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Achieving low-threshold infrared stimulated emission in solution-processed quantum dots is critical to enable real-life application including photonic integrated circuits (PICs), LIDAR application and optical telecommunication. However, realization of low threshold infrared gain is fundamentally challenging due to high degeneracy of the first emissive state (e.g., 8-fold) and fast Auger recombination. In this letter, we demonstrate ultralow-threshold infrared stimulated emission with an onset of 110 uJ.cm-2 employing cascade charge transfer (CT) in Pb-chalcogenide colloidal quantum dot (CQD) solids. In doing so, we investigate this idea in two different architectures including a mixture of multiband gap CQDs and layer-by-layer (LBL) configuration. Using transient absorption spectroscopy, we show ultrafast cascade CT from large band-gap PbS CQD to small band-gap PbS/PbSSe core/shell CQDs in LBL (~ 2 ps) and mixture (~ 9 ps) configuration. These results indicate the feasibility of using cascade CT as an efficient method to reduce optical gain threshold in CQD solid films.

Image sensors hold a pivotal role in society due to their ability to capture vast amounts of information. Traditionally, image sensors are opaque due to light absorption in both the pixels and the read-out electronics that are stacked on top of each other. Making image sensors visibly transparent would have a far-reaching impact in numerous areas such as human-computer interfaces, smart displays, and both augmented and virtual reality. In this paper, we present the development and analysis of the first semi-transparent image sensor and its applicability as an eye-tracking device. The device consists of an 8x8 array of semi-transparent photodetectors and electrodes disposed on a fully transparent substrate. Each pixel of the array has a size of 60 x 140 {\mu}m and an optical transparency of 85-95%. Pixels have a high sensitivity, with more than 90% of them showing a noise equivalent irradiance < 10-4 W/m2 for wavelengths of 637 nm. As the semi-transparent photodetectors have a large amount of built-in gain, the opaque read-out electronics can be placed far away from the detector array to ensure maximum transparency and fill factor. Indeed, the operation and appearance of transparent image sensors present a fundamental shift in how we think about cameras and imaging, as these devices can be concealed in plain sight.

Photodetectors that are sensitive in shortwave infrared (SWIR) range (1 um - 2 um) are of significant interest for applications in 3D, night and adverse weather imaging, machine vision and autonomous driving, among others. Currently available technologies in the SWIR rely on costly epitaxial semiconductors that are not monolithically integrated with CMOS electronics. Solution-processed quantum dots can address this challenge by enabling low-cost manufacturing and simple monolithic integration on silicon in a back-end-of-line (BEOL) process. To date, colloidal quantum dot (CQD) materials to access the SWIR are mostly based on lead sulfide (PbS) and mercury telluride (HgTe) compounds, imposing major regulatory concerns and impeding their deployment in consumer electronics due to toxicity concerns. Here we report a new synthesis method for environmentally-friendly silver telluride (Ag2Te) quantum dots and their application in high-performance SWIR photodetectors. The CQD photodetector stack employs materials compliant with the Restriction of Hazardous Substance (RoHS) directives and is sensitive in the spectral range from 350 nm - 1600 nm. The room-temperature detectivity is of the order 1012 Jones, the 3dB bandwidth is in excess of 0.1 MHz and the linear dynamic range is over 118 dB. We also realize a monolithically integrated SWIR imager based on solution processed, heavy-metal-free materials, thus paving the way of this technology to consumer electronics market.

The use of colloidal quantum dots (CQDs) as a gain medium in infrared laser devices has been underpinned by the need for high pumping intensities, very short gain lifetimes and low gain coefficients.

The creation of white and multicoloured 3D-printed objects with high colour fidelity via powder sintering processes is currently limited by discolouration from thermal sensitizers used in the printing process. Here we circumvent this problem by using switchable, photochromic tungsten oxide nanoparticles, which are colourless even at high concentrations. Upon ultraviolet illumination, the tungsten oxide nanoparticles can be reversibly activated making them highly absorbing in the infrared. Their strong infrared absorption upon activation renders them efficient photothermal sensitizers that can act as fusing agents for polymer powders in sintering-based 3D printing. The WO3 nanoparticles show fast activation times, and when mixed with polyamide powders they exhibit a heating-to-colour-change ratio greatly exceeding other sensitizers in the literature. Upon mixing with coloured inks, powders containing WO3 display identical colouration to a pristine powder. This demonstrates the potential of WO3, and photochromic nanoparticles in general as a new class of material for advanced manufacturing.

