Ultrafast scanning electron microscopy

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Ultrafast scanning electron microscopy (UFSEM) combines two microscopic modalities, Pump-probe microscopy and Scanning electron microscope, to gather temporal and spatial resolution phenomena. The technique uses ultrashort laser pulses for pump excitation of the material and the sample response will be detected by an Everhart-Thornley detector. Acquiring data depends mainly on formation of images by raster scan mode after pumping with short laser pulse at different delay times. The characterization of the output image will be done through the temporal resolution aspect.[ clarification needed ] Thus, the idea is to exploit the shorter DeBroglie wavelength in respect to the photons which has great impact to increase the resolution about 1 nm. [1] That technique is an up-to-date approach to study the dynamic of charge on material surfaces.

Contents

Time resolved in scanning electron spectroscopy

Ernst Ruska was a pioneer German scholar who won the Nobel prize in 1986 for his work on the development of an electron microscope in 1933 in collaboration with Max Knoll. [2] Nowadays, electron microscopy is miscellaneous used tool due to enhancement not only the spatial resolution respect to the optical microscope but also high imaging contrast and remarkable sensitivity due to the fact that the robustness of electrons impact on the matter in comparison with photons.[ clarification needed ]} Proceeding from that concept, the technology of ultrafast scanning electron microscopy has been modified by assistance of Ultrashort pulse laser which allows the scientists to investigate material dynamic in short and ultra-short scale of time. Nowadays, pump-probe microscopy has been improved after Ahmed Zewail's discovery of femtosecond time scale for chemical reaction and has awarded the Nobel Prize for his historical discovery. [3] [4] [5] [6] [7]


Pump probe microscopy

Pump-probe techniques in physics.

Pump-probe microscopy phenomenon, widely known as transient absorption microscopy, is a sort of nonlinear process starting by excitation of the material by very short pulse laser beam (pump), which induces internal transition. [8] A probe beam follows the pump beam to trace the progress that has been done inside the material also in very short time. In reality, that response could be changed by manipulating the time delay between pump and probe and by this way the concept of Time-resolved spectroscopy will be used to trace dynamic process evolution as a function of time. [9] The fascinating in Ultrafast scanning electron microscopy is how powerful it obtains by combining high spatial resolution of the electrons and temporal resolution of ultra-fast pump-probe microscopy. [10]

Measurement methodology

The fundamental idea that measurement has been built to exploit the Spatial resolution of electron microscopy and temporal resolution for ultrafast optical pump probe. [11] The setup simply consists of scanning electron microscopy machine always works in ultra-high vacuum that regarding on electron beam as a probe and ultrashort laser beam as pump. [12] Firstly, Schottky emission gun is almost common to use as source of primary beam due to high beam brightness after passing through electromagnetic lens. Secondly, femtosecond Powerful fibre laser with repetition rates from KHZ to few of MHz splits by nonlinear process into third and fourth harmonic generation 343 nm and 257 nm, respectively. During the measurement, the tip emission is less than thermal emission limit to acquire photoemission mode. That photoemission mode improves by allow forth harmonic generation beam to interact the tip which generates more electrons. On the other hand, another third harmonic generation will be used to excite the sample itself. The time-resolved measurement will be acquired by detecting the secondary electron emission in image shape at different delay time between third and fourth harmonic beam. The final acquired intensity must be normalized by subtraction from the background. It is important to acquire the measurement at different delay time forward and reverse that a good tool for checking the stability and reproducibility. [13]

See also

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<span class="mw-page-title-main">Microscopy</span> Viewing of objects which are too small to be seen with the naked eye

Microscopy is the technical field of using microscopes to view objects and areas of objects that cannot be seen with the naked eye. There are three well-known branches of microscopy: optical, electron, and scanning probe microscopy, along with the emerging field of X-ray microscopy.

<span class="mw-page-title-main">Cathodoluminescence</span> Photon emission under the impact of an electron beam

Cathodoluminescence is an optical and electromagnetic phenomenon in which electrons impacting on a luminescent material such as a phosphor, cause the emission of photons which may have wavelengths in the visible spectrum. A familiar example is the generation of light by an electron beam scanning the phosphor-coated inner surface of the screen of a television that uses a cathode ray tube. Cathodoluminescence is the inverse of the photoelectric effect, in which electron emission is induced by irradiation with photons.

<span class="mw-page-title-main">Transmission electron microscopy</span> Imaging and diffraction using electrons that pass through samples

Transmission electron microscopy (TEM) is a microscopy technique in which a beam of electrons is transmitted through a specimen to form an image. The specimen is most often an ultrathin section less than 100 nm thick or a suspension on a grid. An image is formed from the interaction of the electrons with the sample as the beam is transmitted through the specimen. The image is then magnified and focused onto an imaging device, such as a fluorescent screen, a layer of photographic film, or a detector such as a scintillator attached to a charge-coupled device or a direct electron detector.

