Soluble mid-IR emitters can be used to address many issues in various research fields such as telecommunications, biosensing, gas sensing, unmanned vehicles, etc. Due to such demands for soluble mid-IR emitters, both organic and inorganic chemistry approaches have been rigorously performed. Inorganic material-based colloidal mid-IR emitters are known to be superior to organic molecules-based infrared emitter in the mid-IR range because of a lack of the intrinsic vibrational modes arising from their own structure. Colloidal quantum dots are promising materials to realize the efficient mid-IR emitter since the nanocrystal mainly comprises of inorganic constituents, and its phonon energy is a few hundred wavenumbers, which is one order of magnitude smaller than the vibrational energy of organic molecules. Furthermore, the vibrational mode of organic ligands of the nanocrystal can be suppressed by surface ligand engineering. Especially, self-doped quantum dots, in which excess carriers occupy the lowest quantized electronic state of the conduction band in steady-state, are excellent candidates for the soluble mid-IR emitter with respect to spectral line-shape and wavelength-tunability. This talk will focus on carrier recombination processes occurring in the self-doped quantum dots studied by mid-IR spectroscopy. Additionally, biologically compatible mid-IR emitting nanocrystals and several applications based on the intraband transition will be discussed as well.

There are many different synthetic methods to make infrared (IR) emitting quantum dots (QDs) such as the mercury chalcogenides. These materials have low bulk bandgaps and indeed in the case of HgTe even a zero-bandgap due to the inversion of the conduction and valence bands near the Γ point in the Brillouin zone. Shifting to the nanoparticle size scale, quantum confinement, aided by relatively large Bohr radii, can lead to very wide emission and band edge absorption tuning ranges and this has made these materials of great interest for near infrared (NIR) through to mid-infrared (MIR) photodetection applications. In this paper we describe a synthetic method based on aprotic solvent chemistry, which sits between aqueous QD synthesis and hot injection methods in other organic solvents and which is very suitable for larger scale syntheses.

In their own right, most IR fluorophores suffer from the same basic physics constraints of the interband emission process. The radiative transition rate rapidly decreases as the emission wavelength shifts further into the IR, to such an extent that non-radiative processes dominate hugely. In this range photoluminescence quantum yields (PLQYs) are invariably very much smaller those typically encountered in the visible these days, even where the underlying transition oscillator strengths might be comparable. In the short wavelength IR (SWIR) it is not uncommon to find PLQYs below 1% in the 2-3 µm range and <0.1% above 3 µm, etc. This severe decline in radiative rate can be offset to some degree by using hybrid QD materials and device structures incorporating nanophotonic or nanoplasmonic entities that can counter the slowing in the radiative rate across some spectral ranges, and we will describe some of our collaborations in these areas. In addition, we will describe other collaborative work on compact integrated optical devices that have been aided by advances in photonic and plasmonic waveguide engineering.

In this talk, I will describe recent work in our group developing an understanding of charge transport in thin films made of nanocrystal quantum dots, which will hopefully facilitate their development for use in applications requiring predictive control over both optical and electronic properties. We have studied electronic structure and the origin of electronic traps states experimentally and computationally [1,2]. We have also performed large-scale, ab initio simulations to gain insight into free carrier generation and charge hopping in strongly confined nanocrystal quantum dot-based semiconductors. We have used these findings to build a predictive model for charge transport in these systems, which we validate experimentally using time-of-flight measurements [2]. We have then applied this model to probe the impact of energetic and positional disorder in nanocrystal solids on mobility [3]. These findings, which focus on PbS quantum dots, help identify what to prioritize in terms of synthesis and fabrication in the development of nanocrystal-based devices. 

[1] “Dopants and Traps in Nanocrystal-Based Semiconductor Thin Films: Origins and Measurement of Electronic Midgap States.” S. Volk, N. Yazdani, O. Yarema, M. Yarema, and V. Wood, ACS Applied Electronic Materials 2 (2020).

[2] “Charge Transport in Semiconductors Assembled from Nanocrystal Quantum Dots” N. Yazdani, S. Andermatt, M. Yarema, V. Farto, M. H. Bani-Hashemian, S. Volk, W.M.M. Lin, O. Yarema, M. Luisier, and V. Wood. Nature Communications, 12 (2020).