Selective spectral detection of ultraviolet (UV) radiation is highly important across numerous fields from health and safety to industrial and environmental monitoring applications. Herein, we report a non-toxic, visible-blind, inorganic quantum dot (QD)-based sensing scheme that expands the spectral coverage of Silicon CMOS sensors into the UV, enabling efficient UV detection without affecting the sensor performance in the visible and UV-band discrimination. The reported scheme employs zinc magnesium oxide (ZnMgO) QDs with compositionally tunable absorption across UV and high photoluminescence quantum yield (PLQY) in the visible. The efficient luminescence and large stokes shift of these QDs have been exploited herein to act as an efficient downconverting material that enhances the UV sensitivity of Si-photodetector (Si-PD). A Si-PD integrated with the QDs results in a nine-fold improvement in photoresponsivity from 0.83 mA/W to 7.5 mA/W at 260 nm. Leveraging the tunability of these QDs we further report on a simple UV band identification scheme, using two distinct band gap ZnMgO QDs stacked in a tandem architecture whose spectral emission color depends on the UV-band excitation light. The downconverting stack enables facile discrimination of UV light using a standard CMOS image sensor (camera) or by the naked eye and avoids the use of complex optics.

Broadband infrared photodetectors have profound importance in diverse applications including security, gas sensing, bioimaging, spectroscopy for food quality, and recycling, just to name a few. Yet, these applications can currently be served by expensive epitaxially grown photodetectors, limiting their market potential and social impact. The use of colloidal quantum dots (CQDs) and 2D-materials in a hybrid layout is an attractive alternative to design low-cost CMOS-compatible infrared photodetectors. However, the spectral sensitivity of these conventional hybrid detectors has been restricted to 2.1 um. Herein, we present a hybrid structure comprising MoS2 with PbSe CQDs to extend their sensitivity further towards the mid-wave infrared, up to 3 um. We achieve room temperature responsivity of 137.6 A/W and a detectivity of 7.7 10^10 Jones at 2.55 um owing to highly efficient photoexcited carrier separation at the interface of MoS2 and PbSe in combination with an oxide-coating to reduce dark current; the highest value yet for a PbSe based hybrid device. These findings strongly support the successful fabrication of hybrid devices which may pave the pathway for cost-effective, high performance, next-generation, novel photodetectors.

Pb-chalcogenide colloidal quantum dots (CQDs) are attractive materials to be used as tuneable laser media across the infrared spectrum. However, excessive nonradiative Auger recombination due to the presence of trap states outcompetes light amplification by rapidly annihilating the exciton population, leading to high gain thresholds. Here, we employ a binary blend of CQDs and ZnO nanocrystals in order to passivate the in-gap trap states of PbS-CQD gain medium. Using transient absorption, we measure a five-fold increase in Auger lifetime demonstrating the suppression of trap-assisted Auger recombination. By doing so, we achieve a two-fold reduction in amplified spontaneous emission (ASE) threshold. Finally, by integrating our proposed binary blend to a DFB resonator, we demonstrate single-mode lasing emission at 1650 nm with a linewidth of 1.23 nm (0.62 meV), operating at a low lasing threshold of ~385 {\mu}J.cm-2. The Auger suppression in this system has allowed to achieve unprecedented lasing emission stability for a CQD laser with recorded continuous operation of 5 hours at room temperature and ambient conditions.

Metal selenide nanocrystals have attracted attention as promising materials in photovoltaics and thermoelectrics. However, the expensive and labor-intensive synthesis methods utilized for the production of these nanomaterials have impeded their widespread utilization. The need for air-free environment and high synthesis temperature for crystal nucleation and growth lead as the major factors contributing to the cost of synthesis. In this work, we present a synthesis method for metal selenide nanocrystals at room temperature under ambient conditions that is enabled by a cost-effective selenium precursor.

Unbalanced charge injection is deleterious for the performance of colloidal quantum dot (CQD) light emitting diodes (LEDs) as it deteriorates the quantum efficiency (QE), brightness and operational lifetime. CQD LEDs emitting in the infrared have previously achieved high quantum efficiencies but only when driven to emit in the low radiance regime. At higher radiance levels, required for practical applications, the efficiency decreased dramatically in view of the notorious efficiency droop. Here we report a novel methodology to regulate charge supply in multinary bandgap CQD composites that facilitates improved charge balance. Our approach is based on engineering the energetic potential landscape at the supra-nanocrystalline level that has allowed us to report short-wave infrared (SWIR) PbS CQD LEDs with record-high external QE in excess of 8%, most importantly, at a radiance level of ~ 5 WSr-1m2, an order of magnitude higher than prior reports. Furthermore, the balanced charge injection and Auger recombination reduction has led to unprecedentedly high operational stability with radiance half-life of 26068 hours at a radiance of 1Wsr-1m-2.

