Photocurrent enhancement of HgTe quantum dot photodiodes by plasmonic gold nanorod structures

Nanophotonic structures energized short-wave infrared quantum dot photodetectors and their advancements in imaging and large-scale fabrication techniques

Short-wave infrared (SWIR) photodetectors (PDs) have a wide range of applications in the field of information and communication. Especially in recent years, with the increasing demand for consumer electronics, conventional semiconductor-based PDs alone are unable to cope with the ever-increasing market. Colloidal quantum dots (QDs) have attracted great interest due to their low fabrication cost, solution processability, and promising optoelectronic properties. In addition to advancements in synthesis methods and surface ligand engineering, the photoelectronic performance of QD-based SWIR PDs has been greatly improved due to developments in nanophotonic structural engineering, such as microcavities, localized and propagating surface plasmon resonant structures, and gratings for specific and high-performance detection application. The improvement in the performance of photoconductors, photodiodes, and phototransistors also enhances the performance of SWIR imaging sensors where they have been realized and demonstrated promising potential due to the direct integration of QD PDs with CMOS substrates. In addition, flexible manipulation of the QDs has been realized, thanks to their solution-processable capability. Therefore, a variety of large-scale production process methods have been examined including blade coating, flexible microcomb printing, ink-jet printing, spray deposition, etc. which can effectively reduce the cost and promote commercial application in consumer electronics. Finally, the current challenges and future development prospects of QD-based PDs are reviewed and could provide guidance for future design of the QDs PDs.

Very long wave infrared quantum dot photodetector up to 18 μm

Colloidal quantum dots (CQDs) are of interest for optoelectronic devices because of the possibility of high-throughput solution processing and the wide energy gap tunability from ultraviolet to infrared wavelengths. People may question about the upper limit on the CQD wavelength region. To date, although the CQD absorption already reaches terahertz, the practical photodetection wavelength is limited within mid-wave infrared. To figure out challenges on CQD photoresponse in longer wavelength, would reveal the ultimate property on these nanomaterials. What's more, it motivates interest in bottom-up infrared photodetection with less than 10% cost compared with epitaxial growth semiconductor bulk. In this work, developing a re-growth method and ionic doping modification, we demonstrate photodetection up to 18 μm wavelength on HgTe CQD. At liquid nitrogen temperature, the responsivity reaches 0.3 A/W and 0.13 A/W, with specific detectivity 6.6 × 108 Jones and 2.3 × 109 Jones for 18 μm and 10 μm CQD photoconductors, respectively. This work is a step toward answering the general question on the CQD photodetection wavelength limitation.

Uncooled High Detectivity Mid-Infrared Photoconductor Using HgTe Quantum Dots and Nanoantennas

Using a metal/insulator/metal (MIM) structure with a gold nanoantenna array made by electron beam lithography, the responsivity of a HgTe colloidal quantum dot film is enhanced in the mid-infrared. Simulations indicate that the spatially averaged peak spectral absorption of an 80 nm film is 60%, enhanced 23-fold compared to that of the same film on a bare sapphire substrate. The field intensity enhancement is focused near the antenna tips, being 20-fold 100 nm away, which represents only 1% of the total area and up to 1000-fold at the tips. The simulated polarized absorption spectra are in good agreement with the experiments, with a strong resonance around 4 μm. A responsivity of 0.6 A/W is obtained at a 1 V bias. Noise measurements separate the 1/f noise from the generation-recombination white noise and give a spatially averaged photoconductive gain of 0.3 at 1 V bias. The spatially averaged peak detectivity is improved 15-fold compared to the same film on a sapphire substrate without an MIM structure. The experimental peak detectivity reaches 9 × 109 Jones at 2650 cm-1 and 80 kHz, decreasing at lower frequencies. The MIM structure also enhances the spatially averaged peak photoluminescence of the CQD film by 16-fold, which is a potential Purcell enhancement. The good agreement between simulations and measurements confirms the viability of lithographically designed nanoantenna structures for vastly improving the performance of mid-IR colloidal quantum dot photoconductors. Further improvements will be possible by matching the optically enhanced and current collection areas.

Plasmonic Enhanced Nanocrystal Infrared Photodetectors

Low-dimensional nanomaterials are widely investigated in infrared photodetectors (PDs) due to their excellent optical and electrical properties. To further improve the PDs property like quantum efficiency, metallic microstructures are commonly used, which could squeeze light into sub-diffraction volumes for enhanced absorption through surface plasma exciton resonance effects. In recent years, plasmonic enhanced nanocrystal infrared PDs have shown excellent performance and attracted much research interest. In this paper, we summarize the progress in plasmonic enhanced nanocrystal infrared PDs based on different metallic structures. We also discuss challenges and prospects in this field.

