High green index electromagnetic interference shields with semiconducting Bi2S3 fillers in a PEDOT:PSS matrix.

Conventional metallic electromagnetic interference (EMI) shields, as well as the emerging 2D material-based shields, meet the shielding effectiveness (SE) needs of most applications. However, their shielding performance is dominated by the reflection of incoming radiation due to their high electrical conductivity, which leads to secondary pollution. This problem is getting exacerbated with the proliferation of electronics and communication networks in modern society. Thus, EMI shields that function dominantly by the absorption of incoming radiation are highly desirable. Such shields would be characterized by a green index, which is the ratio of absorbance over reflectance, close to or greater than one. For nonmagnetic materials, the best way to reduce the undesirable large impedance mismatch is to reduce the effective permittivity of the shield material. Here, we present a new EMI shield with a semiconductor Bi2S3 filler in a conducting PEDOT:PSS polymer matrix, instead of the conventional conductive fillers, to reduce the effective permittivity and demonstrate that even a light loading of only 10% Bi2S3 provides high SE of over 40 dB with a green index value of 0.75. Increasing the filler content to 15 wt% increases the green index close to unity while dropping the SE to 30 dB. The shielding mechanism is explained through electromagnetic parameter measurements and supplemented by density functional theory calculations. This work lays the foundation for the advancement of lightweight and ultrathin green EMI shields with minimum secondary pollution.

Role of oxygen functional groups and attachment of Au nanoparticles on graphene oxide sheets for improved photodetection performance.

Integrating low-dimensional graphene oxide (GO) with conventional Si technology offers innovative strategies for developing ultrafast wideband photodetectors. In this study, we synthesized GO and explored its potential application in broadband photodetection alongside silicon heterostructures. The as-synthesized GO contains various oxygen functional groups, as evidenced by X-ray photoelectron and Fourier transform infrared spectroscopy. These functional groups contribute to increased photo absorption, enhancing photodetection performance. The systematic reduction of these functional groups from the GO surface via thermal annealing decreases photo absorption and consequently lowers the photocurrent. This reduction diminishes photo absorption and amplifies the dark current by approximately 25 times, from 20 nA to 496 nA. This dark current increase is attributed to the electron mobility following the reduction of functional groups. However, attaching plasmonic gold nanoparticles (Au NPs) to the GO surface enhances UV-Vis absorption in the visible region, enabling broadband detection. The even distribution of attached Au NPs on the GO surface is confirmed through field emission transmission electron microscopy. While thermal annealing of GO diminishes the responsivity from 4.6 A W-1 to 3.0 A W-1, the attachment of Au NPs augments the responsivity by more than two-fold, reaching 10.0 A W-1. Thus, it highlights the importance of rich oxygen functional groups in GO and the attachment of Au NPs to achieve more efficient photo-sensing properties.

Asymmetric contact-induced selective doping of CVD-grown bilayer WS2 and its application in high-performance photodetection with an ultralow dark current.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are excellent candidates for high-performance optoelectronics due to their high carrier mobility, air stability and strong optical absorption. However, photodetectors made with monolayer TMDs often exhibit a high dark current, and thus, there is a scope for further improvement. Herein, we developed a 2D bilayer tungsten disulfide (WS2) based photodetector (PD) with asymmetric contacts that exhibits an exceptionally low dark current and high specific detectivity. High-quality and large-area monolayer and bilayer WS2 flakes were synthesized using a thermal chemical vapor deposition system. Compared to conventional symmetric contact electrodes, utilizing metal electrodes with higher and lower work functions relative to bilayer WS2 aids in achieving asymmetric lateral doping in the WS2 flakes. This doping asymmetry was confirmed through the photoluminescence spectral profile and Raman mapping analysis. With the asymmetric contacts on bilayer WS2, we find evidence of selective doping of electrons and holes near the Ti and Au contacts, respectively, while the WS2 region away from the contacts remains intrinsic. When compared with the symmetric contact case, the dark current in the WS2 PD with asymmetric (Au, Ti) contact decreases by an order of magnitude under reverse bias with a concomitant increase in the photocurrent, resulting in an improved on/off ratio of ∼105 and overall improved device performance under identical illumination conditions. We explained this improved performance based on the energy band alignment showing a unidirectional charge flow under light illumination. Our results indicate that the planar device structure and compatibility with current nanofabrication technologies can facilitate its integration into advanced chips for futuristic low-power optoelectronic and nanophotonic applications.

