Tungsten Nanotip Research Articles

Field Ion Microscopy of Tungsten Nano-Tips Coated with Thin Layer of Epoxy Resin

This paper presents an analysis of the field ion emission mechanism of tungsten–epoxy nanocomposite emitters and compares their performance with that of tungsten nano-field emitters. The emission mechanism is described using the theory of induced conductive channels. Tungsten emitters with a radius of 70 nm were fabricated using electrochemical polishing and coated with a 20 nm epoxy resin layer. Characterization of the emitters, both before and after coating, was performed using electron microscopy and energy-dispersive X-ray spectroscopy (EDS). The Tungsten nanocomposite emitter was tested using a field ion microscope (FIM) in the voltage range of 0–15 kV. The FIM analyses revealed differences in the emission ion density distributions between the uncoated and coated emitters. The uncoated tungsten tips exhibited the expected crystalline surface atomic distribution in the FIM images, whereas the coated emitters displayed randomly distributed emission spots, indicating the formation of induced conductive channels within the resin layer. The atom probe results are consistent with the FIM findings, suggesting that the formation of conductive channels is more likely to occur in areas where the resin surface is irregular and exhibits protrusions. These findings highlight the distinct emission mechanisms of both emitter types.

InSitu Observation of Field-Induced Nanoprotrusion Growth on a Carbon-Coated Tungsten Nanotip.

Nanoprotrusion (NP) on metal surface and its inevitable contamination layer under high electric field is often considered as the primary precursor that leads to vacuum breakdown, which plays an extremely detrimental effect for high energy physics equipment and many other devices. Yet, the NP growth has never been experimentally observed. Here, we conduct field emission (FE) measurements along with insitu transmission electron microscopy (TEM) imaging of an amorphous-carbon (a-C) coated tungsten nanotip at various nanoscale vacuum gap distances. We find that under certain conditions, the FE current-voltage (I-V) curves switch abruptly into an enhanced-current state, implying the growth of an NP. We then run field emission simulations, demonstrating that the temporary enhanced-current I-V is perfectly consistent with the hypothesis that a NP has grown at the apex of the tip. This hypothesis is also confirmed by the repeatable insitu observation of such a nanoprotrusion and its continued growth during successive FE measurements in TEM. We tentatively attribute this phenomenon to field-induced biased diffusion of surface a-C atoms, after performing a finite element analysis that excludes the alternative possibility of field-induced plastic deformation.

Highly Localized Optical Field Enhancement at Neon Ion Sputtered Tungsten Nanotips.

We present laser-driven rescattering of electrons at a nanometric protrusion (nanotip), which is fabricated with an in situ neon ion sputtering technique applied to a tungsten needle tip. Electron energy spectra obtained before and after the sputtering show rescattering features, such as a plateau and high-energy cutoff. Extracting the optical near-field enhancement in both cases, we observe a strong increase of more than 2-fold for the nanotip. Accompanying finite-difference time-domain (FDTD) simulations show a good match with the experimentally extracted near-field strengths. Additionally, high electric field localization for the nanotip is found. The combination of transmission electron microscope imaging of such nanotips and the determination of the near-field enhancement by electron rescattering represent a full characterization of the electric near-field of these intriguing electron emitters. Ultimately, nanotips as small as single nanometers can be produced, which is of utmost interest for electron diffraction experiments and low-emittance electron sources.

Attosecond field emission

Field emission of electrons underlies great advances in science and technology, ranging from signal processing at ever higher frequencies1 to imaging of the atomic-scale structure of matter2 with picometre resolution. The advancing of electron microscopy techniques to enable the complete visualization of matter on the native spatial (picometre) and temporal (attosecond) scales of electron dynamics calls for techniques that can confine and examine the field emission on sub-femtosecond time intervals. Intense laser pulses have paved the way to this end3,4 by demonstrating femtosecond confinement5,6 and sub-optical cycle control7,8 of the optical field emission9 from nanostructured metals. Yet the measurement of attosecond electron pulses has remained elusive. We used intense, sub-cycle light transients to induce optical field emission of electron pulses from tungsten nanotips and a weak replica of the same transient to directly investigate the emission dynamics in real time. Access to the temporal properties of the electron pulses rescattering off the tip surface, including the duration τ = (53 as ± 5 as) and chirp, and the direct exploration of nanoscale near fields open new prospects for research and applications at the interface of attosecond physics and nano-optics.