Mercury chalcogenide colloidal quantum dots for infrared photodetection: from synthesis to device applications

Commercial infrared (IR) photodetectors based on epitaxial growth inorganic semiconductors, e.g. InGaAs and HgCdTe, suffer from high fabrication cost, poor compatibility with silicon integrated circuits, rigid substrates and bulky cooling systems, which leaves a large development window for the emerging solution-processable semiconductor-based photo-sensing devices. Among the solution-processable semiconductors, mercury (Hg) chalcogenide colloidal quantum dots (QDs) exhibit unique ultra-broad and tuneable photo-responses in the short-wave infrared to far-wave infrared range, and have demonstrated photo-sensing abilities comparable to the commercial products, especially with advances in high operation temperature. Here, we provide a focused review on photodetectors employing Hg chalcogenide colloidal QDs, with a comprehensive summary of the essential progress in the areas of synthesis methods of QDs, property control, device engineering, focus plane array integration, etc. Besides imaging demonstrations, a series of Hg chalcogenide QD photodetector based flexible, integrated, multi-functional applications are also summarized. This review shows prospects for the next-generation low-cost highly-sensitive and compact IR photodetectors based on solution-processable Hg chalcogenide colloidal QDs.

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.

Elucidating the Gain Mechanism in PbS Colloidal Quantum Dot Visible-Near-Infrared Photodiodes

The responsivities of colloidal quantum dot (CQD) photodiodes are not satisfactory (∼0.3 A W^-1) due to the lack of gain. Here, visible-near-infrared PbS CQD photodiodes with a peak responsivity of ∼1 A W^-1 and external quantum efficiencies larger than 100% are demonstrated. The gain is realized by electron tunneling injection through the Schottky junction (PbS-EDT/Au) with barrier height reduced to 0.27 eV, originating from the capture of photogenerated holes at the negatively charged acceptor traps generated in the oxidized hole-transport layer PbS-EDT. The resulting device exhibits a peak detectivity of ∼8 × 10¹¹ jones at -1 V. Additionally, the response speed (400 μs) is not sacrificed by the trap states because of the dominated faster electron drift motion in the fully depleted device. Our results provide an accurate elucidation of the gain mechanism in CQD photodiodes and promise them great potential in weak light detection.

Gold Nanorods: The Most Versatile Plasmonic Nanoparticles

Gold nanorods (NRs), pseudo-one-dimensional rod-shaped nanoparticles (NPs), have become one of the burgeoning materials in the recent years due to their anisotropic shape and adjustable plasmonic properties. With the continuous improvement in synthetic methods, a variety of materials have been attached around Au NRs to achieve unexpected or improved plasmonic properties and explore state-of-the-art technologies. In this review, we comprehensively summarize the latest progress on Au NRs, the most versatile anisotropic plasmonic NPs. We present a representative overview of the advances in the synthetic strategies and outline an extensive catalogue of Au-NR-based heterostructures with tailored architectures and special functionalities. The bottom-up assembly of Au NRs into preprogrammed metastructures is then discussed, as well as the design principles. We also provide a systematic elucidation of the different plasmonic properties associated with the Au-NR-based structures, followed by a discussion of the promising applications of Au NRs in various fields. We finally discuss the future research directions and challenges of Au NRs.

Molecular Main Group Metal Hydrides

This review serves to document advances in the synthesis, versatile bonding, and reactivity of molecular main group metal hydrides within Groups 1, 2, and 12-16. Particular attention will be given to the emerging use of said hydrides in the rapidly expanding field of Main Group element-mediated catalysis. While this review is comprehensive in nature, focus will be given to research appearing in the open literature since 2001.

Integration of Colloidal Quantum Dots with Photonic Structures for Optoelectronic and Optical Devices

Colloidal quantum dot (QD), a solution-processable nanoscale optoelectronic building block with well-controlled light absorption and emission properties, has emerged as a promising material system capable of interacting with various photonic structures. Integrated QD/photonic structures have been successfully realized in many optical and optoelectronic devices, enabling enhanced performance and/or new functionalities. In this review, the recent advances in this research area are summarized. In particular, the use of four typical photonic structures, namely, diffraction gratings, resonance cavities, plasmonic structures, and photonic crystals, in modulating the light absorption (e.g., for solar cells and photodetectors) or light emission (e.g., for color converters, lasers, and light emitting diodes) properties of QD-based devices is discussed. A brief overview of QD-based passive devices for on-chip photonic circuit integration is also presented to provide a holistic view on future opportunities for QD/photonic structure-integrated optoelectronic systems.

