Graphene/nitrogen-functionalized graphene quantum dot hybrid broadband photodetectors with a buffer layer of boron nitride nanosheets

Fluorescence and Nonlinear Optical Response of Graphene Quantum Dots Produced by Pulsed Laser Irradiation in Toluene

Graphene quantum dots (GQDs), the zero dimensional (0D) single nanostructures, have many exciting technological applications in diversified fields such as sensors, light emitting devices, bio imaging probes, solar cells, etc. They are emerging as a functional tool to modulate light by means of molecular engineering due to its merits, including relatively low extend of loss, large outstretch of spatial confinement and control via doping, size and shape. In this article, we present a one pot, facile and ecofriendly synthesis approach for fabricating GQDs via pulsed laser irradiation of an organic solvent (toluene) without any catalyst. It is a promising synthesis choice to prepare GQDs due to its fast production, lack of byproducts and further purification, as well as the control over the product by accurate tuning of laser parameters. In this work, the second (532 nm) and third harmonic (355 nm) wavelengths of a pulsed nanosecond Nd:YAG laser have been employed for the synthesis. It has been found that the obtained GQDs display fluorescence and is expected to have potential applications in optoelectronics and light-harvesting devices. In addition, nonlinear optical absorption of the prepared GQDs was measured using the open aperture z-scan technique (in the nanosecond regime). These GQDs exhibit excellent optical limiting properties, especially those synthesized at 532 nm wavelength.

Selective Fe(ii)-fluorescence sensor with validated two-consecutive working range using N,S,I-GQDs associated with garlic extract as an auxiliary green chelating agent

The goal of this work was to use the pyrolysis process to synthesize graphene quantum dots doped with garlic extract (as N,S-GQDs) and simultaneously co-doped with iodine (as I-GQDs). XPS, HR-TEM, FE-SEM/EDX, FT-IR, fluorescence, and UV-visible absorption spectroscopy were used to characterize the N,S,I-GQDs and analyze their morphological images. The quantum yield of N,S,I-GQDs was found to be 45%, greater than that of undoped GQDs (31%). When stimulated at 363 nm, the N,S,I-GQDs display a strong fluorescence intensity at a maximum wavelength of 454 nm. Using N,S,I-GQDs as a fluorescence quenching sensor for screening tests with various metal ions, it was discovered that they are extremely selective towards Fe2+ over Fe3+ and other ions. Thus, solution pH, concentration of N,S,I-GQDs, quantity of garlic extract, EDTA and AgNO3 concentration as masking agents, reaction duration under ultrasonic aid, and tolerable limit of Fe3+ presence in the target analyte were all optimized for Fe2+ detection. A highly sensitive detection of Fe2+ was obtained using a linear curve with y = 141.34x + 5.5855, R 2 = 0.9961, LOD = 0.11 mg L-1, and LOQ = 0.35 mg L-1. The method precision, given as RSDs, was determined to be satisfactory at 1.04% for intra-day analysis and 3.22% for inter-day analysis, respectively. As a result, the selective determination of trace amounts of Fe2+ in real water samples using such labile multi-element doped GQDs in conjunction with garlic extract as a green chelating agent to maintain its enhanced sensitivity was successfully applied with good recoveries ranging from 89.16 to 121.45%.

Synthesis, Applications, and Prospects of Graphene Quantum Dots: A Comprehensive Review

Graphene quantum dot (GQD) is one of the youngest superstars of the carbon family. Since its emergence in 2008, GQD has attracted a great deal of attention due to its unique optoelectrical properties. Non-zero bandgap, the ability to accommodate functional groups and dopants, excellent dispersibility, highly tunable properties, and biocompatibility are among the most important characteristics of GQDs. To date, GQDs have displayed significant momentum in numerous fields such as energy devices, catalysis, sensing, photodynamic and photothermal therapy, drug delivery, and bioimaging. As this field is rapidly evolving, there is a strong need to identify the emerging challenges of GQDs in recent advances, mainly because some novel applications and numerous innovations on the ease of synthesis of GQDs are not systematically reviewed in earlier studies. This feature article provides a comparative and balanced discussion of recent advances in synthesis, properties, and applications of GQDs. Besides, current challenges and future prospects of these emerging carbon-based nanomaterials are also highlighted. The outlook provided in this review points out that the future of GQD research is boundless, particularly if upcoming studies focus on the ease of purification and eco-friendly synthesis along with improving the photoluminescence quantum yield and production yield of GQDs.

