van der Waals layered structures, notably the transitional metal dichalcogenides (TMDs) and TMD-based heterostructures, have recently attracted immense interest due to their unique physical ...properties and potential applications in electronics, optoelectronics, and energy harvesting. Despite the recent progress, it is still a challenge to perform comprehensive characterizations of critical properties of these layered structures, including crystal structures, chemical dynamics, and interlayer coupling, using a single characterization platform. In this study, we successfully developed a multimodal nonlinear optical imaging method to characterize these critical properties of molybdenum disulfide (MoS2) and MoS2-based heterostructures. Our results demonstrate that MoS2 layers exhibit strong four-wave mixing (FWM), sum-frequency generation (SFG), and second-harmonic generation (SHG) nonlinear optical characteristics. We believe this is the first observation of FWM and SFG from TMD layers. All three kinds of optical nonlinearities are sensitive to layer numbers, crystal orientation, and interlayer coupling. The combined and simultaneous SHG/SFG-FWM imaging not only is capable of rapid evaluation of crystal quality and precise determination of odd–even layers but also provides in situ monitoring of the chemical dynamics of thermal oxidation in MoS2 and interlayer coupling in MoS2–graphene heterostructures. This method has the advantages of versatility, high fidelity, easy operation, and fast imaging, enabling comprehensive characterization of van der Waals layered structures for fundamental research and practical applications.
In this study, we successfully developed a carbon dioxide (CO2)-laser-assisted metal–organic chemical vapor deposition (LMOCVD) approach to fast synthesis of high-quality gallium nitride (GaN) ...epilayers on Al2O3 sapphire(0001) substrates. By employing a two-step growth procedure, high crystallinity and smooth GaN epilayers with a fast growth rate of 25.8 μm/h were obtained. The high crystallinity was confirmed by a combination of techniques, including X-ray diffraction, Raman spectroscopy, transmission electron microscopy, and atomic force microscopy. By optimizing growth parameters, the ∼4.3-μm-thick GaN films grown at 990 °C for 10 min showed a smooth surface with a root-mean-square surface roughness of ∼1.9 nm and excellent thickness uniformity with sharp GaN/substrate interfaces. The full-width at half-maximum values of the GaN(0002) X-ray rocking curve of 313 arcsec and the GaN(101̅2) X-ray rocking curve of 390 arcsec further confirmed the high crystallinity of the GaN epilayers. We also fabricated ultraviolet (UV) photodetectors based on the as-grown GaN layers, which exhibited a high responsivity of 0.108 A W–1 at 367 nm and a fast response time of ∼125 ns, demonstrating its high optical quality with potential in optoelectronic applications. Our strategy thus provides a simple and cost-effective means toward fast and high-quality GaN heteroepitaxy growth suitable for fabricating high-performance GaN-based UV detectors.
Monolithic integration of high-speed, high-efficiency photodiodes with receiver electronics on a single silicon chip is a key to reduce cost and improve the performance of data centers' short reach ...optical interconnects. We report a CMOS-compatible surface illuminated silicon PIN photodiode with integrated micro- and nano-scale holes that trap photons for longer interaction with semiconductors resulting in higher absorption efficiencies. The fabricated photodiode demonstrates more than 55% external quantum efficiency (EQE) at 850 nm, and a measured Full-Width at Half Maximum (FWHM) impulse response of 29 ps, 31 ps, and 34 ps at 15 V, 10 V, and 3 V reverse biases, respectively. Simulation results indicate the possibility of operation up to 12.5 Gb/s (NRZ) with no equalization.
Realization of high efficiency and long transmission range in high-frequency wireless power transfer (WPT) systems has always been hindered by the increased resistance due to the eddy current loss ...occurring in the inductive coils. In this study, multi-walled carbon nanotube-coated copper (MWCNT-Cu) coils are successfully introduced to address this limitation by implementing the frequency-inert MWCNT channels along with using their high-surface areas to realize the electromagnetic shielding of the Cu substrate through multiple reflection mechanisms. At a frequency of 15 MHz, the resistance of the individual MWCNT-Cu coil was reduced to less than 40% of its original value for primitive Cu, leading to more than a four-fold increase in their quality factor. When MWCNT-Cu coils were used as the transmitting component, the transmission efficiency of the WPT system increased from 10.57% to 95.81% at a transmission distance of 4 cm and a frequency of 3.45 MHz. Finally, it was demonstrated that the loss reduction improved as the eddy current loss became more severe in coils with higher inductance values, which makes this approach promising for significantly improving the performance of inductive components in WPT applications.
