Environmental Scanning Ultrafast Electron Microscopy: Structural Dynamics of Solvation at Interfaces

Yang, Ding‐Shyue; Mohammed, Omar F.; Zewail, Ahmed H.

doi:10.1002/ange.201205093

Cited by 9 publications

(13 citation statements)

References 29 publications

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“…The characteristic surface selectivity of S-UEM has been applied to investigate a host of diverse phenomena in photoactive materials, such as the surface charge distribution in semiconductor single crystals. 42 It has also been used to image molecular solvation dynamics on semiconductor materials surfaces, 43 doping and carrier concentration-dependent ultrafast carrier dynamics in single-crystalline semiconductor substrates, 44 and interfacial dynamics at p-n junctions. 45 Recently, the capabilities of this technique were improved through the development of a second-generation S-UEM (Figure 1) with an improved time probing window and slightly higher spatial resolution.…”

mentioning

confidence: 99%

Mapping Carrier Dynamics on Material Surfaces in Space and Time using Scanning Ultrafast Electron Microscopy

Sun

Adhikari

Shaheen

et al. 2016

J. Phys. Chem. Lett.

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Selectively capturing the ultrafast dynamics of charge carriers on materials surfaces and at interfaces is crucial to the design of solar cells and optoelectronic devices. Despite extensive research efforts over the past few decades, information and understanding about surface-dynamical processes, including carrier trapping and recombination remains extremely limited. A key challenge is to selectively map such dynamic processes, a capability that is hitherto impractical by time-resolved laser techniques, which are limited by the laser's relatively large penetration depth and consequently these techniques record mainly bulk information. Such surface dynamics can only be mapped in real space and time by applying four-dimensional (4D) scanning ultrafast electron microscopy (S-UEM), which records snapshots of materials surfaces with nanometer spatial and subpicosecond temporal resolutions. In this method, the secondary electron (SE) signal emitted from the sample's surface is extremely sensitive to the surface dynamics and is detected in real time. In several unique applications, we spatially and temporally visualize the SE energy gain and loss, the charge carrier dynamics on the surface of InGaN nanowires and CdSe single crystal and its powder film. We also discuss the mechanisms for the observed dynamics, which will be the foundation for future potential applications of S-UEM to a wide range of studies on material surfaces and device interfaces.

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mentioning

confidence: 99%

Mapping Carrier Dynamics on Material Surfaces in Space and Time using Scanning Ultrafast Electron Microscopy

Sun

Adhikari

Shaheen

et al. 2016

J. Phys. Chem. Lett.

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“…Unlike UEM, which operates in the transmission mode, scanning UEM techniques exploit the time evolution of secondary electrons (SEs) produced in the specimen, and provide additional marked advantages over the transmission mode. These include a relatively facile sample preparation requirement, an efficient heat dissipation, a lower radiation damage, and an accessibility to low-voltage environmental study (2,3). Since its development this technique has been used to study carrier excitation dynamics in several prototypical semiconducting materials surfaces.…”

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confidence: 99%

Visualization of carrier dynamics in p(n)-type GaAs by scanning ultrafast electron microscopy

Cho

Hwang

Zewail

2014

Proc. Natl. Acad. Sci. U.S.A.

