In this talk, I will discuss our group’s recent experimental and computational work on understanding electronic and phononic structure nanocrystal thin films and charge transport in these thin films. Using electrochemical-based approaches, we show that we can quantify the electronic density of states and also examine charge-transfer processes across interfaces. Using inelastic x-ray scattering, we quantify the phononic denisty of states. We combine density functional theory calculations and kinetic Monte Carlo simulations to develop a first-principles model for charge transport in nanocrystals solids. We show that these simulations explain temperature-dependent time-of-flight measurements of electron and hole mobility performed on lead sulfide nanocrystal thin films. The combination of experimental and computational work highlights the importance of electron-phonon interactions in nanoscale transport and enables us to determine the relative impact of energetic and positional disorder on transport, providing us with design guidelines on parameters to consider when optimizing nanocrystal synthesis, nanocrystal surface treatments, and nanocrystal thin film preparation for different device applications.

For the fabrication of an integrated solar-to-chemical system, different components should be interfaced together in an orchestrated manner. Photoelectrodes need to absorb in the visible range, with a valence and a conduction band suited for the target reaction. Moreover, the presence of catalysts is required to manage the intrinsic energetic hurdle. Herein, we address the study of the major challenges, namely performance, stability, and interfaces to enable fabrication of integrated solar-to chemical systems. Novel scientific directions for the synthesis of functional interfaces and the development of new tools for their characterization will be addressed. Specifically, we will present a methodology for evaluating corrosion mechanisms and apply it to bismuth vanadate, a state-of-the-art photoanode. Analysis of changing morphology and composition under solar water splitting conditions reveals chemical instabilities that are not predicted from thermodynamic considerations of stable solid oxide phases, as represented by the Pourbaix diagram for the system. These findings are confirmed by in situ electrochemical atomic force microscopy (EC-AFM), which reveals that degradation under operating conditions occurs via dissolution of the film, starting at exposed facets of grains in polycrystalline thin films. In addition, we will present the correlation between morphological and functional heterogeneity in this material by photoconductive atomic force microscopy. We demonstrate that contrast in mapping electrical conductance depends on charge transport limitations, and on the contact at the sample/probe interface. We observe no additional recombination sites at grain boundaries, which indicates high defect tolerance in bismuth vanadate.

Insights into corrosion mechanisms and nanoscale heterogeneity aid development of protection strategies and provide information on how local functionality affects the macroscopic performance. 

We use tunable nanoscale defect cavities to create zero-dimensional exciton-polaritons [1]. Data on the strong coupling as well as mode engineering is shown as a function of cavity detuning. At ambient conditions, we observe non-equilibrium exciton-polariton condensation [2] with strong lateral confinement on the wavelength scale [3]. Threshold and line narrowing are analyzed as a function of excitation density. Both, in real and momentum space we observe the distinct signature of strong transversal confinement in the condensation. First order coherence properties are investigated by means of a Michelson interferometer.

As a building block towards extended lattices, we realize two coupled cavities by focused ion beam milling and thermal scanning probe lithography. These photonic molecules are investigated by means of atomic force microscopy and optical measurements, to compare both fabrication methods. Furthermore, we investigate different ways to improve the optical properties of the active material e.g., photo-degradation and inhomogeneity.

These are the initial steps towards studying quantum fluids in extended, arbitrary potential landscapes at ambient conditions.

References

[1] D. Urbonas et al., ACS Photonics 3 (9), 1542–1545 (2016).

[2] J. D. Plumhof et al., Nat. Mater. 13, 247 (2014).

[3] F. Scafirimuto et al., ACS Photonics 5 (1), 85–89 (2018).

Spectroscopy of single colloidal quantum dots (QDs), especially at cryogenic temperature, helps to understand the inherent properties of nanocrystals that are often hidden in ensemble level studies. This applies in particular to InP-based QDs, which attract increasing interest as Cd-free alternative nanocrystals yet were hardly investigated at the single QD level. Here, we discuss the photoluminescence properties of single InP/ZnSe QDs, both at room temperature and at cryogenic temperature. While ensemble level measurements feature a luminescent linewidth of around 50 nm, we find that emission spectrum of single InP/ZnSe QDs can have a linewidth as narrow as 14 nm (50 meV). Hence, the relatively broad emission line that characterizes ensembles of InP-based QDs is by no means an intrinsic material property. In addition, we found that InP/ZnSe QDs combine a nearly blinking free emission with a high purity single photon emission (g²(0)<0.03), also well beyond the saturation intensity.[1] Cryogenic single QD spectra, on the other hand, consist of zero-phonon lines that can be as narrow as 40 µeV. Polarization resolved spectra point to a linearly polarized spectral doublet from the bright exciton. At lower excitation intensities, jitter was negligible and spectra could be integrated for tens of seconds without erasing the doublet splitting of 1.2 meV. At higher excitation intensities, jitter becomes more severe and switching between emission from the exciton-doublet and the trion-singlet can be observed. This indicates that spectral jitter finds its origin in changes of the local electric field caused by the temporal trapping of one charge carrier. In summary, we find conclude that single InP QDs have emission characteristics similar to the extensively studied CdSe-based QDs. Moreover, the narrow emission lines, limited jitter and fluorescence intermittency of single InP/ZnSe QDs holds great promise to further explore these materials as a solution-processable single-photon emitter and improve the ensemble level characteristics of these materials.

