In this talk I will summarize recent efforts in my group around the development of metal chalcogenide CQD Photodetectors based on PbS and Ag2Te CQDs for SWIR imaging applications. In the second part of the talk I will dicuss recent results on the development of CQD based thermistors and bolometers thereof. 

In this talk I will summarize recent efforts in my group around the development of metal chalcogenide CQD Photodetectors based on PbS and Ag2Te CQDs for SWIR imaging applications. In the second part of the talk I will dicuss recent results on the development of CQD based thermistors and bolometers thereof. 

In this talk I will summarize recent efforts in my group around the development of metal chalcogenide CQD Photodetectors based on PbS and Ag2Te CQDs for SWIR imaging applications. In the second part of the talk I will dicuss recent results on the development of CQD based thermistors and bolometers thereof. 

Herein, we propose a new strategy that uses kinetic energy to facilitate charge multiplication in high-detectivity CQD-based IRPDs. Accelerated by strong electric fields within the CQD layer, electrons gain kinetic energy and subsequently transfer this excess energy to neighboring valence electrons through electron–electron collisions, thereby generating multiple charge carriers within the CQD layer. In CQD-based IRPDs with a 585 nm-thick CQD layer, we observed a positive temperature-dependent breakdown voltage behavior, suggesting the occurrence of kinetically pumped charge multiplication. Our multi-physics models support the hypothesis that the primary mechanism of current generation shifts from electron tunneling to impact ionization in CQD-based IRPDs when the CQD layer thickness exceeds 540 nm. Further evidence from density functional theory (DFT) calculations and operando photoluminescence (PL) measurements revealed that an increase in dDtoD, modulated by the ligand chain length, lowers the threshold energy required for charge multiplication. However, this increase in dDtoD simultaneously impedes electron hopping in CQD-based IRPDs. Consequently, we engineered a charge-multipliable CQD-based IRPD that exhibits a maximum gain of 85, a specific detectivity of 1.4×10¹⁴ Jones, and a bandwidth of 1.1×10⁶ Hz at a wavelength of 940 nm, outperforming the emerging solution-processable IRPDs [1].

The urgent demand for high-efficiency infrared photon conversion has underscored the potential of colloidal quantum dots (CQDs) as solution-processable absorbers for advanced optoelectronic applications, including photovoltaics (PVs), thermophotovoltaics (TPVs), and photodetectors (PDs). Nonetheless, the practical deployment of shortwave infrared (SWIR) CQDs has been hindered by persistent challenges in achieving monodisperse particle distributions and complete surface passivation. In this work, we present a ligand-pairing strategy specifically optimized for SWIR CQDs, enabling precise modulation of surface chemistry during both synthesis and post-synthetic ligand exchange. A thiourea-derived precursor with tunable reactivity facilitates the formation of CQDs with enhanced size uniformity and colloidal stability, particularly in the large-size regime. Subsequent surface reconstruction using bifunctional atomic ligands effectively displaces residual amines and passivates the (100) facets, thereby minimizing trap states. The resulting SWIR CQD-based optoelectronic devices demonstrate near-unity internal quantum efficiency and deliver high performance across diverse applications, including PV, TPV, and PD, establishing a new performance benchmark for solution-processed SWIR technologies.

Superlattices of lead chalcogenide colloidal quantum dots (QDs) hold promise to revolutionise the field of infrared optoelectronics due to their unique combination of optical and transport properties. The main challenge remains to form a homogeneous thin-film with long-range order, avoiding the formation of macroscopic cracks caused by the ligand exchange. This problem is particularly evident in 2D superlattices where the interactions driving the self-assembly are limited to a single plane yielding very defective films.

To overcome these issues, we introduce a novel approach where an external lateral pressure during the self-assembly and subsequent ligand exchange, forces the PbS QDs closer compensating crack formation due to volume shrinking. We achieve this with a Langmuir-Schaefer deposition where the degree of compression can be controlled from differential surface pressure measurements and tuned to obtain increasingly compact films.

The Langmuir-Schaefer superlattices present a hexagonal arrangement, long-range order, and partial collective alignment of the nanocrystals. After ligand exchange, the hexagonal arrangement is preserved, resulting in a highly compact film, contrarily to the square superlattices in uncompressed assembly. The highly-compressed superlattices are crack-free over several millimetres square which, to our knowledge, has never been demonstrated. The superlattice formation mechanism is elucidated by atomistic molecular dynamic simulations supporting the positive effect of the external pressure. Transport measurements in an ionic gel-gated field-effect transistor reveal that electron mobilities increase up to 37 cm²V^-1s^-1 with increasing surface pressure  thanks to enhanced compactness, higher number of nearest-neighbours, and degree of ordering.