Materials with optical gain in the infrared are of paramount importance for optical communications, medical diagnostics and silicon photonics. The current technology is based either on costly III-V semiconductors that are not monolithic to silicon CMOS technology or Er-doped fiber technology that does not make use of the full fiber transparency window. Colloidal quantum dots (CQD) offer a unique opportunity as an optical gain medium in view of their tunable bandgap, solution processability and CMOS compatibility. The 8-fold degeneracy of infrared CQDs based on Pb-chalcogenides has hindered the demonstration of low-threshold optical gain and lasing, at room temperature. We demonstrate room-temperature, infrared, size-tunable, band-edge stimulated emission with linewidth of ~14 meV. Leveraging robust electronic doping and charge-exciton interactions in PbS CQD thin films, we reach gain threshold at the single exciton regime representing a four-fold reduction from the theoretical limit of an eight-fold degenerate system, with a net modal gain in excess of 100 cm-1., Comment: arXiv admin note: substantial text overlap with arXiv:1908.03796

Recent approaches to develop infrared photodetectors characterized by high sensitivities, broadband spectral coverage, easy integration with silicon electronics and low cost have been based on hybrid structures of transition metal dichalcogenides (TMDCs) and PbS colloidal quantum dots (CQDs). However, to date, such photodetectors have been reported with high sensitivity up to 1.5 $\mu$m. Here we extend the spectral coverage of this technology towards 2 $\mu$m demonstrating for the first time compelling performance with responsivities 1400 A/W at 1.8 $\mu$m with 1V bias and detectivities as high as $10^{12}$ Jones at room temperature. To do this we studied two TMDC materials as a carrier transport layer, tungsten disulfide (WS$_2$) and molybdenum disulfide (MoS$_2$) and demonstrate that WS$_2$ based hybrid photodetectors outperform those of MoS$_2$ due to a more adequate band alignment that favors carrier transfer from the CQDs., Comment: 24 pages including Supporting Information

Steady-state access to intraband transitions in colloidal quantum dots (CQDs), via heavy doping, allows exploiting the electromagnetic spectrum at energies below the band gap. CQD intraband optoelectronics opens up a new path to cheap mid- and long-wavelength infrared photodetectors and light-emitting devices, which today employ mostly epitaxial materials. As a recent field of experimental research, thorough studies of the basic properties of intraband transitions in CQDs are still lacking. In this work, we investigate the size and temperature dependence of the intraband transition in heavily n-doped PbS quantum dot (QD) films. We measure the absorption coefficient of the intraband transition to be in the order of $10^4$ cm$^{-1}$, which is comparable to the value of the interband absorption coefficient. Additionally, we determine the size-dependence of the oscillator strength of the intraband transition. We demonstrate a negative dependence of the intraband energy with temperature, in contrast to the positive dependence of the interband transition.

The turn-on voltage of an LED is an important parameter as it determined the power consumption of the LED and influences the effective power conversion efficiency.

Harnessing low energy photons is of paramount importance for multi-junction high efficiency solar cells as well as for thermo-photovoltaic applications. However, semiconductor absorbers with bandgap lower than 0.8 eV have been limited to III-V (InGaAs) or IV (Ge) semiconductors that are characterized by high manufacturing costs and complicated lattice matching requirements in their growth and integration with the higher bandgap cells. Here, we have developed solution processed low bandgap photovoltaic devices based on PbS colloidal quantum dots (CQDs) with a bandgap of 0.7 eV suited for both thermo-photovoltaic as well as low energy solar photon harvesting. By matching the spectral response of those cells to that of the infrared solar spectrum, we report a record high short circuit current (JSC) of 37 mA/cm2 under full solar spectrum and 5.5 mA/cm2 when placed at the back of a silicon wafer resulting in power conversion efficiencies (PCE) of 6.4 % and 0.7 % respectively. Moreover, the device reached an above bandgap PCE of ~6 % as a thermo-photovoltaic cell recorded under a 1000 {\deg}C blackbody radiator.