Reference:
[1] Chandrasekaran, V.; Tessier, M. D.; Dupont, D.; Geiregat, P.; Hens, Z.; Brainis, E. Nano Letters 2017, 17 (10), 6104–6109.

Colloidal semiconductor quantum dots (CQDs) have been at the forefront of scientific research for more than two decades, based on their size tunable properties. Although implementation of CQDs in opto-electronic devices already occurs, various fundamental issues with a direct impact on technology are left as open questions. Recent years showed an interest in the investigation of magneto-optical properties of various CQDs with substantial importance for opto-electronic and spin-based devices.

Here we include the study of two different CQD platforms: (1) Synthesis and magneto-optical characterization of spectrally stable pure and diluted magnetic semiconductor CQDs from the II-VI semiconductor family (e.g., CdTe/CdSe and Mn@CdTe/CdSe); (2) Magneto-optical characterization of perovskite CQDs of the type APbBr3 (A - methylamonium or Cs+). Both systems show intriguing spin properties of special scientific and technological interests. The uniqueness of the spin properties and their novelty will be the focus during the talk.

CdTe/CdSe colloidal quantum dots with special composition, including soft boundary (alloying) at the core/shell interface or a giant core or a shell, possess quasi type-II configuration and show blinking-free behavior. The Mn+2 doping induces internal spin interactions between photo-generated species and the dopant spins, leading to giant magnetization or to an internal energy transfer into the dopant orbitals, and consequence emission from host-dopant hybrid- or from dopant atomistic-states. The current study developed a method to position the Mn ions selectively either at the core or at the shell. The magneto-optical measurements, including the use of optically detected magnetic resonance, exhibited resonance transitions related to the coupling of the Mn electron and the nuclear spins with the individual photo-generated carriers. The work was done in collaboration with the laboratory of Prof. Volkan Hilmi Demir from Bilkent and University and NTU.

The perovskites are minerals that have been studied extensively in the past. They are the focus of new interest in recent years, due to their exceptional performance in photovoltaic cells. Perovskites semiconductors possess high absorption coefficients as well as long-range transport properties. Currently, they are also prepared in the form of CQDs with very interesting properties including ferroelectricity, magnetism and exciton effects. The magneto-optical measurements of excitons in CsPbBr3 as individuals were investigated by monitoring the micro-photoluminescence spectra in the presence of an external magnetic field, while monitoring either the circular or linear polarization components.

Fully inorganic cesium lead halide (CsPbX₃, where X = Cl, Br, I) perovskite-type nanocrystals are colloidal quantum dots with bright, narrowband emission that is tunable by composition and size over a wide spectral range with room-temperature photoluminescence quantum yield of up to 90%^[1]. A pecularity of this material is the almost blinking-free emission at low temperature^[2] that originates from a triplet state with exceptionally high oscillator strength^[3].

We use densely packed arrays of up to several millions of these nanocrystals, known as superlattices, produced by means of solvent-drying-induced spontaneous assembly^[4]. Such ensemble of emitters behaves dramatically different from its individual constituents due to coherent interaction enabled by the strong light-matter interaction and the excellent monodispersity of the quantum dots. The collective coupling gives rise to an intriguing many-body quantum phenomenon, resulting in short, intense bursts of light: so-called superfluorescence. We observe a comprehensive set of key signatures like dynamically red-shifted emission with more than twenty-fold accelerated radiative decay, extension of the first-order coherence time by more than a factor of four, photon bunching, and delayed emission pulses with Burnham–Chiao ringing behaviour at high excitation density.

This is the first demonstration of optical collective behaviour and extended coherent states with nanocrystals, opening up new opportunities for high-brightness and multi-photon quantum light sources.

