Real‐Space Visualization of Energy Loss and Carrier Diffusion in a Semiconductor Nanowire Array Using 4D Electron Microscopy

Bose, Riya; Sun, Jingya; Khan, Jafar Iqbal; Shaheen, Basamat S.; Adhikari, Aniruddha; Ng, Tien Khee; Burlakov, V. M.; Parida, Manas R.; Priante, Davide; Goriely, Alain; Ooi, Boon S.; Bakr, Osman M.; Mohammed, Omar F.

doi:10.1002/adma.201600202

Cited by 32 publications

(52 citation statements)

References 59 publications

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“…The surface sensitivity can be further enhanced by reducing the accelerating voltage of the electron beam [47]. This high surface sensitivity enabled, for example, the study of surface states and surface morphology and their effects in photocarrier recombination in indium gallium nitride (InGaN) nanowires [31,49], multinary copper indium gallium selenide (CIGSe) nanocrystals [32] and CdSe [50]. When used in the environmental SEM mode, SUEM can also study photocarrier dynamics on sample surfaces in the presence of water vapor and other gases [29].…”

Section: Recent Resultsmentioning

confidence: 99%

“…SUEM experiences the same charging issue as the normal SEM, so in principle is not suitable to study electrically insulating materials, although the environmentalmode SUEM [29] can be a potential solution. SUEM has been utilized to image ultrafast photocarrier dynamics on the surface of a wide range of materials, including crystalline semiconductors [28,30], semiconducting nanowires [31] and nanocrystals [32], amorphous semiconductors [33], semiconductor p-n junctions [34] and two-dimensional materials [35], and these applications have resulted in intriguing observations such as ballistic transport of photocarriers across a p-n junction [34], superdiffusion of photocarriers in heavily-doped semiconductors [30] and spontaneous spatial separation of electrons and holes in amorphous semiconductors [33]. Whereas there has been an abundance of recent reviews of ultrafast electron microscopy [8,9,[36][37][38], we dedicate this article specifically to SUEM, with an emphasis on the current understanding of various physical processes that contribute to the contrast images observed in SUEM from a users' perspective.…”

Section: Introductionmentioning

confidence: 99%

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Scanning ultrafast electron microscopy: A novel technique to probe photocarrier dynamics with high spatial and temporal resolutions

Liao

Najafi

2017

Materials Today Physics

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The dynamics of photo-excited charge carriers, particularly their transport and interactions with defects and interfaces, play an essential role in determining the performance of a wide range of solar and optoelectronic devices. A thorough understanding of these processes requires tracking the motion of photocarriers in both space and time simultaneously with extremely high resolutions, which poses a significant challenge for previously developed techniques, mostly based on ultrafast optical spectroscopy. Scanning ultrafast electron microscopy (SUEM) is a recently developed photon-pump-electron-probe technique that combines the spatial resolution of scanning electron microscopes (SEM) and the temporal resolution of femtosecond ultrafast lasers. Despite many recent excellent reviews for the ultrafast electron microscopy, we dedicate this article specifically to SUEM, where we review the working principle and contrast mechanisms of SUEM in the secondary-electron-detection mode from a users' perspective and discuss the applications of SUEM to directly image photocarrier dynamics in various materials. Furthermore, we propose future theoretical and experimental directions for better understanding and fully utilizing the SUEM measurements to obtain detailed information about the dynamics of photocarriers. To conclude, we envision the potential of expanding SUEM into a versatile platform for probing photophysical processes beyond photocarrier dynamics.

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Section: Recent Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Scanning ultrafast electron microscopy: A novel technique to probe photocarrier dynamics with high spatial and temporal resolutions

Liao

Najafi

2017

Materials Today Physics

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“…[43][44][45][46][47][48][49][50][51][52][53][54][55] In particular, the invention of scanning ultrafast electron microscopy (S-UEM) provided the unique opportunity to selectively map charge carrier dynamics on the surface of a material with nm spatial and subpicosecond temporal resolutions. [56][57][58][59][60][61][62] In S-UEM, the optical pulse generated from a femtosecond (fs) laser system is used to generate electron packets from the tip of the scanning electron microscope, instead of the continuous electron beam used in the conventional setup. This pulse is synchronized with another optical excitation pulse that initiates carrier dynamics in the sample (Scheme 1).…”

mentioning

confidence: 99%

“…The dark contrast at negative time can be attributed to the diffusion of plasmon-excited carriers that are generated from deeper areas of the sample by electron impact, which can be perturbed by electron-hole pairs generated by the clocking photon pulse, or to different scattering processes. 58,62 At positive time delays, the 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 9 optical pulse promotes a fraction of the valence-band electrons to the conduction band.…”

mentioning

confidence: 99%

“…56 However, the dark contrast observed here suggests that SEs in the conduction band lose energy during transit to the surface and they do not reach the detector because of an energy deficit. 58,[61][62] In this case, scattering processes with photo-generated electron-hole pairs are most likely responsible for the energy loss. As the effective cross-section for the scattering of SEs with conduction electrons is much higher than that with valence electrons, 58, 61-62 a decrease in SE emission and, subsequently, low contrast are observed.…”