Printed MoSe2/GaAs Photodetector Enabling Ultrafast and Broadband Photodetection up to 1.5 µm.

The development of high-performance and low-cost photodetectors (PDs) capable of detecting a broad range of wavelengths, from ultraviolet (UV) to near-infrared (NIR), is crucial for applications in sensing, imaging, and communication systems. This work presents a novel approach for printing a broadband PD based on a heterostructure of two-dimensional (2D) molybdenum diselenide (MoSe2) and gallium arsenide (GaAs). The fabrication process involves a precise technique to print MoSe2 nanoflower (NF) ink onto a prepatterned GaAs substrate. The resulting heterostructure exhibits unique properties, leveraging the exceptional electronic and optical characteristics of both GaAs and 2D MoSe2. The fabricated PD achieves an astounding on-off ratio of ∼105 at 5 V bias while demonstrating an exceptional on-off ratio of ∼104 at 0 V. The depletion region between GaAs and MoSe2 facilitates efficient charge generation and separation and collection of photogenerated carriers. This significantly improves the performance of the PD, resulting in a notably high responsivity across the spectrum. The peak responsivity of the device is 5.25 A/W at 5 V bias under 808 nm laser excitation, which is more than an order of magnitude higher than that of any commercial NIR PDs. Furthermore, the device demonstrates an exceptional responsivity of 0.36 A/W under an external bias of 0 V. The printing technology used here offers several advantages including simplicity, scalability, and compatibility with large-scale production. Additionally, it enables precise control over the placement and integration of the MoSe2 NF onto the GaAs substrate, ensuring uniformity and reliability in device performance. The exceptional responsivity across a broad spectral range (360-1550 nm) and the success of the printing technique make our MoSe2/GaAs heterostructure PD promising for future low-cost and efficient optoelectronic devices.

Evidence of oxygen vacancy-mediated ultrahigh SERS sensitivity of Niobium pentoxide nanoparticles through defect engineering: theoretical and experimental studies.

Oxygen vacancy engineering in metal oxide-based semiconductors has emerged as an important area of research for sensing applications, such as Surface-enhanced Raman scattering (SERS), gas sensing, etc. It has the potential to replace high-cost and unstable noble metal-based substrates in the near future. However, improving the SERS enhancement factor in semiconductor-based substrates remains a challenge. In the present study, we demonstrate that oxygen vacancy engineering in Niobium pentoxide (Nb2O5) enables ultrahigh SERS sensitivity. Oxygen vacancies were induced and manipulated in the Nb2O5 nanoparticles via a facile high-energy ball milling method and post-growth oxygen annealing. A high enhancement factor (EF) of 5.15 × 107 was obtained for the Methylene Blue (MeB) molecule on the oxygen-deficient substrate with the lowest detection limit of 10-8 M, which is 2 orders of magnitude lower than the pristine substrate. Through a careful analysis of the experimental data and theoretical calculations, we investigated the underlying mechanism behind the high EF in SERS and showed that the SERS performance is directly proportional to the oxygen vacancy concentration in the Nb2O5 nanoparticles. Density functional theory (DFT) calculation suggests a strong coupling of the vibronic states and an increased charge transfer (CT) efficiency in the Nb2O5-MeB complex mediated through the vacancy-induced trap states in the defective Nb2O5 structure. Finite element method (FEM)-based simulations revealed a field enhancement factor of ∼4.17 × 102 that contributed to the SERS EF, while the remaining is contributed to the oxygen vacancy-mediated charge transfer, i.e., a factor of ∼1.23 × 105 is due to the high CT efficiency, the highest among the reported values. We believe that these findings offer valuable insights into the fabrication of defect-tailored cost-effective semiconductor-based SERS substrates for ensuing applications, such as trace dye detection.

Fast detection of Staphylococcus aureus using thiol-functionalized WS2 quantum dots and Bi2O2Se nanosheets hybrid through a fluorescence recovery mechanism.

