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2D hybrid perovskites with strong spin–orbit coupling (SOC) and structural asymmetry represent a new class of quantum materials that can potentially display coexisting ferroelectricity and Rashba spin splitting. Studies on these systems provide insight into the coupling between strain, spin, and electronic degrees of freedom, as well as symmetry and order parameters in the bulk and at the interfaces. We review design principles for ferroelectric hybrid perovskites with tunable band gaps, Rashba spin-split properties, and multiple polar axes. Different from traditional inorganic semiconductors, the composition and chirality of the organic cation, along with the strength of the coupling between the organic and inorganic components, play key roles in deciding the structural asymmetry that drives ferroelectricity and the spin orbit field.

Two-dimensional materials provide opportunities for developing semiconductor applications at atomistic thickness to break the limits of silicon technology. Black phosphorus (BP), as a layered semiconductor with controllable bandgap and high carrier mobility, is one of the most promising candidates for transistor devices at atomistic thickness1–4. However, the lack of large-scale growth greatly hinders its development in devices. Here, we report the growth of ultrathin BP on the centimetre scale through pulsed laser deposition. The unique plasma-activated region induced by laser ablation provides highly desirable conditions for BP cluster formation and transportation5,6, facilitating growth. Furthermore, we fabricated large-scale field-effect transistor arrays on BP films, yielding appealing hole mobility of up to 213 and 617 cm2 V−1 s−1 at 295 and 250 K, respectively. Our results pave the way for further developing BP-based wafer-scale devices with potential applications in the information industry. Centimetre-scale growth of few-layer black phosphorous with high crystalline quality and homogeneity is realized by pulsed laser deposition.

Lieb lattice, a two-dimensional edge-centered square lattice, has attracted considerable interest due to its exotic electronic and topological properties. Although various optical and photonic Lieb lattices have been experimentally demonstrated, it remains challenging for an electronic Lieb lattice to be realized in real material systems. Here, based on first-principles calculations and tight-binding modeling, a silver sulfide (Ag2S) monolayer is reported as a long-sought-after inorganic electronic Lieb lattice. This Lieb-lattice Ag2S is further found to be ultrasoft, which enables its electronic properties and topological states near the Fermi level to be finely tuned, as evidenced by the strain-induced topologically non-trivial edge states near the valence band edge. These results not only provide an ideal platform to further explore and harvest interesting quantum properties but also pave a way to pursue other inorganic electronic Lieb lattices in a broader material domain.

Metal halide perovskites are emerging materials for next-generation optoelectronic devices, of which the all-inorganic CsPbBr3 perovskite has attracted increasing attention due to its outstanding stability and excellent photoelectric characteristics compared with organic–inorganic counterparts. However, the electroluminescence (EL) efficiencies of inorganic CsPbBr3 perovskite light-emitting diodes (PeLEDs) are unsatisfactory because of high trap densities, which lead to non-radiative recombination loss. Herein, by introducing potassium bromide (KBr), we successfully prepare zero-dimensional/three-dimensional (0D/3D) Cs4−xKxPbBr6/CsPbBr3 heterostructure perovskite films with suppressed trap density and improved photoluminescence quantum yield (PLQY). The deep energy level of 0D Cs4−xKxPbBr6 phase is confirmed using DFT calculation. Moreover, the formation of the unique heterostructure inhibits the free charge diffusion at the grain boundary from suffering trapping and recombination, which facilitates efficient radiative recombination in the 3D CsPbBr3 phase. Therefore, the optimized green PeLEDs show a high external quantum efficiency (EQE) of 12.8%, which is a six-fold improvement over the pristine one (2.1%), and a maximum brightness of 39 400 cd m−2. Our work provides a rational charge carrier confinement strategy for developing high-performance PeLEDs and can be broadened to other potential perovskite materials, not restricted to the CsPbBr3-based perovskite films.

GeTe is an important narrow bandgap semiconductor material and has found application in the fields of phase change storage as well as spintronics devices. However, it has not been studied for application in the field of infrared photovoltaic detectors working at room temperature. Herein, GeTe nanofilms were grown by magnetron sputtering technique and characterized to investigate its physical, electrical, and optical properties. A high-performance infrared photovoltaic detector based on GeTe/Si heterojunction with the detectivity of 8 × 1011 Jones at 850 nm light irradiation at room temperature was demonstrated.

