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

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Mapping Carrier Dynamics on Material Surfaces in Space and Time using Scanning Ultrafast Electron Microscopy Jingya Sun, Aniruddha Adhikari, Basamat S. Shaheen, Haoze Yang, and Omar F. Mohammed J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b02908 • Publication Date (Web): 25 Feb 2016 Downloaded from http://pubs.acs.org on March 2, 2016

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The Journal of Physical Chemistry Letters

Mapping Carrier Dynamics on Material Surfaces in Space and Time using Scanning Ultrafast Electron Microscopy Jingya Sun†, Aniruddha Adhikari†, Basamat S. Shaheen, Haoze Yang, and Omar F. Mohammed* Solar and Photovoltaics Engineering Research Center, Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia Corresponding Author * E-mail: [email protected] Author Contributions †

These authors contributed equally to this work

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ABSTRACT

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 surfacedynamical 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 they record mainly bulk information. Such surface dynamics can only be mapped in real space and time by applying fourdimensional (4D) scanning ultrafast electron microscopy (S-UEM), which records snapshots of materials surfaces with nanometer spatial and sub-picosecond 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 crystals 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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The desire to observe carrier dynamics following optical excitation at the highest possible spatial and temporal resolutions has been a long-standing goal for scientists from various areas of research. Although the electron microscopy and laser spectroscopy communities have separately made great strides and achieved incredibly high spatial and temporal resolutions, respectively, it is extremely difficult to acquire high spatial and temporal resolutions in tandem and in one experiment. Over the last decade, a significant step in this direction has been taken to establish and develop four-dimensional ultrafast electron microscopy (4D-UEM), which acquires these capabilities and provides dynamical information in space and time.1-6 The concept of 4D-UEM (pioneered by Zewail and coworkers at Caltech) is entirely unique in that the spatial resolution is that of a conventional electron microscope and the time resolution is determined by the two ultrashort laser pulses and the electron pulse width involved in generating pulsed electrons and initiating dynamics; collectively, this design strategy allows an excellent spatiotemporal resolution to be realized simultaneously in a single experimental setup.7-15 Broadly, the working principle of 4D-UEM involves the initiation of dynamics in the sample of interest with a clocking optical pulse and recording time-resolved images of the induced changes at different delay times using photoelectron packets, each packet containing a few electrons per pulse (at most) to eliminate Coulombic repulsion between the electrons, which grants an unprecedented temporal resolution. Such a unique experimental setup has been exploited successfully to address several key issues that were hitherto inaccessible by other purely optical techniques.16-30 These issues include the direct visualization of metal-to-insulator phase transitions in metal oxides,16 structural expansions of lattice structures in metals,17 in situ imaging of the mechanical drumming of nanoscale materials18 and the observation of nanomechanical oscillations of cantilevers.19 In addition, 4D-UEM has enabled the study of Moire fringe dynamics in graphite,20 Marternsitic phase transformation dynamics in iron,21 the nanoscopic


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crystallization of amorphous silicon nitride on carbon nanotubes,22 and atomic motion in nickel-titanium alloy microstructures.23 Other investigations using 4D-UEM have considered a wide range of phenomena, such as magnetization reversal dynamics in ferromagnetic thin films,24 the conformational dynamics of ethylene oxide macromolecules,25 irreversible chemical reactions in copper-based crystals,26 anisotropic atomic motion in carbon nanotubes,27 and the in situ irreversible transformation of individual nanoparticles in molecular frameworks.30 Further structural insights into material behavior have been gained through the visualization of photon-induced nanoscale fields generated between entangled silver nanoparticles28 and layered steps of graphene sheets.29 Additional efforts have been made to realize the coherent quantum state manipulation of free electron populations in electron microscopes.31 Aside from materials, 4D-UEM has also been applied to structural problems relevant to biology,32-35 including efforts to study the minute motions of amyloid proteins in vitrified water at cryogenic temperatures33 and the biomechanics of DNA structures.35 The majority of 4D-UEM studies have relied on the transmission mode of electron microscopy. Although transmission electron microscopy (TEM) offers an excellent spatial resolution, it is not well suited for studying the ultrafast dynamical processes occurring at the surface and interfaces of photoactive materials.36-40 For such scenarios, a new direction and better choice for 4D-UEM has been implemented using scanning electron microscopy (SEM). This technique, called scanning ultrafast electron microscopy (S-UEM), continues to use the ‘photon pump – photoelectron probe’ scheme used in TEM-based 4D-UEM; however, it relies on the secondary electrons (SEs; generated by the pulsed photoelectron probe) from the sample surface to be the relevant signal.41-46 The SEs emitted exclusively from the first few nanometers of the material surface are used to construct time-dependent SE images and provide unique information on the surface dynamics in space and time. The characteristic