Correlating Structure and Detection Properties in HgTe Nanocrystal Films

HgTe nanocrystals (NCs) enable broadly tunable infrared absorption, now commonly used to design light sensors. This material tends to grow under multipodic shapes and does not present well-defined size distributions. Such point generates traps and reduces the particle packing, leading to a reduced mobility. It is thus highly desirable to comprehensively explore the effect of the shape on their performance. Here, we show, using a combination of electron tomography and tight binding simulations, that the charge dissociation is strong within HgTe NCs, but poorly shape dependent. Then, we design a dual-gate field-effect-transistor made of tripod HgTe NCs and use it to generate a planar p-n junction, offering more tunability than its vertical geometry counterpart. Interestingly, the performance of the tripods is higher than sphere ones, and this can be correlated with a stronger Te excess in the case of sphere shapes which is responsible for a higher hole trap density.

Infrared photoconduction at the diffusion length limit in HgTe nanocrystal arrays

Narrow band gap nanocrystals offer an interesting platform for alternative design of low-cost infrared sensors. It has been demonstrated that transport in HgTe nanocrystal arrays occurs between strongly-coupled islands of nanocrystals in which charges are partly delocalized. This, combined with the scaling of the noise with the active volume of the film, make case for device size reduction. Here, with two steps of optical lithography we design a nanotrench which effective channel length corresponds to 5–10 nanocrystals, matching the carrier diffusion length. We demonstrate responsivity as high as 1 kA W⁻¹, which is 10⁵ times higher than for conventional µm-scale channel length. In this work the associated specific detectivity exceeds 10¹² Jones for 2.5 µm peak detection under 1 V at 200 K and 1 kHz, while the time response is as short as 20 µs, making this performance the highest reported for HgTe NC-based extended short-wave infrared detection.

Infrared nanocrystals have become an enabling building block for the design of low-cost infrared sensors. Here, Chu et al. design a nanotrench device geometry at the diffusion length limit in HgTe nanocrystals and demonstrate the record high sensing performance operated in the short-wave infrared.

Integrated Photodetectors Based on Group IV and Colloidal Semiconductors: Current State of Affairs

With the aim to take advantage from the existing technologies in microelectronics, photodetectors should be realized with materials compatible with them ensuring, at the same time, good performance. Although great efforts are made to search for new materials that can enhance performance, photodetector (PD) based on them results often expensive and difficult to integrate with standard technologies for microelectronics. For this reason, the group IV semiconductors, which are currently the main materials for electronic and optoelectronic devices fabrication, are here reviewed for their applications in light sensing. Moreover, as new materials compatible with existing manufacturing technologies, PD based on colloidal semiconductor are revised. This work is particularly focused on developments in this area over the past 5-10 years, thus drawing a line for future research.

Advances of Sensitive Infrared Detectors with HgTe Colloidal Quantum Dots

The application of infrared detectors based on epitaxially grown semiconductors such as HgCdTe, InSb and InGaAs is limited by their high cost and difficulty in raising operating temperature. The development of infrared detectors depends on cheaper materials with high carrier mobility, tunable spectral response and compatibility with large-scale semiconductor processes. In recent years, the appearance of mercury telluride colloidal quantum dots (HgTe CQDs) provided a new choice for infrared detection and had attracted wide attention due to their excellent optical properties, solubility processability, mechanical flexibility and size-tunable absorption features. In this review, we summarized the recent progress of HgTe CQDs based infrared detectors, including synthesis, device physics, photodetection mechanism, multi-spectral imaging and focal plane array (FPA).

Microplasma-Enabled Graphene Quantum Dot-Wrapped Gold Nanoparticles with Synergistic Enhancement for Broad Band Photodetection

Plasmonic nanostructure/semiconductor nanohybrids offer many opportunities for emerging electronic and optoelectronic device applications because of their unique geometries in the nanometer scale and material properties. However, the development of a simple and scalable synthesis of plasmonic nanostructure/semiconductor nanohybrids is still lacking. Here, we report a direct synthesis of colloidal gold nanoparticle/graphene quantum dot (Au@GQD) nanohybrids under ambient conditions using microplasmas and their application as photoabsorbers for broad band photodetectors (PDs). Due to the unique AuNP core and graphene shell nanostructures in the synthesized Au@GQD nanohybrids, the plasmonic absorption of the AuNP core extends the usable spectral range of the photodetectors. It is demonstrated that the Au@GQD-based visible light photodetector simultaneously possesses an extraordinary photoresponsivity of ∼10³ A/W, ultrahigh detectivity of 10¹³ Jones, and fast response time in the millisecond scale (65 ms rise time and 53 ms fall time). We suggest that the synergistic effect can be attributed to the strong fluorescence quenching in Au@GQD coupled with the two-dimensional graphene layer in the device. This work provides knowledge of tailoring the optical absorption in GQDs with plasmonic AuNPs and the corresponding photophysics for broad band response in PD-related devices.