[3] “Understanding the effect of positional disorder in nanocrystal quantum dot thin films on charge transport” Y. Xing et al. in preparation.

Infrared light detection enables diverse applications ranging from night vision to gas analysis. Emerging technologies such as low-cost cameras for self-driving cars require highly-sensitive, low-cost photodetectors with spectral sensitivities up to wavelengths of 10 µm. Colloidal PbS quantum dot (QD) sensitized graphene field-effect transistors may potentially lead to low cost photodetectors; however, the spectral sensitivity of these phototransistors has been limited to about 1.6 µm. Here, we present HgTe QD/graphene phototransistors with specific detectivities of 6×10⁸ Jones at a wavelength of 2.5 µm. At room temperature, the HgTe QD/graphene phototransistor does not show any photoresponse, and had to be cooled to cryogenic temperatures to exhibit detector functionality.

 The photoresponse of QD/graphene phototransistors likely depends on a Schottky-like potential barrier resulting from the band alignment of the two materials. It promotes the transfer of photo induced charge carriers from QDs to graphene. Interestingly, a charge carrier transfer at low temperatures is not frozen-out. We propose that the strength of the surface dipoles at the QD/ligand interface is temperature-dependent. This would effect the conduction- and valence band positions of the QD thin-film in respect to the vacuum level, which ultimately may alter the band alignment with graphene resulting in functional detectors. Besides extending of the spectral sensitivity, the HgTe QD/graphene photodetector exhibit high specific detectivities, in excess of 10⁸ Jones, even at kHz light modulation frequencies, making them suitable for fast video imaging.

Altogether, the simple device architecture, QD film patterning capabilities, and the extended spectral sensitivity make QD/graphene phototransistors potentially suitable for multi-color photodetector cameras.

Beyond their use as light sources for displays, nanocrystals also appear as promising candidates to design low cost infrared sensors. In such devices the carrier density is a key parameter driving the signal-to-noise ratio. The carrier density can be controlled thanks to the gate in a field effect transistor configuration. Most common gates are SiO₂ and electrolyte^[1] which are respectively limited by their low capacitance and (only) room temperature operation. Here, we explore (i) a high capacitance solid state gating from ionic glass (LaF₃) and (ii) a leakage free and low temperature operation gate based on quantum paraelectric SrTiO₃ substrate.^[2]

This high capacitance LaF₃ gate can be coupled to graphene electrodes enabling (i) IR transparency, (ii) tunable work function of the contact and (iii) propagation of the gate induced doping to the film thanks to the large quantum capacitance of graphene. We demonstrate the formation of a p-n junction improving charge extraction.^[3] The latter enable operating condition which simultaneously maximizes the response and reduces the dark current enhancing the detectivity by two orders of magnitude.

In a second part we demonstrate ferroelectric gating of a HgTe NC array with a SrTiO₃ (STO) gate. The divergence of the dielectric constant of STO at low temperature enables a high capacitance gate with a thick (500 µm) insulating substrate making it leakage and breakdown free. This gate is compatible with low temperature (<100 K) operation usually required for narrow band gap NC devices. This gate is then coupled to a plasmonic resonator to obtain a broadband absorption (1.5-3 µm) of 30% of the incident light. The combination of the STO gate with the plasmonic resonator shows a detectivity that can be as high as 10¹² Jones at 30 K.

The recent progress in nanocrystal-based solar cell development is very encouraging and makes this kind of solar cell a promising candidate for next generation photovoltaics. However, understanding the factors that hinder further improvement of the solar cell performance are not trivial due to the complex interlinked parameters of the devices. Despite newly gained understanding of the underlying chemical and physical parameters, the improvements to the nanocrystal-based solar cells have been mostly trial-and-error based. In this work, we use a simulation tool based on 1D drift-diffusion to run full device simulations of simple Schottky as well as more complex heterojunction devices. By only using input parameters, which were either derived from measurements or large-scale ab initio simulations, and no additional fitted parameters, we are able to closely match the characteristics of measured devices. We use these simulations as a tool to understand the influence of interfaces, charge carrier mobility and trap-assisted recombination.