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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 with an improved time probing window and higher spatial resolution.46 In addition, the surface morphology, grains, defects and nanostructural features were found to significantly impact the overall dynamics of the surface of photoactive materials.46 It is worth mentioning that ultrafast transient absorption microscopy techniques can also provide a high spatio-temporal resolution for the study of charge dynamics in various materials.47-53 However, in addition to tens of nanometers spatial resolution instead of several nm in S-UEM, these techniques lack the unique surface specificity that is intrinsic to S-UEM. Hence, the dynamical information obtained from these methods mostly originates from the bulk region of the samples being investigated. Here, we attempt to provide an overview of the current state of knowledge in the emerging field of S-UEM, which is a powerful tool for capturing surface dynamics in space and time. The remainder of this article is organized as follows. First, we provide a brief introduction to the imaging concept using pulsed photoelectrons in a 4D setup versus thermal electron imaging in conventional electron microscopy. Next, the differences and capabilities of the TEM and SEM techniques are highlighted to emphasize the relevance of the latter to visualize the interfaces and surfaces of photoactive materials. The two distinct regimes of probing in S-UEM and their potential applications are also discussed. More specifically, the common regime for photon-initiated dynamics, as probed by electrons, was used to directly visualize and follow the recombination and transport of the photo-excited carriers. The


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second regime of using electrons to initiate dynamics and clocking by photons provides a new approach to study the dynamics of SE emission in real time. Subsequently, the potential applications of S-UEM as a tool of choice for the accessibility of surface dynamics selectively in space and time and an overview on how its strength may be suitably harnessed in various areas of research, including solar, photo-catalysis and optoelectronics is highlighted. Finally, we provide some concluding remarks that provide an outlook for the future prospects of the technique. S-UEM as a method of choice One of the key features that distinguishes 4D-UEM from conventional electron microscopy in both TEM and SEM arrangements is the use of femtosecond optical pulses to generate photoelectrons from the photocathode for time-resolved imaging instead of currentdriven thermal electrons to achieve the requisite static electron images. The photoelectrons generated in this manner propagate down the EM column in the form of ultrafast pulses, which can be synchronized in time, with a clocking optical pulse via a variable delay line, which provides dynamical information in real space and time (see Figure 1). In this scenario, the time resolution of the technique is determined by the ultrashort laser pulses and electron pulse width involved and has no bearing on the speed with which the detection system responds to the signals from the sample. Diligent care is taken to ensure that the electron probe pulse contains the minimum number of photoelectrons (on average one electron per pulse), which ensures that Coulombic inter-electron repulsion is minimized to prevent pulse broadening and acquire a higher temporal resolution. Next, we briefly review the differences between the scanning and transmission modes of operation in electron microscopy to understand the ways in which S-UEM is deemed to be a better choice under certain circumstances.54-55 First, TEM involves the use of highly