The fabrication of van der Waals heterostructures mainly extends to two-dimensional (2D) materials that are exfoliated from their bulk counterparts, which is greatly limited by high-volume manufacturing. Here, we demonstrate multilayered PtS2/PtSe2 heterojunctions covering a large area on the SiO2/Si substrate with a maximum size of 2″ in diameter, offering throughputs that can meet the practical application demand. Theoretical simulation was carried out to understand the electronic properties of the PtS2/PtSe2 heterojunctions. Zero-bias photoresponse in the heterojunctions is observed under laser illumination of different wavelengths (405–2200 nm). The PtS2/PtSe2 heterojunctions exhibit broad band photoresponse and high quantum efficiency at infrared wavelengths with lower bounds for the external quantum efficiencies being 1.2% at 1064 nm, 0.2% at 1550 nm, and 0.05% at 2200 nm, and also relatively fast response time at the dozens of millisecond level. The large area, broad band 2D heterojunction photodet...

Two-dimensional (2D) materials exhibit many unique optical and electronic properties that are highly desirable for application in optoelectronics. Here, we report the study of a photodetector based on 2D Bi2O2Te grown on an n-Si substrate. The 2D Bi2O2Te material was transformed from a sputtered Bi2Te3 ultrathin film after rapid annealing at 400 °C for 10 min under an air atmosphere. The photodetector was capable of detecting a broad wavelength ranging from 210 nm to 2.4 μm with an excellent responsivity of up to 3 × 105 and 2 × 104 AW−1 and a detectivity of 4 × 1015 and 2 × 1014 Jones for deep ultraviolet (UV) and short-wave infrared (SWIR) under weak light illumination, respectively. The effectiveness of 2D materials in weak light detection was investigated by the analysis of the photocurrent density contribution. Importantly, the facile growth process at low annealing temperatures would allow the direct large-scale integration of 2D Bi2O2Te materials with the complementary metal–oxide–semiconductor (CMOS) technology.

Mid-infrared (MIR) photodetection, covering diverse molecular vibrational regions and atmospheric transmission windows, is vital to civil and military purposes. Versatile use of MIR photodetectors is commonly dominated by HgCdTe alloys, InSb, and quantum superlattices, which are limited by strict operation demands, high-cost, and environmental toxicity. Despite the rapid advances of black phosphorus (BP)-based MIR photodetectors, these are subject to poor stability and large-area integration difficulty. Here, the van der Waals (vdW) epitaxial growth of a wafer-scale 2D platinum ditelluride (PtTe2 ) layer is reported via a simple tellurium-vapor transformation approach. The 2D PtTe2 layer possesses a unique mosaic-like crystal structure consisting of single-crystal domains with highly preferential [001] orientation along the normal direction, reducing the influence of interface defects and ensuring efficient out-of-plane carrier transportation. This characteristic, combined with the wide absorption of PtTe2 and well-designed vertical device architecture, makes the PtTe2 /Si Schottky junction photodetector capable of sensing ultra-broadband light of up to 10.6 µm with a high specific detectivity. Also, the photodetector exhibits an excellent room-temperature infrared-imaging capability. This approach provides a new design concept for high-performance, room-temperature MIR photodetection based on 2D layered materials.

Abstract Tantalum disulfide (TaS2) two-dimensional film material has attracted wide attention due to its unique optical and electrical properties. In this work, we report the preparation of 1 T-TaS2 quantum dots (1 T-TaS2 QDs) by top-down method. Herein, we prepared the TaS2 QDs having a monodisperse grain size of around 3 nm by an effective ultrasonic liquid phase exfoliation method. Optical studies using UV-Vis, PL, and PLE techniques on the as-prepared TaS2 QDs exhibited ultraviolet absorption at 283 nm. Furthermore, we found that dimension reduction of TaS2 has led to a modification of the band gap, namely a transition from indirect to direct band gap, which is explained using first-principle calculations. By using quinine as reference, the fluorescence quantum yield is 45.6%. Therefore, our results suggest TaS2 QDs have unique and extraordinary optical properties. Moreover, the low-cost, facile method of producing high quality TaS2 QDs in this work is ideal for mass production to ensure commercial viability of devices based on this material. Graphical abstract TaS2 quantum dots having a monodisperse grain size of around 3 nm have been prepared by an ultrasonic liquid phase exfoliation method, it has been found that the dimension reduction of TaS2 has led to a transition from indirect to direct band gap that results in the unique and extraordinary optical properties (PL QY: 45.6%).

SnOx thin films with tunable optical bandgap were prepared using RF magnetron sputtering. The bandgap was adjustable between 2.16 and 3.96 eV depending on oxygen partial pressure during deposition and heat treatment. The structure and properties of the SnOx films were studied. The optical absorption of the SnOx films covered multiple ultraviolet bands, such as UVA, UVB and UVC. A photodetector based on the annealed SnOx film exhibited a detectivity of 1 × 1011 Jones (under 365 nm UV light) at room temperature, which demonstrates the important potential application of SnOx films for UV detection.