Ultrafast and sensitive detection of Staphylococcus aureus (S. aureus), a harmful Gram-positive human pathogenic bacterium, by two-dimensional layered materials continues to be a challenge. Herein, we have studied the sensing of S. aureus using a tungsten disulfide (WS2) quantum dot (QD) and bismuth oxyselenide (Bi2O2Se) nanosheet (NS) hybrid through their unique optical functionalities. The WS2 QDs of a mean diameter of 2.5 nm were synthesized by liquid exfoliation. Due to the quantum confinement and functional groups, the WS2 QDs exhibit high fluorescence (FL) yield under UV excitation. The addition of Bi2O2Se NSs resulted in the adsorption of WS2 QDs on their surface, resulting in quenching of the FL emission due to nonfluorescent complex formation between the WS2 QDs and Bi2O2Se NSs. A specific sequencing single-standard DNA (ssDNA) aptamer, which identifies and explicitly binds with S. aureus, was attached to the defect sites of the WS2 QDs for selective detection. The thiol-modified ssDNA aptamers attach covalently to the WS2 QD defect sites, which was confirmed by Raman and X-ray photoelectron spectroscopy (XPS). The interaction of S. aureus with the aptamer functionalized WS2 QDs weakens the van der Waals interaction between the WS2 QDs and Bi2O2Se NSs, which results in the detachment of the WS2 QDs from the Bi2O2Se NS surface and restores the FL intensity of the WS2 QDs, thus allowing the efficient detection of S. aureus. Similar measurements with non-targeted bacteria show that the system is quite selective towards S. aureus. Our FL-based biosensor has a linear response in the range of 103-107 CFU mL-1 (colony formation unit mL-1) with a detection limit of 580 CFU mL-1. We have observed a fast response time of 15 minutes for sensing, which is superior to the previous reports. The proposed system was tested in human urine and can detect S. aureus in human urine samples selectively, proving its potential in real-life applications. The reported approach is versatile enough for sensing other biomolecules and metal ions by choosing suitable receptors.

A fully printed ultrafast Si/WS2 quantum dot photodetector with very high responsivity over the UV to near-infrared region.

Two-dimensional (2D) semiconducting material-based photodetectors (PDs) with high responsivity and fast photo-response are of great interest for various applications such as optical communications, biomedical imaging, security surveillance, environmental monitoring, etc. Additive manufacturing such as 2D printing is a potentially less cumbersome and cost-effective alternative to conventional microdevice fabrication processes used in the production of PDs. Here, we have fabricated a Si/WS2 quantum dot-based heterostructure PD with a very short electrode gap of 40 µm by a simple printing process. The printed p-Si/n-WS2 PD shows an excellent photo-to-dark current ratio of 5121 under 405 nm illumination (23.8 mW cm-2). The printed photodetector exhibits a peak responsivity of 126 A W-1 and a peak detectivity of 9.24 × 1012 Jones over a very broad wavelength range (300-1100 nm), which is much superior to commercial Si PDs. A high external quantum efficiency of 3.9 × 104% and an ultrafast photoresponse (7.8 µs rise time and 9.5 µs fall time) make the device an attractive candidate as an efficient photodetector. The origin of high-performance photodetection is traced to a nearly defect-free interface at the heterojunction, leading to highly efficient charge separation and high photocurrent. Finally, the 2D-printed device exhibits good photodetection even in self-powered conditions, which is very attractive.

Two-dimensional bismuth oxyselenide quantum dots as nanosensors for selective metal ion detection over a wide dynamic range: sensing mechanism and selectivity.

