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. There was an early attempt to initiate this technique by Larry D.Flesner in a US patent in 1990, he incorporated the scanning electron microscopy and modulated light to study semiconductor surface photovoltaic in both time and space scale. [3] 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. [4] [5] [6] [7] [8]

Scanning electron microscopy

Illustration shows the phenomena that occur from the interaction of highly energetic electrons with matter, also depicting the pear shape interaction volume which is typically observed in this type of interactions. Electron Interaction with Matter.svg
Illustration shows the phenomena that occur from the interaction of highly energetic electrons with matter, also depicting the pear shape interaction volume which is typically observed in this type of interactions.

Scanning electron microscopy is a powerful technique to give full reliable mapping of sample surface topography and material content in very wide range metal, semiconductor even organic samples and always operates in vacuum environment. [9] The main operation idea depends on the production of a primary beam by cathode which passes through electron column. That column contains a series of electromagnetic lenses that focus the intensity of the beam and by this process, the primary beam reaches the specimen in few nanometers size. In principle, if the thickness of the sample is within few micrometers, the primary beam will be completely attenuated by scattering with other electrons or lattices. In fact, the primary beam interaction process could be elastic or inelastic. For the first case, no loss of energy happens, this is known as a backscattered electron. [10] On the other hand, in case of inelastic interaction process, the emitted electron from the sample from eV to 30 KeV. The excitation might be deep core hole excitation or electron hole production. The shown picture summarizes all kinds of possible interactions and their related depth to the sample. For example, the x-ray generated from long depth or an Auger electron generated at the surface. So, depending on the energy emitted from the sample, specific detectors will be used in accordance with emitted energy. The final image, acquired and reconstructed by raster scan mode, it is acquired in grayscale. Due to the fact that the emission of secondary electrons is less than 50 eV which in fact has been collected due to inelastic collision from all the volume of interaction with the specimen but only ones near to the surface can be detected. Therefore, secondary electron emission considers highly sensitive to the surface due to its generation occurs from just nanometers of depth. The sample itself plays an important role in emission depending on the local work function of the sample. These factors have nominated the SEM to be much more affordable facility in micro and nano scale regimes.

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. [11] 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. [12] Nowadays, the appreciated impact to reach high progress in that phenomenon is directly coming from the nonlinear optics. [13] There are many ways for nonlinear process interaction, for example second-harmonic generation, Coherent anti-Stokes Raman or two-photon-excited fluorescence. 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. [14]

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. [15] 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. [16] 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. [17]

Applications

The powerfulness of that technique meets the requirement for investigation of innovative materials for electronics, sustainable energy harvesting and photonics that enables us to study the charge dynamic in deep for semiconductors materials which have been stimulated by ultrashort laser beam. It has powerful accessibility to carrier recombination and trapping in condensed matter physics that allows more progress in photovoltaics fabrication.

See also

Related Research Articles

<span class="mw-page-title-main">Electron microscope</span> Type of microscope with electrons as a source of illumination

An electron microscope is a microscope that uses a beam of electrons as a source of illumination. They use electron optics that are analogous to the glass lenses of an optical light microscope to control the electron beam, for instance focusing them to produce magnified images or electron diffraction patterns. As the wavelength of an electron can be up to 100,000 times smaller than that of visible light, electron microscopes have a much higher resolution of about 0.1 nm, which compares to about 200 nm for light microscopes. Electron microscope may refer to:

<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">Microscope</span> Scientific instrument

A microscope is a laboratory instrument used to examine objects that are too small to be seen by the naked eye. Microscopy is the science of investigating small objects and structures using a microscope. Microscopic means being invisible to the eye unless aided by a microscope.

<span class="mw-page-title-main">Scanning electron microscope</span> Electron microscope where a small beam is scanned across a sample

A scanning electrode microscope (SEM) is a type of electron microscope that produces images of a sample by scanning the surface with a focused beam of electrons. The electrons interact with atoms in the sample, producing various signals that contain information about the surface topography and composition of the sample. The electron beam is scanned in a raster scan pattern, and the position of the beam is combined with the intensity of the detected signal to produce an image. In the most common SEM mode, secondary electrons emitted by atoms excited by the electron beam are detected using a secondary electron detector. The number of secondary electrons that can be detected, and thus the signal intensity, depends, among other things, on specimen topography. Some SEMs can achieve resolutions better than 1 nanometer.