External compression during fabrication results in crack-free, highly ordered superlattices. The samples’ electron mobilities are on par with the state-of-the-art 2D superlattices obtained with pressure-free methods but with unprecedently higher large-area coverage. These results demonstrate that QD superlattices with high charge mobility can be fabricated over millimetre-square areas. Further extension of this deposition method to 3D superlattices is highly relevant for application in optoelectronic devices like short wavelength infrared photodetectors.

Colloidal quantum dots (CQDs) hold immense promise for next-generation optoelectronic devices, yet their performance remains fundamentally limited by surface defects and poor charge transport within the CQD solid film. Traditional ligand exchange strategies often focus on passivating cationic Pb²⁺ sites, leaving the exposed anionic S^2- sites on the {100} facets of larger PbS CQDs insufficiently coordinated, particularly in polar processing solvents. This deficiency results in high trap-state density, poor colloidal stability, disordered packing, and ultimately, short carrier diffusion lengths (< 150 nm), severely restricting the efficiency and stability of devices.

To address this challenge, we introduce a novel strategy based on Organic Cation Lewis Acid (OCLA) ligands, specifically designed to comprehensively passivate the CQD surface and induce highly ordered film formation. By judiciously manipulating the pKa value of the organic cation (e.g., Benzyl Hydrazine Cation, BH⁺), the OCLA ligand acts as an effective Lewis acid, coordinating with the anionic S^2- sites through strong H⁺-S^2- interactions. Concurrently, the associated anion (Cl^-) passivates the Pb²⁺ sites, achieving dual-site passivation.

Crucially, the aromatic structure incorporated in the OCLA ligand facilitates rapid, self-limiting self-assembly. Utilizing potential pi-pi interactions, this strategy guides the PbS CQDs into a remarkably ordered 3D rhombohedral superlattice structure during a simple, single-step spin-coating process, enabling the fabrication of thick (> 400 nm) films.

Structural and electrical characterization confirms the superiority of the OCLA-CQD films. We observed a significant reduction in trap-state density (from 6.6 × 10¹⁵ cm^-3 to 4.7 × 10¹⁵ cm^-3) and a dramatic sixteen-fold enhancement in photoluminescence lifetime (from 1.2 ns to 18.9 ns). Most significantly, the comprehensive passivation and superior ordering boost the carrier diffusion length to a record-breaking 256 nm, establishing a new benchmark for PbS CQD solids.

This high-quality material platform translated into extraordinary device performance. Infrared photodetectors achieved a record-high specific detectivity of 1.01 × 10¹³ Jones at 1560 nm. Furthermore, infrared light-emitting diodes (LEDs) demonstrated a record radiance of 22.4 W Sr^-1 m^-2—double the previous state-of-the-art—along with excellent operational stability (T₉₀ > 200 h). This work introduces a versatile ligand engineering pathway, also validated on CdSe/ZnS CQDs, paving the way for stable, high-performance solution-processed optoelectronics.

3D superlattices made of colloidal quantum dots are a promising candidate for the next generation of optoelectronic devices as they are expected to exhibit a unique combination of tunable optical properties and coherent electrical transport through minibands. In my presentation I will show the fabrication of 3D superlattices of PbSe and PbS QDs with nanoscale-level controlled ordering over large areas [1, 2], and of outstanding transport properties. The measured electron mobilities for PbSe superlattices are the highest ever reported for a self-assembled solid of fully quantum-confined objects (electron mobility up to 278 cm² V⁻¹ s⁻¹). This ultimately demonstrates that optoelectronic metamaterials with highly tunable optical properties (in this case in the short-wavelength infrared spectral range) and charge mobilities approaching that of bulk semiconductor can be obtained. This finding paves the way toward a new generation of optoelectronic devices.

Thanks to their cost-effective and energy-efficient growth, colloidal nanocrystals have garnered significant interest for the design of infrared sensors. However, this technology faces a tradeoff: while thick films are desirable for absorbing light, charge collection is limited to short distances. To overcome this bottleneck, light resonators can be introduced. These resonators focus light onto a semiconductor slab with a thickness compatible with the material's diffusion length. Here, I will present various examples that have achieved near-unity absorption [1-3].

