High-Performance Visible to Mid-Infrared Photodetectors Based on HgTe Colloidal Quantum Dots under Room Temperature.

HgTe colloidal quantum dots (CQDs) are one of few materials that can realize near-to-midwave infrared photodetection. And the quality of HgTe CQD directly affects the performance of photodetection. In this work, we optimize the method of synthesizing HgTe CQDs to reduce the defect concentration, therefore improving the photoelectric properties. The photodetector based on HeTe CQD can respond to the light from the visible to mid-infrared band. Notably, a photoresponse to 4000 nm light at room temperature is realized. The responsivity and detectivity are 90.6 mA W-1 and 6.9 × 107 Jones under 1550 nm light illumination, which are better than these of most reported HgTe CQD photodetectors. The response speed reaches a magnitude of microseconds with a rising time of τr = 1.9 µs and a falling time of τf = 1.5 µs at 10 kHz under 1550 nm light illumination.

Study of Tunable Dielectric Permittivity of PBDB-T-2CL Polymer in Ternary Organic Blend Thin Films Using Spectroscopic Ellipsometry.

The ellipsometric analyses reported in this paper present a novelty by bringing an in-depth optical investigation of some ternary organic blends. This study focuses on the tunability and control of the relative permittivity of active layers by varying the weight ratio of blended materials spin-coated as thin films. To investigate this, an extensive approach based on spectroscopic ellipsometry was conducted on ternary blend (D:A1:A2) thin films, involving a donor [D = chlorinated conjugated polymer (PBDB-T-2Cl)] and two acceptor materials [A1 = a non-fullerene (ITIC-F) and A2 = a fullerene (PCBM)]. The refractive index constitutes a key parameter that exposes insights into the feasibility of photovoltaic cells by predicting the trajectory of light as it transits the device. In this term, higher obtained refractive indexes support higher absorption coefficients. Notably, the dielectric constant can be rigorously tuned and finely calibrated by modest variations in the quantity of the third element, resulting in considerable modifications. Moreover, the inclusion of fullerene in blends, as the third element, results in a smooth topographical profile, further refining the surface of the film. From an electrical point of view, the ternary blends outperform the polymer thin films. The synergistic interaction of constituents emphasizes their potential to enhance solar conversion devices.

Light response and adsorption interaction of black phosphorus quantum dots and single-layer graphene phototransistor.

Black phosphorus quantum dots (BPQDs) are synthesized and combined with graphene sheet. The fabricated BPQDs/graphene devices are capable of detecting visible and near infrared radiation. The adsorption effect of BPQDs in graphene is clarified by the relationship of the photocurrent and the shift of the Dirac point with different substrate. The Dirac point moves toward a neutral point under illumination with both SiO2/Si and Si3N4/Si substrates, indicating an anti-doped feature of photo-excitation. To our knowledge, this provides the first observation of photoresist induced photocurrent in such systems. Without the influence of the photoresist the device can respond to infrared light up to 980 nm wavelength in vacuum in a cryostat, in which the photocurrent is positive and photoconduction effect is believed to dominate the photocurrent. Finally, the adsorption effect is modeled using a first-principle method to give a picture of charge transfer and orbital contribution in the interaction of phosphorus atoms and single-layer graphene.

Photoconduction and Electroluminescence of Copper (II) Protoporphyrin and Chlorin Cu-C-e6.

Cu (II) protoporphyrin Cu-PP-IX and chlorin Cu-C-e6 were found to have both thin solid film formation and charge carrier transport abilities. In the layers deposited by resistive thermal evaporation, the mobilities of holes and electrons are on the order of 10-5 cm2 V-1 s-1. Organic light-emitting diodes incorporating the dye molecules as emitting dopants demonstrate electroluminescence in the UV and near-IR ranges.

Helmholtz Resonator Applied to Nanocrystal-Based Infrared Sensing.

While the integration of nanocrystals as an active medium for optoelectronic devices progresses, light management strategies are becoming required. Over recent years, several photonic structures (plasmons, cavities, mirrors, etc.) have been coupled to nanocrystal films to shape the absorption spectrum, tune the directionality, and so on. Here, we explore a photonic equivalent of the acoustic Helmholtz resonator and propose a design that can easily be fabricated. This geometry combines a strong electromagnetic field magnification and a narrow channel width compatible with efficient charge conduction despite hopping conduction. At 80 K, the device reaches a responsivity above 1 A·W-1 and a detectivity above 1011 Jones (3 µm cutoff) while offering a significantly faster time-response than vertical geometry diodes.

Circularly Polarized Light Detection by Chiral Photonic Cellulose Nanocrystal with ZnO Photoconductive Layer in Ultraviolet Region.