Our study demonstrates the ability to simulate nanocrystal-based solar cells, independent of device architecture and without relying on fitting. We can use this to systematically simulate improvements to devices and guide further development of nanocrystal-based solar cells.

Optical antennas have become ubiquitous tools to enhance and tailor the spontaneous emission of quantum emitters [1]. The designs rules which have been established over the years are based on the understanding that optical antennas operate through the Purcell effect—the dependence of the fluorescence rate of point-source emitters to their surrounding environment. Here, we experimentally show that this paradigm fails for ensembles of interacting emitters (such as quantum dot solids) and that a different rule governs their interactions with optical antennas—the local Kirchhoff law recently introduced by Greffet et al [2].

We discuss the specificities of this regime by considering assemblies of PbS nanocrystals in direct contact with arrays of metal nanoparticles [3-5]. We illustrate the new opportunities of these findings by showing:

1) how to overcome the limiting trade-off between high electroluminescence (which occurs for PbS nanocrystals separated by nanometre-long ligands) and high carrier mobilities (which requires PbS nanocrystal capped with much shorter molecules) [3].

2) how to turn the isotropic and unpolarised luminescence of PbS nanocrystals into vector beams and other non-trivial light streams associated with fruitful developments in fluorescence imaging, optical trapping, high-speed telecommunications and quantum technologies.

Regarding low-cost infrared photodetection, colloidal quantum dots (CQDs), thanks to their large tunability, appear to be a new interesting building-block.[1] However, due to hopping transport, the diffusion length of the carriers in CQD film is short (typically few 10-100 nm). The absorption depth of the light is much larger (few µm). As a result, there is a trade-off between transport and optical absorption: usually thin films are used then, and only few % of the incident light is absorbed.[2] Light-matter coupling based on sub-wavelength resonators are used to tackle this issue.

Our device relies on guided mode resonators (GMR) and is made of a slab of CQDs (waveguide) onto a gold grating. The latter has two roles: it focuses the light into the nanocrystal film increasing its absorption, and it plays the role of electrode. The device is designed to induce a resonance and to achieve 100% of absorption at the targeted wavelength, for one of the polarization.[3] This particular design also enables photoconductive gain to occur. Both those effects generate a boost of responsivity of few orders of magnitude. This method is versatile and can be applied at different wavelengths (1.55 µm SWIR and 2.5 µm extended SWIR) with different materials (HgTe, PbS and a mix of perovskite/PbS).[3,4]

The introduction of nano-resonators not only generates a responsivity enhancement but enable spectral shaping oh this responsivity. First, it is possible to tune the position peak (of few hundreds of nm) by changing geometrical parameters such as the period of the grating.[3] Secondly, polarized devices can be made by inducing unmatched resonances in TE and TM polarizations. Then it is possible to achieve broadband absorption by introducing multi-resonances.

Devices based on small-gap mercury chalcogenide semiconductor nanocrystal inks are demonstrating increasingly high performance short and mid-wave infrared photodetection. These systems have the potential to eliminate cryogenic cooling needs and vastly reduce device costs compared to the current single-crystal devices. To achieve this goal and develop these materials as mid-infrared lasers, more detailed understandings of the exciton and carrier dynamics are required. Here we describe mid-infrared picosecond absorption and photoluminescence studies on HgTe and HgSe nanocrystal quantum dots. Comparisons between interband and intraband transitions in intrinsic and n-type systems reveal interesting new phenomena such as slow or absent Auger relaxations in n-type systems, phonon bottlenecks, and brighter emission from intraband versus interband transitions at the same wavelength. Yet, the measured lifetimes are limited by other nonradiative processes unique to small-gap materials. Investigations of the temperature- and surface-dependence of the luminescence in novel HgX/CdX core/shell structures help unravel such mechanisms. In parallel to these fundamental spectroscopic studies, we discuss progress towards harnessing the Auger suppression in n-HgSe to achieve mid-infrared lasing in this system. The deeper understanding of nonradiative relaxation in small-gap nanocrystals afforded by these experiments provides a path towards realizing high performance infrared photodetection near room temperature and mid-infrared lasing with nanocrystal quantum dots.