<span class="mw-page-title-main">Femtochemistry</span> Chemistry of reactions on 10^-15 second timescales

Femtochemistry is the area of physical chemistry that studies chemical reactions on extremely short timescales in order to study the very act of atoms within molecules (reactants) rearranging themselves to form new molecules (products). In a 1988 issue of the journal Science, Ahmed Hassan Zewail published an article using this term for the first time, stating "Real-time femtochemistry, that is, chemistry on the femtosecond timescale...". Later in 1999, Zewail received the Nobel Prize in Chemistry for his pioneering work in this field showing that it is possible to see how atoms in a molecule move during a chemical reaction with flashes of laser light.

<span class="mw-page-title-main">Streak camera</span> Instrument for measuring variation in light pulse intensity over time

A streak camera is an instrument for measuring the variation in a pulse of light's intensity with time. They are used to measure the pulse duration of some ultrafast laser systems and for applications such as time-resolved spectroscopy and LIDAR.

Photoemission electron microscopy is a type of electron microscopy that utilizes local variations in electron emission to generate image contrast. The excitation is usually produced by ultraviolet light, synchrotron radiation or X-ray sources. PEEM measures the coefficient indirectly by collecting the emitted secondary electrons generated in the electron cascade that follows the creation of the primary core hole in the absorption process. PEEM is a surface sensitive technique because the emitted electrons originate from a shallow layer. In physics, this technique is referred to as PEEM, which goes together naturally with low-energy electron diffraction (LEED), and low-energy electron microscopy (LEEM). In biology, it is called photoelectron microscopy (PEM), which fits with photoelectron spectroscopy (PES), transmission electron microscopy (TEM), and scanning electron microscopy (SEM).

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<span class="mw-page-title-main">Scanning transmission electron microscopy</span> Scanning microscopy using thin samples and transmitted electrons

A scanning transmission electron microscope (STEM) is a type of transmission electron microscope (TEM). Pronunciation is [stɛm] or [ɛsti:i:ɛm]. As with a conventional transmission electron microscope (CTEM), images are formed by electrons passing through a sufficiently thin specimen. However, unlike CTEM, in STEM the electron beam is focused to a fine spot which is then scanned over the sample in a raster illumination system constructed so that the sample is illuminated at each point with the beam parallel to the optical axis. The rastering of the beam across the sample makes STEM suitable for analytical techniques such as Z-contrast annular dark-field imaging, and spectroscopic mapping by energy dispersive X-ray (EDX) spectroscopy, or electron energy loss spectroscopy (EELS). These signals can be obtained simultaneously, allowing direct correlation of images and spectroscopic data.

Ultrafast laser spectroscopy is a category of spectroscopic techniques using ultrashort pulse lasers for the study of dynamics on extremely short time scales. Different methods are used to examine the dynamics of charge carriers, atoms, and molecules. Many different procedures have been developed spanning different time scales and photon energy ranges; some common methods are listed below.

<span class="mw-page-title-main">European XFEL</span>

The European X-Ray Free-Electron Laser Facility is an X-ray research laser facility commissioned during 2017. The first laser pulses were produced in May 2017 and the facility started user operation in September 2017. The international project with twelve participating countries; nine shareholders at the time of commissioning, later joined by three other partners, is located in the German federal states of Hamburg and Schleswig-Holstein. A free-electron laser generates high-intensity electromagnetic radiation by accelerating electrons to relativistic speeds and directing them through special magnetic structures. The European XFEL is constructed such that the electrons produce X-ray light in synchronisation, resulting in high-intensity X-ray pulses with the properties of laser light and at intensities much brighter than those produced by conventional synchrotron light sources.

<span class="mw-page-title-main">Raman microscope</span> Laser microscope used for Raman spectroscopy

The Raman microscope is a laser-based microscopic device used to perform Raman spectroscopy. The term MOLE is used to refer to the Raman-based microprobe. The technique used is named after C. V. Raman, who discovered the scattering properties in liquids.

The technique of vibrational analysis with scanning probe microscopy allows probing vibrational properties of materials at the submicrometer scale, and even of individual molecules. This is accomplished by integrating scanning probe microscopy (SPM) and vibrational spectroscopy. This combination allows for much higher spatial resolution than can be achieved with conventional Raman/FTIR instrumentation. The technique is also nondestructive, requires non-extensive sample preparation, and provides more contrast such as intensity contrast, polarization contrast and wavelength contrast, as well as providing specific chemical information and topography images simultaneously.