GeTe is an important narrow band gap semiconductor material, which has found application in the fields of thermoelectricity, phase change storage as well as switch. However, it has not been studied for application in the field of photodetectors. Here, GeTe thin films were grown by magnetron sputtering and their material structure, optical and electrical properties were compared before and after annealing. High-performance photodetectors with detectivity of ∼ 10 13 Jones at 850 nm light were demonstrated. Thus the novel, to the best of our knowledge, application of GeTe in optoelectronic devices is reported in this work.

Ultraviolet (UV) photodetector has attracted extensive interests due to its wide-ranging applications from defense technology to optical communications. The use of wide bandgap metal oxide semiconductor materials is of great interest in the development of UV photodetector due to their unique electronic and optical properties. In this work, deep UV photodetector based on NiO/β-Ga2O3 heterojunction was developed and investigated. The β-Ga2O3 layer was prepared by magnetron sputtering and exhibited selective orientation along the family of ($$ \overline{2} $$2¯ 01) crystal plane after annealing. The photodetector demonstrated good performance with a high responsivity (R) of 27.43 AW−1 under a 245-nm illumination (27 μWcm−2) and the maximum detectivity (D*) of 3.14 × 1012 cmHz1/2 W−1, which was attributed to the p-NiO/n-β-Ga2O3 heterojunction.

Methylammonium lead iodide perovskite (MAPbI 3), as a new type of light absorber for optoelectronic devices, has attracted much interest and served as foundation for new device concepts. However, most studies to date are mainly focusing on the fabrication of MAPbI3 -based solar cells and photodetectors . Here we report a facile method to prepare a large-scale MAPbI 3 photocatalyst through atmospheric pressure physical vapor deposition . The photocurrent density of the MAPbI 3 coated with an Au layer can reach 30.8 and 85.5 μA/cm2 in ethanol and H2SO4 electrolytes, respectively. Meanwhile, the strong interaction between H2O orbitals and the conduction band of MAPbI3 is revealed by density functional theory calculations. Our study opens a new pathway for the development of perovskite-based photo-electrochemical reaction systems.

Group‐10 transition metal dichalcogenides (TMDs) with distinct optical and tunable electrical properties have exhibited great potential for various optoelectronic applications. Herein, a self‐powered photodetector is developed with broadband response ranging from deep ultraviolet to near‐infrared by combining FA1−xCsxPbI3 perovskite with PdSe2 layer, a newly discovered TMDs material. Optoelectronic characterization reveals that the as‐assembled PdSe2/perovskite Schottky junction is sensitive to light illumination ranging from 200 to 1550 nm, with the highest sensitivity centered at ≈800 nm. The device also shows a large on/off ratio of ≈104, a high responsivity (R) of 313 mA W−1, a decent specific detectivity (D*) of ≈1013 Jones, and a rapid response speed of 3.5/4 µs. These figures of merit are comparable with or much better than most of the previously reported perovskite detectors. In addition, the PdSe2/perovskite device exhibits obvious sensitivity to polarized light, with a polarization sensitivity of 6.04. Finally, the PdSe2/perovskite detector can readily record five “P,” “O,” “L,” “Y,” and “U” images sequentially produced by 808 nm. These results suggest that the present PdSe2/perovskite Schottky junction photodetectors may be useful for assembly of optoelectronic system applications in near future.