Extreme lightwave electron field emission from a nanotip

We report on sub-cycle terahertz light-field emission of electrons from tungsten nanotips under extreme conditions corresponding to a Keldysh parameter $\gamma_K\approx10^{-4}$. Local peak THz fields up to 40~GV/m are achieved at the apex of an illuminated nanotip, causing sub-cycle cold-field electron emission and acceleration in the quasi-static field. By simultaneous measurement of the electron bunch charge and energy distribution, we perform a quantitative test of quasi-static Fowler-Nordheim tunnelling theory under field conditions that completely suppress the tunnel barrier. Very high bunch charges of $\sim10^6$ electrons/pulse are observed, reaching maximum energies of 3.5~keV after acceleration in the local field. The energy distribution and emission current show good agreement with Fowler-Nordheim theory even in this extreme field regime. Extending this model to the single-shot regime under these conditions predicts peak electron distributions with a spectral purity of $10^{-4}$. THz field-induced reshaping and sharpening of the nanotip is observed, reducing the tip radius from 120~nm to 35~nm over roughly $10^9$ THz shots. These results indicate THz-driven nanotips in the extreme field limit are promising electron sources for ultrafast electron diffraction and microscopy.

Ideality factor and barrier height for a GaN nanomembrane electrically contacted with a tungsten nano-tip in a TEM

In the present article, the electrical characteristics of a freestanding gallium nitride nanomembrane in contact with a tungsten nanoprobe are evaluated using scanning tunneling microscopy in an aberration-corrected transmission electron microscope without any lithographic patterning. We report here barrier height (ΦB=0.33±0.05eV and ideality factor (ηW/GaN−NM=1.620±0.07) parameters as extracted from I–V characteristic curve. Our experimental findings, combined with analytical calculations, show that the use of nanosized edge contacts results in a reduced barrier height, which is very promising for achieving a high ‘on’ current, large photoresponse, and high-frequency operation in FET devices.

Laser-induced field emission from a tungsten nanotip by circularly polarized femtosecond laser pulses

We have investigated emission patterns and energy spectra of electrons from a tungsten nanotip induced by circularly polarized femtosecond laser pulses. Variations of emission patterns were observed for different helicities of circular polarization while the energy spectra remained almost identical. The physics behind this difference in emission patterns is the change in propagation directions of surface electromagnetic waves on the tip apex. The energy spectra showed the same spectroscopic signatures as the linearly polarized laser in a strong-field regime, which are a low-energy peak and a plateau feature. The low-energy peak is due to a delayed electron emission with respect to a prompt emission. The experimental data and plasmonic simulations support our previous conclusion, where the observed delayed emission processes originate from an inelastic rescattering process. This work demonstrates that the use of circular polarization is an easy means to add extra knobs to control the spatial and temporal emission from a nanotip at the nanometer and femtosecond scale. It could find applications as a helicity-driven subcycle optical switch.

Electrochemical Etching of Tungsten for Fabrication of Sub-10-nm Tips with a Long Taper and a Large Shank

From the perspective of maximizing the practicability of tungsten nano-tips, a sharp tip, long taper, and large shank are usually expected. However, simultaneously satisfying the requirements for tip radius, taper length, and shank diameter is theoretically impossible with the conventional drop-off tip fabrication method, which is based on lamellae electrochemical etching. In this study, a two-step etching method is proposed to fabricate a sub-10-nm tungsten tip directly from a 1-mm rod. First, a floating electrolyte-based drop-off process is carried out to fabricate a tungsten needle with a long length taper of 10 mm. Then, an inversed lamellae drop-off process is conducted to realize fine etching of the needlepoint. As a result, an ultra-sharp tungsten nano-tip with a radius of 5.5 nm and taper length of 10 mm is successfully fabricated from a 1-mm tungsten rod.

Electrochemical etching of lightweight nanotips for high quality‐factor quartz tuning fork force sensor: atomic force microscopy applications

Commercially available quartz tuning forks (QTFs) can be transformed into self-sensing and actuating force sensors by micro-assembling a sharp tip on the apex of a tine. Mass of the tip is critical in determining the quality (Q)-factor of the sensor, therefore, fabrication of the lightweight nanotips is a precondition for high Q-factor QTF sensors. The work reports fabrication of very lightweight tungsten nanotips with a two-step electrochemical etching technique which can be used to develop high Q-factor QTF force sensor. First, a tungsten wire with protective coating at one end (1–2 mm) is etched with a trapezoidal waveform to form a lengthy (∼2–5 mm) and slender (diameter ∼10–40 μm) micro-needle. In the second step, sharp tip apex is fabricated with a direct current etching. High Q-factor (6600–8000) QTF force sensors have been developed with the fabricated nanotips. Atomic force microscope scanning of nano-grating and a triblock copolymer surface validates the scanning performance of the developed sensors.

