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

Bose, Riya; Bera, Ashok; Parida, Manas R.; Adhikari, Aniruddha; Shaheen, Basamat S.; Alarousu, Erkki; Sun, Jingya; Wu, Tom; Bakr, Osman M.; Mohammed, Omar F.

doi:10.1021/acs.nanolett.6b01553

Cited by 26 publications

(46 citation statements)

References 75 publications

(158 reference statements)

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“…To understand the charge carrier dynamics at MoS 2 /g‐C 3 N 4 interfaces and correlate it to the photodetection performance, we employed femtosecond transient absorption (fs‐TA) spectroscopy with broadband capability. It is worth pointing out that TA spectroscopy has proven to be a critical method to study such interfacial carrier dynamics . Here, we used it to monitor the carrier dynamics at the interface between MoS 2 and g‐C 3 N 4 .…”

Section: Resultsmentioning

confidence: 99%

2D Organic–Inorganic Hybrid Thin Films for Flexible UV–Visible Photodetectors

Velusamy

Haque

Parida

et al. 2017

Adv Funct Materials

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129

106

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as hybridization of MoS 2 with quantum dots and heterostructures with other 2D materials to suppress the dark current and enhance the broad spectral range in the NIR region were attempted. While convenient, these approaches are not scalable for large-scale implementation due to the complicated procedures as well as expensive material and instrument-related costs. [17][18][19] To overcome the aforementioned issues, a promising approach is to create multifunctional hybrid photodetectors by simple solution mixing of MoS 2 with other 2D materials. The different band gaps and dark currents of 2D materials and the low-cost solution process offers a convenient route to engineering and controlling the photodetection properties. [20][21][22] In addition, the arrays of hybrid photodetectors fabricated on flexible substrates, such as plastic and paper, would be beneficial for future wearable applications. [23,24] Recently, 2D organic semiconductors, such as graphitic carbon nitride (g-C 3 N 4 ), have emerged as promising UV-and visible-light-active photocatalysts in the arena of solar energy conversion and environmental applications. [25][26][27][28] Their unique electrical and optical properties, wide band gap (≈2.7 eV), stability in ambient conditions, and low dark current make them attractive for UV light photodetection, but very little work has been carried out so far. Compared to other 2D materials, by considering its merits, g-C 3 N 4 is a promising candidate for hybridizing with MoS 2 because not only does it suppress the dark current of MoS 2 , but also allows hybrid broadband photodetection from UV to visible region. In addition, the crystal lattice matching [29,30] and the ultrafast charge transfer between MoS 2 and g-C 3 N 4 at the interface may lead to efficient separation of photogenerated carriers, as predicted by a recent computational study. [31] Herein we report, to the best of our knowledge for the first time, mechanically flexible 2D organic-inorganic hybrid thin film photodetectors consisting of inorganic MoS 2 and organic g-C 3 N 4 nanosheets for broadband photodetection. Simple but robust solution mixing of MoS 2 and g-C 3 N 4 offers an extremely convenient route to controlling their composition in the hybrid films and thus allows for tuning the optoelectronic properties. Hybrid thin films with 5:5 ratio of MoS 2 and g-C 3 N 4 (henceforth denoted as 5:5 hybrid films) exhibited excellent photodetection performance in terms of ON/OFF photocurrent ratio, specific detectivity, responsivity, and response time upon both Flexible 2D inorganic MoS 2 and organic g-C 3 N 4 hybrid thin film photodetectors with tunable composition and photodetection properties are developed using simple solution processing. The hybrid films fabricated on paper substrate show broadband photodetection suitable for both UV and visible light with good responsivity, detectivity, and reliable and rapid photoswitching characteristics comparable to monolayer devices. This excellent performance is retained even after the films are severely...

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Section: Resultsmentioning

confidence: 99%

2D Organic–Inorganic Hybrid Thin Films for Flexible UV–Visible Photodetectors

Velusamy

Haque

Parida

et al. 2017

Adv Funct Materials

Self Cite

129

106

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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%

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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“…Consequently, the contrast should be bright in the SEs timeresolved difference images, as detected for the clean surfaces of CdSe and Si. [18][19] However, a dark image contrast is also observed after surface excitation in a number of photovoltaic materials, including GaAs, 21 CIGSe 22 and InGaN NWs [22][23] . This dark contrast suggests that the density of SEs decreases after optical excitation (fewer SEs escape from the material surface).…”

Section: Introductionmentioning

confidence: 99%

Real-Space Mapping of Surface-Oxygen Defect States in Photovoltaic Materials Using Low-Voltage Scanning Ultrafast Electron Microscopy

Shaheen

El‐Zohry

Zhao

et al. 2020

ACS Appl. Mater. Interfaces

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Ultrathin layers of native oxides on the surface of photovoltaic materials may act as very efficient carrier trapping/recombination centers, thus significantly affecting their interfacial photophysical properties. How ultrathin oxide layers affect the surface and interface carrier dynamics cannot be selectively accessed by conventional ultrafast transient spectroscopic methods due to the deep penetration depth (10s-100s nm) of the pump/probe laser pulses. Herein, scanning ultrafast electron microscopy (S-UEM) at a low voltage of 1 keV electrons was recently developed at KAUST to selectively map the ultrafast charge carrier dynamics of a few layers on the top surfaces of photovoltaic materials. Unlike high voltage 30 keV experiments, at 1 keV, the depth of detected secondary electrons (SEs) underneath the surface is significantly reduced five times, thus making it possible to visualize the dynamics of charge carrier from the uppermost surface of photoactive layers. More specifically, this new approach has been employed to explore and decipher the tremendous impact of native oxide layers and oxygen defect states on charge carrier dynamics in space and time simultaneously at sub-nm levels on several photovoltaic material surfaces, including Si, GaAs, CdTe and CdZnTe single crystals. Interestingly, the contrast in the SEs timeresolved difference images switched from a dark "energy-loss mechanism" to a bright "energygain mechanism" only by removing the layers of surface oxides. Moreover, the charge carrier recombination was estimated and found to be dramatically affected by the native oxide layers. The density functional theory (DFT) calculations demonstrate that the work function of oxygenterminated Si surface also increases slightly upon optical excitation and makes for less SEs intensity, providing another piece of evidence that the origin of the dark contrast observed on these material surfaces should be assigned to the surface oxide formation, mainly oxygen defect states in the bandgap, and/or work function increment. Our findings provide a new method and pave the way for evaluating the effect of surface morphology and defects, including but not limited to native oxide, on charge carrier dynamics and interfacial properties of photovoltaic absorber layers.

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

Cited by 26 publications

References 75 publications

2D Organic–Inorganic Hybrid Thin Films for Flexible UV–Visible Photodetectors

2D Organic–Inorganic Hybrid Thin Films for Flexible UV–Visible Photodetectors

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

Real-Space Mapping of Surface-Oxygen Defect States in Photovoltaic Materials Using Low-Voltage Scanning Ultrafast Electron Microscopy

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