References:
[1] Protesescu et al., Nano Lett. 15, 3692–3696 (2015)
[2] Rainò et al., ACS Nano 10, 2485–2490 (2016)
[3] Becker et al., Nature, 553, 187-193 (2018)
[4] Rainò et al., arXiv 1804.01873 (2018)

Coherent quantum dynamics of excitons in semiconductor quantum dots (QDs) are of key interest, besides fundamental physics, for many applications ranging from quantum computing to advanced photonic devices. With the advances in colloidal synthesis, high-quality semiconductor nanocrystals can be fabricated at lower cost, and more flexibility in size, shapes and compositions. Despite its importance, measuring the exciton dephasing time in colloidal nanocrystals is technically challenging. Using three-beam transient resonant four-wave mixing technique in heterodyne detection not affected by spectral diffusion, we have measured the temperature-dependent ground-state exciton dephasing dynamics in CdSe/ZnS wurtzite QDs, CdSe/CdS spherical zincblende and rod-shape wurtzite QDs with variable core diameter and shell thickness / rod length, CdSe nanoplatelets, PbS QDs, InP/ZnSe QDs, and perovskite (CsPbBr₂Cl) QDs.

In these structures, the importance of phonon-assisted transitions, and the zero-phonon line (ZPL) dephasing by phonon-mediated spin-relaxation and radiative decay at low temperature are vastly varying. In PbS QDs, the peculiar band structure allows coupling with phonons at the zone edge (X-point), resulting in dominating phonon assisted transitions [1] even at low temperatures with sub-picosecond dephasing and a ZPL weight of less than 7%. In CdSe QDs [2,3] and dots in rods of similar size, the ZPL weight instead is above 50% since only zone-center phonons can couple, and the ZPL dephasing is limited by spin-relaxation into dark states on a 10-1000ps time scale, faster than the radiative lifetime of about 10ns. In InP QDs, a non-toxic alternative to CdSe, a similar behaviour is observed. In CdSe nano-platelets, the quasi two-dimensional confinement leads to quantum-well type behaviour, with a large exciton coherence area, such that the exciton dephasing is dominated by a fast radiative decay in the 1ps range [4]. In perovskite CsPbBr₂Cl QDs, the exciton ground state is bright, and a radiatively limited dephasing in the 10-100ps range is observed at low temperatures.  

[1] F. Masia et. al., Phys. Rev. B 83, 201309(R) (2011) DOI:10.1103/PhysRevB.83.201309
[2] F. Masia et al. Phys. Rev. Lett. 108, 087401 (2012) DOI:10.1103/PhysRevLett.108.087401
[3] N. Accanto et al., ACS Nano 6, 5227-5233 (2012) DOI:10.1021/nn300992a
[4] A. Naeem et al., Phys. Rev. B 91, 121302(R) (2015)  DOI 10.1103/PhysRevB.91.121302

Recent progress in colloidal chemistry have led to the synthesis of new types of two-dimensional (2D) lattices of PbX (X=Se, S, Te) nanocrystals [1,2]. In these lattices, each nanocrystal is epitaxially connected to its neighbors, the 2D materials are single crystalline in absence of disorder. Remarkably, square and honeycomb lattices can be synthesized using the same initial nanocrystals, allowing to investigate the role of the lattice geometry on the electronic properties. In addition, cation exchange processes can be used to transform PbX lattices into CdX ones. Theoretical studies accompanying these experiments have already demonstrated that these 2D lattices are characterized by very interesting band structures, including in some cases Dirac and non-trivial flat bands. In the present talk, theoretical works on the optical properties of 2D lattices will be presented. The effects of the epitaxial bonds between neighbor nanocrystals on the optical properties will be discussed. The theoretical calculations will be compared to recent experimental studies [3]. The differences between IV-VI (PbX) and II-VI (CdX) materials, between square and honeycomb lattices, will be reviewed.

[1] W. H. Evers, B. Goris, S. Bals, M. Casavola, J. de Graaf, R. van Roij, M. Dijkstra, and D. Vanmaekelbergh, Nano Lett. 13, 2317 (2013).

[2] M. P. Boneschanscher, W. H. Evers, J. J. Geuchies, T. Altantzis, B. Goris, F. T. Rabouw, S. A. P. van Rossum, H. S. J. van der Zant, L. D. A. Siebbeles, G. Van Tendeloo, I. Swart, J. Hilhorst, A. V. Petukhov, S. Bals, and D. Vanmaekelbergh, Science 344, 1377-1380 (2014).

[3] M. Alimoradi Jazi et al., Nano Lett. 17, 9 5238-5243 (2017).