Development of a high brightness ultrafast Transmission Electron Microscope based on a laser-driven cold field emission source

We report on the development of an ultrafast Transmission Electron Microscope based on a cold field emission source which can operate in either DC or ultrafast mode. Electron emission from a tungsten nanotip is triggered by femtosecond laser pulses which are tightly focused by optical components integrated inside a cold field emission source close to the cathode. The properties of the electron probe (brightness, angular current density, stability) are quantitatively determined. The measured brightness is the largest reported so far for UTEMs. Examples of imaging, diffraction and spectroscopy using ultrashort electron pulses are given. Finally, the potential of this instrument is illustrated by performing electron holography in the off-axis configuration using ultrashort electron pulses.

Electron Spectrum of Nonlinear Cold Emission from a Metal under the Action of a Laser Shot

The nonlinear emission of electrons from a metal under the action of a femtosecond moderate-intensity laser pulse (laser shot) has been studied. A theoretical model of the process has been constructed based on the 1D nonstationary Schrodinger equation in the vacuum half-space with given boundary conditions for the electron wavefunction. This equation has been solved using the Laplace transformation. It has been assumed that the states of free electrons in a metal, which are described by the Sommerfeld theory of metals, are insignificantly influenced by the laser field. The energy spectrum of emitted electrons has been obtained, and its dependence on the parameters of the lased shot has been found. The calculated spectrum of nonlinear electron emission from a tungsten nanotip under the action of a 6.5-fs-long laser shot generating a field of 9.26 V/nm agrees with the experimental data.

Optical Control of Young\u2019s Type Double-slit Interferometer for Laser-induced Electron Emission from a Nano-tip

Interference experiments with electrons in a vacuum can illuminate both the quantum and the nanoscale nature of the underlying physics. An interference experiment requires two coherent waves, which can be generated by splitting a single coherent wave using a double slit. If the slit-edge separation is larger than the coherence width at the slit, no interference appears. Here we employed variations in surface barrier at the apex of a tungsten nano-tip as slits and achieved an optically controlled double slit, where the separation and opening-and-closing of the two slits can be controlled by respectively adjusting the intensity and polarization of ultrashort laser pulses. Using this technique, we have demonstrated interference between two electron waves emitted from the tip apex, where interference has never been observed prior to this technique because of the large slit-edge separation. Our findings pave the way towards simple time-resolved electron holography on e.g. molecular adsorbates employing just a nano-tip and a screen.

Coherent control of two-color above-threshold photoemission from tungsten nanotips

SynopsisWe demonstrate coherently controlled two-color above-threshold photoemission from a single crystal tungsten nanotip. With optimized fundamental and second harmonic intensities near perfect phase contrast can be obtained.

Surface Desorption Dielectric-Barrier Discharge Ionization Mass Spectrometry.

A variant of dielectric-barrier discharge named surface desorption dielectric-barrier discharge ionization (SDDBDI) mass spectrometry was developed for high-efficiency ion transmission and high spatial resolution imaging. In SDDBDI, a tungsten nanotip and the inlet of the mass spectrometer are used as electrodes, and a piece of coverslip is used as a sample plate as well as an insulating dielectric barrier, which simplifies the configuration of instrument and thus the operation. Different from volume dielectric-barrier discharge (VDBD), the microdischarges are generated on the surface at SDDBDI, and therefore the plasma density is extremely high. Analyte ions are guided directly into the MS inlet without any deflection. This configuration significantly improves the ion transmission efficiency and thus the sensitivity. The dependence of sensitivity and spatial resolution of the SDDBDI on the operation parameters were systematically investigated. The application of SDDBDI was successfully demonstrated by analysis of multiple species including amino acids, pharmaceuticals, putative cancer biomarkers, and mixtures of both fatty acids and hormones. Limits of detection (S/N = 3) were determined to be 0.84 and 0.18 pmol, respectively, for the analysis of l-alanine and metronidazole. A spatial resolution of 22 μm was obtained for the analysis of an imprinted cyclophosphamide pattern, and imaging of a "T" character was successfully demonstrated under ambient conditions. These results indicate that SDDBDI has high-efficiency ion transmission, high sensitivity, and high spatial resolution, which render it a potential tool for mass spectrometry imaging.

Two-color multiphoton emission from nanotips

Two-color multiphoton emission from polycrystalline tungsten nanotips has been demonstrated using two-color laser fields. The two-color photoemission is assisted by a three-photon multicolor quantum channel, which leads to a twofold increase in quantum efficiency. Weak-field control of two-color multiphoton emission was achieved by changing the efficiency of the quantum channel with pulse delay. The result of this study complements two-color tunneling photoemission in strong fields, and has potential applications for nanowire-based photonic devices. Moreover, the demonstrated two-color multiphoton emission may be important for realizing ultrafast spin-polarized electron sources via optically injected spin current.