Bismuth oxyselenide (Bi2O2Se) nanosheets, a new 2D non-van der Waals nanomaterial having unique semiconducting properties, could be favorable for various sensing applications. In the present report, a top-down chemical approach was adopted to synthesize ultrathin Bi2O2Se quantum dots (QDs) in an appropriate solution. The as-prepared 2D Bi2O2Se QDs with an average size of ∼3 nm, exhibiting strong visible fluorescence, were utilized for heavy-metal ion detection with high selectivity. The QDs show a high optical band gap and a reasonably high fluorescence quantum yield (∼4%) in the green region without any functionalization. A series of heavy metal ions were detected using these QDs. The as-prepared QDs exhibit selective detection of Fe3+ over a wide dynamic range with a high quenching ratio and a low detection limit (<0.5 µM). The mechanism of visible fluorescence and Fe3+ ion-induced quenching was investigated in detail based on a model involving adsorption and charge transfer. Density functional theory (DFT) first principles calculations show that fluorescence quenching occurred selectively due to the efficient trapping of electrons in the bandgap states created by the Fe atoms. This work presents a sustainable and scalable method to synthesize 2D Bi2O2Se QDs for heavy metal ion sensing over a wide dynamic range and these 2D QDs could find potential uses in gas sensors, biosensors and optoelectronics.

Room temperature exciton formation and robust optical properties of CVD-grown ultrathin Bi2O2Se crystals on arbitrary substrates.

The appealing success of non-van der Waals (non-VdW) two-dimensional (2D) bismuth oxyselenide (Bi2O2Se) crystals in optoelectronics provides an exciting avenue to investigate their fundamental physical properties. To date, the majority of efforts have focused on understanding the properties of 2D Bi2O2Se, usually grown on a mica substrate. However, a gap exists in realizing the origin of photoluminescence (PL) of new age non-VdW Bi2O2Se at visible and near-infrared (NIR) wavelengths and the effect of growth substrates on the structure and optical properties. Herein, we report that the formation of multiple excitons in momentum valleys is responsible for broadband absorption and visible PL from a few layer thick 2D Bi2O2Se. The effect of growth substrates on the structure and optical properties is investigated in detail. Our studies unfold that the growth substrates (mica, sapphire, quartz, SiO2, glass) introduce strain/doping in chemical vapor deposition (CVD)-grown Bi2O2Se crystals, and consequently, the morphology, lattice constant, absorption coefficient, optical bandgap, refractive index, and PL properties are modulated. In addition, the possible direct/indirect multiple exciton formation at the valence band to the conduction band at different symmetry points of Bi2O2Se is analyzed from experimental data on different growth substrates and corroborated with the density functional theory (DFT) calculation of the electronic band structure. Furthermore, temperature-dependent photo-carrier dynamics discloses an A/Γ-exciton activation energy of 209.6 meV in Bi2O2Se. These findings are significant for the futuristic optoelectronic applications of Bi2O2Se and the choice of growth substrates on directly fabricated nanodevices.

Two-Dimensional Transition Metal Dichalcogenide Based Biosensors: From Fundamentals to Healthcare Applications.

There has been an exponential surge in reports on two-dimensional (2D) materials ever since the discovery of graphene in 2004. Transition metal dichalcogenides (TMDs) are a class of 2D materials where weak van der Waals force binds individual covalently bonded X-M-X layers (where M is the transition metal and X is the chalcogen), making layer-controlled synthesis possible. These individual building blocks (single-layer TMDs) transition from indirect to direct band gaps and have fascinating optical and electronic properties. Layer-dependent opto-electrical properties, along with the existence of finite band gaps, make single-layer TMDs superior to the well-known graphene that paves the way for their applications in many areas. Ultra-fast response, high on/off ratio, planar structure, low operational voltage, wafer scale synthesis capabilities, high surface-to-volume ratio, and compatibility with standard fabrication processes makes TMDs ideal candidates to replace conventional semiconductors, such as silicon, etc., in the new-age electrical, electronic, and opto-electronic devices. Besides, TMDs can be potentially utilized in single molecular sensing for early detection of different biomarkers, gas sensors, photodetector, and catalytic applications. The impact of COVID-19 has given rise to an upsurge in demand for biosensors with real-time detection capabilities. TMDs as active or supporting biosensing elements exhibit potential for real-time detection of single biomarkers and, hence, show promise in the development of point-of-care healthcare devices. In this review, we provide a historical survey of 2D TMD-based biosensors for the detection of bio analytes ranging from bacteria, viruses, and whole cells to molecular biomarkers via optical, electronic, and electrochemical sensing mechanisms. Current approaches and the latest developments in the study of healthcare devices using 2D TMDs are discussed. Additionally, this review presents an overview of the challenges in the area and discusses the future perspective of 2D TMDs in the field of biosensing for healthcare devices.