The newly discovered Group-10 transition metal dichalcogenides (TMDs) like PtSe2 have promising applications in high-performance microelectronic and optoelectronic devices due to their high carrier mobilities, widely tunable bandages and ultrastabilities. However, the optoelectronic performance of broadband PtSe2 photodetectors integrated with silicon remains undiscovered. Here, we report the successful preparation of large-scale, uniform and vertically grown PtSe2 films by simple selenization method for the design of a PtSe2/Si nanowire array heterostructure, which exhibited a very good photoresponsivity of 12.65 A/W, a high specific detectivity of 2.5 × 1013 Jones at −5 V and fast rise/fall times of 10.1/19.5 μs at 10 kHz without degradation while being capable of responding to high frequencies of up to 120 kHz. Our work has demonstrated the compatibility of PtSe2 with the existing silicon technology and ultrabroad band detection ranging from deep ultraviolet to optical telecommunication wavelengths, which can largely cover the limitations of silicon detectors. Further investigation of the device revealed pronounced photovoltaic behavior at 0 V, making it capable of operating as a self-powered photodetector. Overall, this representative PtSe2/Si nanowire array-based photodetector offers great potential for applications in next-generation optoelectronic and electronic devices. Aligning ultra-thin semiconductors with silicon nanowires enables high-speed sensing of an unusually broad range of ultraviolet, visible, and infrared light frequencies. The shapes of silicon nanowires enable quicker and more effective light detection than conventional thin films, but their spectral response still falls outside the parameters needed for various applications, including optical telecommunication. Researchers led by Di Wu from China’s Zhengzhou University and Yuen Hong Tsang at Hong Kong Polytechnic University turned to the broadband absorption of graphene-like platinum selenide (PtSe2) films to extend the light sensitivity. To parallel the geometry of nanowires, the team used precision deposition techniques to grow 2D PtSe2 films into vertically-oriented layers, some tens of nanometers thick. Direct transfer of the PtSe2 film onto a large-scale nanowire array produced a microsecond-fast device sensitive to multiple optical bands. Platinum diselenide (PtSe2) is a newly discovered Group-10 transition metal dichalcogenide (TMD) which has unique electronic properties, in particular a semimetal-to-semiconductor transition. In this work, we have demonstrated the proposed vertically standing layered structure PtSe2 nanofilms based on hybrid heterojunction with high overall performance was realized for broadband light photodetection ranging from 200 nm to 1550 nm. The high-performance broadband photodetector will open up a new pathway for the development of next-generation two dimensional Group-10 materials based optoelectronic devices.

AgBiS2 nanocrystals are emerging optoelectronic materials due to their solution-processability, earth abundance and non-toxic properties. We report a facile method to prepare AgBiS2 nanocrystals in ambient conditions. The nanocrystals are of high crystallinity and without byproducts, which make them suitable for solution processable optoelectronic devices. They were incorporated into graphene transistors for their near infrared detection application. Photodetectors with a high photo-responsivity of 105 A W−1 for 895 nm wavelength at a low operation voltage of 0.1 V were demonstrated.

Layered transition-metal dichalcogenides (TMDCs) have received great interest due to their potential applications in many fields including electronics, optoelectronics, electrochemical hydrogen production and so on. Recent research effort on the development of effective hydrogen evolution reaction (HER) is to modulate the active edge sites through controlling surface structure at the atomic scale. Here we firstly demonstrate a facile strategy to synthesize large-area and edge-rich platinum diselenide (PtSe 2 ) via selenization of Pt films by magnetron sputtering physical deposition method. The edge site density of the PtSe 2 can be effectively controlled by tuning the thickness of Pt films. The HER activity of the PtSe 2 can be enhanced significantly as the active edge site density increases. The maximum cathodic current density of 227 mA/cm 2 can be obtained through increasing the edge density, which well agrees with the density functional theory calculations. Our work provides a fundamental insight on the effect of active edge site density towards HER.

Metals have the best dielectric screening capability among all materials; however, it is usually difficult to fabricate continuous and uniform ultrathin (few-atomic-layer thickness) metal films. Conversely, high-quality atomic-thick semiconductor or semimetal materials (so called two-dimensional materials) such as graphene or MoS2 can be readily obtained and robust in ambient conditions; however, their dielectric screening capabilities are greatly reduced by their reduced dimensionality. Particularly, in the vertical direction, the dielectric screening of two-dimensional materials is insufficient; thus, the performances of devices by two-dimensional materials were easily affected by the coulomb-scattering or other kind of sources. Herein, we propose that with a screw dislocation connecting the van der Waals layers in two-dimensional MoS2 spiral structures, excellent dielectric screening in the vertical direction can be achieved. Our Kelvin force microscopy directly demonstrates that the external impurity ...

Methods and mechanisms for improvement of photocatalytic activity, are important and popular research topics for renewable energy production and waste water treatment. Here, we demonstrate a facile laser drilling method for engineering well-aligned pore arrays on magnetron-sputtered WS2 nanofilms with increased active edge sites; the proposed method promotes partial oxidation to fabricate WS2/WO3 heterojunctions that enhance the separation of photogenerated electron-hole pairs. The WS2 film after one, two, and three treatments exhibited photocurrent density of 3.9, 6.2, and 8 μA/cm2, respectively, reaching up to 31 times larger than that of pristine WS2 film along with greatly improved charge recombination kinetics. The unprecedented combinational roles of laser drilling revealed in this study in regards to geometric tailoring, chemical transformation, and heterojunction positioning for WS2-based composite nanomaterials create a foundation for further enhancing the performance of other 2D transition metal dichalcogenides in photocatalysis via laser treatment.

