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Recent Submissions

  • Polarization-induced photocurrent switching effect in heterojunction photodiodes

    Chen, Dingbo; Chen, Yu Chang; Zeng, Guang; Li, Yu Chun; Li, Xiao Xi; Li, Dong; Shen, Chao; Chi, Nan; Ooi, Boon S.; Zhang, David Wei; Lu, Hong Liang (Nano Research, Springer Science and Business Media LLC, 2022-11-14) [Article]
    The unipolar photocurrent in conventional photodiodes (PDs) based on photovoltaic effect limits the output modes and potential versatility of these devices in photodetection. Bipolar photodiodes with photocurrent switching are emerging as a promising solution for obtaining photoelectric devices with unique and attractive functions, such as optical logic operation. Here, we design an all-solid-state chip-scale ultraviolet (UV) PD based on a hybrid GaN heterojunction with engineered bipolar polarized electric field. By introducing the polarization-induced photocurrent switching effect, the photocurrent direction can be switched in response to the wavelength of incident light at 0 V bias. In particular, the photocurrent direction exhibits negative when the irradiation wavelength is less than 315 nm, but positive when the wavelength is longer than 315 nm. The device shows a responsivity of up to −6.7 mA/W at 300 nm and 5.3 mA/W at 340 nm, respectively. In particular, three special logic gates in response to different dual UV light inputs are demonstrated via a single bipolar PD, which may be beneficial for future multifunctional UV photonic integrated devices and systems.
  • Maritime Communications: A Survey on Enabling Technologies, Opportunities, and Challenges

    Alqurashi, Fahad S.; Trichili, Abderrahmen; Saeed, Nasir; Ooi, Boon S.; Alouini, Mohamed-Slim (IEEE Internet of Things Journal, Institute of Electrical and Electronics Engineers (IEEE), 2022-11-04) [Article]
    Water covers 71% of the Earth’s surface, where the steady increase in oceanic activities has promoted the need for reliable maritime communication technologies. The existing maritime communication systems involve terrestrial, aerial, and space networks. This paper presents a holistic overview of the different forms of maritime communications and provides the latest advances in various marine technologies. The paper first introduces the different techniques used for maritime communications over the radio frequency (RF) and optical bands. Then, we present the channel models for RF and optical bands, modulation and coding schemes, coverage and capacity, and radio resource management in maritime communications. After that, the paper presents some emerging use cases of maritime networks, such as the Internet of Ships and the ship-to-underwater Internet of things. Finally, we highlight a few exciting open challenges and identify a set of future research directions for maritime communication, including bringing broadband connectivity to the deep sea, using terahertz and visible light signals for on-board applications, and data-driven modeling for radio and optical marine propagation.
  • Ultrasensitive UV-C detection based on MOCVD-grown highly crystalline ultrawide bandgap orthorhombic κ-Ga2O3

    Lim, Namsoo; Min, Jungwook; Min, Jung Hong; Hong Kang, Chun; Li, Kuang Hui; Park, Tae Yong; Kim, Woochul; Davaasuren, Bambar; Ng, Tien Khee; Ooi, Boon S.; Ha Woo, Deok; Park, Ji Hyeon; Pak, Yusin (Applied Surface Science, Elsevier BV, 2022-10-25) [Article]
    Orthorhombic κ-Ga2O3, as one of the Ga2O3 polymorphs, is considered a promising as ultrawide bandgap material for extreme environment devices. It is considered more superior than the conventional group III-V compound semiconductors and silicon carbides in extreme environments demanding material/device characteristics of high-voltage, high-temperature, high-pressure, high-impact, and high-radiation. In this study, we demonstrate ultrasensitive ultraviolet-C (UV-C) detection using Si-doped orthorhombic κ-Ga2O3 photodetectors. A 150 nm thick κ-Ga2O3 film was grown on a 2-inch diameter sapphire (α–Al2O3) wafer via metal organic chemical vapor deposition (MOCVD) method. The orthorhombic crystallinity of κ-Ga2O3 film was investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The ultrawide bandgap of approximately 4.9 eV was confirmed by UV transmittance measurement. For UV-C detection analysis, a planar device with a channel length of 20 μm was fabricated using Au/Ti metal contacts on the orthorhombic κ-Ga2O3 film. The device doped under 15 sccm SiH4 flow rate showed ultrahigh photoresponse of ∼72.1 A/W, Ion/Ioff of ∼14, and decent rise (∼0.35 s) and decay (∼1.79 s). Our results will contribute to the understanding on a new material phase of κ-Ga2O3, as well as on developing optoelectronics devices with high radiation hardness suitable for operation in extreme environments.
  • Optical image rotation based on orbital angular momentum decomposition and combination