Emerging doping strategies in two-dimensional hybrid perovskite semiconductors for cutting edge optoelectronics applications.

The past decade has witnessed tremendous progress in metal halide perovskites, particularly in lead (Pb) halide perovskites, because of their extraordinary performance in cutting-edge optoelectronic devices. However, the toxicity of Pb and the environmental stability of the perovskites are two major issues that this field is currently facing. In recent years, 2D layered perovskites have emerged as a promising alternative to the traditional 3D perovskites due to their structural flexibility and higher environmental stability, though they lack the desired level of device efficiency. Doping with target ions can drastically tune the crystal structure, optical properties, charge recombination dynamics, and electronic properties of the 2D perovskite. Although the field of doping in 2D perovskites has seen substantial growth in recent times, no comprehensive review is available on the recent advances in doping of 2D perovskites and its effect on the optoelectronic properties. In this review, we summarize the progress in doping in 2D perovskites based on different doping sites including progress in different synthesis strategies and their impact on crystal structures and various optoelectronic properties. We then highlight the recent achievements in doped 2D perovskites for photovoltaic, LED and other emerging applications. Finally, we conclude with the challenges and the future scope in the doping studies of 2D layered perovskites, which need to be addressed for further developments of next-generation 2D perovskite-based optoelectronic devices.

Recent advances in lead-free double perovskites for x-ray and photodetection.

Over the last decade, lead halide perovskites have attracted significant research attention in the field of photovoltaics, light-emitting devices, photodetection, ionizing radiation detection, etc, owing to their outstanding optoelectrical properties. However, the commercial applications of lead-based perovskite devices are restricted due to the poor ambient stability and toxicity of lead. The encapsulation of lead-based devices can reduce the possible leakage of lead. However, it is hard to ensure safety during large-scale production and long-term storage. Recently, considerable efforts have been made to design lead-free perovskites for different optoelectronic applications. Metal halide double perovskites with the general formula of A2MIMIIIX6or A2MIVX6could be potentially considered as green and stable alternatives for different optoelectronic applications. In this review article, we focus on the recent progress and findings on lead-free halide double perovskites for x-ray and UV-vis photodetection applications. Lead-free halide double perovskite has recently drawn a great deal of attention for superior x-ray detection due to its high absorption coefficient, large carrier mobility-lifetime product, and large bulk resistance. In addition, these materials exhibit good performance in photodetection in the UV-vis region due to high photocarrier generation and efficient carrier separation. In this review, first, we define the characteristics of lead-free double perovskite materials. The fundamental characteristics and beneficial properties of halide perovskites for direct and indirect x-ray detection are then discussed. We comprehensively review recent developments and efforts on lead-free double perovskite for x-ray detection and UV-vis photodetection. We bring out the current challenges and opportunities in the field and finally present the future outlook for developing lead-free double perovskite-based x-ray and UV-vis photodetectors for practical applications.

Experimental and theoretical study of europium-doped organometal halide perovskite nanoplatelets for UV photodetection with high responsivity and fast response.