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energetic electron beams (in the range of 30–300 kV) that are sufficiently strong to pass through material objects. After its journey through the material interior, the transmitted electron beam is subsequently recorded by a CCD camera as a suitable detector, providing insights into the changes in the lattice structure of the specimen. In contrast, SEM, which involves accelerating voltages in the range of 1–30 kV, focuses on the surface topology and composition of the material under study. This necessitates the use of electron beams with less energy than in TEM, which can be swept across the sample to scan the entire surface. Secondary or backscattered electron detectors are used to construct an image of the surface in 3D via the ejected (secondary) electrons from within the first few nanometers of the material by the impingement of the electron beam. TEM and SEM also differ in that the former approach requires the probe electron beam to be transmitted through the sample, which places a constraint on the thickness of the specimen. Such a limit on sample thickness is not a concern for SEM because all of the relevant collected signals are generated from the topmost surface of the bulk material that is exposed to the probing electrons. This configuration allows considerably for better heat dissipation for dynamic studies, resulting in a reduction in radiation damage. Moreover, the ease of sample handling procedures for SEM imaging provides it a greater advantage over TEM in terms of its utility, simplicity and versatility. Another major benefit of SEM is the possibility of using it in the environmental scanning mode, which relaxes the requirement that vacuum conditions be maintained in the sample chamber.43 This approach allows various types of sample surfaces, including wet surfaces, to be imaged under environmental conditions. In addition, the laser-microscope integration and electron microscope modification cost to establish 4D imaging in SEM is considerably lower than in TEM. This is because the optics used in a 4D-TEM column for generation of pulsed photoelectrons is much more elaborate than that used in a 4D-SEM. Although, TEMs reach much higher spatial resolution (sub-Angstrom) compared to the SEMs, and have diffraction


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and EELS capabilities, SEM’s resolution is often adequate for many of the systems studied. Considering these differences, one can expect S-UEM to be the method of choice in future studies of surface dynamics in chemistry, materials science and other disciplines.

Figure 1. Schematic of the experimental setup with the femtosecond laser integrated with SEM. The left inset shows an image of Au nanoparticles in the presence and absence of the photoelectron-generating laser irradiation. The right inset shows the characterization of the instrument’s temporal resolution obtained by fitting the change in contrast for time-resolved difference images of a CdSe single crystal.

A description of the 4D S-UEM experimental design would help to comprehend the manner in which the measurements are realized. Details of the experimental setup are provided elsewhere.46 Briefly, our 4D S-UEM experimental design is comprised of a femtosecond Clark-MXR fiber laser operating at a central wavelength of 1030 nm with a pulse width of 270 fs integrated with a modified FEI Quanta 650 SEM (Figure 1). The fundamental laser output is divided by a beam splitter and directed to two independent harmonic generators (HGs) to produce the second and third harmonic signals at 515 and 343 nm, respectively. The output of the first HG (343 nm) is directed with precision through a pyrometric quartz window and is tightly focused on a cooled Schottky field-emitter tip (zirconium oxide-coated tungsten) to generate the pulsed electrons for imaging. The output of


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the second HG (515 nm, excitation clocking pulse) enters the microscope at an angle of 50° relative to the surface normal through a viewport and is focused onto the specimen surface to initiate the dynamics. Finally, a computer-controlled optical delay line is used to vary the relative clocking between the electron and photon pulses. Upon illumination of the specimen with the pulsed photoelectrons, SEs are emitted from the sample surface and detected by a positively biased Everhart-Thornley detector. All experiments were conducted at a laser repetition rate of 2–8 MHz to ensure full relaxation of the material dynamics to its equilibrium state prior to the arrival of the subsequent excitation pulse. To enhance the signal-to-noise ratio, the SE images were obtained as an integration of 64 frames with a dwell time of 300 ns at each panel. To determine the spatial resolution of our setup, gold nanoparticles were imaged using pulsed photoelectrons generated by the laser pulse from the field emission gun (Figure 1). High-quality images were obtained that provided a spatial resolution of approximately 5 nm. The same image has been taken in the absence of laser irradiation. In this case, a negligible background originating from the thermal electrons is observed, thus confirming the use of pulsed photoelectrons to construct the images. The temporal resolution of the instrument was determined by fitting the time-dependent intensity change of the SEs (generated by photoexcitation of a CdSe single crystal) to an error function. In this manner, a temporal resolution of 650 fs ± 100 fs was obtained with a photoelectron generating pulse of energy 0.21 nJ. In this regime, the temporal resolution degrades as the pulse energy is increased. This observation can be easily understood in terms of the inter-electron Coulombic repulsion, which causes considerable pulse broadening as the number of photoelectrons per pulse impinging upon the sample specimen increases (approximately 5 electrons/pulse for the best obtainable temporal resolution).46