High Photon Absorptivity of Quantum Dot Infrared Photodetectors Achieved by the Surface Plasmon Effect of Metal Nanohole Array

With the increasing demand for small-scale photodetector devices, quantum dot-based infrared photodetectors have attracted more and more attention in the past decades. In this work, periodic metal nanohole array structures are introduced to the quantum dot infrared photodetectors to enhance the photon absorptivity performance via the surface plasmon enhancement effect in order to overcome the bottleneck of low optical absorption efficiency that exists in conventional photodetectors. The results demonstrate that the optimized metal nanohole array structures can greatly enhance the photon absorptivity up to 86.47% in the specific photodetectors, which is 1.89 times than that of conventional photodetectors without the metal array structures. The large enhancement of the absorptivity can be attributed to the local coupling surface plasmon effect caused by the metal nanohole array structures. It is believed that the study can provide certain theoretical guidance for high-performance nanoscale quantum dot-based infrared photodetectors.

Hybridizing Plasmonic Materials with 2D-Transition Metal Dichalcogenides toward Functional Applications

Recently, 2D transition metal dichalcogenides (TMDs) have become intriguing materials in the versatile field of photonics and optoelectronics because of their strong light-matter interaction that stems from the atomic layer thickness, broadband optical response, controllable optoelectronic properties, and high nonlinearity, as well as compatibility. Nevertheless, the low optical cross-section of 2D-TMDs inhibits the light-matter interaction, resulting in lower quantum yield. Therefore, hybridizing the 2D-TMDs with plasmonic nanomaterials has become one of the promising strategies to boost the optical absorption of thin 2D-TMDs. The appeal of plasmonics is based on their capability to localize and enhance the electromagnetic field and increase the optical path length of light by scattering and injecting hot electrons to TMDs. In this regard, recent achievements with respect to hybridization of the plasmonic effect in 2D-TMDs systems and its augmented optical and optoelectronic properties are reviewed. The phenomenon of plasmon-enhanced interaction in 2D-TMDs is briefly described and state-of-the-art hybrid device applications are comprehensively discussed. Finally, an outlook on future applications of these hybrid devices is provided.

Tailoring spontaneous infrared emission of HgTe quantum dots with laser-printed plasmonic arrays

Chemically synthesized near-infrared to mid-infrared (IR) colloidal quantum dots (QDs) offer a promising platform for the realization of devices including emitters, detectors, security, and sensor systems. However, at longer wavelengths, the quantum yield of such QDs decreases as the radiative emission rate drops following Fermi's golden rule, while non-radiative recombination channels compete with light emission. Control over the radiative and non-radiative channels of the IR-emitting QDs is crucially important to improve the performance of IR-range devices. Here, we demonstrate strong enhancement of the spontaneous emission rate of near- to mid-IR HgTe QDs coupled to periodically arranged plasmonic nanoantennas, in the form of nanobumps, produced on the surface of glass-supported Au films via ablation-free direct femtosecond laser printing. The enhancement is achieved by simultaneous radiative coupling of the emission that spectrally matches the first-order lattice resonance of the arrays, as well as more efficient photoluminescence excitation provided by coupling of the pump radiation to the local surface plasmon resonances of the isolated nanoantennas. Moreover, coupling of the HgTe QDs to the lattice plasmons reduces the influence of non-radiative decay losses mediated by the formation of polarons formed between QD surface-trapped carriers and the IR absorption bands of dodecanethiol used as a ligand on the QDs, allowing us to improve the shape of the emission spectrum through a reduction in the spectral dip related to this ligand coupling. Considering the ease of the chemical synthesis and processing of the HgTe QDs combined with the scalability of the direct laser fabrication of nanoantennas with tailored plasmonic responses, our results provide an important step towards the design of IR-range devices for various applications.

Enhanced performance of all solid-state quantum dot-sensitized solar cells via synchronous deposition of PbS and CdS quantum dots

The synchronous deposition of PbS and CdS affords band-structure tailoring and surface recombination passivation for efficient and stable solid-state QDSCs.

HgTe Nanocrystals for SWIR Detection and Their Integration up to the Focal Plane Array

Infrared applications remain too often a niche market due to their prohibitive cost. Nanocrystals offer an interesting alternative to reach cost disruption especially in the short-wave infrared (SWIR, λ < 1.7 μm) where material maturity is now high. Two families of materials are candidate for SWIR photoconduction: lead and mercury chalcogenides. Lead sulfide typically benefits from all the development made for a wider band gap such as the one made for solar cells, while HgTe takes advantage of the development relative to mid-wave infrared detectors. Here, we make a fair comparison of the two material detection properties in the SWIR and discuss the material stability. At such wavelengths, studies have been mostly focused on PbS rather than on HgTe, therefore we focus in the last part of the discussion on the effect of surface chemistry on the electronic spectrum of HgTe nanocrystals. We unveil that tuning the capping ligands is a viable strategy to adjust the material from the p-type to ambipolar. Finally, HgTe nanocrystals are integrated into multipixel devices to quantize spatial homogeneity and onto read-out circuits to obtain a fast and sensitive infrared laser beam profile.