Materials with optical gain in the infrared are of paramount importance for optical communications, medical diagnostics1 and silicon photonics2,3 . The current technology is based either on costly III-V semiconductors that are not monolithic to silicon CMOS technology or Er-doped fiber technology that does not make use of the full fiber transparency window. Colloidal quantum dots (CQD) offer a unique opportunity as an optical gain medium4 in view of their tunable bandgap, solution processability and CMOS compatibility. Their potential for narrower linewidths5 and the lower-than-bulk degeneracy6 has led to dramatic progress towards successful demonstration of optical gain4, stimulated emission7 and lasing8,9,10 in the visible part of spectrum utilizing CdSe-based CQDs. Infrared Pb-chalcogenide colloidal quantum dots however exhibit higher state degeneracy and as a result the demonstration of optical gain has imposed very high thresholds.11,12 Here we demonstrate room-temperature, infrared stimulated emission, tunable across the optical communication band, based on robust electronically doped PbS CQDs, that reach gain threshold at the single exciton regime, representing a four-fold reduction from the theoretical limit of an eight-fold degenerate system and two orders of magnitude lower than prior reports.

Conventional semiconductors such as silicon and InGaAs based photodetectors have encountered a bottleneck in modern electronics and photonics in terms of spectral coverage, low resolution, non-transparency, non-flexibility and CMOS-incompatibility. New emerging 2D materials such as graphene, TMDs and their hybrid systems thereof, however, can circumvent all these issues benefitting from mechanically flexibility, extraordinary electronic and optical properties, as well as wafer-scale production and integration. Heterojunction-based photodiodes based on 2D materials offer ultrafast and broadband response from visible to far infrared range. Phototransistors based on 2D hybrid systems combined with other material platforms such as quantum dots, perovskites, organic materials, or plasmonic nanostructures yield ultrasensitive and broadband light detection capabilities. Notably the facile integration of 2D-photodetectors on silicon photonics or CMOS platforms paves the way towards high performance, low-cost, broadband sensing and imaging modalities.

Two-terminal tandem cell architectures are believed to be an effective way to further improve the power conversion efficiency in solution processed photovoltaics. To design an efficient tandem solar cell, two key issues need to be considered. Firstly, subcells with well-matched currents and complementary absorption characteristics are a prerequisite for high efficiency. Secondly identifying the appropriate intermediate layer (IML) to connect the subcells is necessary to minimize the optical and electronic losses. PbS colloidal quantum dots (CQDs) are a notable choice for the subcells due to their low cost, solution processibility and remarkable wide range band gap tunability. Single layer Graphene (Gr) has been proposed to be a promising IML due to its high transparency and conductivity. Here, as a proof of concept, we demonstrate a solution processed two terminal PbS CQDs tandem solar cell employing chemical vapor deposited Gr as the IML. In doing so, we report a PbS CQD cell comprising subcells with bandgaps of 1.4 and 0.95 eV that delivers power conversion efficiency in excess of 7%, substantially higher than previously reported CQD tandem cells.

Defects play an important role in tailoring the optoelectronic properties of materials. Here we demonstrate that sulphur vacancies are able to engineer sub-band photoresponse into the short-wave infrared range due to formation of in-gap states in Bi2S3 single crystals supported by density functional (DF) calculations. Sulfurization and subsequent refill of the vacancies results in faster response but limits the spectral range to the near infrared as determined by the bandgap of Bi2S3. A facile chemical treatment is then explored to accelerate the speed of sulphur deficient (SD)-based detectors on the order of 10 ms without sacrificing its spectral coverage into the infrared, while holding a high D* close to 10^15 Jones in the visible-near infrared range and 10^12 Jones at 1.6 um. This work also provides new insights into the role sulphur vacancies play on the electronic structure and, as a result, into sub-bandgap photoresponse enabling ultrasensitive, fast and broadband photodetectors.

Lead halide perovskite nanocrystals (NCs) are currently emerging as one of the most interesting solution processed semiconductors since they possess high photoluminescence quantum yield (PLQY), and colour tunability through anion exchange reactions or quantum confinement. Here, we show efficient solar cells based on mixed halide (CsPbBrI2) NCs obtained via anion exchange reactions in ambient conditions. We performed anion exchange reactions in concentrated NC solutions with I-, thus inducing a PL red-shift up to 676 nm, and obtaining a high PLQY in film (65%). Solar cell devices operating in the wavelength range 350-660 nm were fabricated in air with two different deposition methods. The solar cells display a photo-conversion efficiency of 5.3% and open circuit voltage (Voc) up to 1.31V, among the highest reported for perovskite based solar cells with band gap below 2eV, clearly demonstrating the potential of this material.