Ever since the exfoliation of graphite into an atomically thin monolayer, known today as graphene [1], two dimensional (2D) materials have been of central interest for a variety of electronic applications. 2D materials belong to a large family of anisotropically active compounds which have strong, covalent bonds within a layer while in between layers there are only weak van der Waals interactions that can be overcome, obtaining molecularly thin sheets. Such a reduction of dimensionality has a profound impact on properties, known to vary strongly with respect to the number of atomic layers.

As graphene applications in electronics has thus far been hindered by its non-existent band gap, layered semiconductors are studied as potential candidates for future devices. Many Transition Metal Dichalcogenides (TMDs) including MoS₂, MoSe_2, WS₂, WSe₂ have been thoroughly investigated, however in order to meet rising demands new families of 2D semiconductors are studied. One family of such is the transition metal thiophosphates, denoted MPS_x, for x=3 or 4; for example bulk crystals of CrPS₄ - chromium thiophosphate – which has been examined in the past for applications in lithium batteries. Nowadays, this compound has once again gained scientific interest due to its optical anisotropic properties and the possibility to obtain and study its few- and monolayer systems [2].

In this work, bulk crystals of CrPS₄ were obtained by vapor transport synthesis (furnace method), followed by structure and composition confirmation via different techniques, for example Scanning Electron Microscopy with Energy-Dispersive X-ray Spectroscopy (SEM/EDX), Powder X-Ray Diffraction (PXRD) and Raman spectroscopy. Optical properties, such as band gap and optical transitions were investigated by Solid State UV-VIS Spectroscopy, PhotoAcoustic Spectroscopy (PAS) [3] and Modulated Spectroscopy (MS) [3]. Later, bulk crystals of chromium thiophosphate (CrPS₄) were exfoliated in liquid to obtain few layers systems and photoconductivity measurements were used to ascertain photoactive properties, both of re-stacked films and bulk crystals.

Acknowledgments:
This work was supported by the European Comission via the Marie-Sklodowska Curie action Phonsi (H2020-MSCA-ITN-642656)
This work was performed within the grant of the National Science Centre Poland (OPUS 11 no. 2016/21/B/ST3/00482).
S.J.Z. also acknowledges the support within the ETIUDA 5 grant from National Science Center Poland (no. 2017/24/T/ST3/00257).

References:
[1] Novoselov et.al., Science, vol 306, no. 5696 (2004)
[2] Lee et.al., ACS Nano vol. 11, no. 11 (2017)
[3] Zelewski & Kudrawiec, Scientific Reports, vol. 7 no. 15365 (2017)

The emission of fully inorganic cesium lead halide (CsPbX₃, where X = I,Br,Cl) perovskite-type nanocrystals is tunable over a wide energy range with ultrahigh photoluminescence quantum yields of up to 90%^[1] and exhibits narrow emission lines. Due to their facile solution processability and their potential for high-efficiency photovoltaics and light sources they have gained enormous interest.

Experiments on single perovskite quantum dots reveal a unique energetic level structure with a lowest bright triplet state^[2], thus enabling photon emission rates ~20 and ~1000 times higher compared to any other conventional semiconductor nanocrystals at room and cryogenic temperatures, respectively. We investigate the nature of this exceptionally fast photon emission by temperature dependent quantum yield measurements. Furthermore we discriminate it from composition dependent “A-type” blinking behaviour in intensity-decay time correlation measurements and demonstrate stable, narrowband emission, with suppressed blinking and small spectral diffusion^[3] for single CsPbBr₂Cl nanocrystals. By means of polarization dependent high resolution spectroscopy, the complex nature of the exciton fine structure splitting and charged exciton emission has been characterized.

Based on these measurements, supported by effective-mass models and group theory calculations, we conclude that the triplet exciton state is responsible for the extraordinary photon emission properties of lead halide perovskites. Our results can assist to identify other semiconductors that exhibit bright triplet excitons, with potential implications for improved optoelectronic devices.

References:

[1] Protesescu et al., Nano Lett. 15, 3692–3696 (2015)

[2] Becker et al., Nature, 553, 187-193 (2018)

[3] Rainò et al., ACS Nano 10, 2485–2490 (2016)