Field-Effect Transistor and Photo-Transistor of Narrow-Band-Gap Nanocrystal Arrays Using Ionic Glasses

The gating of nanocrystal films is currently driven by two approaches: either the use of a dielectric such as SiO₂ or the use of electrolyte. SiO₂ allows fast bias sweeping over a broad range of temperatures but requires a large operating bias. Electrolytes, thanks to large capacitances, lead to the significant reduction of operating bias but are limited to slow and quasi-room-temperature operation. None of these operating conditions are optimal for narrow-band-gap nanocrystal-based phototransistors, for which the necessary large-capacitance gate has to be combined with low-temperature operation. Here, we explore the use of a LaF₃ ionic glass as a high-capacitance gating alternative. We demonstrate for the first time the use of such ionic glasses to gate thin films made of HgTe and PbS nanocrystals. This gating strategy allows operation in the 180 to 300 K range of temperatures with capacitance as high as 1 μF·cm^-2. We unveil the unique property of ionic glass gate to enable the unprecedented tunability of both magnitude and dynamics of the photocurrent thanks to high charge-doping capability within an operating temperature window relevant for infrared photodetection. We demonstrate that by carefully choosing the operating gate bias, the signal-to-noise ratio can be improved by a factor of 100 and the time response accelerated by a factor of 6. Moreover, the good transparency of LaF₃ substrate allows back-side illumination in the infrared range, which is highly valuable for the design of phototransistors.

A colloidal quantum dot infrared photodetector and its use for intraband detection

Wavefunction engineering using intraband transition is the most versatile strategy for the design of infrared devices. To date, this strategy is nevertheless limited to epitaxially grown semiconductors, which lead to prohibitive costs for many applications. Meanwhile, colloidal nanocrystals have gained a high level of maturity from a material perspective and now achieve a broad spectral tunability. Here, we demonstrate that the energy landscape of quantum well and quantum dot infrared photodetectors can be mimicked from a mixture of mercury selenide and mercury telluride nanocrystals. This metamaterial combines intraband absorption with enhanced transport properties (i.e. low dark current, fast time response and large thermal activation energy). We also integrate this material into a photodiode with the highest infrared detection performances reported for an intraband-based nanocrystal device. This work demonstrates that the concept of wavefunction engineering at the device scale can now be applied for the design of complex colloidal nanocrystal-based devices.

The field of wavefunction engineering using intraband transition to design infrared devices has been limited to epitaxially grown semiconductors. Here the authors demonstrate that a device with similar energy landscape can be obtained from a mixture of colloidal quantum dots made of HgTe and HgSe.

Towards Infrared Electronic Eyes: Flexible Colloidal Quantum Dot Photovoltaic Detectors Enhanced by Resonant Cavity

Electronic eye cameras are receiving increasing interest due to their unique advantages such as wide field of view, low aberrations, and simple imaging optics compared to conventional planar focal plane arrays. However, the spectral sensing ranges of most electronic eyes are confined to the visible, which is limited by the energy gaps of the sensing materials and by fabrication obstacles. Here, a potential route leading to infrared electronic eyes is demonstrated by exploring flexible colloidal quantum dot (CQD) photovoltaic detectors. Benefitting from their tunable optical response and the ease of fabrication as solution processable materials, mercury telluride (HgTe) CQD detectors with mechanical flexibility, wide spectral sensing range, fast response, and high detectivity are demonstrated. A strategy is provided to further enhance the light absorption in flexible detectors by integrating a Fabry-Perot resonant cavity. Integrated short-wave IR detectors on flexible substrates have peak D^* of 7.5 × 10¹⁰ Jones at 2.2 µm at room temperature and promise the development of infrared electronic eyes with high-resolution imaging capability. Finally, infrared images are captured with the flexible CQD detectors at varying bending conditions, showing a practical approach to sensitive infrared electronic eyes beyond the visible range.

Road Map for Nanocrystal Based Infrared Photodetectors

Infrared (IR) sensors based on epitaxially grown semiconductors face two main challenges which are their prohibitive cost and the difficulty to rise the operating temperature. The quest for alternative technologies which will tackle these two difficulties requires the development of new IR active materials. Over the past decade, significant progresses have been achieved. In this perspective, we summarize the current state of the art relative to nanocrystal based IR sensing and stress the main materials, devices and industrial challenges which will have to be addressed over the 5 next years.