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Four-dimensional scanning ultrafast electron microscopy is used to investigate doping-and carrier-concentration-dependent ultrafast carrier dynamics of the in situ cleaved single-crystalline GaAs(110) substrates. We observed marked changes in the measured timeresolved secondary electrons depending on the induced alterations in the electronic structure. The enhancement of secondary electrons at positive times, when the electron pulse follows the optical pulse, is primarily due to an energy gain involving the photoexcited charge carriers that are transiently populated in the conduction band and further promoted by the electron pulse, consistent with a band structure that is dependent on chemical doping and carrier concentration. When electrons undergo sufficient energy loss on their journey to the surface, dark contrast becomes dominant in the image. At negative times, however, when the electron pulse precedes the optical pulse (electron impact), the dynamical behavior of carriers manifests itself in a dark contrast which indicates the suppression of secondary electrons upon the arrival of the optical pulse. In this case, the loss of energy of material's electrons is by collisions with the excited carriers. These results for carrier dynamics in GaAs(110) suggest strong carrier-carrier scatterings which are mirrored in the energy of material's secondary electrons during their migration to the surface. The approach presented here provides a fundamental understanding of materials probed by four-dimensional scanning ultrafast electron microscopy, and offers possibilities for use of this imaging technique in the study of ultrafast charge carrier dynamics in heterogeneously patterned micro-and nanostructured material surfaces and interfaces.scanning electron microscopy | ultrafast phenomena | charge dynamics imaging R ecent advances in four-dimensional (4D) ultrafast electron microscopy (UEM) have made it possible to investigate nonequilibrium electronic and structural dynamics with atomicscale spatial resolution and femtosecond temporal resolution (1). Unlike UEM, which operates in the transmission mode, scanning UEM techniques exploit the time evolution of secondary electrons (SEs) produced in the specimen, and provide additional marked advantages over the transmission mode. These include a relatively facile sample preparation requirement, an efficient heat dissipation, a lower radiation damage, and an accessibility to low-voltage environmental study (2, 3). Since its development this technique has been used to study carrier excitation dynamics in several prototypical semiconducting materials surfaces. In these studies, image contrast was monitored as a function of time, and it was found that Si exhibits a bright contrast in the image at positive times without appreciable dynamics at negative times, whereas CdSe displays bright contrast at positive times and dark contrast at negative times (2). However, the correlation between the measured time-dependent SE intensity and electronic structure of the material of interes...

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“…𝑇 * (𝑡) = 𝑇 * (0)𝑒 F\ q ⁄ + 𝑇 S (6) where 𝑇 S is the room temperature. From the Einstein relations for transport equation, we write…”

Section: Manuscript Textmentioning

confidence: 99%

“…This potentially leads to misleading conclusions when analyzing samples with reduced dimensions such as thin films and nanomaterials. With the recent development of scanning ultrafast electron microscopy (SUEM), which combines the temporal resolution of the femtosecond laser with the spatial resolution and the surface sensitivity of the electron probe (5)(6)(7)(8)(9), we are now able to overcome these limitations by "directly" imaging carriers and mapping their evolution in space and time.…”

Section: Introductionmentioning

confidence: 99%

Carrier density oscillation in the photoexcited semiconductor

Najafi¹,

Jafari²,

Liao³

2021

J. Phys. D: Appl. Phys.

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The perturbation of a semiconductor from the thermodynamic equilibrium often leads to the display of nonlinear dynamics and formation of spatiotemporal patterns due to the spontaneous generation of competing processes. Here, we describe the ultrafast imaging of nonlinear carrier transport in silicon, excited by an intense femtosecond laser pulse. We use scanning ultrafast electron microscopy (SUEM) to show that, at a sufficiently high excitation fluence, the transport of photoexcited carriers slows down by turning into an oscillatory process. We attribute this nonlinear response to the electric field, generated by the spatial separation of these carriers under intrinsic and photo-induced fields; we then provide an advection-diffusion model that mimics the experimental observation. Our finding provides a direct imaging evidence for the electrostatic oscillation of hot carriers in highly excited semiconductors and offers new insights into their spatiotemporal evolution as the equilibrium is recovered. Significance Statement This work uses time-resolved electron microscopy to study the nonlinear carrier transport on the semiconductor surface at ultrafast timescales. Furthermore, it provides, to our knowledge for the first time, direct "imaging "evidence for the spatiotemporal oscillation of hot carriers, which hinders their long-range transport. This study elucidates the excitation and relaxation mechanisms in semiconductors at high excitation regimes, an information potentially critical for the design of fast and efficient electronic, thermoelectric and optoelectronic devices.

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Environmental Scanning Ultrafast Electron Microscopy: Structural Dynamics of Solvation at Interfaces

Cited by 9 publications

References 29 publications

Mapping Carrier Dynamics on Material Surfaces in Space and Time using Scanning Ultrafast Electron Microscopy

Mapping Carrier Dynamics on Material Surfaces in Space and Time using Scanning Ultrafast Electron Microscopy

Visualization of carrier dynamics in p(n)-type GaAs by scanning ultrafast electron microscopy

Carrier density oscillation in the photoexcited semiconductor

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