Substrate Effects on the Bandwidth of CdSe Quantum Dot Photodetectors

We investigate the time-resolved photocurrent response of CdSe quantum dot (QD) thin films sensitized with zinc β-tetraaminophthalocyanine (Zn4APc) (Kumar , ACS Appl. Mater. Interfaces, 2019, 11, 48271-48280) on three different substrates, namely, silicon with 230 nm SiO₂ dielectric, glass, and polyimide. While Si/SiO₂ (230 nm) is not suitable for any transient photocurrent characterization due to an interfering photocurrent response of the buried silicon, we find that polyimide substrates invoke the larger optical bandwidth with 85 kHz vs 67 kHz for the same quantum dot thin film on glass. Upon evaluation of the transient photocurrent, we find that the photoresponse of the CdSe quantum dot films can be described as a combination of carrier recombination and fast trapping within 2.7 ns followed by slower multiple trapping events. The latter are less pronounced on polyimide, which leads to the higher bandwidth. We show that all devices are resistance-capacitance (RC)-time limited and that improvements of photoresistance are the key to further increasing the bandwidth.

Liquid-Exfoliated 2D Materials for Optoelectronic Applications

Two-dimensional (2D) materials have attracted tremendous research attention in recent days due to their extraordinary and unique properties upon exfoliation from the bulk form, which are useful for many applications such as electronics, optoelectronics, catalysis, etc. Liquid exfoliation method of 2D materials offers a facile and low-cost route to produce large quantities of mono- and few-layer 2D nanosheets in a commercially viable way. Optoelectronic devices such as photodetectors fabricated from percolating networks of liquid-exfoliated 2D materials offer advantages compared to conventional devices, including low cost, less complicated process, and higher flexibility, making them more suitable for the next generation wearable devices. This review summarizes the recent progress on metal-semiconductor-metal (MSM) photodetectors fabricated from percolating network of 2D nanosheets obtained from liquid exfoliation methods. In addition, hybrids and mixtures with other photosensitive materials, such as quantum dots, nanowires, nanorods, etc. are also discussed. First, the various methods of liquid exfoliation of 2D materials, size selection methods, and photodetection mechanisms that are responsible for light detection in networks of 2D nanosheets are briefly reviewed. At the end, some potential strategies to further improve the performance the devices are proposed.

A review of molybdenum disulfide (MoS2) based photodetectors: from ultra-broadband, self-powered to flexible devices

Two-dimensional transition metal dichalcogenides (2D TMDs) have attracted much attention in the field of optoelectronics due to their tunable bandgaps, strong interaction with light and tremendous capability for developing diverse van der Waals heterostructures (vdWHs) with other materials. Molybdenum disulfide (MoS2) atomic layers which exhibit high carrier mobility and optical transparency are very suitable for developing ultra-broadband photodetectors to be used from surveillance and healthcare to optical communication. This review provides a brief introduction to TMD-based photodetectors, exclusively focused on MoS2-based photodetectors. The current research advances show that the photoresponse of atomic layered MoS2 can be significantly improved by boosting its charge carrier mobility and incident light absorption via forming MoS2 based plasmonic nanostructures, halide perovskites-MoS2 heterostructures, 2D-0D MoS2/quantum dots (QDs) and 2D-2D MoS2 hybrid vdWHs, chemical doping, and surface functionalization of MoS2 atomic layers. By utilizing these different integration strategies, MoS2 hybrid heterostructure-based photodetectors exhibited remarkably high photoresponsivity raging from mA W-1 up to 1010 A W-1, detectivity from 107 to 1015 Jones and a photoresponse time from seconds (s) to nanoseconds (10-9 s), varying by several orders of magnitude from deep-ultraviolet (DUV) to the long-wavelength infrared (LWIR) region. The flexible photodetectors developed from MoS2-based hybrid heterostructures with graphene, carbon nanotubes (CNTs), TMDs, and ZnO are also discussed. In addition, strain-induced and self-powered MoS2 based photodetectors have also been summarized. The factors affecting the figure of merit of a very wide range of MoS2-based photodetectors have been analyzed in terms of their photoresponsivity, detectivity, response speed, and quantum efficiency along with their measurement wavelengths and incident laser power densities. Conclusions and the future direction are also outlined on the development of MoS2 and other 2D TMD-based photodetectors.