Herein, we investigate the role of Eu3+ doping on CH3NH3PbBr3 nanoplatelets (NPLs) in terms of their optoelectronic properties and photodetection application through a combined experimental and theoretical approach. The introduction of EuCl3 in the CH3NH3PbBr3 crystal structure by a facile solvothermal method enabled the tuning of the lateral and vertical dimensions of the NSs to form large-area NPLs and finally monolayer nanocrystals. The appearance of low-angle diffraction peaks with Eu doping, which are observed in layered perovskite structures, confirms the formation of quasi-2D NPLs. The bandgap of the Eu-doped mixed halide perovskite systematically increases from 2.39 eV to 2.94 eV with increasing doping concentration. Interestingly, 10 mol% EuCl3 doping in the pure CH3NH3PbBr3 crystal dramatically enhances its absorbance and photosensitivity, resulting in high-performance photodetection. Under 405 nm excitation, the CH3NH3Pb0.9Eu0.1Br2.7Cl0.3 photodetector exhibits self-biased behavior with an on/off ratio >103, which is very significant. The planar device achieves a responsivity as high as 5.29 A W- and a detectivity of 1.06 × 1012 Jones under 405 nm with a power density of 0.14 mW cm-2 at 5 V. In addition, the device exhibits very fast response time with a rise/fall time of 17.5/38.5 µs, which is ∼4 times faster than the pristine CH3NH3PbBr3 counterpart. A linear relationship of photocurrent with light intensity in the CH3NH3Pb0.9Eu0.1Br2.7Cl0.3 photodetector signifies low recombination or charge trapping loss. High-performance photodetection in the Eu-doped device is ascribed to the elimination of trap states and the fast charge transfer process. To obtain better insight into the doped system, DFT analysis of the electronic structure of EuCl3-doped CH3NH3PbBr3 was performed and the results are fully consistent with the experimental findings. It was revealed that EuCl3 doping increases the density of states near the conduction band along with a blue shift in the bandgap as compared to that of the pristine perovskite, which in turn increases the built-in potential of the fabricated device, resulting in self-biased photodetection. This work paves the way for deeper understanding of lanthanide doping in perovskites and self-biased photodetection applications of a new family of Eu-doped mixed halide perovskite nanostructures.

Understanding the interfacial charge transfer in the CVD grown Bi2O2Se/CsPbBr3 nanocrystal heterostructure and its exploitation in superior photodetection: experiment vs. theory.

Efficient charge transfer in a 2D semiconductor heterostructure plays a crucial role in high-performance photodetectors and energy harvesting devices. Non-van der Waals 2D Bi2O2Se has enormous potential for high-performance optoelectronics, though very little is known about the interfacial charge transport at the corresponding 2D heterojunction. Herein, we report a combined experimental and theoretical investigation of interfacial charge transfer in the Bi2O2Se/CsPbBr3 heterostructure through various microscopic and spectroscopic tools corroborated with density functional theory calculations. The CVD-grown few-layer Bi2O2Se nanosheet possesses high crystallinity and a high absorption coefficient in the visible-near IR region. We integrated the few-layer Bi2O2Se nanosheet possessing superior electron mobility and CsPbBr3 nanocrystals with high light-harvesting capability for efficient broadband photodetection. The band alignment reveals a type-I heterojunction, and the device under reverse bias reveals a fast response time of 12 µs/24 µs (rise time/fall time) and an improved responsivity in the 390 to 840 nm range due to the effective interfacial charge transfer and efficient interlayer coupling at the Bi2O2Se/CsPbBr3 interface. Notably, a photodetector with a better light on/off ratio and a peak responsivity of ∼103 A W-1 was achieved in the Bi2O2Se/CsPbBr3 heterostructure due to the synergistic effects in the heterostructure under ambient conditions. The DFT analysis of the density of states and charge density plots in the heterostructure revealed a net transfer of electrons/holes from perovskite nanocrystals to Bi2O2Se layers and additional density of states in Bi2O2Se. These results are significant for the development of non-van der Waals heterostructure based high-performance low-powered photodetectors.

Exciton-plasmon coupling and giant photoluminescence enhancement in monolayer MoS2through hierarchically designed TiO2/Au/MoS2ternary core-shell heterostructure.

Enhancing the light coupling efficiency of large-area monolayer molybdenum disulfide (1L-MoS2) is one of the major challenges for its successful applications in optoelectronics and photonics. Herein, we demonstrate a dramatically enhanced photoluminescence (PL) emission from direct chemical vapor deposited monolayer MoS2on a fluorine-doped TiO2/Au nanoparticle plasmonic substrate, where the PL intensity is enhanced by nearly three orders of magnitude, highest among the reported values. The formation of TiO2/Au/1L-MoS2ternary core-shell heterojunction is evidenced by the high-resolution transmission electron microscopy and Raman analyses. Localized surface plasmon resonance induced enhanced absorption and improved light coupling in the system was revealed from the UV-vis absorption and Raman spectroscopy analyzes. Our studies reveal that the observed giant PL enhancement in 1L-MoS2results from two major aspects: firstly, the heavy p-doping of the MoS2lattice is caused by the transfer of the excess electrons from the MoS2to TiO2at the interface, which enhances the neutral exciton emissions and restrains the trion formation. Secondly, the localized surface plasmon in Au NPs underneath the 1L-MoS2film initiates exciton-plasmon coupling between excitons of the 1L-MoS2and surface plasmons of the Au NPs at the MoS2/Au interface. The PL and Raman analyses further confirm the p-doping effect. We isolate the contributions of plasmon enhancement from the theoretical calculation of the field enhancement factor using the effective medium approximation of plasmonic heterostructure, which is in excellent agreement with the experimental data. This work paves a way for the rational design of the plasmonic heterostructure for the effective improvement in the light emission efficiency of 1L-MoS2, and may enable engineering the different contributions to enhance the optoelectronic performance of 2D heterostructures.

