect and metal electrodes in a well controlled layout settings.

Special Issue of the Journal NanoBiotechnology “Physical Methods of Nanophotonics”

A sharp metal tip, or conducting nanoparticles, can concentrate light and electric field. This is a primary method by which photonics systems can be adapted for characterization and fabrication at the nanoscale: nanophotonics. In the process of confining the light, new processes have been identified, and novel schemes for enhancing the effects have been invented. Our understanding and ability to manipulate light and therefore materials at the nanoscale is now blossoming and is of great interest to a variety of disciplines. Our goal with this special issue is to present the current state of understanding, key insights that permit a qualitative grasp of the phenomena, and recent ideas to improve the localization and signal level. As the number of molecules sampled decreases, as it must for increased resolution, the signal level is also reduced. One of the most important realizations in this field to date is that it is possible to obtain sufficient enhancement that measurements are still possible. The articles in this issue can be broadly categorized as theoretical modeling, experimental characterization, or fabrication, although there is, and must be, an interplay between these in all the papers. In the theoretical paper by Tsukerman, Cajko, and Dai, the power of state of the art calculation methods for designing nanoscale optical concentrators is illustrated, and the plasmon that underlies much of the novel capabilities makes its appearance. Such advances are key to attaining measurable signal levels from small volumes. The paper by Dhawan, Gerhold, and VoDinh describes the fabrication (and modeling) of nanotip arrays, in this case for surface-enhanced Raman use. Raman spectroscopy is one of the most powerful optical techniques for identifying species, for measuring stress or confinement, and for quantitative analysis. It is therefore not surprising that it is used as a demonstration vehicle for nanoscale studies. The paper by Neacsu, Berweger, and Raschke show how it can be taken to the limit of single molecule Raman, by the use of tip-enhanced illumination. The paper by Hallen describes the plasmon effects in the detection (rather than illumination) part of the process. This caps our understanding of the various differences between nanoRaman and far-field Raman, mostly due to the presence of the tip, which have been measured over the past 15 or so years. The other commonly used optical technique is fluorescence. The paper by Cade, Culfaz, Eligal, RitmanMeer, Huang, Festy, and Richards describes the current capabilities available for this technique at the nanoscale, again tied to modeling studies so that an understanding of the processes is possible. The other two papers describe some of the most interesting efforts currently underway to fabricate nanoscale materials in a controlled fashion. Myhra uses tip-induced local anodic oxidation as a means to modify or etch nanoscale patterns into a surface, and Leggett describes how self-assembled monolayers can be used or modified with the optical fields of a sharp tip to produce patterns for their own use or as templates for further growth. The breadth and capabilities described in this special issue show the vitality of activities in nanophotonics. The amazing achievements to date are harbingers of the applications that will be developed over the next several years. Nanobiotechnol (2007) 3:147 DOI 10.1007/s12030-009-9019-3

Imaging a coupled quantum dot-quantum point contact system

We have quantitatively studied the effect of charge traps on the electrical conductance of a quantum dot and a capacitively coupled quantum point contact. Using the sharp metallic tip of a low-temperature scanning force microscope as a scanned gate, we could localize the traps. The quantum point contact served as a charge detector and allowed us to distinguish single electron charging events in several traps from charging events on the dot. We used the quantum dot to analyze the tip-induced potential quantitatively and found its shape to be independent of the voltage applied to the tip within a certain range of parameters. We estimate that the trap density is below 0.1% of the doping density, that the charging energy of a trap is three times higher than that of the quantum dot, and that the interaction energy between the quantum dot and a trap is a significant portion of the dot’s charging energy. Possibly, such charge traps are the reason for frequently observed parametric charge rearrangements.

Effects of process parameters on the electrochemical etching of sharp metallic tips with an attached mass

The electrochemical etching of a metal wire with an attached mass at the end of the immersed wire is a new technique to enhance the yield rate of sharp tips. Unlike conventional electrochemical etching, the yield rate of sharp tips with subhundred nanometer apex could be increased up to around 60% with the attached mass method. In this article, the effects of the magnitude of attached mass and the taping material used for attachment on the yield rate and tip shape are investigated. Also, the variation of tip shape with respect to the temporal shape of applied electric field, constant or pulsed dc, is examined.

Localized Multiphoton Emission of Femtosecond Electron Pulses from Metal Nanotips

Intense multiphoton electron emission is observed from sharp (approximately 20 nm radius) metallic tips illuminated with weak 100-pJ, 7-fs light pulses. Local field enhancement, evidenced by concurrent nonlinear light generation, confines the emission to the tip apex. Electrons are emitted from a highly excited nonequilibrium carrier distribution, resulting in a marked change of the absolute electron flux and its dependence on optical power with the tip bias voltage. The strong optical nonlinearity of the electron emission allows us to image the local optical field near a metallic nanostructure with a spatial resolution of a few tens of nanometers in a novel tip-enhanced electron emission microscope.