Despite great improvements in traditional inorganic photodetectors and photovoltaics, more progress is needed in the detection/collection of light at low-level conditions. Traditional photodetectors tend to suffer from high noise when operated at room temperature; therefore, these devices require additional cooling systems to detect weak or dim light. Conventional solar cells also face the challenge of poor light-harvesting capabilities in hazy or cloudy weather. The real world features such varying levels of light, which makes it important to develop strategies that allow optical devices to function when conditions are less than optimal. In this work, we report an organic/inorganic hybrid device that consists of graphene quantum dot-modified poly(3,4-ethylenedioxythiophene) polystyrenesulfonate spin-coated on Si for the detection/harvest of weak light. The hybrid configuration provides the device with high responsivity and detectability, omnidirectional light trapping, and fast operation speed. To demonstrate the potential of this hybrid device in real world applications, we measured near-infrared light scattered through human tissue to demonstrate noninvasive oximetric photodetection as well as characterized the device's photovoltaic properties in outdoor (i.e., weather-dependent) and indoor weak light conditions. This organic/inorganic device configuration demonstrates a promising strategy for developing future high-performance low-light compatible photodetectors and photovoltaics.

Large-scale synthesis of two-dimensional (2D) materials is one of the significant issues for fabricating layered materials into practical devices. As one of the typical III-VI semiconductors, InSe has attracted much attention due to its outstanding electrical transport property, attractive quantum physics characteristics, and dramatic photoresponse when it is reduced to atomic scale. However, scalable synthesis of single phase 2D InSe has not yet been achieved so far, greatly hindering further fundamental studies and device applications. Here, we demonstrate the direct growth of wafer-scale layered InSe nanosheets by pulsed laser deposition (PLD). The obtained InSe layers exhibit good uniformity, high crystallinity with macro texture feature, and stoichiometric growth by in situ precise control. The characterization of optical properties indicates that PLD grown InSe nanosheets have a wide range tunable band gap (1.26-2.20 eV) among the large-scale 2D crystals. The device demonstration of field-effect transistor shows the n-type channel feature with high mobility of 10 cm

Harvesting wave energy based on waving potential, a newly found electrokinetic effect, is attractive but limited mainly to monolayer graphene. Here we demonstrate that moving a transparent flexible ZnO nanofilm across the surface of ionic solutions can generate electricity. The generated electricity increases linearly with the moving velocity with an open-circuit voltage up to tens of millivolt and a short-circuit current at the order of microampere. The harvested electricity can be efficiently scaled up through series and parallel connections. Theoretical simulations show that it is the proper electrical property that endows the ZnO nanofilm with the outstanding capacity in harvesting the wave energy.

Sulphur-doped graphene quantum dots (S-GQDs) were prepared in this work by a novel and facile method using the co-combustion (T-X-J method) of a liquid mixture of paraffin oil and carbon disulphide (CS2). Ultra-high performance (R: 307 A W−1; D*: 1.5 × 1014 Jones) ultraviolet photodetectors based on S-GQDs were fabricated under ambient conditions, shedding light on the fabrication of graphene based high-performance optoelectronic devices.

Low-dimensional semiconductors exhibit remarkable performances in many device applications because of their unique physical, electrical, and optical properties. In this paper, we report a novel and facile method to synthesize In2S3 quantum dots (QDs) at atmospheric pressure and room temperature conditions. This involves the reaction of sodium sulfide with indium chloride and using sodium dodecyl sulfate (SDS) as a surfactant to produce In2S3 QDs with excellent crystal quality. The properties of the as-prepared In2S3 QDs were investigated and photodetectors based on the QDs were also fabricated to study the use of the material in optoelectronic applications. The results show that the detectivity of the device stabilizes at ~ 1013 Jones at room temperature under 365 nm ultraviolet light irradiation at reverse bias voltage.

Group-III-nitride semiconductors, including AlN, GaN, InN and their ternary, quaternary compounds, are promising electronic and optoelectronic materials for the applications in light emitting diodes, lasers, field emitters, photodetectors, artificial photosynthesis, and solar cells. Owing to their direct bandgaps ranging from near infra-red to deep ultraviolet. In recent years, the growth of group-III nitride nanostructures has been extensively explored. Herein, we provide a comprehensive review on the rational synthesis, fundamental properties and promising applications of group-III nitride nanostructures. Group-III nitride nanostructures with diverse morphologies, their corresponding synthesis methods and formation mechanisms involved are systematically compared and discussed, as well as the detailed factors that influence the optical and electrical properties of the nanostructures. The recent achievements gained in the fields of III-nitride nanostructures are highlighted, including light emitting diodes, laser diodes, photodetectors, solar cells, artificial photocatalysis, nanosensors, and nanogenerators. Finally, some perspectives and outlook on the future developments of III-nitride nanostructures are commented.