    Al Ibrahim, Redha H.; Zheng, Shuiqin; Ng, Tien Khee; Ooi, Boon S. (JOURNAL OF OPTICS, IOP Publishing, 2022-10-21) [Article]
    This paper theoretically presents an optical image rotation system based on orbital angular momentum (OAM) manipulation. This system consists of two parts corresponding to OAM decomposition and OAM combination functions. OAM decomposition transforms the optical field into the OAM spectrum by log-polar transformation. OAM manipulation is applied on the OAM spectrum plane of the system, and then the OAM combination transforms the manipulated spectrum back into an optical image. By applying a linear phase shift modulation to the OAM spectrum, the image rotates due to the modulation. This paper demonstrates that the OAM spectrum manipulation for image rotation is equivalent to the one-dimensional movement of one of the optical elements. It is suggested that the rotation angle adjustment of an optical image rotation system can be realized, not only by the rotation of the elements, but also made possible by the linear translation of the optical elements. This means that the OAM dimension can bring new features to optical design.
  • Full-scale exfoliation of InGaN-based light-eMitting diodes via Microcavity-assisted crack propagation by using tensile-stressed Ni layers

    Min, Jung-Hong; Jeong, Tae-Hoon; Lee, Kwang Jae; Min, Jung-Wook; Park, Tae-Yong; Ng, Tien Khee; Ooi, Boon S. (IEEE, 2022-10-16) [Conference Paper]
    We demonstrated microcavity-assisted crack propagation for the full-scale exfoliation of a InGaN-based light-emitting diode (LED) membrane by using nanoporous structures and tensile-stressed Ni layers. The blue LED membrane was transferred on a glass slide by using an adhesive bonding and showed good performance.
  • Vertical profiling of ultrafast carrier dynamics in partially strain relaxed and strained InGaN grown on GaN/sapphire template of different In composition

    Park, Kwangwook; Min, Jung Wook; Lopatin, Sergei; Davaasuren, Bambar; Park, Tae Yong; Ooi, Boon S.; Kim, Hyeongmun; Yim, Sang Youp; Cheol Park, Gyeong; Kang, Chul (Applied Surface Science, Elsevier BV, 2022-10-10) [Article]
    InGaN is one of the important ternary alloys which enables the nitrides to have bandgap energy in a wide range of 0.77–3.44 eV corresponding to the photon wavelength of 360–1610 nm by simply adjusting In content. However, the research on the InGaN has primarily focused on low-dimensional structures though knowledge of the physical properties of bulk InGaN thin-film is important in designing optoelectronic applications that utilize thick InGaN layers. In this study, we revisited partially strain relaxed bulk InGaN thin-films of different In compositions. We found that fast carrier decay was caused by hot carrier cooling due to the presence of oxygen impurity, and the slow carrier decay was governed by the carrier localization initiated by V-shape defects which concentrated at the bottom of the InGaN layer starting at InGaN/GaN interface. We also observed that the V-shape defects became severe, and the slow decay carrier lifetime decreased as In composition of the InGaN layer increased. Our observation gives guidance in designing optoelectronic applications using partially relaxed thick InGaN layers, such as buffer layers for long-wavelength InGaN light emitting diodes as well as solar absorber layer for InGaN photovoltaic cells.
  • Interfacial band parameters of ultrathin ALD–Al2O3, ALD–HfO2, and PEALD–AlN/ALD–Al2O3 on c-plane, Ga-face GaN through XPS measurements