Parietal cortex involvement in the localization of tactile and noxious mechanical stimuli: A transcranial magnetic stimulation study

The cortical system underlying perceptual ability to localize tactile and noxious cutaneous stimuli in humans is still incompletely understood. We used transcranial magnetic stimulation (TMS) to transiently interfere with the function of the parietal cortex, at different times after the beginning of noxious or non-noxious mechanical stimulation of the hairy skin overlying the dorsal surface of the first metacarpal of the contralateral hand. Peripheral stimuli consisted of rounded (1 mm diameter) or sharp (0.2 mm) metal tips; skin contact lasted on average 242 ms (noxious) and 228 ms (non-noxious). Brief (80 ms, 25 Hz) TMS trains, given at 150 ms after the onset of cutaneous stimulation, significantly impaired subjects’ ability in localizing non-nociceptive, tactile input, an effect which was not observed when TMS was applied at 300 ms after cutaneous stimulation. In contrast, brief TMS trains given at 300 ms after the onset of cutaneous stimulation significantly impaired subjects’ ability in localizing nociceptive input, an effect which was not observed when TMS was applied at 150 ms after cutaneous stimulation. No impairment in stimulus detection was found in comparison with control sham TMS. The timing of parietal TMS interference with the ability to localize tactile and painful stimuli is compatible with known time differences in the arrival of non-noxious and noxious information in the postcentral gyrus. On these grounds, our findings support the existence of overlapping cortical populations in the contralateral parietal lobe, exerting a role in spatial discriminative aspects of touch and mechanically induced pain.

Efficient electrochemical etching method to fabricate sharp metallic tips for scanning probe microscopes

A new technique based on electrochemical etching for the fabrication of sharp metallic tips for scanning probe microscopes is introduced. In the proposed method, a small Teflon mass is attached to the end of an immersed tungsten wire using an aluminum tape, which leads to a significant enhancement of yield rate of sharp tungsten tips with an apex size below 100nm to over 60%. The functionality of the tungsten tips fabricated by the proposed method is verified by measuring the topography of a standard sample using a shear-force scanning probe microscope.

Generation of a directed nanolocalized photoelectron beam by means of femtosecond laser pulses

A directed photoelectron beam is obtained when photoelectrons from a sharp metal tip irradiated by femtosecond laser pulses are passed through a quartz nanocapillary. Such a nanolocalized photoelectron train makes it possible to conduct experiments with simultaneous femtosecond time and nanometer spatial resolutions.

The growth mechanism and field-emission properties of single carbon nanotips

We study the self-aligned growth of a single carbon nanotip on a sharp metal tip via thefield-emission-induced growth method. Under typical growth conditions, a micron longnanotip can be formed in around 10 s by field emission in the presence of acetylene gas at∼10−2 mbar. The tip radius of the carbon nanotip can be as small as 5 nm, while its length is determinedby the growth duration. Electron energy loss spectroscopy (EELS) analysis in a transmissionelectron microscope revealed that the carbon nanotip is amorphous carbon with predominantsp2 bonding. The growth mechanism of the carbon nanotip is discussed to explain theformation of the nanotip and its carbon matrix. Field-emission measurements showed thatemission from the carbon nanotip follows the Fowler–Nordheim equation, suggesting thatthe tip shows metallic behaviour as far as field emission is concerned. A high emissioncurrent stress cycle is found to improve the emission current stability significantly.

In situ transmission electron microscopy investigation of the structural changes in carbonnanotubes during electron emission at high currents

The structural changes in carbon nanotubes under electron emission conditions wereinvestigated in situ in a transmission electron microscope (TEM). The measurements wereperformed on individually mounted free-standing multi-walled carbon nanotubes (CNTs).It was found that the structure of the carbon nanotubes did not change gradually, as is thecase with field emission electron sources made of sharp metal tips. Instead, changesoccurred only above a current level of a few microamperes, which was different for eachnanotube. Above the threshold current, carbon nanotubes underwent either structuraldamage, such as shortening and splitting of the apex of the nanotube, or closing of theiropen cap. The results are discussed on the basis of several models for degradationmechanisms.

Near-field photonics: tip-enhanced microscopy and spectroscopy on the nanoscale

Any detailed study of the interaction of electromagnetic radiation with nanoscale systems is limited by diffraction effects in classical optical systems. Near-field microscopy extends conventional imaging beyond this self-imposed barrier and is used to perform microscopy and spectroscopy with ultra-high spatial resolution. In this article we will discuss the use of the enhanced electric field created at the apex of a sharp laser-irradiated metal tip as a means of producing a truly nanoscale light source. This confined light source can be used to excite locally vibrational modes along carbon nanotubes or to investigate surface charge oscillations in optically resonant nanoparticles. We report the use of such a technique to demonstrate localized photofluorescence and Raman imaging with sub 20 nm spatial resolution.

Electromagnetic modelling of Raman enhancement from nanoscale substrates: a route to estimation of the magnitude of the chemical enhancement mechanism in SERS

Despite widespread use for more than two decades, the SERS phenomenon has defied accurate physical and chemical explanation. The relative contributions from electronic and chemical mechanisms are difficult to quantify and are often not reproduced under nominally similar experimental conditions. This work has used electromagnetic modelling to predict the Raman enhancement expected from three configurations: metal nanoparticles, structured metal surfaces, and sharp metal tips interacting with metal surfaces. In each case, parameters such as artefact size, artefact separation and incident radiation wavelength have been varied and the resulting electromagnetic field modelled. This has yielded an electromagnetic description of these configurations with predictions of the maximum expected Raman enhancement, and hence a prediction of the optimum substrate configuration for the SERS process. When combined with experimental observations of the dependence of Raman enhancement with changing ionic strength, the modelling results have allowed a novel estimate of the size of the chemical enhancement mechanism to be produced.