February 23, 2021 feature
High-resolution, terahertz-driven atom probe tomography
Materials scientists must be able to exert ultrafast control of matter using a strong electromagnetic field on the atomic scale to understand the ionization dynamics and excitations in solids. Researchers can couple picosecond duration terahertz pulses to metallic nanostructures to generate extremely localized and intense electric fields. In a new report now on Science Advances, Angela Vella and a research team at the CNRS and the University Institute of France controlled field ion emission across from metallic nanotips. The terahertz near-field induced an athermal ultrafast evaporation of surface atoms as ions on the subpicosecond timescale with the tip acting as a field amplifier. The ultrafast terahertz-ion interaction offered unprecedented control on ultrafast free-ion pulses to image, analyze and manipulate matter at atomic scales. In this work, Vella et al. demonstrated terahertz atom probe microscopy as a new platform for microscopy with atomic and chemical resolution.
The basics of atom probe tomography
The ability to couple electromagnetic fields to solid-state nanostructures to control the basic properties of matter at the nanoscale is increasingly attracting interest for a variety of applications including chemistry, catalysis, gas-sensing and ultrafast electron microscopy and imaging. The basic principle of atom probe tomography (APT) involves the field emission of positive ions from a sharp tip as an imaging technique based on controlled field evaporation of atoms from a nanometric needle-shaped sample under a strong electric field. The technique was attractive due to its capability to provide sub-nanometer spatial resolution in the three dimensions of space, with high chemical sensitivity across whole periodic elements and their isotopes.
Laser-assisted atom probe tomography
At first, the atom probe tomography method was restricted to conductive materials due to the use of high-voltage pulses to trigger ion evaporation. The development of the laser-assisted atom probe tomography (La-APT) allowed the analysis of semiconducting and dielectric materials. During La-APT, the scientists evaporated the sample atom by atom via the combined actions of a high DC field and an ultrashort laser pulse. Due to existing limits, the potential for terahertz-based APT to perform high-resolution imaging was very promising although essential to acquire deeper awareness of the underlying physics of terahertz-pulse-matter interactions. The researchers showed the enhancement of terahertz fields in positively biased nanotips to trigger the emission of positively charged ions from the nanostructure surface to present a high chemical and space resolution terahertz-assisted APT instrument.
During the experiments, the team focused on a single-cycle strong terahertz field generated from two-color air plasma on an aluminum tip-biased at several kilovolts. They combined a near-infrared (NIR) pulse with the terahertz pulse and colinearly focused it on an aluminum tip biased at several kilovolts. Using time-of-flight measurements, they retrieved the mass/charge ratio, and from the impact position on the detector system they reconstructed the evaporated volume using a reverse projection law. The researchers noted the temporal traces of the generated terahertz pulses for two inverted field directions or polarities measured by electro-optic sampling outside the atom probe chamber. Vella et al. measured the terahertz field at the apex of the sample using this field to drive electron emission from the negatively biased aluminum tip under NIR illumination to show how the tip functioned as an ultrafast rectifying diode. The team noted the same deviation from the incident terahertz pulse due to the antenna response to the tip. The results indicated the amplitude of the terahertz pulse to be about 2000 times higher than the incident terahertz field. To compare the field enhancement factor, the team used finite-difference time-domain commercial software Lumerical to take the tip geometry into account. The team increased the amplitude of the terahertz field to its maximum of 5.5 V/nm to perform ion field emission using terahertz pulses. They then experimentally checked this value of the terahertz near-field using electron energy filtering.
Studying the aluminum tip in terahertz-assisted APT and its dual-frequency excitation
To perform ion field evaporation using terahertz pulses, Vella et al. positively biased the aluminum tip at 9 kV and set the terahertz pulse with positive polarity to its maximum amplitude of 5.5 V/µm corresponding to a near field of 10.5 V/nm. The scientists presented the mass spectra obtained at the same bias using terahertz and NIR laser pulses. The 3-D reconstruction of the evaporated volume showed well-resolved atomic planes for three crystallographic directions as discerned using NIR analysis. The team obtained the image reconstruction of APT using field erosion and calculated the spatial resolution of 3-D images using the Fourier transform approach. Using dual-frequency excitation of the aluminum tip, they recorded the evaporation rate as a function of the delay between the NIR and terahertz laser pulses.
In situations where the terahertz pulse preceded the NIR pulse, the evaporation rate was deemed stable and its value equal to that obtained only by terahertz pulses and therefore unaffected by NIR laser excitation. Temporal overlap between the NIR and the THz pulse maintained unchanged evaporation rates. When the NIR pulses preceded the terahertz pulse, the evaporation rate increased up to its maximum in less than 0.5 picoseconds. The underlying physical evaporation mechanism contributed to the chemical and space resolution of the atom probe assisted by terahertz pulses compared to NIR pulses. The results of the dual-frequency excitation in the AI nanotip contributed experimental proof of the athermal ion evaporation by terahertz pulses.
In this way, Angela Vella and colleagues showed how ultrafast, nonthermal field evaporation of surface atoms as ions by tip-enhanced single-cycle terahertz pulses paved the path for material analysis with spatial and chemical resolutions. The method can also facilitate time-resolved chemistry in high electric fields to open new ways in field-induced chemistry. The narrow energy spread of the field-evaporated ions by single-cycle terahertz pulses will open the way to use charged particle beams for imaging, analysis and matter modification from the microscale to the nanoscale.
Sadeghian B. et al. Ultralow-voltage field-ionization discharge on whiskered silicon nanowires for gas-sensing applications. Nature Materials, doi.org/10.1038/nmat2944
Blavette D. et al. An atom probe for three-dimensional tomography. Nature, doi.org/10.1038/363432a0
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