    Gong, Jiarui; Zheng, Zheyang; Vincent, Daniel; Zhou, Jie; Kim, Jisoo; Kim, Donghyeok; Ng, Tien Khee; Ooi, Boon S.; Chen, Kevin J.; Ma, Zhenqiang (Journal of Applied Physics, AIP Publishing, 2022-10-03) [Article]
    Ultrathin oxides (UOs) and ultrathin nitrides (UNs) play a crucial role in forming lattice-mismatched semiconductor heterostructures that are fabricated by using semiconducting grafting approach. The grafting approach has shown its great potential to realize GaN-based heterojunction bipolar transistors by fulfilling the missing high-performance p-type nitrides with other p-type semiconductors. A handful of UO and UN dielectrics readily available by atomic layer deposition (ALD) satisfy the requirements of double-sided surface passivation and quantum tunneling for semiconductor grafting. Due to the states existing between the UO or UN conduction band and that of the GaN, the ALD deposited UO or UN layer can generate significant effects on the surface band-bending of GaN. Understanding the band parameters of the interface between UO or UN and c-plane Ga-face GaN can guide the selection of interfacial dielectrics for grafted GaN-based devices. In this study, we performed x-ray photoelectron spectroscopy measurements to obtain the band-bending properties on c-plane, Ga-face GaN samples coated by different ALD cycles of ultrathin-HfO2 or ultrathin AlN. The valence band spectra of GaN coated with ultrathin-ALD–Al2O3, ALD–HfO2, or PEALD–AlN/ALD–Al2O3 were further analyzed to calculate the valence and conduction band offsets between the ALD dielectrics and the Ga-face GaN under different thicknesses and post-deposition annealing conditions of the dielectrics.
  • Deep-Ultraviolet LEDs Incorporated with SiO2-Based Microcavities Toward High-Speed Ultraviolet Light Communication

    Yu, Huabin; Memon, Muhammad Hunain; Jia, Hongfeng; Ding, Yifan; Xiao, Shudan; Liu, Xin; Kang, Yang; Wang, Danhao; Zhang, Haochen; Fang, Shi; Gong, Chen; Xu, Zhengyuan; Ooi, Boon S.; Sun, Haiding (Advanced Optical Materials, Wiley, 2022-09-16) [Article]
    Optical wireless communication (OWC) in the deep-ultraviolet (DUV) band requires an efficient DUV light source with large bandwidth characteristics. In this work, a feasible approach is reported to enlarge the light output power as well as the bandwidth of a DUV light-emitting diode (LED) by embedding a SiO2-based microcavity on which an aluminum (Al) reflector is simultaneously deposited. Consequently, on the one hand, the microcavity with the Al-reflector can facilitate photon escape from the LED to increase the light extraction efficiency, thus enhancing the light output power of the devices. On the other hand, the LED incorporated with a microcavity structure exhibits a reduced resistance–capacitance time constant, leading to an increase in the modulation bandwidth of the LED. Strikingly, the DUV LED incorporated with microcavities represents a significant enhancement of light output power by nearly 30% at 80 mA while exhibiting a higher modulation bandwidth of 12% in comparison to the conventional LED without microcavities. Thus, the implementation of the microcavity and Al reflector on top of a classic LED can enlarge the light output power and modulation bandwidth, eventually facilitating to establish viable high-speed OWC systems.
  • Cl2-Doped CuSCN Hole Transport Layer for Organic and Perovskite Solar Cells with Improved Stability

    Liang, Jian Wei; Firdaus, Yuliar; Azmi, Randi; Faber, Hendrik; Kaltsas, Dimitrios; Kang, Chun Hong; Nugraha, Mohamad Insan; Yengel, Emre; Ng, Tien Khee; De Wolf, Stefaan; Tsetseris, Leonidas; Ooi, Boon S.; Anthopoulos, Thomas D. (ACS Energy Letters, American Chemical Society (ACS), 2022-08-30) [Article]
    Copper(I) thiocyanate (CuSCN) is a wide bandgap and solution-processable p-type semiconductor with tremendous potential for large-area optoelectronic applications. In this work, chlorine-doped CuSCN (Cl2–CuSCN) was utilized to form a hole transport layer (HTL) for different organic solar cells (OSCs) and inverted perovskite solar cells (PSCs). Chlorine doping into CuSCN thin films is found to improve the device performance of different OSCs, to a level comparable to that of PEDOT:PSS-based OSCs. Notably, the inverted PSCs with Cl2–CuSCN showed a better performance than those with pristine CuSCN or PEDOT:PSS-based inverted PSC devices. Moreover, Cl2–CuSCN-based OSCs and PSCs also reveal significantly better stability than pristine CuSCN and PEDOT:PSS-based devices. Our results show how Cl2–CuSCN thin films act as a universally applicable HTL for emerging solar cell technologies, improving both device performance and stability.
  • CNN–Aided Optical Fiber Distributed Acoustic Sensing for Early Detection of Red Palm Weevil: A Field Experiment