Two-dimensional (2D) materials have received much attention in the past years for a wide variety of photonic applications due to their pronounced excitonic features leading to unique properties in terms of light emission. However, only a few studies focus on the use of these materials for light amplification or net optical gain development and the ensuing high carrier density photo-physics. The beneficial nature of the strong excitonic effects on optical gain remain hence unquantified and , despite the large binding energies, it remains unclear what the involvement of is at the concomitant high carrier densities. Here, we use colloidal 2D CdSe nanoplatelets as a model system and show, using a quantitative and combinatory approach to ultrafast spectroscopy, that several distinct and carrier density-dependent optical gain regimes exist for these materials. At low density, optical gain is found to originate from excitonic molecules delivering large material gains up to 20.000 cm^-1, yet with an Auger limited lifetime of few hundred picoseconds. At increasing pair density, we observe a surprising transition to a combined regime of blue-shifted and disruptively large optical gain, combined with the typical exciton mediated gain. We show that this peculiar situation originates from a carrier cooling bottleneck at high density. Surprisingly, the insulating (multi-)exciton gas is found to co-exist with the conductive phase in a density regime nearly one order of magnitude beyond the expected Mott transition.  The ensuing exciton ground state absorption even counter-acts the development of net optical gain in certain spectral regions. Our results shed a new light on the disruptive photo-physics of high binding energy excitons in strongly excited 2D materials and pave the way for the development of more efficient broadband optical gain media and/or high density excitonic devices such as polariton lasers.

Colloidal lead halide perovskite nanocrystals (APbX_3, NCs, A=Cs⁺, FA⁺, FA=formamidinium; X=Cl, Br, I) emerge as promising materials for optoelectronic applications such as in television displays, light-emitting devices, and solar cells. The sponaneous and stimulated emission spectra of these NCs are readily tunable over the entire visible spectral region of 410-700 nm [1-2]. The photoluminescence of these NCs is characterized by narrow emission line-widths of 12-42 nm, wide color gamut covering up to 140% of the NTSC color standard, and high quantum yields of up to 100%. Cs_1-xFA_xPbI₃ and FAPbI₃ reach the near-infrared wavelengths of 800 nm [3].  A particularly difficult challenge lies in warranting the practical utility of such semiconductor NCs in the red and infrared spectral regions. A promising approach lies in the formation of multinary compositions such as Cs_xFA_1–xPb(Br_1–yI_y)₃ NCs. We show that droplet-based microfluidics can successfully guide the synthesis of such complex compositions [4]. We could fine-tune the photoluminescence maxima of such multinary NCs between 700 and 800 nm, minimize their emission linewidths (to below 40nm), and maximize their photoluminescence quantum efficiencies (up to 89%) and phase/chemical stabilities. Most importantly, we demonstrate the excellent transference of reaction parameters from microfluidics to a conventional flask-based environment, thereby enabling up-scaling and further implementation in optoelectronic devices. As an example, Cs_xFA_1–xPb(Br_1–yI_y)₃ NCs with an emission maximum at 735 nm were integrated into light-emitting diodes, exhibiting high external quantum efficiency of 5.9% and very narrow electroluminescence spectral bandwidth of 27 nm.

The processing and optoelectronic applications of perovskite NCs are, however, hampered by the loss of colloidal stability and structural integrity due to the facile desorption of surface capping molecules during isolation and purification. To address this issue, we have developed a new ligand capping strategy utilizing common and inexpensive long-chain zwitterionic molecules, resulting in much improved chemical durability [5].

             Perovskite NCs also readily form long-range ordered asssemblies known as superlattices. These assemblies exhibit accelerated coherent emission (superfluorescence), not observed before in semiconductor nanocrystal superlattices [6].

L. Protesescu et al. Nano Letters 2015, 15, 3692–3696

M. V. Kovalenko et al. Science 2017, 358, 745-750

L. Protesescu et al. ACS Nano 2017, 11, 3119–3134

I. Lignos et al. ACS Nano 2018, DOI: 10.1021/acsnano.8b01122

F. Krieg et al. ACS Energy Letters 2018, 3, 641–646.

Raino, M. Becker, M. Bodnarchuk et al. 2018, submitted

Controlling the flow of light at length scales below the diffraction limit is the quest of nanophotonics. The experimental mapping of electromagnetic modes could provide fundamental insights into nanophotonic systems and facilitate their rational design.

We propose NaYF₄ nanocrystals doped with Eu³⁺ ions as nanoscopic probes for electromagnetic modes at optical frequencies. Eu³⁺ ions feature several electronic transitions throughout the visible spectrum with electric- or magnetic-dipole character. This enables the nanoprobes to sense both the electric and magnetic components of optical modes at the probes’ location. We verify our concept by mapping the photonic modes close to a metallic mirror. Further, we adapt the method to study surface plasmon polaritons (SPPs), electromagnetic modes confined to metal–dielectric interfaces. By placing the nanoprobes locally at varying distances from a plasmonic reflector, we study plasmonic modes with a resolution well beyond the diffraction limit of light. Our results highlight how a well-designed plasmonic environment can be utilized to control the emission directionality of SPP sources and to selectively enhance electric-dipole-forbidden optical transitions of quantum emitters.