Fast nonradiative relaxation in narrow gap semiconductor quantum dots (QDs) is a major bottleneck for their application in mid-infrared detection, LEDs and lasing. Nonradiative decay in the mid-IR is widely attributed to relaxation to surface vibrations via a Forster-type near-field energy transfer [1][2][3]. Given the extremely low photoluminescence quantum yields (PLQY) of mid-IR QDs ~10^-4 [3] and the long range of Forster coupling, it is necessary to grow a giant shell (>~5nm in thickness) with type-I alignment to observe a significantly slowed relaxation. Though efforts have been made in growing shells of wide-gap material on HgTe and HgSe QDs [4][5][6], limited success has been observed in growing thick shells, largely due to the poor thermal stability of the cores.

We have recently developed the synthesis of giant HgSe/CdS QDs (>15 layers) to slow the nonradiative decay. The use of single-source precursors allows the growth of thick shells at a relatively low temperature, without independent nucleation or interface alloying. The synthetic strategy provides a uniform shell coverage, along with a Cd-rich surface that is necessary for observing mid-IR intraband PL. Preliminary results on PLQY and PL lifetime measurements show that the giant HgSe/CdS QDs exhibit a nonradiative decay rate 2 orders of magnitude slower than the cores. The PLQY at 5µm is ~1%, which is 10 times brighter than previous reports of mid-IR emitting QDs. These results shed light on the nonradiative relaxation processes in HgSe-based QDs, and pave the path for developing solution-processed mid-IR LEDs and lasers.

Image sensors fabricated using colloidal quantum dot photodetectors have the potential for combining the infrared sensitivity of traditional III-V and II-IV detector materials with the resolution and scalability of CMOS image sensors. SWIR Vision Systems has developed a family of CQD-based image sensors and cameras that has begun to realize this potential.  We present our uncooled extended shortwave infrared (eSWIR) 1920 x 1080-format cameras sensitive from 300 nm to 2100 nm wavelengths and describe the performance of these cameras using the EMVA1288 testing standard. We also present our standard SWIR cameras and show a variety of use cases for this technology in industrial machine vision applications.

Metal oxide nanocrystals doped with a few percent of aliovalent dopants become electronically conducting and support strong light-matter interactions in the infrared due to localized surface plasmon resonance (LSPR). In the prototypical material tin-doped indium oxide (Sn:In₂O₃), we explored the influence of the spatial distribution of Sn dopants on optical and electronic properties. Colloidal synthesis by slow addition of precursors allows precise control over the radial distribution of Sn, which dictates the electrostatic potential landscape and, in turn, the radial density of free electrons. Sequestering dopants in either the core or shell of the nanocrystals leads to multimodal optical spectra that respond strongly to changes in the dielectric environment. In thin films of nanocrystals, electronic conductivity is greatly enhanced in shell-doped nanocrystals wherein the barrier to nanocrystal-nanocrystal electron transfer is minimized.

We studied the coupling and impact of ligands on the QD optical and electrical properties.  We demonstrated that the bandedge energies can be shifted by over 2 eV for a QD absorber with 1 eV bandgap.  We also demonstrated that the addition of ligands causes the optical absorption of the QD/ligand complex to increase due to electronic coupling between the QD and ligands.  The coupling increases for smaller ligand optical gaps.  We utilize the enhanced absorbance of the QD/ligand to construct ligand adsorption isotherms. We model these isotherms with a 2-d square lattice model, which allows us to extract differences in trends of binding free energies and nearest neighbor coupling. As expected oleate binds more strongly than any of the functionalized cinnamates, but the binding preference is mitigated by dipole-dipole interactions for both large positive and negative dipoles. We explain this trend in binding free energy as a function of dipole moment via a collective electrostatic interaction with the lattice. For cinnamic acids with electron withdrawing molecular dipoles (negative dipoles), the isotherms show behavior associated with strong nearest neighbor association that causes the ligand exchange reaction to display a phase transition from all oleate coverage to all cinnamate coverage as more cinnamate is added, with a sharpness dictated by the ligand dipole moment: more negative dipole moments leads to sharper order-disorder phase transitions than those observed with positive dipole moments, as a function of ligand addition.  Using these observations, we prepared PbS QD with Janus-shell ligands.