Circularly polarized light (CPL) detection and polarization state recognition are required for a wide range of applications. Conventional polarization detection with optical components causes difficulties for miniaturization and integration. An effective design strategy is proposed for direct CPL detection with chiral material. Here, we realized direct CPL detection based on the combination of chiral photonic cellulose nanocrystal (CNC) and ultraviolet-sensitive ZnO photoconductive material. The CNC layer deposited by evaporation-induced self-assembly established the left-handed chiral nematic structure with a photonic bandgap (PBG) to recognize left-handed CPL (LCPL) and right-handed CPL (RCPL) at specific wavelengths. The PBG of CNC layer has been modulated by the adjustment of chiral nematic pitch to match the semiconductor bandgap of ZnO film in ultraviolet region. The photocurrents under RCPL and LCPL are 2.23 × 10-6 A and 1.77 × 10-6 A respectively and the anisotropy factor Δgpc of 0.23 is acquired for the CPL detection based on the chiral photonic CNC. This design provides a new approach to the detection of CPL polarization state with competitive performance.

Reduction of Persistent Photoconduction with IGZO/ZnON-Tandem-Structure Visible-Near-Infrared Phototransistors.

Indium-gallium-zinc oxide- and zinc oxynitride-based heterojunction phototransistors were successfully demonstrated to control the persistent photoconduction (PPC) effect and be also responded sensitively at the range from visible to near-infrared. ZnON plays a key role in extending the spectral response at various frequencies of operation. The devices show significantly different photoresponse and photorecovery characteristics depending on the number of stacked layers of IGZO and ZnON. After negative bias and illumination stress was applied to the devices for 1 h, tandem-structure-based phototransistors recovered remarkably better than single-component IGZO devices. We suggest that the improvements to photoresponse and photorecovery result from the presence of potential wells between two IGZO layers and the energy band alignment of the tandem structure.

Sensitive and Repeatable Photoinduced Luminescent Radicals from A Simple Organic Crystal.

Photoinduced organic radicals are important for chemical and physical processes of organic materials, which are extensively investigated and applied in organic synthesis, photoelectronic devices and biotechnology. However, there are rare reports of the luminescence for these photoinduced radicals, especially in the condensed state. Herein, an unexpected and interesting luminescent radical is described, which can be rapidly and reversibly generated from a simple organic crystal by gentle light irradiation in air. It was revealed that the twist and asymmetric conformation of isolated molecule in its crystal with only weak C-H⋅⋅⋅π intermolecular interactions, which led to the generation of such photoinduced luminescent radicals. In addition, dual-channel photosensitive device with rapid response and well repeatability can be obtained based on the thin film of this organic crystal, showing both photoswitching on luminescence and conducting.

Extrinsic Photoconduction Induced Short-Wavelength Infrared Photodetectors Based on Ge-Based Chalcogenides.

2D layered photodetectors have been widely researched for intriguing optoelectronic properties but their application fields are limited by the bandgap. Extending the detection waveband can significantly enrich functionalities and applications of photodetectors. For example, after breaking through bandgap limitation, extrinsic Si photodetectors are used for short-wavelength infrared or even long-wavelength infrared detection. Utilizing extrinsic photoconduction to extend the detection waveband of 2D layered photodetectors is attractive and desirable. However, extrinsic photoconduction has yet not been observed in 2D layered materials. Here, extrinsic photoconduction-induced short-wavelength infrared photodetectors based on Ge-based chalcogenides are reported for the first time and the effectiveness of intrinsic point defects are demonstrated. The detection waveband of room-temperature extrinsic GeSe photodetectors with the assistance of Ge vacancies is broadened to 1.6 µm. Extrinsic GeSe photodetectors have an excellent external quantum efficiency (0.5%) at the communication band of 1.31 µm and polarization-resolved capability to subwaveband radiation. Moreover, room-temperature extrinsic GeS photodetectors with a detection waveband to the communication band of 1.55 µm further verify the versatility of intrinsic point defects. This approach provides design strategies to enrich the functionalities of 2D layered photodetectors.

Large Switchable Photoconduction within 2D Potential Well of a Layered Ferroelectric Heterostructure.

The coexistence of large conductivity and robust ferroelectricity is promising for high-performance ferroelectric devices based on polarization-controllable highly efficient carrier transport. Distinct from traditional perovskite ferroelectrics, Bi2 WO6 with a layered structure shows a great potential to preserve its ferroelectricity under substantial electron doping. Herein, by artificial design of photosensitive heterostructures with desired band alignment, three orders of magnitude enhancement of the short-circuit photocurrent is achieved in Bi2 WO6 /SrTiO3 at room temperature. The microscopic mechanism of this large photocurrent originates from separated transport of electrons and holes in [WO4 ]-2 and [Bi2 O2 ]+2 layers respectively with a large in-plane conductivity, which is understood by a combination of ab initio calculations and spectroscopic measurements. The layered electronic structure and appropriately designed band alignment in this layered ferroelectric heterostructure provide an opportunity to achieve high-performance and nonvolatile switchable electronic devices.

Photoconductivity in Metal-Organic Framework (MOF) Thin Films.

