RESUMEN
The development of time-resolved transmission electron microscopy (TEM), ultrafast electron spectroscopy and pulsed X-ray sources relies on the realization of stable and high brightness sources of ultra-short electron bunches with a long service time. The flat photocathodes implanted in thermionic electron guns have been replaced by Schottky-type or cold-field emission sources driven by ultra-fast laser. Recently, lanthanum hexaboride (LaB6) nanoneedles have been reported to have high brightness and high emission stability when working in a continuous emission mode. Here, we prepare nano-field emitters from bulk LaB6 and we report on their use as ultra-fast electron sources. Using a high repetition rate laser in the infrared range, we present different field emission regimes as a function of the extraction voltage and laser intensity. The properties of the electron source (brightness, stability, energy spectrum and emission pattern) are determined for the different regimes. Our results show that LaB6 nanoneedles can be used as ultrafast and ultra-bright sources for time-resolved TEM, with better performances as compared to metallic ultra-fast field-emitters.
RESUMEN
Dissociation processes involving phosphorus cations were investigated during laser-assisted atom probe tomography of crystalline indium phosphide (InP). This technique not only allows the formation of medium-sized phosphorus cations by means of femtosecond laser pulses under ultrahigh vacuum and high electric field conditions but also allows one to study the time-resolved dissociation dynamics. Data reveal the formation of cations up to P232+ and their subsequent dissociation into two smaller Pk+ cations (k > 2). The use of a time- and position-sensitive detector combined with numerical calculations provided information related to the molecule orientation, decay time, and kinetic energy release during dissociation phenomena. Results suggest that the dissociation processes are most likely due to the emission of Pk2+ cations in excited states and their subsequent decay in low field regions during their flight toward the detector. This study provides operative guidelines to obtain information on dissociation processes using a tomographic atom probe as a reaction microscope and indicates the current capabilities and limitations of such an approach.
RESUMEN
Laser enhanced field evaporation of surface atoms in laser-assisted Atom Probe Tomography (APT) can simultaneously excite photoluminescence in semiconductor or insulating specimens. An atom probe equipped with appropriate focalization and collection optics has been coupled with an in situ micro-photoluminescence (µPL) bench that can be operated during APT analysis. The photonic atom probe instrument we have developed operates at frequencies up to 500 kHz and is controlled by 150 fs laser pulses tunable in energy in a large spectral range (spanning from deep UV to near IR). Micro-PL spectroscopy is performed using a 320 mm focal length spectrometer equipped with a CCD camera for time-integrated and with a streak camera for time-resolved acquisitions. An example of application of this instrument on a multi-quantum well oxide heterostructure sample illustrates the potential of this new generation of tomographic atom probes.
RESUMEN
Practical realization of stable and high brightness sources of ultra-short electron pulses is an important issue in the development of time-resolved electron microscopy for the study of ultra-fast dynamics in materials. Here, we report on the experimental investigation of static (in the dark) and pulsed (under illumination by sub-picosecond laser pulses at 1040 nm) electron emission from single-crystal diamond needles. A significant increase of electron emission current was detected under laser illumination. The nonlinear dependence of the emission current on the laser intensity and on the angle between the needle and the laser beam polarization axis suggests multi-photon emission processes. This interpretation is in agreement with electron spectroscopy measurements performed for electrons emitted at different bias voltages and different laser power levels and repetition rates. The remarkable feature of the diamond emitters is their stability under high average power of laser radiation. This provides a new highly efficient source of photoemitted electrons based on single-crystal diamond.
RESUMEN
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.
RESUMEN
A systematic study of the biases occurring in the measurement of the composition of GaN by Atom Probe Tomography was carried out, in which the role of surface electric field and laser pulse intensity has been investigated. Our data confirm that the electric field is the main factor influencing the measured composition, which exhibits a deficiency of N at low field and a deficiency of Ga at high field. The deficiency of Ga at high field is interpreted in terms of preferential evaporation of Ga. The detailed analysis of multiple evaporation events reveals that the measured composition is not affected by pile-up phenomena occurring in detection system. The analysis of correlation histograms yields the signature of the production of neutral N2 due to the dissociation of GaN32+ ions. However, the amount of N2 neutral molecules that can be detected cannot account for the N deficiency found at low field. Therefore, we propose that further mechanisms of neutral N evaporation could be represented by dissociation reactions such as GaN+â Ga++â¯N and GaN2+â Ga2++â¯N.
RESUMEN
The application of a high electrostatic field at the apex of monocrystalline diamond nanoscale needles induces an energy splitting of the photoluminescence lines of color centers. In particular, the splitting of the zero-phonon line of the neutral nitrogen-vacancy complex (NV0) has been studied within a laser-assisted tomographic atom probe equipped with an in situ microphotoluminescence bench. The measured quadratic dependence of the energy splitting on the applied voltage corresponds to the stress generated on the metal-like apex surface by the electrostatic field. Tensile stress up to 7 GPa has thus been measured in the proximity of the needle apex. Furthermore, the stress scales along the needle shank inversely proportionally to its axial cross section. We demonstrate thus a method for contactless piezo-spectroscopy of nanoscale systems by electrostatic field regulation for the study of their mechanical properties. These results also provide an experimental confirmation to the models of dielectrics surface metallization under high electrostatic field.
RESUMEN
The laser assisted field-evaporation of metals and oxides strongly depends on the illumination conditions. Here we study the effect of laser pulse width using two different laser systems, with a pulse duration of a few tens of femtoseconds and a few tens of picoseconds, respectively. Adjusting the laser wavelength by nonlinear optical conversion systems, we study the evaporation behavior of FeCu and MgO samples. We prove that the laser pulse width does not affect the evaporation behavior, in the range of duration explored. These results are explained taking into account the absorption behavior of nanometric samples and their thermal properties.