We developed a facile method to prepare n and p-doped PbSe QDs via a post-synthetic cation exchange technique. Quantitative XRD analysis suggests a substitutional doping mechanism, with the lattice parameters decreasing upon either Ag⁺ or In³⁺ incorporation. A significant bleach of the first excitonic transition is observed, which is coupled with the appearance of a size-dependent intraband absorption in the NIR, indicating a successful introduction of electron/hole impurities dopants. We also observe a decrease of PLQY and a faster exciton decay with higher cation incorporation. Spectroelectrochemical measurements show a characteristic n-type behavior, which agrees with the substitutional doping mechanism of In³⁺ in PbSe. We proposed a model whereby the majority of the added cations remains at the QD surface and do not interact with the PbSe QD core states.  Small amounts of excess cations diffuse into the lattice and establish equilibrium between surface-bound and lattice-incorporated cation dopants.

Optoelectronic applications in the short wave infrared (SWIR) address a number of societal and technology challenges, including safety and security, food and process quality inspection, night vision, automotive safety, biological and environmental monitoring, just to name a few. However despite the huge impact and market potential currently available technologies based on costly III-V semiconductors impose commercialization challenges. That said, CQDs offer a unique opportunity to address this in view of their unique optoelectronic properties, low cost and CMOS compatibility. In this talk I will present recent lines of activities at ICFO towards high performance light emitters in the SWIR.

I will present record quantum efficiency and power conversion efficiency LEDs based on PbS CQDs enabled by engineering the energetic landscape and density of states of the active layers towards high PLQY, ultra-low trap-state density and improved charge balance [1]. Further, band engineering at the supra-nanocrystalline level has led to droop suppression and thereby to the achievement of record EQEs of 8% at high radiance conditions and exceptional stability of devices [2]. The talk will be concluded by recent results demonstrating for the first time tunable stimulated emission across the optical telecommunication band with high modal gain in excess of 100 cm^-1 and record low threshold in the single exciton regime, despite the 8-fold degeneracy of PbS CQDs, paving the way towards infrared CQD lasing [3].

References

[1] S. Pradhan et al., High-efficiency colloidal quantum dot infrared light-emitting diodes via engineering at the supra-nanocrystalline level. Nature Nanotechnol. 14, 72-79 (2019)

[2] S. Pradhan et al., Highly efficient, Bright and Stable SWIR CQD LEDs, Adv. Fun. Mat. Accepted 2020.

[5] S. Christodoulou et al. Single-Exciton Gain and Stimulated Emission Across the Infrared Optical Telecom Band from Robust Heavily-doped PbS Colloidal Quantum Dots. arXiv preprint arXiv:1908.03796.

Colloidal nanocrystals from PbS are most prominent materials for applications in optoelectronic devices operating in near to mid-infrared. Traditionally, they are synthesized by a hot injection method based on bis(trimethylsilyl) sulfide used as sulfur precursor. More recently Hendricks et al. [1] introduced a library of substituted thio-ureas as sulfur precursors of varying reactivity that can be used for size tuning. We combined these two approaches and selected a disubstituted thiourea compound as sulfur precursor and show the growth [2] and overgrowth of colloidal nanocrystals with this precursor. The advantage of this method is, that we can obtain controlled growth over infinitely large substrates, over all the nanocrystal size range towards bulk material (Figure 1). With the thio-urea precursor homo-epitaxial growth is achieved as well as heteroepitaxial growth, certainly related to the lattice matching between the substrate and the overgrowing PbS crystal. The obtained materials are of excellent quality and based on the nanocrystals grown from the thiourea precursor, photoconducting devices are demonstrated with band gap energies reaching those of the bulk material. The nanocrystals were also applied in photovoltaic devices providing record like behavior, especially for relatively long wavelengths [3]. Thus the growth of optoelectronic structures form thiourea precursors in organic solvents represents a versatile and promising rout for the low cost fabrication of infrared-optoelectronic devices.