A vertical graphene heterostructure field-effect transistor (VGHFET) using an ultrathin ferroelectric film as a tunnel barrier is developed. The heterostructure is capable of providing new degrees of tunability and functionality via coupling between the ferroelectricity and the tunnel current of the VGHFET, which results in a high-performance device. The results pave the way for developing novel atomic-scale 2D heterostructures and devices.

We report a simple approach to fabricate high performance energy storage devices based on aqueous inorganic ink comprised of hexagonal MnO2 nanosheets. The MnO2 ink exhibits long term stability. Continuous thin films can be formed on various substrates without using any binder. To obtain a flexible electrode for capacitive energy storage, we printed the MnO2 ink on commercially available A4 paper pre-treated by multi-walled carbon nanotubes. The electrode exhibited a maximum specific capacitance of 90.8 mF/cm2. The electrode could maintain 98.7% capacitance retention for 1,000 cycles at 10 mV/s. The MnO2 ink could be a potential candidate for large-scale production of flexible and printable electronic devices for energy storage and conversion.

Solution processable high-performance, large-area, low-cost infrared organic photodetectors (OPDs) have been receiving more and more attention for their important applications both in scientific and technological fields. The search for a simple method to upgrade device performance for OPDs becomes increasingly important. Here, the performance of an OPD in the near-infrared (NIR) region is tremendously improved by doping iodine into the device's active layer (P3HT:PCBM:I2), 2.7 wt% iodine doping may increase the absorption by 31.3% for the active film and result in a ∼11 000-fold increase in responsivity for the detector. A high detectivity (D*) of ∼1.6 × 1012 cm Hz1/2 W−1, a good specific responsivity (R) of ∼80 A W−1 and a large EQE (external quantum efficiency) of 120% are achieved under illumination (λ = 850 nm) at room temperature. Systematic characterizations reveal that iodine-doping can introduce acceptor states in the energy band gap for the polymer layer, and thus increase the harvesting to long wavelength photons. A small dose of iodine doping can significantly induce improvement in device performance. This work demonstrates a simple but feasible method to enhance an NIR optoelectronics device.

By employing graphene quantum dots (GQDs), we have achieved a high efficiency of 16.55% in n-type Si heterojunction solar cells. The efficiency enhancement is based on the photon downconversion phenomenon of GQDs to make more photons absorbed in the depletion region for effective carrier separation, leading to the enhanced photovoltaic effect. The short circuit current and the fill factor are increased from 35.31 to 37.47 mA/cm(2) and 70.29% to 72.51%, respectively. The work demonstrated here holds the promise for incorporating graphene-based materials in commercially available solar devices for developing ultrahigh efficiency photovoltaic cells in the future.

Metal nanostructures with conical shape, vertical alignment, large ratio of cone height and curvature radius at the apex, controlled cone angle, and single-crystal structure are ideal candidates for enhancing field electron-emission efficiency with additional merits, such as good mechanical and thermal stability. However, fabrication of such nanostructures possessing all these features is challenging. Here, we report on the controlled fabrication of large scale, vertically aligned, and mechanically self-supported single-crystal Cu nanocones with controlled cone angle and enhanced field emission. The Cu nanocones were fabricated by ion-track templates in combination with electrochemical deposition. Their cone angle is controlled in the range from 0.3° to 6.2° by asymmetrically selective etching of the ion tracks and the minimum tip curvature diameter reaches down to 6 nm. The field emission measurements show that the turn-on electric field of the Cu nanocone field emitters can be as low as 1.9 V/μm at current density of 10 μA/cm(2) (a record low value for Cu nanostructures, to the best of our knowledge). The maximum field enhancement factor we measured was as large as 6068, indicating that the Cu nanocones are promising candidates for field emission applications.