Quantitative Understanding of Charge-Transfer-Mediated Fe3+ Sensing and Fast Photoresponse by N-Doped Graphene Quantum Dots Decorated on Plasmonic Au Nanoparticles

The formation of a heterostructure with plasmonic nanoparticles drastically alters the optoelectronic properties of graphene quantum dots (GQDs), resulting in exceptional properties. In the present work, we prepare nitrogen-doped GQDs decorated on gold nanoparticles (Au@N-GQDs) by a one-step green reduction method and study its extraordinary fluorescence and photoresponse characteristics. The as-prepared Au@N-GQDs show more than one order of magnitude enhancement in the fluorescence intensity as compared to the bare N-GQDs, which is attributed to hot electron generation and improved absorption in N-GQDs by local field enhancement and the modification of the edge functional groups. Because of the selective coordination to Fe³⁺ ions, the Au@N-GQDs exhibit extraordinary quenching of fluorescence, with ultrahigh sensitivity for the detection of Fe³⁺ (<1 nM). A new model for the charge-transfer dynamics is developed involving the Langmuir's law of adsorption to explain the unusual quenching, which strongly deviates from the known models of static/dynamic quenching. The proposed sensor is successfully implemented for the ultrasensitive detection of Fe³⁺ ions in human serum and Brahmaputra river water samples, representing its high potential applications in clinical as well as environmental diagnosis. Additionally, because of its high absorption in the UV-vis-NIR region and high charge density with long life excitons, the Au@N-GQDs are utilized as photodetectors with ∼10⁴ times faster response than that of bare N-GQDs. The Au@N-GQD-based photodetector possesses a high responsivity of ∼1.36 A/W and a remarkably high external quantum efficiency of ∼292.2%, which is much superior to the GQD-based photodetectors reported till date. The underlying mechanism of ultrafast photoresponse is ascribed to the transfer of hot electrons along with the tunneling of the electrons from Au NPs to N-GQDs as well as the defect reduction of N-GQDs by the incorporation of Au NPs. Without the use of any charge transporting layer, the outstanding performance of N-GQD-based plasmonic photodetector opens up unique opportunities for future high-speed optoelectronic devices.

Recent Advances on Graphene Quantum Dots: From Chemistry and Physics to Applications

Graphene quantum dots (GQDs) that are flat 0D nanomaterials have attracted increasing interest because of their exceptional chemicophysical properties and novel applications in energy conversion and storage, electro/photo/chemical catalysis, flexible devices, sensing, display, imaging, and theranostics. The significant advances in the recent years are summarized with comparative and balanced discussion. The differences between GQDs and other nanomaterials, including their nanocarbon cousins, are emphasized, and the unique advantages of GQDs for specific applications are highlighted. The current challenges and outlook of this growing field are also discussed.

Fabrication and application of BN nanoparticles, nanosheets and their nanohybrids

Smart implementation of novel advanced nanomaterials is the key for the solution of many complex problems of modern science. In recent years, there has been a great interest in the synthesis and application of boron nitride (BN) nanotubes because of their unique physical, chemical, and mechanical properties. By contrast, the synthesis, characterization and exploration of other morphological types of BN nanostructure - BN nanoparticles and BN nanosheets - have received less attention. However, the detailed investigations on advantages of every morphological BN type for specific applications have only recently been started. One of the promising directions is the utilization of BN-based nanohybrids. This review is dedicated to the in-depth analysis of recently published works on the fabrication and application of BN nanoparticles, nanosheets, and their nanohybrids. It covers a variety of developed synthetic methods toward fabrication of such nanostructures, and their specific application potentials in catalysis, drug delivery, tribology and structural materials. Finally, the review focuses on the theoretical aspects of this quickly emerging field.