    Ashry, Islam; Wang, Biwei; Mao, Yuan; Sait, Mohammed; Guo, Yujian; Al-Fehaid, Yousef; Al-Shawaf, Abdulmoneim; Ng, Tien Khee; Ooi, Boon S. (Sensors, MDPI AG, 2022-08-29) [Article]
    Red palm weevil (RPW) is a harmful pest that destroys many date, coconut, and oil palm plantations worldwide. It is not difficult to apply curative methods to trees infested with RPW; however, the early detection of RPW remains a major challenge, especially on large farms. In a controlled environment and an outdoor farm, we report on the integration of optical fiber distributed acoustic sensing (DAS) and machine learning (ML) for the early detection of true weevil larvae less than three weeks old. Specifically, temporal and spectral data recorded with the DAS system and processed by applying a 100–800 Hz filter are used to train convolutional neural network (CNN) models, which distinguish between “infested” and “healthy” signals with a classification accuracy of ∼97%. In addition, a strict ML-based classification approach is introduced to improve the false alarm performance metric of the system by ∼20%. In a controlled environment experiment, we find that the highest infestation alarm count of infested and healthy trees to be 1131 and 22, respectively, highlighting our system’s ability to distinguish between the infested and healthy trees. On an outdoor farm, in contrast, the acoustic noise produced by wind is a major source of false alarm generation in our system. The best performance of our sensor is obtained when wind speeds are less than 9 mph. In a representative experiment, when wind speeds are less than 9 mph outdoor, the highest infestation alarm count of infested and healthy trees are recorded to be 1622 and 94, respectively.
  • Current Trend in Optical Internet of Underwater Things

    Guo, Yujian; Kong, Meiwei; Alkhazragi, Omar; Sait, Mohammed; Kang, Chun Hong; Ashry, Islam; Yang, Qunhui; Ng, Tien Khee; Ooi, Boon S. (IEEE Photonics Journal, Institute of Electrical and Electronics Engineers (IEEE), 2022-08-02) [Article]
    Our Earth is a “blue planet” that 70% of the surface is covered by the oceans, but most area of oceans remain largely unexplored. Besides supporting the Earth's ecosystem and moderating climate change, oceans are rich in economically relevant natural resources ready for harvesting, such as fishery, oil and gas, and mineral resources. Ocean observation and monitoring are therefore essential for environmental preservation and sea exploration. With the availability of advanced communication techniques, researchers began to look into distributed data acquisition and ocean interconnectivity, which engendered the concepts of intelligent ocean and the Internet-of-Underwater-Things (IoUT) framework. The framework is gaining traction since one could incorporate fiber sensing, acoustic, radio frequency, and optical wireless communication technologies to establish stable, broad-coverage, and massive ocean networks. The development of underwater internet beyond acoustic communication is still in its relative infancy, and therefore more aggregated research efforts from the related communities will be required to eventually achieve breakthroughs in comprehensive IoUT technologies. This review sheds light on the practical considerations and solutions to the challenges and robustness of the optical IoUT network in terms of channel characterization, turbulence studies, mobility, receiver optimization, and the application layer.
  • Optical and interfacial characteristics of a heterojunction between (2¯01)-oriented single-domain β-(In0.072Ga0.928)2O3 and α-Al2O3 crystals

    Alfaraj, Nasir; Li, Kuang-Hui; Braic, Laurentiu; Hedhili, Mohamed N.; Guo, Zaibing; Ng, Tien Khee; Ooi, Boon S. (Optical Materials Express, Optica Publishing Group, 2022-08-01) [Article]
    In this article, we determine the band alignment at the thermodynamically stable heterointerface between a (2¯01)-oriented single-domain β-(In0.072Ga0.928)2O3 crystal and bulk c-plane sapphire, namely, (0001)-oriented α-Al2O3. The β-(In0.072Ga0.928)2O3 layer was deposited on the bulk sapphire crystal using pulsed laser deposition. The β-(In0.072Ga0.928)2O3 and α-Al2O3 valence and conduction band offsets (VBO and CBO, respectively) were found to be 0 ± 0.1 and 4.87 ± 0.1 eV, respectively. Accordingly, we identified a type-I α-Al2O3/β-(In0.072Ga0.928)2O3 heterojunction. X-ray diffraction measurements confirmed (2¯01)-oriented single-domain β-(In0.072Ga0.928)2O3 high-quality films with in-plane rotations of every 120∘, whereas Rutherford backscattering spectrometry was employed to verify the bulk composition. We employed high-resolution X-ray photoelectron spectroscopy to measure the core level binding energies of Al 2p and Ga 2p3/2 with respect to the valence band maxima of the β-(In0.072Ga0.928)2O3 and α-Al2O3 layers, respectively. Then, we measured the energy separation between the Al 2p and Ga 2p3/2 core levels at the interface of the heterojunction. β-(InGa)2O3 is a wide-bandgap semiconductor, while α-Al2O3 is a well-known dielectric. Together, they can be employed to fabricate reliable and efficient power electronic devices. We also combined high-resolution transmission electron microscopy, X-ray diffraction, and fast Fourier transform algorithms to characterize the dislocations at the interface.
  • Aggregation-Induced Fluorescence Enhancement for Efficient X-ray Imaging Scintillators and High-Speed Optical Wireless Communication