The formation of bound excitons via the spatial localization of charge carriers has long been a goal for luminescent semiconductors. Often, this has been accomplished through the formation of nanocrystals either through top-down or bottom-up methods. Alternatively, zero dimensional (0D) materials structurally impose carrier localization and result in the formation of highly localized Frenkel excitons. Recent works on perovskite-derived, hybrid organic-inorganic, 0D Sn(II) materials have demonstrated that high quantum yield emission from self-trapped excitons is possible when octahedra are isolated. As a new entry to the family of luminescent 0D materials, the fully-inorganic, perovskite-derived Cs₄SnBr₆ exhibits broad-band photoluminescence centred at 540 nm with a quantum yield of 15±5% at room temperature.[1] A compositional series, following the general formula Cs_4-xA_xSn(Br_1-yI_y)₆ (A = Rb, K; x ≤ 1,y ≤ 1), can be synthesized by solid-state methods. Furthermore, the emission of these materials ranges from 500 nm – 620 nm with the possibility to compositionally tune the Stokes shift and the self-trapped exciton emission bands. Finally, utilizing density functional theory calculations, the self-trapped exciton was ascribed to pseudo-Jahn-Teller distorted octahedra.

[1] Benin, B.M.,*; Dirin, D.N.*; Morad, V.; Woerle, M.; Yakunin, S.; Raino, G.; Nazarenko, O.; Fischer, M.; Infante, I.; Kovalenko, M.V. submitted.

The excellent optoelectronic properties of perovskite nanocrystals (NCs) such as enhanced photoluminescence quantum yield (PLQY) and tunable emission wavelength has stimulated a widespread investigation of this class of semiconductors.^[1] Very recently, it has been demonstrated that CsPbBr₃ NCs can reach near-unity PLQY in solution. Yet, retaining the PLQY in film is not trivial; since the NCs are not as well passivated as in solution and close packing can lead to energy-transfer to trap-states and increased self-absorption.

Here, a room temperature synthesis of perovskite NCs displaying near-unity PLQY in solid state films is presented.^[2]  Spin-coated films of the obtained CsPbBr₃ NCs show PLQY values approaching unity (>95%), thanks to the combination of a novel synthesis at room temperature, and a post synthetic treatment. The as-obtained NCs show PLQY = 80% in spin-coated films. Further enhancement of the PL efficiency is obtained via addition of PbBr₂. Following the synthesis, the obtained NCs were employed in optoelectronic devices. Efficient solar cells based on mixed-halide (CsPbBrI₂) NCs obtained via anion exchange reactions under ambient conditions were fabricated.^[3]. Solar cell devices operating in the wavelength range 350−660 nm were fabricated in air with two different deposition methods: single step (SP) and layer-by-layer (LbL). The solar cells display a photoconversion efficiency of 5.3%, independently of the active-layer fabrication method, and open circuit voltage (V_oc) up to 1.31 V, among the highest reported for perovskite-based solar cells with bandgap below 2 eV, clearly demonstrating the potential of this material.

The high potential of the material is further tested in light-emitting diodes (LEDs) employing an inverted structure comprising of ZnO nanoparticles as an electron-transport layer and a conjugated polymer hole-transport layer. The LEDs demonstrate an external-quantum-efficiency of 6.04%, with luminance of 12998 Cd/m² and low efficiency droop (around 10%). Importantly, such high efficiency was achieved by substituting Cesium with Formamidinium in line with our synthetic procedure. These results show the versatility of our synthetic protocol while the material quality is pointed out by the high performance of the optoelectronic devices. 

References

[1]       L. Protesescu, S. Yakunin, M. I. Bodnarchuk, F. Krieg, R. Caputo, C. H. Hendon, R. X. Yang, A. Walsh, M. V Kovalenko, Nano Lett. 2015, 15, 3692.

[2]       F. Di Stasio, S. Christodoulou, N. Huo, G. Konstantatos, Chem. Mater. 2017, 29, 7663.

[3]       S. Christodoulou, F. Di Stasio, S. Pradhan, A. Stavrinadis, G. Konstantatos, J. Phys. Chem. C 2018, 122, 7621-7626.