The benefits of light resonators, however, extend beyond increased absorption. I will also demonstrate how photonic structures can transform nanocrystals into a platform for active photonics [4-6], a field traditionally driven by phase-change materials and MEMS. Specifically, I will highlight the two key ingredients that enable bias-tunable spectra: inhomogeneous absorption and a bias-dependent diffusion coefficient. The latter is an inherent property of disordered solids, where transport occurs via hopping. Interestingly, while hopping is often perceived as a limitation due to low mobility values, it can also be an opportunity to enable novel optoelectronic functionalities [7].

By synthetically controlling colloidal semiconductor nanocrystal heterostructure we have made significant progress toward meeting the demands of an ideal quantum emitter – achieving on-demand (blinking- and bleaching-free), high-purity, room-temperature, spectrally tunable (blue-visible to telecommunications wavelengths) single-photon sources. More recently, we have further aimed to influence brightness (emission speed and directionality), chirality, polarization and photon indistinguishability using external environmental effects, a strategy generally outside the control of the synthetic chemist. Here, I will describe our efforts to combine advances in synthesis with integration into nanoantennas and/or plasmonic cavities for added functionality. In particular, I will discuss “giant” or thick-shell core/shell quantum dots (gQDs) based on Cd-, Pb- and Hg-chalcogenide compositions that enable access to robust sources of single photons from ~750 nm through the telecommunications C-band (~1550 nm) (e.g., doi.org/10.1021/acsnano.0c05907). I will highlight differences in intrinsic brightness between the systems, as well as the distinct blinking/bleaching behaviors associated with different giant-shell motifs that rely on either type II band alignment for carrier separation or alloying for band engineering. Beyond synthesis, I will show work with collaborators that demonstrates a strategy for shaping light into highly directional, radially polarized and/or fiber-coupled photons streams. Here, we employ a scanning-probe “direct-write” technique to precisely place single nanocrystals into nano/meso-structured surfaces, e.g., hybrid metal-dielectric bullseye antennas (doi.org/10.1063/5.0034863; doi.org/10.1021/acs.nanolett.3c03672). Similar hybrid, coupled gQD-antenna systems have even been used to implement a superior quantum key distribution (QKD) protocol (DOI: https://doi.org/10.1103/7fdd-m92n). Alternatively, in a separate collaboration, we have realized for the first time ultrafast (to 65 ps) and ultrabright (to ~12.6 MHz) room-temperature single-photon emission in the O and C telecommunications wavelength bands via coupling colloidal QDs to solution-processed plasmonic nanoparticle-on-mirror cavities (doi.org/10.1021/acsnano.4c18261). Taken together, progress in synthetic chemistry provides stable quantum emitters for integration into a range of photonic and plasmonic cavities/antennas to push the limits of room-temperature quantum light emission.  

Quantum dots (QDs) are a new generation of solution-processable optoelectronic materials. They can be synthesized in large batches through solution-phase methods and formulated into conductive inks compatible with scalable printing techniques such as doctor-blading and spray-coating for the deposition of large-area thin films. Owing to their strong quantum-confinement effect, the bandgap of typical PbS QDs can be tuned by size, enabling broadband absorption that spans the short-wave infrared (SWIR, 300–2500 nm). This makes them ideal candidates for low-cost SWIR photodetectors and imaging devices. Furthermore, QDs can serve as bottom-cell absorbers in tandem architectures with crystalline silicon or perovskites, compensating for the transmission losses of low-energy photons.

However, current QD photovoltaic and photodetection materials still face several technical bottlenecks, including high fabrication cost, high defect density, and poor colloidal stability. To address these challenges, we propose a new strategy of directly synthesizing conductive QD inks in polar solvents using inorganic ion ligands. This approach completely eliminates the ligand-exchange process, substantially simplifying device fabrication; meanwhile, it enables in-situ passivation during QD nucleation and growth, offering the potential to further enhance their optoelectronic properties.

This presentation will introduce our recent progress on the direct synthesis of infrared QD conductive inks and the fabrication of their optoelectronic conversion devices, aiming to provide new insights for the scalable production of QD optoelectronic materials and devices.