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Regimes of dynamical probing in S-UEM experiments The pump-probe scheme employed in the time-resolved S-UEM measurements offers an interesting scenario in terms of probing dynamical processes on material surfaces in space and time. As shown in Figure 2, in addition to the commonly used photon-electron dynamical probing, where a clocking optical pulse arrives first to initiate dynamics and is probed by pulsed electrons, there is another regime of probing (referred to as electron-photon dynamical probing) when the electron pulse arrives before the clocking pulse near t0. Such an observation in the regime of electron-photon dynamical probing is only possible if the laterarriving photons, which can be as late as >100 ps, cause a change in the non-instantaneous SE emission.

Figure 2. Schematic of the dynamical information obtained in the CdSe single crystals and InGaN nanowires in the electron-photon and photon-electron dynamical probing regimes. Three time-resolved images of a CdSe single crystal or InGaN nanowires at selected times are displayed to highlight the energy gain and energy loss. The dashed ellipses indicate the footprint (1/e of the maximum intensity) of the clocking optical beam on the specimen. One can determine the regime of the dynamical probing depending on the time delay between these two pulses (i.e., clocking photon pulse initiating the sample dynamics and the


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photoelectron probing pulse generated by photoexcitation of the field emission gun). Experimentally, the time axis of the acquired SE images defines using a computer-controlled optomechanical delay line that covers the range from -0.6 to 6.0 ns. Contrast-enhanced differences of the SE images from the laser (pump pulse)-irradiated and non-irradiated regions can be extracted by reference to a negative time frame before the arrival of the excitation photon pulse. As discussed below, the two probing regimes have a significant bearing on the nature of the image contrast that can be obtained through such time-resolved measurements; one might observe either ‘bright’ or ‘dark’ contrast depending on the number of SEs collected with respect to a reference image (Figure 3). A variety of signals may be generated upon the interaction of the photoelectron pulse with the specimen sample, including SEs, backscattered electrons (BSEs) and X-rays. These signals can be distinguished in terms of their energies and filtered out via suitable electron detectors. Our major concern is the SE signal. Because of their low energy, SEs generated by the impingement of the photoelectron pulse have a short escape depth and are emitted exclusively from the first few nanometers of a material’s surface. Thus, imaging SEs provides an excellent means of probing the surface dynamics of materials. a. Regime of photon-electron dynamical probing An interband carrier transition occurs in S-UEM experiments when the clocking optical pulse excites the surface of the specimen and a fraction of the valence-band electrons are promoted to the conduction band (Figure 3, middle panel). Consequently, in contrast to an unperturbed specimen, the promoted electrons have a higher probability of emitting SEs above the vacuum level when scattered by the energetic primary electron beam. This results in an enhancement (“SE energy gain”) (Figure 3, right panel) of the image contrast, which appears as a bright contrast in the difference images. Additionally, the energy added to the


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sample by the photoexcitation pulse may enhance the escape depth of the SEs slightly, leading to an overall bright contrast.42 In contrast, the appearance of dark contrast at positive delay times (photoelectron probe pulse arriving after the photon pump pulse) in different semiconductor materials, including InGaN nanowires, can be observed, indicating a suppression of the emitted SEs through scattering processes (“SE energy loss”) (Figure 3, left panel). In other words, the dark contrast observed in some cases suggests a different mechanism in which the SEs in the conduction band lose their energy while travelling to the surface to come out. In this case, scattering processes with photo-generated electron-hole pairs are likely responsible for the energy loss. Because the effective cross-section for the scattering of SEs with conduction band electrons is considerably higher than that with valence electrons, a decrease in SE emission and thus a low contrast are observed.44, 46

Figure 3. Mechanisms for the dynamics observed in S-UEM, where the valence band electrons are promoted to the conduction band upon optical excitation. The dashed ellipse indicates the location of the laser on the specimen. Several time-resolved images at selected times are displayed to indicate the contrast development. High contrast (right panel) is recorded because of the energy gain, and dark contrast (left panel) is recorded because of the energy loss at the center of the excited region.