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confidence: 99%

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Real-Space Mapping of Surface Trap States in CIGSe Nanocrystals Using 4D Electron Microscopy

et al. 2016

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Surface trap states in copper indium gallium selenide semiconductor nanocrystals (NCs), which serve as undesirable channels for nonradiative carrier recombination, remain a great challenge impeding the development of solar and optoelectronics devices based on these NCs. In order to design efficient passivation techniques to minimize these trap states, a precise knowledge about the charge carrier dynamics on the NCs surface is essential. However, selective mapping of surface traps requires capabilities beyond the reach of conventional laser spectroscopy and static electron microscopy; it can only be accessed by using a one-of-a-kind, second-generation four-dimensional scanning ultrafast electron microscope (4D S-UEM) with subpicosecond temporal and nanometer spatial resolutions. Here, we precisely map the collective surface charge carrier dynamics of copper indium gallium selenide NCs as a function of the surface trap states before and after surface passivation in real space and time using S-UEM. The time-resolved snapshots clearly demonstrate that the density of the trap states is significantly reduced after zinc sulfide (ZnS) shelling. Furthermore, the removal of trap states and elongation of carrier lifetime are confirmed by the increased photocurrent of the self-biased photodetector fabricated using the shelled NCs.

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Visualization of Surface Charge Carrier Diffusion Lengths in Different Perovskite Crystal Orientations Using 4D Electron Imaging

Nughays

Yang

Nematulloev

et al. 2023

Advanced Optical Materials

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to their outstanding optical and transport properties, including a low concentration of defect states, tunable bandgap, and ultralong charge carrier diffusion lengths. [1][2][3][4][5][6][7] Therefore, perovskite single crystals are considered to be an excellent candidate for competing with, if not replacing, polycrystalline perovskite absorber layers, which exhibit impressive solar cell device performance with a world-record photoconversion efficiency of 25.7%. [8] However, the performance of such devices has plateaued due to the large density of trap states at grain boundaries. [9,10] This obstacle can be overcome in perovskite single crystals that are free of grains, and enormous studies have been done to improve surface quality by different passivation methods for better device performance. [11,12] Hence, the correlation between the charge carrier dynamics and the surface orientation of these crystals at the femtosecond and nanometer scales needs to be fully established. Note that the charge carrier dynamics at the interface with electron and hole transporting layers is extremely difficult, if not impossible, to obtain using conventional time-resolved pump-probe techniques due to the large penetration depth Understanding charge carrier dynamics on the surface of materials at the nanometer and femtosecond scales is one of the key elements to optimizing the performance of light-conversion devices, including solar cells. Unfortunately, most of the pump-probe characterization techniques are surfaceinsensitive and obtain information from the bulk due to the large penetration depth of the pulses. However, ultrafast scanning electron microscopy (USEM) is superior in visualizing carrier dynamics at the surface with high spatialtemporal resolution. Here, the authors successfully used USEM to uncover the tremendous effect of surface orientations and termination on the charge carrier of MAPbI 3 perovskite single crystals. Time-resolved secondary electrons snapshots and density functional theory calculations clearly demonstrate that charge carrier diffusion, surface trap density, surface work function, and carrier concentration are strongly facet-dependent. The results display a diffusion length of 22 micrometers within 6.0 nanoseconds along (001) orientation. While (100) facet forms defect states that prevent carrier diffusion and shows an increase in the surface work function leading to dark contrast and fast charge carrier recombination. These findings provide a new key component to optimizing the surface of perovskites, thus paving the way for even more efficient and stable solar-cell devices based on perovskite single crystals.

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Real‐Space Visualization of Energy Loss and Carrier Diffusion in a Semiconductor Nanowire Array Using 4D Electron Microscopy

Cited by 32 publications

References 59 publications

Scanning ultrafast electron microscopy: A novel technique to probe photocarrier dynamics with high spatial and temporal resolutions

Scanning ultrafast electron microscopy: A novel technique to probe photocarrier dynamics with high spatial and temporal resolutions

Real-Space Mapping of Surface Trap States in CIGSe Nanocrystals Using 4D Electron Microscopy

Visualization of Surface Charge Carrier Diffusion Lengths in Different Perovskite Crystal Orientations Using 4D Electron Imaging

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