    Wang, Jian-Xin; Wang, Yue; Nadinov, Issatay; Yin, Jun; Gutierrez Arzaluz, Luis; Alkhazragi, Omar; He, Tengyue; Ng, Tien Khee; Eddaoudi, Mohamed; Alshareef, Husam N.; Bakr, Osman; Ooi, Boon S.; Mohammed, Omar F. (ACS Materials Letters, American Chemical Society (ACS), 2022-07-29) [Article]
    Aggregation of some chromophores generates very strong fluorescence signals due to the tight molecular packing and highly restricted vibrational motions in the electronically excited states. Such an aggregation-induced emission enhancement enables great strides in biomedical imaging, security screening, sensing, and light communication applications. Here, we realized efficient utilization of a series of aggregation-induced emission luminogens (AIEgens) in X-ray imaging scintillators and optical wireless communication (OWC) technology. Ultrafast time-resolved laser spectroscopic experiments and high-level density functional theory (DFT) calculations clearly demonstrate that a significant increase in the rotational energy barrier in the aggregated state of AIEgens is observed, leading to highly restricted molecular vibrations and suppressed nonradiative processes. AIEgen-based scintillators exhibit a high X-ray imaging resolution of 16.3 lp mm–1, making them excellent candidates for X-ray radiography and security inspections. In addition, these AIEgens show a broad -3-dB modulation bandwidth of ∼110 MHz and high net data rates of ∼600 Mb/s, demonstrating their high potential for application in the field of high-speed OWC.
  • Two-Dimensional Hybrid Organic-Inorganic Perovskite Nanosheets for Gb/s Visible-Light Communication

    Wang, Yue; Wang, Hong; Alkhazragi, Omar; Mohammed, Zyad O. F.; Gutierrez Arzaluz, Luis; Kang, Chun Hong; Ng, Tien Khee; Ooi, Boon S. (IEEE Photonics Technology Letters, Institute of Electrical and Electronics Engineers (IEEE), 2022-06-23) [Article]
    With the rapid development of solid-state lighting and the congestion of radio-frequency communication data traffic, visible-light communication (VLC) has emerged as a versatile technology for simultaneous illumination and communication. However, the conventional color-converting phosphors integrated with light-emitting diodes (LEDs), or laser diodes (LDs) usually have limited optical modulation bandwidth due to the long carrier recombination lifetime, which is not suitable for high-speed multiple-wavelength data transfer based on phosphor-conversion VLC systems. Herein, we demonstrate a hybrid organic-inorganic perovskite nanosheets (NSs), i.e., (C<sub>8</sub>N<sub>9</sub>NH<sub>3</sub>)<sub>2</sub>PbI<sub>4</sub> (PEPI), passivated by polymer, as a fast-acting color-converting phosphor for VLC. Compared to the PEPI micro-plates (μPs), the NSs exhibit a stronger excitonic effect with a shorter fluorescence lifetime of 877 ± 4.7 ps, leading to a broad –3-dB bandwidth of 192.8 MHz. Given the large bandwidth, a net data rate of 0.93 Gb/s was achieved based on an orthogonal frequency-division multiplexing (OFDM) modulation scheme. These investigations verified the feasibility of using two-dimensional hybrid organic-inorganic perovskite materials as a promising phosphor for future multi-Gb/s color-pure wavelength-division multiplexing systems.
  • Ultrafast transient infrared spectroscopy for probing trapping states in hybrid perovskite films