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b. Regime of electron-photon dynamical probing As a result of the electron impact by the pulsed primary electrons generated at the emitter source, the dynamics of the SE emission and dark contrast from a specimen are observed through the clocking by timed photon pulses. Such information cannot be accessed by time-resolved spectroscopy or 4D imaging in transmission mode, which highlights a distinct advantage of S-UEM to study the surface-sensitive SE dynamics of condensed matter at localized regions in real time. The SE emission from the surface of a specimen may contain two components: an immediate peak originating mainly within the escape depth of the SEs upon the impact of primary electrons, and a non-instantaneous decaying component following the main peak. The time scale of the former component is expected to be on the order of 1−10 fs because of the fast speed of SEs; thus, it is not resolvable in the current SUEM design. The non-instantaneous SE component can be described by an exponential decay and will depend on the material’s thickness and the penetration depth of the primary electron beam. At negative time delays (electron-photon probing regime), the observation of energy loss (dark contrast) can be attributed to one of the following mechanisms. First, electron-hole pairs created by the subsequent photon pulse can perturb the diffusion of plasmon-excited carriers generated by the electron impact from deeper areas within the specimen. Second, inelastic processes, such as electron-electron and electron-phonon scattering, may also contribute to energy loss. These latter scattering events are extremely fast and vary in their timescales, ranging from a few tens of femtoseconds to a few picoseconds.56-57 However, they are inconsistent with the relatively slower timescales on which the dark contrast is observed in S-UEM experiments. Our experiments on InGaN nanowires clearly suggest that the dark contrast at negative times is consistent with plasmon-excited carriers rather than ultrafast


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inelastic scattering processes. Moreover, if the inelastic scattering processes near the surface are responsible for the dark contrast, it would suggest that dark contrast formation does not depend strongly on the escape depth of the SE signal. To clarify this issue and provide clear experimental evidence that scattering processes are unlikely to be the mechanism for the dark contrast formation at negative time delays, we performed S-UEM experiments on CdSe specimens by varying the working acceleration voltage of our SEM from 30 kV to 1 kV. By changing the accelerating voltage, one can tune the energy of the primary photoelectron pulse that is incident on the sample surface and thus alter the escape depth of the SEs. Hence, SEs generated using weakly accelerated primary photoelectrons should display feeble or no dark contrast in the electron-photon dynamical probing regime. This is precisely what is observed in our experiments with CdSe, thus supporting the importance of plasmon-excited carrier diffusion in generating the dark contrast at negative time delays (Figure 4). As an alternative, one may vary the thickness of the sample specimen to determine whether the above mechanism is valid. Thin samples with minimal escape depths for the emitted SEs (i.e., negligible scope of energy loss through scattering) should exhibit bright contrast. In contrast, a thick sample is expected to induce energy loss via scattering, as the plasmon-excited carriers diffuse from the sample’s bulk toward the surface. This scenario should result in dark contrast at negative delay times and bright contrast at positive delay times. Experiments such as these are currently underway to further investigate these concepts.


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Figure 4. Time-resolved SE difference images exhibiting the contrast evolution in a CdSe single crystal at different acceleration voltages of the EM column. The top four images are obtained with 30 keV primary electrons, whereas the bottom four images are obtained with 1 keV primary electrons.

To summarize this section, the material-dependent energy gain or loss of SEs in each of the two regimes of dynamical probing result in either bright or dark contrast for the timeresolved snapshots obtained via S-UEM experiments. Table 1 provides an overview of the scenarios as observed through experiments reported across various research groups. Further experimental and theoretical efforts are needed to understand the finer details of the mechanisms that lead to such observations.