Low-Dimensional Materials and State-of-the-Art Architectures for Infrared Photodetection

Infrared photodetectors are gaining remarkable interest due to their widespread civil and military applications. Low-dimensional materials such as quantum dots, nanowires, and two-dimensional nanolayers are extensively employed for detecting ultraviolet to infrared lights. Moreover, in conjunction with plasmonic nanostructures and plasmonic waveguides, they exhibit appealing performance for practical applications, including sub-wavelength photon confinement, high response time, and functionalities. In this review, we have discussed recent advances and challenges in the prospective infrared photodetectors fabricated by low-dimensional nanostructured materials. In general, this review systematically summarizes the state-of-the-art device architectures, major developments, and future trends in infrared photodetection.

Quantum Dot Solar Cells: Small Beginnings Have Large Impacts

From a niche field over 30 years ago, quantum dots (QDs) have developed into viable materials for many commercial optoelectronic devices. We discuss the advancements in Pb-based QD solar cells (QDSCs) from a viewpoint of the pathways an excited state can take when relaxing back to the ground state. Systematically understanding the fundamental processes occurring in QDs has led to improvements in solar cell efficiency from ~3% to over 13% in 8 years. We compile data from ~200 articles reporting functioning QDSCs to give an overview of the current limitations in the technology. We find that the open circuit voltage limits the device efficiency and propose some strategies for overcoming this limitation.

Thermal Imaging with Plasmon Resonance Enhanced HgTe Colloidal Quantum Dot Photovoltaic Devices

Thermal imaging in the midwave infrared plays an important role for numerous applications. The key functionality is imaging devices in the atmospheric window between 3 and 5 μm, where disturbance from fog, dust, and other atmospheric influence could be avoided. Here, we demonstrate sensitive thermal imaging with HgTe colloidal quantum dot (CQD) photovoltaic detectors by integrating the HgTe CQDs with plasmonic structures. The responsivity at 5 μm is enhanced 2- to 3-fold over a wide range of operating temperatures from 295 to 85 K. A detectivity of 4 × 10¹¹ Jones is achieved at cryogenic temperature. The noise equivalent temperature difference is 14 mK at an acquisition rate of 1 kHz for a 200 μm pixel. Thermal images are captured with a single-pixel scanning imaging system.

Fast and Sensitive Colloidal Quantum Dot Mid-Wave Infrared Photodetectors

Colloidal quantum dots (CQDs) with a band gap tunable in the mid-wave infrared (MWIR) region provide a cheap alternative to epitaxial commercial photodetectors such as HgCdTe (MCT) and InSb. Photoconductive HgTe CQD devices have demonstrated the potential of CQDs for MWIR photodetection but face limitations in speed and sensitivity. Recently, a proof-of-concept HgTe photovoltaic (PV) detector was realized, achieving background-limited infrared photodetection at cryogenic temperatures. Using a modified PV device architecture, we report up to 2 orders of magnitude improvement in the sensitivity of the HgTe CQD photodetectors. A solid-state cation exchange method was introduced during device fabrication to chemically modify the interface potential, leading to an order of magnitude improvement of external quantum efficiency at room temperature. At 230 K, the HgTe CQD photodetectors reported here achieve a sensitivity of 10⁹ Jones with a cutoff wavelength between 4 and 5 μm, which is comparable to that of commercial photodetectors. In addition to the chemical treatment, a thin-film interference structure was devised using an optical spacer to achieve near unity internal quantum efficiency upon reducing the operating temperature. The enhanced sensitivity of the HgTe CQD photodetectors reported here should motivate interest in a cheap, solution-processed MWIR photodetector for applications extending beyond research and military defense.

Color rendering based on a plasmon fullerene cavity

Fullerene in the plasmon fullerene cavity is utilized to propagate plasmon energy in order to break the confinement of the plasmonic coupling effect, which relies on the influential near-field optical region. It acts as a plasmonic inductor for coupling gold nano-islands to the gold film; the separation distances of the upper and lower layers are longer than conventional plasmonic cavities. This coupling effect causes the discrete and continuum states to cooperate together in a cavity and produces asymmetric curve lines in the spectra, producing a hybridized resonance. The effect brings about a bright and saturated displaying film with abundant visible colors. In addition, the reflection spectrum is nearly omnidirectional, shifting by only 5% even when the incident angle changes beyond ± 60°. These advantages allow plasmon fullerene cavities to be applied to reflectors, color filters, visible chromatic sensors, and large-area display.