Colloidal quantum dots (CQDs) offer size-, shape-, and composition-tunable electronic and optical properties thus being used in a wide array of devices ranging from solar cells to light-emitting diodes. However, most of these applications are limited to the visible and near infrared (IR) region of the electromagnetic spectrum. In this talk, we will present our recent efforts to push the envelope of the applicability of these CQDs towards short-wave and extended short wave IR applications. These CQD-based photodetectors present a promising path toward fabricating IR imagers at significantly reduced cost and complexity compared to state-of-the-art epitaxial grown quantum-well IR devices. However, their progress has been constrained by reliance on restricted heavy metals and by manufacturing bottlenecks that limit quality control and throughput. Herein, we report advances in CQD chemistry, patterning, deposition and device design that collectively overcome some of these challenges. We develop a solution-phase RoHS compliant IR-sensitive Ag₂Se CQD ink that forms crack-free photodiodes with competitive sensitivity and microsecond response times. A direct CQD lithography platform achieves μm resolution and enables patterned devices, including a quad-band detector for multispectral NIR–SWIR imaging. To address process scalability, we introduce an automated, materials-efficient, layer-by-layer fabrication method that yields uniform, large-area films compatible with high-throughput manufacturing. Finally, we demonstrate a hybrid organic/inorganic electrode with low sheet resistance and broadband IR transparency, surpassing conventional oxide contacts and allowing for unique top illuminated IR devices. Together, these developments chart a scalable and environmentally responsible pathway toward next-generation IR photodetectors, illustrating how nanoscale design principles can reshape technologies once constrained by complex epitaxial growth.

Colloidal quantum dots (CQDs) are highly valued for their wavelength-tunability and transition selectivity. The origin of wavelength tunability is affected mainly by the quantum-confinement effect, which increases the bandgap energy. Also, the transition selectivity, the feature of an artificial atom, is related to the angular momentum of the states. As demand for environmentally friendly materials for sustainable technologies has steadily increased, it is necessary to explore new CQD materials. Here, I will present infrared silver chalcogenide (AgₓE, x > 2, E=S, Se, Te) CQDs with adjustable optical properties and lower toxicity. These silver chalcogenide CQDs can exhibit both intraband transition and interband transitions by varying stoichiometry, surface ligands, and conduction energy levels. We also found that precise control of composition, surface passivation strategies, and additional shell growth significantly enhances the eSWIR-MWIR photoluminescence quantum yield, increases structural stability, and improves photodetector performance, including responsivity and detectivity. Additionally, I will discuss the Ag2Se and Ag2Te CQDs synthesized at room temperature, which can reduce manufacturing costs.

Colloidal semiconductor quantum dots (CQDs) have widely applied in various fields, range from optoelectronic devices to biological imaging, owing to their unique physicochemical properties like size-tunable absorption and fluorescence emission, high photoluminescence (PL) brightness, facile surface functionalization, and well stability. Thereinto, silver chalcogenide CQDs have garnered a considerable interest because of their narrow bandgap and the absence of toxic heavy metal components. However, synthesizing silver chalcogenide CQDs with high absolute PL quantum yields (PLQYs) remains a great challenge. Here, we have developed gold atom doping and surface ligand passivation strategies for silver chalcogenide CQDs, which effectively suppress their nonradiative recombination of excitons, achieving the CQDs with absolute PLQYs exceeding 87%. In addition, we have further explored the CQDs in near-infrared optoelectronic devices. The CQD integrated into light-emitting diodes (LEDs) achieve an external quantum efficiency (EQE) of 15.8% with electroluminescence extending beyond 1000 nm, while their application in photodiodes yields a specific detectivity exceeding 10¹³ Jones.

Colloidal quantum dots (CQDs) are solution-processed semiconductor nanocrystals that exhibit size-tunable optical and electronic properties due to strong quantum confinement effects. Their ability to be synthesised with precise control over composition, structure, and surface chemistry has enabled the creation of high-performance optoelectronic devices. In the NIR range, they are being investigated for a wide variety of infrared applications, such as light detection and ranging (LiDAR), telecommunications, quantum technologies, photodetectors, cameras, biosensors, deep tissue imaging and therapy. In this study, a microelectronics-based method is employed for the selective deposition of nanomaterials with micrometric resolution. This method relies on UV-patternable composites incorporating Ag2S nanocrystals, where photolithography compatible with the electronics industry is used to create the patterns. The Ag2S nanocrystals were synthesized through a coprecipitation method, and their morphology and optical characteristics were subsequently analyzed. An optimal formulation for the lithographic with different types of masks to achieve a wide range of patterns. process was then developed. Finally, the micro-luminescence of the resulting patterns was evaluated using a confocal microscope coupled to an FLS1000 spectrofluorometer from Edinburgh Instruments.