Graphene dots (GDs) are used for enhancing the performance of the poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS)-based counter electrodes in Pt-free dye-sensitized solar cells (DSSCs). As compared to PEDOT:PSS CEs, GD–PEDOT:PSS films possess a rough surface morphology, high conductivity and electrocatalytic activity, and low charge-transfer resistance toward I − /I 3 − redox reaction, pushing cell efficiency to 7.36%, which is 43% higher than that of the cell with PEDOT:PSS CEs (5.14%). Without much impact on efficiency, the DSSCs with GD–PEDOT:PSS CEs work well under low-light conditions (light intensity −2 and angle of incidence >60°), such as indoor and low-level outdoor lighting and of the sun while the other traditional cells would fail to work. The concurrent advantage in low cost in Pt-free materials, simple fabrication processes, comparable efficiency with Pt CEs, and high performance under low-light conditions makes the DSSC with GD–PEDOT:PSS CEs suitable to harvest light for a diverse range of indoor and low-level outdoor lighting locations.

Zinc sulfide (ZnS) quantum dots (QDs) were synthesized using a facile, low-cost and environmentally friendly method at room temperature and ambient pressure. The structural, optical and electrical properties of the as-prepared ZnS QDs were investigated. The monodispersed crystalline ZnS QDs with an average size of 3.8 nm has been prepared, an absorption peak at 292 nm in the ultra-violet (UV) range was observed. The maximum responsivity (R) and detectivity (D*) of the ZnS QDs based photodetector under 365 nm UV light illumination were 5.8 A W−1 and 1.97 × 1013 Jones, respectively, which shows important potential application in UV detection.

Palladium diselenide (PdSe2) films exhibit a high charge carrier mobility and sensitivity in photodetection. In this work, wafer-scale PdSe2 thin films with controllable thickness have been synthesized by the selenization of Pd films. A PdSe2-based photodetector can detect a broad wavelength ranging from 420 nm to 1200 nm. The responsivity and detectivity can reach 1.96 × 103 A W−1 and 1.72 × 1010 W Hz−1/2 at VSD = 3 V, respectively. The figure of merit of the photodetection are comparable to the mechanically exfoliated PdSe2 based photodetector. This work demonstrated that selenization is a facile method to synthesize PdSe2 films in large scale and the films are promising for broadband photodetection.

Black phosphorus (BP) is a very promising material for telecommunication due to its direct bandgap and strong resonant absorption in near-infrared wavelength range. However, ultrafast nonlinear photonic applications relying on the ultrafast photocarrier dynamics as well as optical nonlinearity in black phosphorus remain unexplored. In this work, nonlinear optical properties of solution exfoliated BP are investigated and the usage of BP as a new saturable absorber for high energy pulse generation in fiber laser is demonstrated. In order to avoid the oxidization and degradation, BP is encapsulated by polymer matrix which is optically transparent in the spectrum range of interest to form a composite. Two fabrication approaches are demonstrated to produce BP-polymer composite films which are further incorporated into fiber laser cavity as nonlinear media. BP shows very fast carrier dynamics and BP-polymer composite has a modulation depth of 10.6%. A highly stable Q-switched pulse generation is achieved and the single pulse energy of 194 nJ is demonstrated. The ease of handling of such black phosphorus-polymer composite thin films affords new opportunities for wider applications such as optical sensing, signal processing, and light modulation.

Amorphous black phosphorus (a-BP) ultrathin films are deposited by pulsed laser deposition. a-BP field-effect trans-istors, exhibiting high carrier mobility and moderate on/off current ratio, are demonstrated. Thickness dependence of the bandgap, mobility, and on/off ratio are observed. These results offer not only a new nanoscale member in the BP family, but also a new opportunity to develop nano-electronic devices.

To control the optical properties of two-dimensional (2D) materials is a long-standing goal, being of both fundamental and technological significance. Tuning nonlinear optical absorption (NOA) properties of 2D transition metal dichalcogenides in a cost effective way has emerged as an important research topic because of its possibility to custom design NOA properties, implying enormous applications including optical computers, communications, bioimaging, and so on. In this study, WS2 with different size and thickness distributions was fabricated. The results demonstrate that both NOA onset threshold, FON, and optical limiting threshold, FOL, of WS2 under the excitation of a nanosecond pulsed laser can be tuned over a wide range by controlling its size and thickness. The FON and FOL show a rapid decline with the decrease of size and thickness. Due to the edge and quantum confinement effect, WS2 quantum dots (2.35 nm) exhibit the lowest FON (0.01 J cm−2) and FOL (0.062 J cm−2) among all the samples, which are comparable to the lowest threshold achieved in graphene based materials, showing great potential as NOA materials with tunable properties.