Band Edge Dynamics and Multiexciton Generation in Narrow Band Gap HgTe Nanocrystals

Mercury chalcogenide nanocrystals and especially HgTe appear as an interesting platform for the design of low cost mid-infrared (mid-IR) detectors. Nevertheless, their electronic structure and transport properties remain poorly understood, and some critical aspects such as the carrier relaxation dynamics at the band edge have been pushed under the rug. Some of the previous reports on dynamics are setup-limited, and all of them have been obtained using photon energy far above the band edge. These observations raise two main questions: (i) what are the carrier dynamics at the band edge and (ii) should we expect some additional effect (multiexciton generation (MEG)) as such narrow band gap materials are excited far above the band edge? To answer these questions, we developed a high-bandwidth setup that allows us to understand and compare the carrier dynamics resonantly pumped at the band edge in the mid-IR and far above the band edge. We demonstrate that fast (>50 MHz) photoresponse can be obtained even in the mid-IR and that MEG is occurring in HgTe nanocrystal arrays with a threshold around 3 times the band edge energy. Furthermore, the photoresponse can be effectively tuned in magnitude and sign using a phototransistor configuration.

Superior Plasmonic Photodetectors Based on Au@MoS2 Core-Shell Heterostructures

Integrating plasmonic materials into semiconductor media provides a promising approach for applications such as photosensing and solar energy conversion. The resulting structures introduce enhanced light-matter interactions, additional charge trap states, and efficient charge-transfer pathways for light-harvesting devices, especially when an intimate interface is built between the plasmonic nanostructure and semiconductor. Herein, we report the development of plasmonic photodetectors using Au@MoS₂ heterostructures-an Au nanoparticle core that is encapsulated by a CVD-grown multilayer MoS₂ shell, which perfectly realizes the intimate and direct interfacing of Au and MoS₂. We explored their favorable applications in different types of photosensing devices. The first involves the development of a large-area interdigitated field-effect phototransistor, which shows a photoresponsivity ∼10 times higher than that of planar MoS₂ transistors. The other type of device geometry is a Si-supported Au@MoS₂ heterojunction gateless photodiode. We demonstrated its superior photoresponse and recovery ability, with a photoresponsivity as high as 22.3 A/W, which is beyond the most distinguished values of previously reported similar gateless photodetectors. The improvement of photosensing performance can be a combined result of multiple factors, including enhanced light absorption, creation of more trap states, and, possibly, the formation of interfacial charge-transfer transition, benefiting from the intimate connection of Au and MoS₂.

HgSe Self-Doped Nanocrystals as a Platform to Investigate the Effects of Vanishing Confinement

Self-doped colloidal quantum dots (CQDs) attract a strong interest for the design of a new generation of low-cost infrared (IR) optoelectronic devices because of their tunable intraband absorption feature in the mid-IR region. However, very little remains known about their electronic structure which combines confinement and an inverted band structure, complicating the design of optimized devices. We use a combination of IR spectroscopy and photoemission to determine the absolute energy levels of HgSe CQDs with various sizes and surface chemistries. We demonstrate that the filling of the CQD states ranges from 2 electrons per CQD at small sizes (<5 nm) to more than 18 electrons per CQD at large sizes (≈20 nm). HgSe CQDs are also an interesting platform to observe vanishing confinement in colloidal nanoparticles. We present lines of evidence for a semiconductor-to-metal transition at the CQD level, through temperature-dependent absorption and transport measurements. In contrast with bulk systems, the transition is the result of the vanishing confinement rather than the increase of the doping level.

Improved UV response of ZnO nanotubes by resonant coupling of anchored plasmonic silver nanoparticles

In this study, plasmonic silver (Ag) nanoparticle-(NP) anchored ZnO nanorods (NRs) and nanotube-(NT) based UV photodetectors are demonstrated. Here, Ag NPs are synthesized and anchored by using a room-temperature photochemical method by exposing the precursor solution in UV radiation. In order to achieve a stronger surface plasmon resonance (SPR) and minimum agglomeration, the photochemical method is optimized with a precursor concentration of 5 mmol, a UV intensity of 0.4 mW · cm^-2, and an exposure time of 30 min. An asymmetry around 380 nm in the absorption spectra of the NP solution indicates the presence of plasmonic resonance in that region. Upon anchoring the Ag NPs, ZnO NRs show enhanced band edge emission (380-400 nm) and the emission is further significantly increased in Ag NP-anchored ZnO NTs. The on/off ratio and photoresponse properties of the UV photodetectors are enhanced significantly after anchoring Ag NPs on the ZnO nanostructures. It is believed that the near-field coupling of SPR causes an optical enhancement of ZnO, whereas the bridging effect and hot-electron transfer to the conduction band of ZnO by plasmonic Ag NPs, anchored in close proximity, gives rise to a faster response of the photodetectors.

Formation of uniform PbS quantum dots by a spin-assisted successive precipitation and anion exchange reaction process using PbX2 (X = Br, I) and Na2S precursors

We devised a straightforward spin-assisted successive precipitation and anion exchange reaction (spin-SPAER) process in order to deposit relatively uniform PbS quantum dots (QDs) on mesoporous TiO₂ (mp-TiO₂).