Facile synthetic route to exfoliate high quality and super-large lateral size graphene-based sheets and their applications in SERS and CO2 gas sensing.

A combination of low-cost synthetic route and simplified exfoliation technique to develop high-quality graphene-based sheets with very large lateral dimensions, which are viable to scale up, remains a challenging problem. Herein, super-large graphene oxide (GO) sheets with lateral size up to 104 µm with a surface area of 6831 µm2 have been developed based on a simple approach using mild heating conditions, and subsequent deoxygenation yields reduced graphene oxide (rGO) sheets. With the decrease in number of layers (<10, <5, bi-layer and mono-layer) in GO, the Raman intensity ratio, I D/I G value increases systematically from 0.73 to 0.97. The efficacy of reducing oxygen-containing functional groups from GO to rGO is confirmed from Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, UV-visible absorption spectroscopy, photoluminescence, and thermogravimetric analysis. Current-voltage measurements revealed substantial improvement of current by three orders of magnitude upon reduction of GO to rGO, which is consistent with the significant decrease in charge transfer resistance in rGO, as revealed from the electrochemical impedance spectra. The large-area GO and rGO sheets when applied in surface-enhanced Raman scattering (SERS) exhibited a large enhancement factor of 104 and high detection capability down to a concentration of 10 nM for Rhodamine B. Furthermore, the rGO incorporated hybrid rGO-SnO2 demonstrated ∼50% improvement in sensitivity for CO2 gas sensing as compared to the commercial SnO2 based gas sensor. The higher sensitivity in the rGO case is ascribed to its high surface area, as revealed from the BET analysis. Therefore, the present simplified and economical approach of large-area graphene oxide could potentially open up a new strategy for industrial-scale production in the future.

Precise Tuning of the Thickness and Optical Properties of Highly Stable 2D Organometal Halide Perovskite Nanosheets through a Solvothermal Process and Their Applications as a White LED and a Fast Photodetector.

Precise control of the thickness of large-area two-dimensional (2D) organometal halide perovskite layers is extremely challenging owing to the inherent instability of the organic component. Herein, a novel, highly reproducible, and facile solvothermal route is reported to synthesize and tailor the thickness and optical band gap of the organic-inorganic halide perovskite nanosheets (NSs). Our study reveals that self-assembly of randomly oriented perovskite nanorods leads to the growth of multilayered perovskite NSs at ∼100 °C, while at higher temperature, large-area few-layer to bilayer 2D NSs (CH3NH3PbBr3) are obtained through lattice expansion and layer separation depending precisely on the temperature. Interestingly, the thickness of the 2D NSs shows a linear dependence on the reaction temperature and thus enables precise tuning of the thickness from 14 layers to 2 layers, giving rise to a systematic increase in the band gap and appearance of excitonic absorption bands. Quantitative analysis of the change in the band gap with thickness revealed a strong quantum confinement effect in the 2D layers. The perovskite 2D NSs exhibit tunable color and a high photoluminescence (PL) quantum yield (QY) up to 84%. Through a careful analysis of the steady-state and time-resolved PL spectra, the origin of the lower PL QY in thinner NSs is traced to surface defects in the 2D layers, for the first time. A white light converter was fabricated using the composition-tuned 2D CH3NH3PbBrI2 NS on a blue light-emitting diode chip. The 2D perovskite photodetector exhibits a stable and very fast rise/fall time (24 µs/103 µs) along with high responsivity and detectivity of ∼1.93 A/W and 1.04 × 1012 Jones, respectively. Storage, operational, and temperature-dependent stability studies reveal high stability of the 2D perovskite NSs under the ambient condition with high humidity. The reported method is highly promising for the development of large-area stable 2D perovskite layers for various cutting-edge optoelectronic applications.