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Table 1: Contrast imaged at the two probing regimes (i.e., positive and negative times) for some examined specimens. Specimen

Contrast

Regime

Reference

Si

Bright

Positive

Ref. 42

Si

No change

Negative

Ref. 42

CdSe (single crystal and powder)

Bright

Positive

Ref. 46

CdSe (single crystal and powder)

Dark

Negative

Ref. 46

GaAs

Dark

Positive

Ref. 44

GaAs

Dark

Negative

Ref. 44

Silicon p-n junction

Bright

Positive

Ref. 45

Silicon p-n junction

No change

Negative

Ref. 45

InGaN Nanowires

Dark

Positive

Ref. 46

S-UEM applications and outlook The ultrafast dynamics of charge carriers on a material’s surface plays a pivotal role in controlling the applicability of nanoscale materials for different optoelectronic and solar cell devices. For instance, it is crucial to understand non-radiative energy losses at surfaces via carrier-carrier and carrier-phonon interactions following pulsed-laser excitation, especially for wide bandgap semiconductors.58-62 The S-UEM technique is well suited to address such issues and provide exclusive experimental insights previously unattainable through other conventional time-resolved laser spectroscopic or static electron imaging techniques. Moreover, inferring the energy gain/loss mechanisms, as revealed through time delay dependent electron-photon dynamical probing, is a vital step toward the development


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of semiconductor microelectronic and optoelectronic devices, such as light-emitting diodes and laser diodes. The extracted kinetics of the time-resolved SE intensities reveal important information regarding carrier dynamics on different materials surfaces, which can be correlated with the surface quality. For instance, the presence of impurities, morphological steps and surface defects may act as additional sources or centers for carrier trapping and recombination of carrier dynamics. More specifically, the extracted kinetics of the SE intensity change have revealed a fast recovery of the bright contrast on a powder film compared to its bulk single crystal (Figure 5 inset, right and left panels). The presence of surface defects on powder films can act as surface trapping centers to remove a substantial portion of the excited carriers that create high bright contrast. Clearly, One can confirm that morphology, grains, and surface defects are key components for controlling the carrier dynamics on the surfaces of semiconductors.46


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Figure 5. Time-resolved SE difference images exhibiting the contrast evolution in a CdSe single crystal (left) and powder film (right) at selected time delays. The insets in the two topmost images depict the SEM images, showing distinct morphologies for the single crystal and powder film. The middle inset shows the kinetic traces of the change in contrast (fitted to exponentials) in both the samples as a function of the time delays.

As stated in the introduction, ultrafast pump-probe transient absorption microscopy has been developed to examine charge carrier dynamics of nanostructures with high spatial and temporal resolutions. However, they are mainly limited to a spatial resolution of tens of nanometers and by the laser’s relatively large penetration depth, and they primarily record bulk information.47-49 S-UEM does not suffer from such constraints and can be successfully utilized for surface selective investigations. To illustrate, S-UEM was employed in the first


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real-space imaging of the charge carrier dynamics on the surface of InGaN nanowire arrays. The time-resolved snapshots of SE emission were used to distinguish between the kinetics of signal intensity bleach and recovery at the center of the laser spot illumination. The possible deactivation channels and mechanism of SE energy loss in the system were identified. Unlike the case of CdSe and Si, where bright contrast is observed at positive time delays, the measured contrast is dark in this instance. In this case, at positive times, the InGaN nanowire is excited across the band gap (1.62 eV) by the optical pulse and an increase in the electron population of the conduction band will be achieved. However, the promoted electrons are still subject to the scattering processes in the conduction band. Because the effective cross section for the scattering of SEs with conduction electrons is expected to be much higher than that with valence electrons, a decrease in SE emission will give rise to a low contrast.46 Another example of a system studied by S-UEM is the slow fading of the dark contrast on InGaN nanowires after surface passivation with a long chain organic moiety (i.e., octadecanethiol, ODT) compared to the as-grown nanowires (Unpublished results). Passivation is known to eliminate undesirable surface defects that decrease the density of free electrons toward the surface and improve the efficiency of photoactive material-based optoelectronic devices. The recovery of the SE intensity, which is a measure of the charge carrier dynamics, is approximately 40% for the as-grown InGaN nanowire array, whereas it is approximately 15% for the ODT-treated array within the observed time window (Figure 6, inset, left and right panels). The longer lifetime of the excited carriers after ODT passivation was attributed to the removal of surface states, which in turn minimized the non-radiative decay pathways that become significant in nanowire array-based devices characterized by a high surface-to-volume ratio of nanowires. Thus, the effect of surface modification on the carrier dynamics was observed through time-resolved real-space imaging. Subsequently, the