Colloidal Synthesis and Applications of Plasmonic Metal Nanoparticles

Plasmonic metal nanoparticles attract intense research attention because of their fascinating surface plasmon resonance properties and their potential applications in diverse fields. Here, some of the recent research efforts on the synthesis and applications of plasmonic metal nanoparticles are highlighted. Starting from the colloidal synthesis of metal nanoparticles, various shaped silver and gold nanostructures are discussed. The applications of plasmonic nanoparticles in photocatalysis, surface-enhanced Raman spectroscopy (SERS), and devices are used as excellent examples showcasing the advantages of these nanoparticles. The report closes with a brief summary and discussion on the challenges and future direction in this research field.

Scalable Fabrication of Multiplexed Plasmonic Nanoparticle Structures Based on AFM Lithography

A controllable and scalable strategy is developed to fabricate multiplexed plasmonic nanoparticle structures by mechanical scratching with AFM lithography, which exhibit multiplex plasmonic properties and surface-enhanced Raman scattering responses. It offers an intuitive way to explore the plasmonic effects on the performance of an organic light-emitting diode device integrating with multiplexed plasmonic nanostructures.

Nanostructured Photodetectors: From Ultraviolet to Terahertz

Inspired by nanoscience and nanoengineering, numerous nanostructured materials developed by multidisciplinary approaches exhibit excellent photoelectronic properties ranging from ultraviolet to terahertz frequencies. As a new class of building block, nanoscale elements in terms of quantum dots, nanowires, and nanolayers can be used for fabricating photodetectors with high performance. Moreover, in conjunction with traditional photodetectors, they exhibit appealing performance for practical applications including high density of integration, high sensitivity, fast response, and multifunction. Therefore, with the perspective of photodetectors constructed by diverse low-dimensional nanostructured materials, recent advances in nanoscale photodetectors are discussed here; meanwhile, challenges and promising future directions in this research field are proposed.

PbSe quantum dot films with enhanced electron mobility employed in hybrid polymer/nanocrystal solar cells

We explore two strategies to improve the performance of hybrid solar cells fabricated using poly(3-hexylthiophene) (P3HT) and colloidal PbSe nanocrystals, which have reached a 1 sun power conversion efficiency of 2.9%.

A Resonance-Shifting Hybrid n-Type Layer for Boosting Near-Infrared Response in Highly Efficient Colloidal Quantum Dots Solar Cells

A new configuration of a plasmonic quantum dots solar structure is proposed. Gold-silver core-shell metal nanoparticles (Au@Ag NCs) are incorporated into the TiO2 layer (Au@Ag NCs-HL) of PbS-based solar cells. The TiO2 layer enables the Au@Ag NCs to have broad plasmonic responses and the external quantum efficiency and absorption of the plasmonic devices are significantly enhanced. The electrical performance of the solar cells is also improved.

Efficiency Enhancement of PbS Quantum Dot/ZnO Nanowire Bulk-Heterojunction Solar Cells by Plasmonic Silver Nanocubes

For improvement of solar cell performance, it is important to make efficient use of near-infrared light, which accounts for ∼40% of sunlight energy. Here we introduce plasmonic Ag nanocubes (NCs) to colloidal PbS quantum dot/ZnO nanowire (PbS QD/ZnO NW) bulk-heterojunction solar cells, which are characterized by high photocurrents, for further improvement in the photocurrent and power conversion efficiency (PCE) in the visible and near-infrared regions. The Ag NCs exhibit strong far field scattering and intense optical near field in the wavelength region where light absorption of PbS QDs is relatively weak. Photocurrents of the solar cells are enhanced by the Ag NCs particularly in the range 700-1200 nm because of plasmonic enhancement of light absorption and possible facilitation of exciton dissociation. As a result of the optimization of the position and amount of Ag NCs, the PCE of PbS QD/ZnO NW bulk-heterojunction solar cells is improved from 4.45% to 6.03% by 1.36 times.

Plasmon-enhanced light harvesting: applications in enhanced photocatalysis, photodynamic therapy and photovoltaics

This review article summarizes the recent progress on surface plasmon-enhanced light harvesting and its applications toward enhanced photocatalysis, photodynamic therapy, chemical transformations and photovoltaics.

Plasmon-enhanced light harvesting: applications in enhanced photocatalysis, photodynamic therapy and photovoltaics

This review article summarizes the recent progress on surface plasmon-enhanced light harvesting and its applications toward enhanced photocatalysis, photodynamic therapy, chemical transformations and photovoltaics.

Plasmon-enhanced light harvesting: applications in enhanced photocatalysis, photodynamic therapy and photovoltaics

This review article summarizes the recent progress on surface plasmon-enhanced light harvesting and its applications toward enhanced photocatalysis, photodynamic therapy, chemical transformations and photovoltaics.