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.

Origin and tunability of dual color emission in highly stable Mn doped CsPbCl3 nanocrystals grown by a solid-state process.

Herein, we report on a facile, bulk scale room temperature solid-state synthesis of highly stable and luminescent Mn-doped CsPbCl3 nanocrystal (NCs) with high Mn substitution, and investigate the origin and tunability of its dual-color emission. The structural and compositional analyses confirm the successful doping of Mn (1-40%) ion in CsPbCl3 NCs. The Mn-doped CsPbCl3 NCs exhibit distinct excitonic photoluminescence (PL) emission at ~412 nm (Iex) and Mn related emission at ~590 nm (IMn) with high PL quantum yield (up to 35%). Our study reveals that the intensity ratio IMn/Iex could be widely tuned by a factor of 64 and 23 by changing the Mn concentration and the measurement temperature, respectively. The stronger Mn emission at higher temperature is attributed to the higher rate of transfer of photoexcited carriers from the excitonic state of CsPbCl3 host to Mn related state. Further, excitation intensity-dependent PL along with time-resolved PL studies revealed important insights into the origin and tunability of Mn related orange-red PL in the dual color emitting Mn-doped CsPbCl3 NCs. The PL line shape analysis revealed additional PL peak in the range ~634-666 nm, which grows stronger with higher doping concentration and it is attributed, for the first time, to structural defects in the NCs. Remarkably, the Mn-doped CsPbCl3 NCs synthesized by our method exhibit about 10 times higher storage stability than those reported earlier due to the high Mn doping efficiencies achieved due to the strain induced enhanced doping process. Finally, white light emitting diodes (LEDs) were fabricated by employing blue-emitting CsPbCl1.5Br1.5 NCs, green-emitting CsPbBr3 NCs and orange-red emitting 10% Mn-doped CsPbCl3 NCs all grown by the same method. Our work demonstrates a low-cost synthesis strategy for high Mn doping in CsPbCl3 NCs with superior ambient stability and highly tunable emission.

Understanding the excitation wavelength dependent spectral shift and large exciton binding energy of tungsten disulfide quantum dots and its interaction with single-walled carbon nanotubes.

Herein, we investigate the origin of excitation wavelength dependent spectral features and high fluorescence quantum yield in fluorescent 2D tungsten disulfide (WS2) quantum dots (QDs) of average size 2.4 nm. The as-prepared WS2 QDs possess high optical bandgap and reasonably high fluorescence quantum yield ~15.4% in the green region without any functionalization. The broad photoluminescence (PL) spectrum consists of multiple peaks owing to emissions from excitonic transitions and surface defect-related transitions. The excitation wavelength-dependent spectral redshift and narrowing of line shape in the PL peak are analyzed carefully, and it is attributed to the selective excitation/recombination of carriers from different energy levels. The temperature-dependent PL analysis yields an exciton binding energy of ~301 meV in the QDs. Furthermore, we study the interaction between fluorescent WS2 QDs and single-walled carbon nanotubes (SWCNTs) and explore the mechanism of systematic quenching of PL of QDs by SWCNTs. The nature of the Stern-Volmer plot is found to be linear, and the time-resolved fluorescence measurements reveal that the quenching follows primarily the static behavior. Our study further reveals that defect sites in SWCNTs primarily act as the binding sites for WS2 QDs and form non-fluorescent complexes for effective quenching of the PL. The strong interaction between the WS2 QDs and the SWCNTs is evidenced from the spectral shift in the X-ray photoelectron spectroscopy and Raman peaks. Our study reveals the origin of excitation wavelength dependent PL emission from WS2 QDs and the nature of the interaction between WS2 QDs and SWCNTs, which are important for their applications in biomedical imaging and sensing, such as surface-enhanced Raman scattering, etc.