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insights gained from the study were successfully correlated with the enhanced optoelectronic performance of a passivated InGaN nanowire-based device.

Figure 6. Time-resolved SE difference images exhibiting the contrast evolution in InGaN nanowires before (left) and after (right) ODT passivation at selected time delays. The insets in the two topmost images depict the cross-sectional SEM images. The middle inset shows the kinetic traces of the change in contrast (fitted to exponentials) in both the samples as a function of the time delays.

Because the SE signal in S-UEM is emitted from the surface of the sample in a manner that is sensitive to the local electron/hole density, which provides direct and controllable dynamical information about the localization of electrons and holes near interfaces and heterojunctions, another major impetus for S-UEM-based studies is the possibility of accessing p-n junctions in silicon diodes to observe interfacial carrier dynamics within the depletion region in real space and time.45 For example, CdSe single crystals partially sputtered with ZnO can be used to study carrier dynamics in various regions of the material surface by a selective spatial focusing of the exciting photon pulse (Figure 7A). By focusing


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on bulk CdSe and its interface with ZnO, the kinetics extracted from such a specimen clearly reveals a difference between the two regimes (Figure 7B). The CdSe/ZnO interface indicates a considerable shorter lifetime for the excited carriers compared to the CdSe bulk, which may be attributed to the carrier injection from the CdSe layer to the ZnO layer. By suitable design, such studies can be extended to any p-n junction in semiconductor devices. One can envisage a situation in which the electron-hole pair dynamics can be examined while the device is in situ operation (Figure 7C). Such applications will enable scientists to gain insights into the performance of various optoelectronic devices and provide them with clues as to how to improve their efficiency.

Figure 7. (A) SEM image of a CdSe single crystal and its interface with a ZnO film. Regions denoted by “X” and “Y” represent the “bulk” and “interface” regions on the surface of the mixed array, respectively. (B) Kinetic traces of the change in contrast (fitted to exponentials) in the respective regions. (C) Schematic of a semiconductor-based device showing the charge depletion region at the p-n junction, as probed by the photon pump-electron probe scheme of S-UEM. The inset shows the SEM image of a representative p-n junction (circled in green).


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Summary and Future Outlook The ability to access ultrafast carrier dynamics selectively on semiconductor material surfaces in real space and time in a photo-induced reaction is a particularly challenging task that was inconceivable until recently. This perspective describes the breakthrough in the development of 4D S-UEM for the real-time and real-space imaging of material surfaces. Overall, this approach provides access to an area of study that cannot be investigated using either static imaging or time-resolved laser spectroscopy. We show clearly how surface morphology, defects and passivation can control the overall carrier recombination on photoactive material surfaces. In addition, the ability to combine two different regimes of dynamical probing in a single experiment, as in S-UEM, provides a new means with which to study the dynamics of SE emission and carrier dynamics on material surfaces in real space and time. With the capability of environmental and low-voltage in situ imaging and taking advantage of the various types of signals from electron impact, the recent development of this methodology is projected to greatly influence studies of complex materials, solar cell devices, optoelectronics, and bio-systems.