Perovskite nanocrystal light-emitting diodes (LEDs) employing architecture comprising a ZnO nanoparticles electron-transport layer and a conjugated polymer hole-transport layer have been fabricated. The obtained LEDs demonstrate a maximum external-quantum-efficiency of 6.04%, luminance of 12998 Cd/m2 and stable electroluminescence at 519 nm. Importantly, such high efficiency and brightness have been achieved by employing solution processed transport layers, formamidinium lead bromide nanocrystals (CH(NH2)2PbBr3 NCs) synthesized at room-temperature and in air without the use of a Schlenk line, and a procedure based on atomic layer deposition to insolubilize the NC film. The obtained NCs show a photoluminescence quantum yield of 90% that is retained upon film fabrica-tion. The results show that perovskite NC LEDs can achieve high-performance without the use of transport layers deposited through evapo-ration in ultra-high-vacuum.

Integrated circuits based on CMOS (complementary metal-oxide semiconductors) are at the heart of the technological revolution of the past 40 years, as these have enabled compact and low cost micro-electronic circuits and imaging systems. However, the diversification of this platform into applications other than microcircuits and visible light cameras has been impeded by the difficulty to combine other semiconductors than silicon with CMOS. Here, we show for the first time the monolithic integration of a CMOS integrated circuit with graphene, operating as a high mobility phototransistor. We demonstrate a high-resolution image sensor and operate it as a digital camera that is sensitive to UV, visible and infrared light. The demonstrated graphene-CMOS integration is pivotal for incorporating 2d materials into the next generation microelectronics, sensor arrays, low-power integrated photonics and CMOS imaging systems covering visible, infrared and even terahertz frequencies.. The demonstrated graphene-CMOS integration is pivotal for incorporating 2d materials into the next generation microelectronics, sensor arrays, low-power integrated photonics and CMOS imaging systems covering visible, infrared and even terahertz frequencies., Comment: 11 pages, 4 figures

Organolead trihalide perovskites have drawn substantial interest for applications in photovoltaic and optoelectronic devices due to their low processing cost and remarkable physical properties. However, perovskite thin films still suffer from low carrier mobility, limiting their device performance and application potential. Here we report that embedding single-walled carbon nanotubes into halide perovskite films can significantly enhance the hole and electron mobilities to record-high values of 595.3 and 108.7 cm2 V-1 s-1, respectively. In the ambipolar phototransistors with such hybrid channels, photo-carriers generated in the light-absorbing perovskite matrix are transported by the carbon nanotubes, leading to ultrahigh detectivity of 6 * 1014 Jones and responsivity of 1 * 104 A W-1. We find that the perovskite precursor in dimethylformamide solution serve as an excellent stabilizer for the dispersion of carbon nanotubes, which potentially extend the scope of applications of perovskites in solution-processed functional composites. The unprecedented high performances underscore the perovskite/carbon nanotubes hybrids as an emerging class of functional materials in optoelectronic and other applications., Comment: We have to withdraw the submission of our paper (Identifier: 1512.03893), because we found that we have not yet studied this work completely, and we certainly should pay much time to do new experiments and add the new data in the revised paper

Optoelectronic functionalities of photodection and light harnessing rely on the band-to-band excitation of semiconductors, thus the spectral response of the devices is dictated and limited by their bandgap. A novel approach, free from this restriction, is to harvest the energetic electrons generated by the relaxation of a plasmonic resonance in the vicinity of a metal-semiconductor junction. In this configuration, the optoelectronic and spectral response of the detectors can be designed ad hoc just by tailoring the topology of metal structures, which has tremendous applications in solar energy harvesting and photodetection. Fully exploiting hot electron based optoelectronics yet requires a platform that combines their exotic spectral capabilities with large scale manufacturing and high performance. Herein we report the first implementation of a large area, low cost quasi 3D plasmonic crystal (PC) for hot electron photodetection, showcasing multiband selectivity in the VIS-NIR and unprecedented responsivity of 70 mA/W., Comment: 22 pages, 5 figures

Graphene has emerged as a novel platform for opto-electronic applications and photodetector, but the inefficient conversion from light to current has so far been an important roadblock. The main challenge has been to increase the light absorption efficiency and to provide a gain mechanism where multiple charge carriers are created from one incident photon. Here, we take advantage of the strong light absorption in quantum dots and the two-dimensionality and high mobility of graphene to merge these materials into a hybrid system for photodetection with extremely high sensitivity. Exploiting charge transfer between the two materials, we realize for the first time, graphene-based phototransistors that show ultrahigh gain of 10^8 and ten orders of magnitude larger responsivity compared to pristine graphene photodetectors. These hybrid graphene-quantum dot phototransistors exhibit gate-tunable sensitivity, spectral selectivity from the shortwave infrared to the visible, and can be integrated with current circuit technologies., Comment: 15 pages text, 3 figures