<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">Atomic force microscopy</span> Type of microscopy

Atomic force microscopy (AFM) or scanning force microscopy (SFM) is a very-high-resolution type of scanning probe microscopy (SPM), with demonstrated resolution on the order of fractions of a nanometer, more than 1000 times better than the optical diffraction limit.

In physics and physical chemistry, time-resolved spectroscopy is the study of dynamic processes in materials or chemical compounds by means of spectroscopic techniques. Most often, processes are studied after the illumination of a material occurs, but in principle, the technique can be applied to any process that leads to a change in properties of a material. With the help of pulsed lasers, it is possible to study processes that occur on time scales as short as 10−16 seconds. All time-resolved spectra are suitable to be analyzed using the two-dimensional correlation method for a correlation map between the peaks.

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).

<span class="mw-page-title-main">Confocal microscopy</span> Optical imaging technique

Confocal microscopy, most frequently confocal laser scanning microscopy (CLSM) or laser scanning confocal microscopy (LSCM), is an optical imaging technique for increasing optical resolution and contrast of a micrograph by means of using a spatial pinhole to block out-of-focus light in image formation. Capturing multiple two-dimensional images at different depths in a sample enables the reconstruction of three-dimensional structures within an object. This technique is used extensively in the scientific and industrial communities and typical applications are in life sciences, semiconductor inspection and materials science.

<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.

<span class="mw-page-title-main">Two-photon excitation microscopy</span> Fluorescence imaging technique

Two-photon excitation microscopy is a fluorescence imaging technique that is particularly well-suited to image scattering living tissue of up to about one millimeter in thickness. Unlike traditional fluorescence microscopy, where the excitation wavelength is shorter than the emission wavelength, two-photon excitation requires simultaneous excitation by two photons with longer wavelength than the emitted light. The laser is focused onto a specific location in the tissue and scanned across the sample to sequentially produce the image. Due to the non-linearity of two-photon excitation, mainly fluorophores in the micrometer-sized focus of the laser beam are excited, which results in the spatial resolution of the image. This contrasts with confocal microscopy, where the spatial resolution is produced by the interaction of excitation focus and the confined detection with a pinhole.

<span class="mw-page-title-main">Focused ion beam</span> Device

Focused ion beam, also known as FIB, is a technique used particularly in the semiconductor industry, materials science and increasingly in the biological field for site-specific analysis, deposition, and ablation of materials. A FIB setup is a scientific instrument that resembles a scanning electron microscope (SEM). However, while the SEM uses a focused beam of electrons to image the sample in the chamber, a FIB setup uses a focused beam of ions instead. FIB can also be incorporated in a system with both electron and ion beam columns, allowing the same feature to be investigated using either of the beams. FIB should not be confused with using a beam of focused ions for direct write lithography. These are generally quite different systems where the material is modified by other mechanisms.

Ultrafast laser spectroscopy is a spectroscopic technique that uses 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.

Super-resolution microscopy is a series of techniques in optical microscopy that allow such images to have resolutions higher than those imposed by the diffraction limit, which is due to the diffraction of light. Super-resolution imaging techniques rely on the near-field or on the far-field. Among techniques that rely on the latter are those that improve the resolution only modestly beyond the diffraction-limit, such as confocal microscopy with closed pinhole or aided by computational methods such as deconvolution or detector-based pixel reassignment, the 4Pi microscope, and structured-illumination microscopy technologies such as SIM and SMI.

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 (UED), also known as femtosecond electron diffraction (FED), is a pump-probe experimental method based on the combination of optical pump-probe spectroscopy and electron diffraction. UED 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 UED technique, a femtosecond (fs) 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 fs 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 CCD camera. Specifically, after the electron pulse diffracts from the sample, the scattered electrons will form a diffraction pattern (image) on a CCD 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. UED can provide a wealth of dynamics on charge carriers, atoms, and molecules.

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.

The operation of a photon scanning tunneling microscope (PSTM) is analogous to the operation of an electron scanning tunneling microscope, with the primary distinction being that PSTM involves tunneling of photons instead of electrons from the sample surface to the probe tip. A beam of light is focused on a prism at an angle greater than the critical angle of the refractive medium in order to induce total internal reflection within the prism. Although the beam of light is not propagated through the surface of the refractive prism under total internal reflection, an evanescent field of light is still present at the surface.

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.

References

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