    El-Zohry, Ahmed; Turedi, Bekir; Alsalloum, Abdullah; Maity, Partha; Bakr, Osman; Ooi, Boon S.; Mohammed, Omar F. (COMMUNICATIONS CHEMISTRY, Springer Science and Business Media LLC, 2022-05-30) [Article]
    Studying the charge dynamics of perovskite materials is a crucial step to understand the outstanding performance of these materials in various fields. Herein, we utilize transient absorption in the mid-infrared region, where solely electron signatures in the conduction bands are monitored without external contributions from other dynamical species. Within the measured range of 4000 nm to 6000 nm (2500–1666 cm−1), the recombination and the trapping processes of the excited carriers could be easily monitored. Moreover, we reveal that within this spectral region the trapping process could be distinguished from recombination process, in which the iodide-based films show more tendencies to trap the excited electrons in comparison to the bromide-based derivatives. The trapping process was assigned due to the emission released in the mid-infrared region, while the traditional band-gap recombination process did not show such process. Various parameters have been tested such as film composition, excitation dependence and the probing wavelength. This study opens new frontiers for the transient mid-infrared absorption to assign the trapping process in perovskite films both qualitatively and quantitatively, along with the potential applications of perovskite films in the mid-IR region.
  • Color-Converting Phosphor with Metal-Organic Frameworks Based Mixed-Matrix Membrane for High-Speed Ultraviolet-light Optical Wireless Communication

    Wang, Yue; Wang, Jian-Xin; Alkhazragi, Omar; Kang, Chun Hong; Mohammed, Zyad O. F.; Ng, Tien Khee; F. Mohammed, Omar; Ooi, Boon S. (IWUMD 2022, 2022-05-26) [Conference Paper]
    Ultraviolet-A (UVA) to X-ray radiation detection is important for outdoor health monitoring and medical imaging applications. While group-III-V and group-III-VI semiconductors are robust media for detection of the above spectral range, researchers continue to innovate and develop new materials, including halide perovskite (CsPbBr3) for solar-blind UV communication, polymer-based fluorophores for wide field-of-view (FoV) opticalspectrum detection, and metal-organic frameworks (MOF) for X-ray imaging. We focus on UV-to-visible color-conversion to, firstly, extend the usable band of high-speed optical wireless communication (OWC) into the UV wavelength regime, which is the largely unexplored communication spectrum, to alleviate the congestion of the radio frequency (RF) bandwidth. Secondly, the color-conversion schemes effectively circumvent the étendue limit (area of coverage and optical gain trade-off) of the conventional photodetection system. However, the existing color-converting phosphors still suffer from limited modulation bandwidth, low stability, and therefore the race is ongoing in materials search. Developing a wide-FoV lightconcentrator-based detector using a luminescent medium that can be shaped into various forms and dimensions while easily processable without the need for heavy capital equipment investment motivates our search and development of new materials for UV-OWC .
  • Protecting and Enhancing the Photoelectrocatalytic Nitrogen Reduction to Ammonia Performance of InGaN Nanowires using Mo2C Nanosheets and GaN Buffer Layer

    Gnanasekar, Paulraj; Peramaiy, Karthik; Zhang, Huafan; Ng, Tien Khee; Huang*, Kuo-Wei; Kulandaivel*, Jeganathan; Ooi, Boon S. (arXiv, 2022-05-26) [Preprint]
    Photoelectrocatalytic (PEC) reduction of N2 to ammonia (NH3) is emerging as the potential alternative to overcome the standard Haber-Bosch approach. In this communication, solar N2 reduction was demonstrated with molybdenum carbide (Mo2C) co-catalyst assisted indium gallium nitride (InGaN) nanowires. The effect of aiding Mo2C on InGaN NWs arrests the dark current and demonstrated the saturation current under illumination was briefly elucidated. Large NH3 production of 7.2 gh-1cm-2 with high Faradaic efficiency of 12.6 % was realized at -0.2 V vs. reversible hydrogen electrode for the Mo2C/GaN/InGaN heterostructure. Notably, the proposed heterostructure also exemplifies excellent stability and reproducibility with excellent selectivity in the long-term chronoamperometry analysis. Further, the incorporation of GaN buffer layer in between Mo2C and InGaN NWs was deeply investigated. From Density Functional Theory (DFT) analysis, the incorporation of GaN buffer layer aids the suitable band edge position for the transfer of photogenerated charge carrier from InGaN to Mo2C co-catalyst, and unique 3d orbital of Mo2C is highly suitable to hold N2 for effective reduction to NH3.
  • Wide-field-of-view optical detectors based on fused fiber-optic tapers for high-speed optical wireless communication