Gallium selenide, an important second-order nonlinear semiconductor, has received much scientific interest. However, the nonlinear properties in its two-dimensional (2D) form are still unknown. A strong second harmonic generation (SHG) in bilayer and multilayer GaSe sheets is reported. This is also the first observation of SHG on 2D GaSe thin layers. The SHG of multilayer GaSe above five layers shows a quadratic dependence on the thickness; while that of a sheet thinner than five layers shows a cubic dependence. The discrepancy between the two SHG responses is attributed to the weakened stability of non-centrosymmetric GaSe in the atomically thin flakes where a layer-layer stacking order tends to favor centrosymmetric modification. Importantly, two-photon excited fluorescence has also been observed in the GaSe sheets. Our free-energy calculations based on first-principles methods support the observed nonlinear optical phenomena of the atomically thin layers.

Integration of organic/inorganic hybrid perovskites with metallic or semiconducting phases of 2D MoS2 nanosheets via solution processing is demonstrated. The results show that the collection of charge carriers is strongly dependent on the electronic properties of the 2D MoS2 with metallic MoS2 showing high responsivity and the semiconducting phase exhibiting high on/off ratios.

Graphene quantum dot–polyaniline (PANI–GQD) composite films were synthesized by a chemical oxidation polymerization process. The optical properties of the PANI–GQD composite were studied by varying the mole concentration of PANI and the size of the GQDs. The Au/PANI–GQDs/ITO sandwich device was fabricated in order to investigate the transport properties of the composite. A stable hysteresis loop was observed in response to the applied voltage. By varying the PANI content and size of the GQDs, the area within the hysteresis loop and electrical conductance behavior of the device can be tuned in a controlled manner. Both the tunable luminescence and electrical hysteresis behavior are attributed to surface states of the GQDs. The PANI–GQD composite films are expected to find application in photonic devices.

Graphene oxide (GO) was synthesized by a hydrothermal method using glucose solution as the sole reagent. The wavelength-dependent photoresponse of GO was investigated by fabricating metal–GO–metal photodetectors. The devices demonstrated a broadband photoresponse from 290 to 1610 nm covering deep ultraviolet (UV) to near-infrared (NIR), which is the broadest spectral range yet demonstrated on GO. The response times of the photodetectors in the UV and visible range are about 100 ms, which are at least one order of magnitude faster than photodetectors based solely on GO reported previously. The responsivity of the photodetector can be as high as 23.6 mA W−1 in the visible range. The wavelength-dependent photoresponse is closely related to the absorption characteristics of GO. Potential for a self-powered GO based photodetector is first demonstrated, and the device shows a prominent photoresponse at zero bias. The GO based photodetectors pave the way for developing low-cost, broadband, self-powered as well as spectrally tuneable photodetectors.

Tin sulfide quantum dots (SnS2 QDs) are n-type wide band gap semiconductor. They exhibit a high optical absorption coefficient and strong photoconductive property in the ultraviolet and visible regions. Therefore, they have been found to have many potential applications, such as gas sensors, resistors, photodetectors, photocatalysts, and solar cells. However, the existing preparation methods for SnS2 QDs are complicated and require a high temperature and high pressure environments; hence they are unsuitable for large-scale industrial production. An effective method for the preparation of monodispersed SnS2 QDs at normal temperature and pressure will be discussed in this paper. The method is facile, green, and low-cost. In this work, the structure, morphology, optical, electrical, and photoelectric properties of SnS2 QDs are studied. The synthesized SnS2 QDs are homogeneous in size and exhibit good photoelectric performance. A photoelectric detector based on the SnS2 QDs is fabricated and its J–V and C–V characteristics are also studied. The detector responds under λ = 365 nm light irradiation and reverse bias voltage. Its detectivity approximately stabilizes at 1011 Jones at room temperature. These results show the possible use of SnS2 QDs in photodetectors.

Solid-state graphene oxide (GO) homogeneously incorporated silica gel glasses have been successfully fabricated, and the good nonlinear optical (NLO) response shown indicates its high potential as the optical limiting material to protect optical detectors and human eyes from damage caused by high power lasers. The GO sheets are covalently functionalized with 3-aminopropyltriethoxysilane and connected to the silica glass matrix through Si–O bonding, resulting in an increase in GO incorporation concentration up to 20.2 mol % (C/SiO2) without reaching the saturation limit. Digital images and element mapping analysis of carbon have confirmed the homogeneous dispersion of GO in the silica gel glass. Because of the higher two-photon absorption coefficient, lower NLO starting threshold, and optical limiting threshold shown at 532 nm wavelength, the NLO performance at this wavelength is better than at 1064 nm. At an input fluence as low as 1.3 J cm–2 at 532 nm and 2.3 J cm–2 at 1064 nm, the glass incorporated wit...