Semiconductor colloidal quantum dots (QDs) offer a realm of opportunities especially, in terms of tuning the band gap, manipulating the trap states in a very precise manner, controlling their electronic doping character and their ease of manufacturing and processing in devices. Moreover, when combined with 2D materials, the resultant structures can be used for a diverse range of applications offering photodetectors with exceptionally high responsivity and sensitivity.¹ During my presentation, I will talk about quantum dot-graphene hybrid photodetector devices for food inspection, spectrometers, imaging purposes, monitoring vital health parameters and night vision leveraging the unique opportunity of detecting photons from the UV up to the short-wave infrared in a single material platform.  In this photodetector, we make use of the large absorption cross section of QDs and high carrier mobility of graphene to demonstrate our complementary metal-oxide–semiconductors (CMOS) compatible infrared image sensors.² Unlike commercial detectors, the hybrid detector can be used simultaneously in UV, visible and infrared light conditions at room temperature with measured detectivity upto 10¹² Jones. Such detectors exhibit response times of 0.1–1 ms that make them suitable for video frame rate as Stijn Goossenswell as spectrometry applications.

[1] G. Konstantatos, et al., Nature Nanotechnol., 7 (June 2012)

[2] Goossens et al., Nat. Phot. 11 (June 2017)

Optical switches are key components for data processing on the basis of “silicon photonics”, in which they perform the crucial conversion of a photonic information from an optical fiber into an electric information for a silicon-based processing unit. The status of the switch is controlled by an external light source, emitting at a wavelength suitable to be absorbed by the conductive channel to photo-induce additional charge carriers and modulate the current output of the switch in close analogy to a classic transistor. This presentation details how hybrid superlattices of semiconducting nanocrystals and organic pi-systems with long-range order are applied as active layers in functional optical switches. The particular novelty for optical switching is an activated absorption mechanism, in which stimulation with one optical signal sensitizes the material towards an amplified recognition of a second optical stimulus. Several examples with different material combinations are presented and the importance of exciton formation as well as charge transfer across the inorganic-organic interface is discussed.

The rapid growth of colloidal quantum dot (CQD) opto-electronics establishes it as one of the most promising new generation technology related to photo-detection, photovoltaic (PV) and light emitting diodes (LEDs) [1]. Although CQD based LEDs enjoy tremendous success in the visible range [2], not so can be claimed about their infrared counterpart, which has a number of applications, including night vision, remote sensing, spectroscopy and biological imaging. The main reason for the poor performance in the infrared is the low photoluminescence quantum yield (PLQY) of ligand exchanged CQD solids. Several techniques like synthesis of core-shell structures [3], chemical passivation with perovskite matrix [4] etc. improve the efficiency but still they are far from their potential. We report here a passivation technique based on suprananocrystalline matrix engineering which leads to record external quantum efficiency (EQE).

Mixed ligand treatment on short-wave infrared (SWIR) based PbS quantum dot solar cell leads to record high PV performance [5]. Yet, the ligand treated SWIR QD solids showed a mere 2% PLQY due to a large amount of non-radiative recombination. The matrix presented here, passivates the non-radiative recombination channels and improve the PLQY over 60%. Careful optimization of the matrix and device architecture leads to record peak EQE of ~7.9% and peak power conversion efficiency of ~9.3% with emission at 1400 nm. Furthermore, PV devices based on this passivation technique showed record high open circuit voltage (0.69 V corresponding to 0.92 eV QD bandgap) confirming the effectiveness of the passivation.

References:

[1] Kagan, C. R., Lifshitz, E., Sargent, E. H., & Talapin, D. V. Buliding devices from colloidal quantum dots. Science 353, aac5523 (2016).

[2] Shirasaki, Y., Supran, G. J., Bawendi, M. G. & Bulović, V. Emergence of colloidal quantum-dot light-emitting technologies. Nat. Photonics 7, 13–23 (2013).

[3] Supran, G. J. et al. High-performance shortwave-infrared light-emitting devices using core–shell (PbS–CdS) colloidal quantum dots. Adv. Mater. 27, 1437–1442 (2015).

[4] Gong, X. et al. Highly efficient quantum dot near-infrared light-emitting diodes. Nat. Photonics 10, 253–257 (2016).

[5] Bi, Y. et al. Infrared solution‐processed quantum dot solar cells reaching external quantum efficiency of 80% at 1.35 µm and J_SC in excess of 34 mA cm^-2. Adv. Mater. 30, 1704928 (2018).

Pushing nanocrystals optical features toward the infrared range can be a material challenge. Two strategies can be explored. Either the use of narrow band gap materials for the design of narrow interband transitions, either the use of doped semiconductor presenting intraband transition in the mid infrared [1]. Here, I will focus on this second strategy while using mercury chalcogenides compounds as optically active material.

At first, I will present our strategy to explore the synthesis of HgSe [2] and HgTe [3] self-doped nanocrystals which has been used to tune absorption up to the THz range, typically from 3 µm to 60 µm for peak absorption and up to 200µm for cut-off wavelength.