High visibility in two-color above-threshold photoemission from tungsten nanotips in a coherent control scheme

In this article, we present coherent control of above-threshold photoemission from a tungsten nanotip achieving nearly perfect modulation. Depending on the pulse delay between fundamental () and second harmonic () pulses of a femtosecond fiber laser at the nanotip, electron emission is significantly enhanced or depressed during temporal overlap. Electron emission is studied as a function of pulse delay, optical near-field intensities, DC bias field and final photoelectron energy. Under optimized conditions modulation amplitudes of the electron emission of 97.5% are achieved. Experimental observations are discussed in the framework of quantum-pathway interference supported by local density of states simulations.

A novel method for formation of single crystalline tungsten nanotip

A point electron source is desired to improve performance of high brightness electron beam instruments. It is valuable to create nano-sized tungsten (W) tip from sharp ordinary polycrystalline W needle. The sharp W needle, which is manufactured by electrochemical etching, has been practically utilized as a cold field emission electron source. A novel method for formation of single crystalline W nanotip on the top of h-BN coated conventional polycrystalline tungsten, by supplying high voltage, has been found. The W nanotip with an apex radius as small as a few times 10 nm would be grown on the top of the polycrystalline W needle. Field emission characteristics of obtained W nanotip are measured, and the field emission microscopic (FEM) and transmission emission microscopic (TEM) images are observed. The emission current from the W nanotip is measured to exceed 0.1 mA. The FEM image shows significant electron emission from the crystallographic facets of the W single crystal. From these results, the present method for formation of the single crystalline W nanotip would be expected as a key technology to realize a point electron source with a nano-sized apex which makes it possible to improve the performance of high brightness electron beam instruments, especially tiny X-ray tubes for medical use, as well as a cantilever of scanning probe microscope.

Nanotip Ambient Ionization Mass Spectrometry.

A method called nanotip ambient ionization mass spectrometry (NAIMS) is described, which applies high voltage between a tungsten nanotip and a metal plate to generate a plasma in which ionized analytes on the surface of the metal plate are directed to the inlet and analyzed by a mass spectrometer. The dependence of signal intensity is investigated as a function of the tip-to-plate distance, the tip size, the voltage applied at the tip, and the current. These parameters are separately optimized to achieve sensitivity or high spatial resolution. A partially observable Markov decision process is used to achieve a stabilized plasma as well as high ionization efficiency. As a proof of concept, the NAIMS technique has been applied to phenanthrene and caffeine samples for which the limits of detection were determined to be 0.14 fmol for phenanthrene and 4 amol for caffeine and to a printed caffeine pattern for which a spatial resolution of 8 ± 2 μm, and the best resolution of 5 μm, was demonstrated. The limitations of NAIMS are also discussed.

A miniaturized electron source based on dielectric laser accelerator operation at higher spatial harmonics and a nanotip photoemitter

Here we propose a miniaturized electron source driven by recent experimental results of laser-triggered electron emission from tungsten nanotips and dielectric laser acceleration of sub relativistic electrons with velocities as low as or energies as low as 9.6 keV, less than 20% of the speed of light. The recently observed laser-triggered emission of coherent low-emittance electron pulses from tungsten nanotips naturally lends itself towards incorporation with subrelativistic dielectric laser accelerators (DLAs). These structures have previously been shown to accelerate 28 keV electrons and here we report on the utilization of the 4th and 5th spatial harmonics of near fields in the single grating DLA to achieve acceleration of electrons with kinetic energies of 15.2 and 9.6 keV. We then propose the combination of needle tip emitters with subrelativistic accelerators to form a mm-scale device capable of producing electrons with arbitrary energies.

Large optical field enhancement for nanotips with large opening angles

We theoretically investigate the dependence of the enhancement of optical near-fields at nanometric tips on the shape, size, and material of the tip. We confirm the strong dependence of the field enhancement factor on the radius of curvature. In addition, we find a surprisingly strong increase of field enhancement with increasing opening angle of the nanotips. For gold and tungsten nanotips in the experimentally relevant parameter range (radius of curvature at laser wavelength), we obtain field enhancement factors of up to for Au and for W for large opening angles. We confirm this strong dependence on the opening angle for many other materials featuring a wide variety in their dielectric response. For dielectrics, the opening angle dependence is traced back to the electrostatic force of the induced surface charge at the tip shank. For metals, the plasmonic response strongly increases the field enhancement and shifts the maximum field enhancement to smaller opening angles.