Ultrafast electron diffraction, also known as femtosecond electron diffraction, is a pump-probe experimental method based on the combination of optical pump-probe spectroscopy and electron diffraction. Ultrafast electron diffraction provides information on the dynamical changes of the structure of materials. It is very similar to time resolved crystallography, but instead of using X-rays as the probe, it uses electrons. In the ultrafast electron diffraction technique, a femtosecond (10–15 second) laser optical pulse excites (pumps) a sample into an excited, usually non-equilibrium, state. The pump pulse may induce chemical, electronic or structural transitions. After a finite time interval, a femtosecond electron pulse is incident upon the sample. The electron pulse undergoes diffraction as a result of interacting with the sample. The diffraction signal is, subsequently, detected by an electron counting instrument such as a charge-coupled device camera. Specifically, after the electron pulse diffracts from the sample, the scattered electrons will form a diffraction pattern (image) on a charge-coupled device camera. This pattern contains structural information about the sample. By adjusting the time difference between the arrival of the pump and probe beams, one can obtain a series of diffraction patterns as a function of the various time differences. The diffraction data series can be concatenated in order to produce a motion picture of the changes that occurred in the data. Ultrafast electron diffraction can provide a wealth of dynamics on charge carriers, atoms, and molecules.

<span class="mw-page-title-main">Scanning transmission X-ray microscopy</span>

Scanning transmission X-ray microscopy (STXM) is a type of X-ray microscopy in which a zone plate focuses an X-ray beam onto a small spot, a sample is scanned in the focal plane of the zone plate and the transmitted X-ray intensity is recorded as a function of the sample position. A stroboscopic scheme is used where the excitation is the pump and the synchrotron X-ray flashes are the probe. X-ray microscopes work by exposing a film or charged coupled device detector to detect X-rays that pass through the specimen. The image formed is of a thin section of specimen. Newer X-ray microscopes use X-ray absorption spectroscopy to heterogeneous materials at high spatial resolution. The essence of the technique is a combination of spectromicroscopy, imaging with spectral sensitivity, and microspectroscopy, recording spectra from very small spots.

<span class="mw-page-title-main">Infrared Nanospectroscopy (AFM-IR)</span> Infrared microscopy technique

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Wide-field multiphoton microscopy refers to an optical non-linear imaging technique tailored for ultrafast imaging in which a large area of the object is illuminated and imaged without the need for scanning. High intensities are required to induce non-linear optical processes such as two-photon fluorescence or second harmonic generation. In scanning multiphoton microscopes the high intensities are achieved by tightly focusing the light, and the image is obtained by beam scanning. In wide-field multiphoton microscopy the high intensities are best achieved using an optically amplified pulsed laser source to attain a large field of view (~100 μm). The image in this case is obtained as a single frame with a CCD without the need of scanning, making the technique particularly useful to visualize dynamic processes simultaneously across the object of interest. With wide-field multiphoton microscopy the frame rate can be increased up to a 1000-fold compared to multiphoton scanning microscopy. Wide-field multiphoton microscopes are not yet commercially available, but working prototypes exist in several optics laboratories.

<span class="mw-page-title-main">Nano-FTIR</span> Infrared microscopy technique

Nano-FTIR is a scanning probe technique that utilizes as a combination of two techniques: Fourier transform infrared spectroscopy (FTIR) and scattering-type scanning near-field optical microscopy (s-SNOM). As s-SNOM, nano-FTIR is based on atomic-force microscopy (AFM), where a sharp tip is illuminated by an external light source and the tip-scattered light is detected as a function of tip position. A typical nano-FTIR setup thus consists of an atomic force microscope, a broadband infrared light source used for tip illumination, and a Michelson interferometer acting as Fourier-transform spectrometer. In nano-FTIR, the sample stage is placed in one of the interferometer arms, which allows for recording both amplitude and phase of the detected light. Scanning the tip allows for performing hyperspectral imaging with nanoscale spatial resolution determined by the tip apex size. The use of broadband infrared sources enables the acquisition of continuous spectra, which is a distinctive feature of nano-FTIR compared to s-SNOM. Nano-FTIR is capable of performing infrared (IR) spectroscopy of materials in ultrasmall quantities and with nanoscale spatial resolution. The detection of a single molecular complex and the sensitivity to a single monolayer has been shown. Recording infrared spectra as a function of position can be used for nanoscale mapping of the sample chemical composition, performing a local ultrafast IR spectroscopy and analyzing the nanoscale intermolecular coupling, among others. A spatial resolution of 10 nm to 20 nm is routinely achieved.