Enhanced performance of a graphene/GaAs self-driven near-infrared photodetector with upconversion nanoparticles

Near-infrared photodetectors (NIRPDs) have attracted great attention because of their wide range of applications in many fields. Herein, a novel self-driven NIRPD at the wavelength of 980 nm is reported based on the graphene/GaAs heterostructure. Extraordinarily, its sensitivity to light illumination (980 nm) is far beyond the absorption limitation of GaAs (874 nm). This means that the photocurrent originates from the separation of photo-induced carriers in graphene, which is caused by the vertically built-in electric field formed through the high quality van der Waals contact between graphene and GaAs. Moreover, after introducing NaYF4:Yb3+/Er3+ upconversion nanoparticles (UCNPs) onto the graphene/GaAs heterojunction, the responsivity increases to be as superior as 5.97 mA W-1 and the corresponding detectivity is 1.1 × 1011 cm Hz0.5 W-1 under self-driven conditions. This dramatic improvement is mainly ascribed to the radiative energy transfer from UCNPs to the graphene/GaAs heterostructure. The high-quality and self-driven UCNPs/graphene/GaAs heterostructure NIRPD holds significant potential for practical application in low-consumption and large-scale optoelectronic devices.

Chemical modification of group IV graphene analogs

Mono-elemental two-dimensional (2D) crystals (graphene, silicene, germanene, stanene, and so on), termed 2D-Xenes, have been brought to the forefront of scientific research. The stability and electronic properties of 2D-Xenes are main challenges in developing practical devices. Therefore, in this review, we focus on 2D free-standing group-IV graphene analogs (graphene quantum dots, silicane, and germanane) and the functionalization of these sheets with organic moieties, which could be handled under ambient conditions. We highlight the present results and future opportunities, functions and applications, and novel device concepts.

Bilayer PbS Quantum Dots for High-Performance Photodetectors

Due to their wide tunable bandgaps, high absorption coefficients, easy solution processabilities, and high stabilities in air, lead sulfide (PbS) quantum dots (QDs) are increasingly regarded as promising material candidates for next-generation light, low-cost, and flexible photodetectors. Current single-layer PbS-QD photodetectors suffer from shortcomings of large dark currents, low on-off ratios, and slow light responses. Integration with metal nanoparticles, organics, and high-conducting graphene/nanotube to form hybrid PbS-QD devices are proved capable of enhancing photoresponsivity; but these approaches always bring in other problems that can severely hamper the improvement of the overall device performance. To overcome the hurdles current single-layer and hybrid PbS-QD photodetectors face, here a bilayer QD-only device is designed, which can be integrated on flexible polyimide substrate and significantly outperforms the conventional single-layer devices in response speed, detectivity, linear dynamic range, and signal-to-noise ratio, along with comparable responsivity. The results which are obtained here should be of great values in studying and designing advanced QD-based photodetectors for applications in future flexible optoelectronics.

2D/0D graphene hybrids for visible-blind flexible UV photodetectors

Nitrogen-functionalized graphene quantum dots (NGQDs) are attractive building blocks for optoelectronic devices because of their exceptional tunable optical absorption and fluorescence properties. Here, we developed a high-performance flexible NGQD/graphene field-effect transistor (NGQD@GFET) hybrid ultraviolet (UV) photodetector, using dimethylamine-functionalized GQDs (NMe₂-GQDs) with a large bandgap of ca. 3.3 eV. The NMe₂-GQD@GFET photodetector exhibits high photoresponsivity and detectivity of ca. 1.5 × 10⁴ A W^–1 and ca. 5.5 × 10¹¹ Jones, respectively, in the deep-UV region as short as 255 nm without application of a backgate voltage. The feasibility of these flexible UV photodetectors for practical application in flame alarms is also demonstrated.

Recent progress in the preparation and application of quantum dots/graphene composite materials

Quantum dots/graphene (QDs/GR) composite materials show a distinct synergistic effect between the QDs and graphene, which has aroused vast attention toward their unique characteristics in the last few decades.

Enhanced broadband photoresponse of substrate-free reduced graphene oxide photodetectors

Broadband responsivity enhancement of substrate-free device is achieved from the ultraviolet to near-infrared range just by removing the substrate of rGO film device.