Quantum dot (QD) solar cells prepared by low-temperature solution process have attracted great attention due to their low cost, ease of mass production, and excellent stability. In particular, the emergence of perovskite QD materials in recent years has raised the efficiency of single-junction solar cells to over 18%. Compared with organic and perovskite thin film materials, QDs do not require precise crystallographic and morphological control during film formation and exhibit superior stability, offering potential advantages in the fabrication of large-area printed devices, multilayer device structures, and flexible devices. This report will introduce the progress of our research group in two types of PbS and perovskite QD solar cells, including 1) the one-step preparation of PbS quantum dot ink, which significantly reduces the preparation cost of QD ink and simplifies device fabrication processes; 2) through the surface passivation of perovskite QDs, the charge transfer in the QD film is enhanced, resulting in highly efficient and stable photovoltaic devices.

Bismuth-based semiconductors have gained increasing attention as potential nontoxic alternatives to lead-halide perovskites [1]. Whilst most attention has been on bismuth-halide-based compounds, there is growing interest in broader families of materials, including chalcogenides, such as ABZ₂ materials (A = monovalent cation; B = Bi³⁺ or Sb³⁺; Z = chalcogen) [2]. This talk discusses our work on two such compounds: NaBiS₂ and AgBiS₂.

We show NaBiS₂ nanocrystals to have a steep absorption onset, with absorption coefficients reaching >10⁵ cm^-1 just above its pseudo-direct bandgap of 1.4 eV. Surprisingly, we also observe an ultrafast (picosecond-timescale) photoconductivity decay and long-lived charge-carrier population persisting for over one microsecond in NaBiS₂ nanocrystals. These unusual features arise due to cation disorder, with inhomogeneous disorder leading to localised S p states forming that contribute to the formation of small hole polarons [3]. Whilst this severely reduces charge-carrier mobilities, we find that it is still possible to extract charge-carriers in photovoltaic devices, with external quantum efficiencies >50% achievable [4].

The second half of this talk covers our recent work on AgBiS₂, which also has high absorption strength, such that films only 50 nm thick are required to achieve adequate light absorption. Given the small bandgap of 1.2 eV, we demonstrate the utility of this material in near-infrared photodetectors. We achieve high cut-off frequencies reaching 0.5 MHz at 940 nm wavelength, along with >1 MHz cut-off frequencies in the visible wavelength range. Through detailed characterisation, we reveal the electronic-ionic transport properties of this material, and how these properties can be controlled to achieve fast NIR photodetectors. Finally, we demonstrate the practical application of these devices for heart beat monitoring [5].

Overall, in this talk, the critical role of cation disorder in these ternary chalcogenide systems is revealed, especially how they influence optical absorption and charge-carrier kinetics.

Colloidal nanocrystals (NCs) are a Nobel-prize winning class of nanomaterials generating widespread attention due to their size-tunable properties and solution-processable nature, making them promising for opto-electronic applications such as LEDs, photo-detectors and lasers. Their potential has been well-established in the visible to near-infrared (NIR) range (400 - 1600 nm) with commercial impact realized via displays and NIR photo-detectors today¹.

Until recently, applications of NCs for the mid-infrared spectral range (MIR, 2 - 10 μm) focused on their use in photo-detection based on Pb- and Hg-chalcogenides. When it comes to MIR stimulated emission of light, the premise to build printable lasers, no reports thus far indicated that this would be feasible with NCs. In part, this originates from a lack in development of techniques capable of probing NC electronic structure and ultrafast carrier dynamics in the challenging MIR range. One such technique is infrared transient absorption spectroscopy, a pump-probe method that enables us to study light-matter interactions on a femto - to nanosecond time scales. As used extensively in the visible, such a toolbox would not only allow us to understand how the excited states of narrow MIR band gap NCs fill up and deplete via potentially fast recombination pathways, but would also enable the direct and quantitative observation of stimulated emission and net optical gain.

Here, we implemented quantitative ultrafast transient absorption spectroscopy in the MIR spectrum to study the ultrafast carrier dynamics of multi-excitons in chalcogenide PbS and HgTe NCs with band gaps in the MIR range (2000 – 4000 nm). First, we demonstrated that PbS nanocrystals, when pushed into the bulk regime with sizes up to 28 nm, exhibited stimulated emission up to 2.9 μm with gain lifetimes up to 400 ps and intrinsic gain coefficients reaching up to 10³ cm^-1. Our findings indicate that PbS NCs are potentially suitable as gain media for MIR lasers. We also showed a first exploration of HgTe MIR NCs, demonstrating their potential as well. Our results show that chalcogenide NCs can impact the MIR spectrum through optical gain, as the latter can outcompete non-radiative energy transfer pathways limiting their spontaneous emission quantum yield.