Because the doping plays a key role in this material, I will discuss the origin of self-doping and how surface chemistry can be used to tune accurately its magnitude [4].

In the last part of the talk, I will present some results relative to the use of this intraband transition for mid IR photodetection. I will show how an heterostructure made of HgSe and HgTe can be used to uncouple the problem of absorption and charge transport [5]. This paves the way for the design of even more complex colloidal heterostructure on the model of quantum cascade system develloped for III-V material.

[1] Emergence of intraband transitions in colloidal nanocrystals, A. Jagtap, C. Livache, B. Martinez, J. Qu, Audrey Chu, C. Gréboval, N. Goubet, E. Lhuillier, Opt. Mater. Express 8(5), 1174-1183 (2018)

[2] Infrared photo-detection based on colloidal quantum-dot films with high mobility and optical absorption up to the THz, E. Lhuillier, M. Scarafagio, P. Hease, B. Nadal, H. Aubin, X. Z. Xu, N. Lequeux, G. Patriache, S. Ithurria, B. Dubertret, Nano Lett 16, 1282 (2016)

 [3] Terahertz HgTe nanocrystals: beyond confinement, N. Goubet, A. Jagtap, C. Livache, B. Martinez, H. Portales, X. Zhen Xu, R.P.S.M. Lobo, B. Dubertret, E. Lhuillier, J. Am. Chem. Soc. 140, 5053 (2018).

[4] Surface Control of Doping in self-doped Nanocrystals, A. Robin, C. Livache, S. Ithurria, E. Lacaze, B. Dubertret, E. Lhuillier, ACS Appl. Mat. Interface 8, 27122−27128 (2016).

 [5] Wavefunction engineering in HgSe/HgTe colloidal heterostructures to enhance mid infrared photoconductive properties, N. Goubet, C. Livache, B. Martinez, X. Z. Xu, S. Ithurria, S. Royer, H. Cruguel, G. Patriarche, A. Ouerghi, M. Silly, B. Dubertret, E. Lhuillier, Nano Lett 18 (2018)

State-of-the-art imagers use silicon circuitry for pixel readout, in combination with either a silicon absorber (visible region) or flip-chip bonded III-V materials (infrared region). Thin-film layers show a promise to replace these absorber layers, as the optical cross-talk can be reduced, and a heterogeneous integration on silicon chips enables further downscaling of the pixel size. In this talk we will discuss our progress and the challenges to incorporate colloidal quantum dot materials into a fab compatible process flow. Challenges lay in translating the chemical vocabulary and incorporating the silicon production fab restrictions into the device optimization. We will show initial results of incorporating infrared PbS based materials (950 nm and 1450 nm quantum peak absorption) into a silicon fab compatible stack. From a device aspect, the focus lays on high EQE values in combination with low noise (= dark current limited). From an integration aspect, the available contact materials are limited, and all layers need patterning using photo-lithography to enable processing on 200 or 300 mm wafers. These two aspects (device and integration) should be looked at jointly. A screening and optimization method will be presented, including a full opto-electronic characterization to determine the optimal stack constitution. This method enables a quick uptake of next generation quantum dot (or other thin-film) materials into wafer scale imager production.

Our constantly increasing society’s need for energy has triggered a pressing need for the development of new materials for renewable sources. Concerning materials for energy harvesting, the most promising approaches for high-performance and low-cost photovoltaics rely in inhomogeneous compounds, such as perovskites and polycrystalline thin films (e.g. CIGS and CdTe). Thus, resolving their electrical and optical behavior at the nanoscale is imperative to advance their understanding. In this talk, I will share our scientific findings to image and quantify the local voltage response of nano- and mesoscale inhomogeneities in perovskites [1,2], CIGS [3], and CdTe through a variant of KPFM and NSOM [4-6]. By submitting the samples to illumination and humidity treatments under controlled conditions, we map the dynamic physical behavior of MAPI and triple-cation perovskites.

References:

[1] J. M. Howard et al. J. Phys Chem Letters, in press (2018)

[2] J. L. Garrett et al. Nano Letters 17, 2554 (2017).

[3] E. M. Tennyson et al. ACS Energy Letters 1, 899 (2016).

[4] E. M. Tennyson et al. ACS Energy Letters, 2, 2761 (2017). Invited Review

[5] E. M. Tennyson et al. ACS Energy Letters 2, 1825 (2017). Invited Perspective

[6] E. M. Tennyson et al. Advanced Energy Materials 5, 1501142 (2015).