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Low-temperature growth of crystalline gallium nitride (GaN) films on c-plane sapphire (α-Al2O3) substrates was achieved by laser-assisted metalorganic chemical vapor deposition (LMOCVD) and coupling ...laser energy into the chemical reactions. Trimethylgallium (TMGa) and ammonia (NH3) were used as precursors for the growth of GaN films. Through the resonant excitation of rotational–vibrational transition (1084.71 cm–1) of the NH-wagging mode (v 2) in NH3 molecules using a wavelength-tunable CO2 laser tuned at 9.219 μm, highly c-axis oriented GaN films were deposited on sapphire at low substrate temperatures from 250 to 600 °C. GaN films with a large thickness of 12 μm were obtained within 1 h at a substrate temperature of 600 °C. The GaN films deposited by LMOCVD showed a higher degree of crystallinity, higher growth rate, and lower defect densities as compared to those synthesized by MOCVD without resonant excitation of NH3 molecules. This low-temperature synthesis technique opens a promising approach to growing nitrides with low adverse effects.
In this paper, we present a rigorous coupled-wave analysis (RCWA) of absorption enhancement in all-silicon (Si) photodiodes with integrated hole arrays of different shapes and dimensions. The RCWA ...method is used to analyze the light propagation and trapping in the photodiodes on both Si-on-insulator (SOI) and bulk Si substrates for the datacom wavelength at about 850 nm. Our calculation and measurement results show that funnel-shaped holes with tapered sidewalls lead to low back-reflection. A beam of light undergoes a deflection subsequent to the diffraction in the hole array and generates laterally propagating waves. SOI substrates with oxide layers play an important role in reducing the transmission loss, especially for deflected light with higher-order diffraction from the hole array. Owing to laterally propagating modes and back-reflection on the SiO
film, light is more confined in the thin Si layer on the SOI substrates compared to that on the bulk Si substrates. Experimental results based on fabricated devices support the predictions of the RCWA. Devices are designed with a 2-μm-thick intrinsic layer, which ensures ultrafast impulse response (full-width at half-maximum) of 30 ps. Measurements for integrated photodiodes with funnel-shaped holes indicate an enhanced external quantum efficiency of more than 55% on the SOI substrates. This represents more than 500% improvement compared to photodiodes without integrated phototrapping nanoholes.
Carbon soot (CS) has the advantages of cost-effectiveness and production scalability over other carbons (i.e., graphene, CNTs) in their synthesis. However, little research has been conducted to ...explore the potential applications of CS. In this study, we demonstrated that a common daily wasteCScan be used for developing a cost-effective absorbent (CS-sponge) to remove oil contaminants from water. The CS was synthesized by an ethylene-oxygen combustion flame. The CS-sponge was prepared via a dip-coating method. Without further surface modification and pretreatments, the CS-sponge demonstrates high absorption capacities (up to 80 times its own weight) for a broad spectrum of oils and organic solvents with a recyclability of more than 10 times. These research results show evidence that the CS-sponge is promising in environmental remediation for large-scale, low-cost removal of oils from water.
It is known that defects strongly influence the properties of two-dimensional (2D) materials. The controlled creation and removal of defects can be utilized to tailor the optical and electronic ...responses of these 2D materials for optoelectronic and nanoelectronic applications. In this study, we developed an efficient approach to reversibly control the defect states in mechanically exfoliated graphene and molybdenum disulfide (MoS2) monolayers. The defects were created by aluminium oxide (Al2O3) plasmas and removed by moderate thermal annealing at up to 300 °C. We employed Raman and photoluminescence (PL) as well as electrical characterization to monitor the variation of the defect level in graphene and MoS2. For graphene, Raman spectra indicate that the Al2O3 plasma induced sp3-type defects with a controlled concentration, which have been substantially removed after thermal annealing. A similar trend was also observed in monolayer MoS2, as revealed by the defect-related emission peak (Xb) in the PL spectra. We further showed that the defects induced by the Al2O3 plasma in both 2D materials can be restored to any intended level via annealing under well-controlled conditions. Our work presents a new route to the functional design of the optical and electronic properties of graphene and MoS2-based devices through defect engineering.
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