    Ooi, Boon S.; Alkhazragi, Omar; Trichili, Abderrahmen (SPIE, 2022-05-24) [Conference Paper, Presentation]
    Optical wireless communication (OWC) is envisioned to become an indispensable technology in future wireless networks. However, one of the main issues hindering the widespread of OWC systems is the strict alignment required to maintain connectivity. This is due to the tradeoff between the receiver’s active area and its response speed, which necessitates the use of a lens to focus the light, limiting the field of view (FOV). Taking inspiration from the wide-FOV eyes of horseflies to address this issue, we propose the use of a convex-surface fused fiber-optic taper (FFOT) that can effectively expand a planar array of photodetectors and project it onto a spherical dome, respectively improving the light collection of individual photodetectors and expanding the overall FOV of the array. In our proof-of-concept demonstration, we show an optical receiver with a FOV semi-angle of around 25° and optical power density gain up to 120 in a 1-GHz link whose bandwidth is limited only by the photodetector. Moreover, reducing of the FOV of each individual fiber that results from tapering and the extra-mural absorption material incorporated around the fibers’ cores reduce the crosstalk between them, preserving the image quality. Therefore, unlike non-imaging light focusing elements, FFOTs can potentially be used in applications in which preserving the image is necessary, such as in imaging multiple-input and multiple-output systems and light detection and ranging (LiDAR). We also show the performance of FFOTs in collecting light from color-converting materials, a technique used in expanding the FOV beyond the étendue limit.
  • Plasmonic Nb2CTx MXene-MAPbI3 Heterostructure for Self-Powered Visible-NIR Photodiodes

    Liu, Zhixiong; El Demellawi, Jehad K.; Bakr, Osman; Ooi, Boon S.; Alshareef, Husam N. (ACS Nano, American Chemical Society (ACS), 2022-05-01) [Article]
    The ability of MXenes to efficiently absorb light is greatly enriched by the surface plasmons oscillating at their two-dimensional (2D) surfaces. Thus far, MXenes have shown impressive plasmonic absorptions spanning the visible and infrared (IR) regimes. However, their potential use in IR optoelectronic applications, including photodiodes, has been marginally investigated. Besides, their relatively low resistivity has limited their use as photosensing materials due to their intrinsic high dark current. Herein, heterostructures made of methylammonium lead triiodide (MAPbI3) perovskite and niobium carbide (Nb2CTx) MXene are prepared with a matching band structure and exploited for self-powered visible-near IR (NIR) photodiodes. Using MAPbI3 has expanded the operation range of the MAPbI3/Nb2CTx photodiode to the visible regime while suppressing the relatively large dark current of the NIR-absorbing Nb2CTx. In consequence, the fabricated MAPbI3/Nb2CTx photodiode has responded linearly to white light illumination with a responsivity of 0.25 A/W and a temporal photoresponse of <4.5 μs. Furthermore, when illuminated by NIR laser (1064 nm), our photodiode demonstrates a higher on/off ratio (∼103) and faster response times (<30 ms) compared to that of planar Nb2CTx-only detectors (<2 and 20 s, respectively). The performed space-charge-limited current (SCLC) and capacitance measurements reveal that such an efficient and enhanced charge transfer depends on the coordinate bonding between the surface groups of the MXene and the undercoordinated Pb2+ ions of the MAPbI3 at the passivated MAPbI3/Nb2CTx interface.
  • Wide-field-of-view Perovskite Quantum-dots Fibers Array for Easing Pointing, Acquisition and Tracking in Underwater Wireless Optical Communication

    Kang, Chun Hong; Alkhazragi, Omar; Sinatra, Lutfan; Alshaibani, Sultan; Wang, Yue; Li, Kuang-Hui; Kong, Meiwei; Lutfullin, Marat; Bakr, Osman; Ng, Tien Khee; Ooi, Boon S. (IEEE, 2022-04-13) [Conference Paper]
    We demonstrated, for the first time, perovskite quantum-dots optical fibers array successfully eases the pointing, acquisition and tracking requirement facing visible-laser-based underwater wireless optical communication.

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