Pump–probe microscopy is a non-linear optical imaging modality used in femtochemistry to study chemical reactions. It generates high-contrast images from endogenous non-fluorescent targets. It has numerous applications, including materials science, medicine, and art restoration.

<span class="mw-page-title-main">Fabrizio Carbone</span> Italian and Swiss physicist

Fabrizio Carbone is an Italian and Swiss physicist and currently an Associate Professor at École Polytechnique Fédérale de Lausanne (EPFL). His research focuses on the study of matter in out of equilibrium conditions using ultrafast spectroscopy, diffraction and imaging techniques. In 2015, he attracted international attention by publishing a photography of light displaying both its quantum and classical nature.

Photon-Induced Near-field Electron Microscopy (PINEM) is a variant of the Ultrafast Transmission Electron Microscopy technique and is based on the inelastic coupling between electrons and photons in presence of a surface or a nanostructure. This method allows one to investigate time-varying nanoscale electromagnetic fields in an electron microscope.

References

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  2. ↑ Knoll, M.; Ruska, E. (May 1932). "Das Elektronenmikroskop". Zeitschrift für Physik. 78 (5–6): 318–339. Bibcode:1932ZPhy...78..318K. doi:10.1007/BF01342199. S2CID   186239132.
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  5. ↑ Barwick, Brett; Zewail, Ahmed H. (21 October 2015). "Photonics and Plasmonics in 4D Ultrafast Electron Microscopy". ACS Photonics. 2 (10): 1391–1402. doi: 10.1021/acsphotonics.5b00427 .
  6. ↑ Vanacore, Giovanni M.; Hu, Jianbo; Liang, Wenxi; Bietti, Sergio; Sanguinetti, Stefano; Zewail, Ahmed H. (12 November 2014). "Diffraction of Quantum Dots Reveals Nanoscale Ultrafast Energy Localization". Nano Letters. 14 (11): 6148–6154. Bibcode:2014NanoL..14.6148V. doi:10.1021/nl502293a. ISSN   1530-6984. PMID   25099123.
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  8. ↑ Fischer, Martin C.; Wilson, Jesse W.; Robles, Francisco E.; Warren, Warren S. (March 2016). "Invited Review Article: Pump-probe microscopy". Review of Scientific Instruments. 87 (3): 031101. Bibcode:2016RScI...87c1101F. doi:10.1063/1.4943211. PMC   4798998 . PMID   27036751.
  9. ↑ Davydova, Dar’ya; de la Cadena, Alejandro; Demmler, Stefan; Rothhardt, Jan; Limpert, Jens; Pascher, Torbjörn; Akimov, Denis; Dietzek, Benjamin (January 2016). "Ultrafast transient absorption microscopy: Study of excited state dynamics in PtOEP crystals". Chemical Physics. 464: 69–77. Bibcode:2016CP....464...69D. doi:10.1016/j.chemphys.2015.11.006.
  10. ↑ Yang, D.-S.; Mohammed, O. F.; Zewail, A. H. (9 August 2010). "Scanning ultra-fast electron microscopy". Proceedings of the National Academy of Sciences. 107 (34): 14993–14998. Bibcode:2010PNAS..10714993Y. doi: 10.1073/pnas.1009321107 . PMC   2930532 . PMID   20696933.
  11. ↑ Carbone, F. (1 June 2011). "Modern electron microscopy resolved in space, energy and time" . The European Physical Journal Applied Physics. 54 (3): 33503. Bibcode:2011EPJAP..5433503C. doi:10.1051/epjap/2010100354. ISSN   1286-0042.
  12. ↑ Zani, Maurizio; Sala, Vittorio; Irde, Gabriele; Pietralunga, Silvia Maria; Manzoni, Cristian; Cerullo, Giulio; Lanzani, Guglielmo; Tagliaferri, Alberto (April 2018). "Charge dynamics in aluminum oxide thin film studied by ultrafast scanning electron microscopy". Ultramicroscopy. 187: 93–97. doi:10.1016/j.ultramic.2018.01.010. hdl: 11311/1042380 . PMID   29427914.
  13. ↑ Irde, Gabriele; Pietralunga, Silvia Maria; Sala, Vittorio; Zani, Maurizio; Ball, James M.; Barker, Alex J.; Petrozza, Annamaria; Lanzani, Guglielmo; Tagliaferri, Alberto (June 2019). "Imaging photoinduced surface potentials on hybrid perovskites by real-time Scanning Electron Microscopy". Micron. 121: 53–65. doi:10.1016/j.micron.2019.03.002. hdl: 11311/1080079 . PMID   30947034. S2CID   96432776.
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