Photoconductivity is a characteristic property of semi-conductors. Herein, we present a photo-conducting crystalline metal-organic framework (MOF) thin film with an on-off photocurrent ratio of two orders of magnitude. These oriented, surface-mounted MOF thin films (SURMOFs), contain porphyrin in the framework backbone and C60 guests, loaded in the pores using a layer-by-layer process. By comparison with results obtained for reference MOF structures and based on DFT calculations, we conclude that donor-acceptor interactions between the porphyrin of the host MOF and the C60 guests give rise to a rapid charge separation. Subsequently, holes and electrons are transported through separate channels formed by porphyrin and by C60 , respectively. The ability to tune the properties and energy levels of the porphyrin and fullerene, along with the controlled organization of donor-acceptor pairs in this regular framework offers potential to increase the photoconduction on-off ratio.

Field Effect and Photoconduction in Au25 Nanoclusters Films.

Quantum-confined Au nanoclusters exhibit molecule-like properties, including atomic precision and discrete energy levels. The electrical conductivity of Au nanocluster films can vary by several orders of magnitude and is determined by the strength of the electronic coupling between the individual nanoclusters in the film. Similar to quantum-confined, semiconducting quantum dots, the electrical coupling in films is dependent on the size and structure of the Au core and the length and conjugation of the organic ligands surrounding it. Unlike quantum dots, however, semiconducting transport has not been reported in Au nanocluster films. Here, it is demonstrated that through a simple yet careful choice of cluster size and organic ligands, stable Au nanocluster films can electronically couple and become semiconducting, exhibiting electric field effect and photoconductivity. The molecule-like nature of the Au nanoclusters is evidenced by a hopping transport mechanism reminiscent of doped, disordered organic semiconductor films. These results demonstrate the potential of metal nanoclusters as a solution-processed material for semiconducting devices.

Air-Stable Room-Temperature Mid-Infrared Photodetectors Based on hBN/Black Arsenic Phosphorus/hBN Heterostructures.

Layered black phosphorus (BP) has attracted wide attention for mid-infrared photonics and high-speed electronics, due to its moderate band gap and high carrier mobility. However, its intrinsic band gap of around 0.33 electronvolt limits the operational wavelength range of BP photonic devices based on direct interband transitions to around 3.7 µm. In this work, we demonstrate that black arsenic phosphorus alloy (b-As xP1- x) formed by introducing arsenic into BP can significantly extend the operational wavelength range of photonic devices. The as-fabricated b-As0.83P0.17 photodetector sandwiched within hexagonal boron nitride (hBN) shows peak extrinsic responsivity of 190, 16, and 1.2 mA/W at 3.4, 5.0, and 7.7 µm at room temperature, respectively. Moreover, the intrinsic photoconductive effect dominates the photocurrent generation mechanism due to the preservation of pristine properties of b-As0.83P0.17 by complete hBN encapsulation, and these b-As0.83P0.17 photodetectors exhibit negligible transport hysteresis. The broad and large photoresponsivity within mid-infrared resulting from the intrinsic photoconduction, together with the excellent long-term air stability, makes b-As0.83P0.17 alloy a promising alternative material for mid-infrared applications, such as free-space communication, infrared imaging, and biomedical sensing.

Structural, Optical and Electrical Properties of Zinc Oxide Layers Produced by Pulsed Laser Deposition Method.

The structural, optical, and electrical properties of zinc oxide (ZnO) layers manufactured at different process conditions were investigated. ZnO epitaxial layers were grown on silicon, glass, and ITO/glass substrates by pulsed laser deposition (PLD) technique. The influence of power beam, substrate temperature, and deposition time on films properties was analysed. Morphological features of the film surface were investigated by scanning electron microscopy. A structural study shown planar orientation of films at low temperatures of substrate, but the columnar type of growth originated in temperature enhances. Electrical properties were determined in the temperature range 300-500 K. It was shown that the type of films conductivity is metallic and it is limited by charge transfer across grain boundaries.

Chemical Cleavage of Layered Carbon Nitride with Enhanced Photoluminescent Performances and Photoconduction.

Graphene quantum dots (GQDs) and carbon dots (C-dots) have various alluring properties and potential applications, but they are often limited by unsatisfied optical performance such as low quantum yield, ambiguous fluorescence emission mechanism, and narrow emission wavelength. Herein, we report that bulk polymeric carbon nitride could be utilized as a layered precursor to prepare carbon nitride nanostructures such as nanorods, nanoleaves and quantum dots by chemical tailoring. As doped carbon materials, these carbon nitride nanostructures not only intrinsically emitted UV lights but also well inherited the explicit photoluminescence mechanism of the bulk pristine precursor, both of which were rarely reported for GQDs and C-dots. Especially, carbon nitride quantum dots (CNQDs) had a photoluminescence quantum yield (QY) up to 46%, among the highest QY for metal-free quantum dots so far. As examples, the CNQDs were utilized as a photoluminescence probe for rapid detection of Fe(3+) with a detection limit of 1 µM in 2 min and a photoconductor in an all-solid-state device. This work would open up an avenue for doped nanocarbon in developing photoelectrical devices and sensors.