Notes The authors declare no competing financial interest. Biographies Jingya Sun obtained her Ph.D. at National University of Singapore in physics in 2012. She is currently working as a postdoctoral fellow at KAUST. Her research now focuses on fundamental understanding of carrier dynamics on material surface by using fourdimensional scanning ultrafast electron microscopy. Aniruddha Adhikari obtained his Ph.D. in ultrafast laser spectroscopy from the Indian Association for the Cultivation of Science. Following a stay at RIKEN, Japan, he is presently working as a postdoctoral fellow at KAUST. His research area focuses on studying ultrafast charge recombination dynamics on semiconductor material surfaces.


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Basamat S. Shaheen received her bachelor in Materials Engineering from Ain Shams University. She obtained her M. SC in Nanotechnology from the American University in Cairo, Egypt, 2014. She is currently working towards her Ph.D in the group of Omar F. Mohammed, studying the surface carrier dynamics of photoactive materials using the 4DSUEM. Haoze Yang is a graduate student at King Abdullah University of Science and Technology. He obtained his B.S. in Chemistry from the University of Delaware. He is completing his Ph.D. in Chemical science under the supervision of Prof. Omar F. Mohammed, where he is working on ultrafast electron microscopy. Omar F. Mohammed is an Assistant Professor in the Division of Physical Sciences and Engineering. He is the principal investigator of ultrafast laser spectroscopy and fourdimensional electron imaging laboratory. His research activities are focused on the development of highly efficient solar cells with the aid of cutting-edge laser spectroscopy and electron microscopy. https://femto.kaust.edu.sa/Pages/Home.aspx. ACKNOWLEDGMENT The work reported here was supported by the King Abdullah University of Science and Technology.


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Quotes: 1. The SEs images in S-UEM provide an excellent means of probing selectively the carrier dynamics on materials surfaces. 2. Spatiotemporal resolutions of 5 nm and 650 fs can be achieved in second generation S-UEM established recently at KAUST. 3. Two regimes of dynamical probing can be identified in a single experiment, one at negative time delays and the other at the positive. 4. One can expect S-UEM to be the method of choice in future studies of surface dynamics in chemistry, materials science and other disciplines.


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

Scanning electron microscopy of dissociated pancreatic acinar cell surfaces.

Accurate Nanoscale Crystallography in Real-Space Using Scanning Transmission Electron Microscopy.

DNA sequence mapping using electron microscopy.

Ultrafast Carrier Dynamics and Hot Electron Extraction in Tetrapod-Shaped CdSe Nanocrystals.

Analysis of leaf surfaces using scanning ion conductance microscopy.

The effects of scanning electron microscopy desiccation preparation on demineralized dentin surfaces.

Bovine olfactory and nasal respiratory epithelium surfaces. High-voltage and scanning electron microscopy, and cryo-ultramicrotomy.

Liquid phase electron-beam-induced deposition on bulk substrates using environmental scanning electron microscopy.

Ultrafast carrier thermalization and cooling dynamics in few-layer MoS2.

Ultrafast carrier dynamics and coherent acoustic phonons in bulk CdSe.

High-resolution scanning electron microscopy.

Imaging friction tracks at diamond surfaces using reflection electron microscopy.

High resolution scanning electron microscopy of cells using dielectrophoresis.

Morphological classification of bioaerosols from composting using scanning electron microscopy.

Probing ultrafast spin dynamics with optical pump-probe scanning tunnelling microscopy.

Ultrafast electron microscopy integrated with a direct electron detection camera.

Correlative photoactivated localization and scanning electron microscopy.

High-resolution and specific detection of bacteria on complex surfaces using nanoparticle probes and electron microscopy.

Scanning electron microscopy of the subarachnoid space in the dog. IV. Subarachnoid macrophages.

Real-time mapping of salt glands on the leaf surface of Cynodon dactylon L. using scanning electrochemical microscopy.

Scanning electron microscopy studies in muscular dystrophy.

Conditions critical for optimal visualization of bacteriophage adsorbed to bacterial surfaces by scanning electron microscopy.

Scanning electron-microscopy of abscission-zone surfaces of Valencia-orange fruit.