Bias Tunable Spectral Response of Nanocrystal Array in a Plasmonic Cavity.

Nanocrystals (NCs) have gained considerable attention for their broadly tunable absorption from the UV to the THz range. Nevertheless, their optical features suffer from a lack of tunability once integrated into optoelectronic devices. Here, we show that bias tunable aspectral response is obtained by coupling a HgTe NC array with a plasmonic resonator. Up to 15 meV blueshift can be achieved from a 3 µm absorbing wavelength structure under a 3 V bias voltage when the NC exciton is coupled with a mode of the resonator. We demonstrate that the blueshift arises from the interplay between hopping transport and inhomogeneous absorption due to the presence of the photonic structure. The observed tunable spectral response is qualitatively reproduced in simulation by introducing a bias-dependent diffusion length in the charge transport. This work expands the realm of existing NC-based devices and paves the way toward light modulators.

Electroluminescence from HgTe Nanocrystals and Its Use for Active Imaging.

Mercury telluride (HgTe) nanocrystals are among the most versatile infrared (IR) materials with the absorption of lowest energy optical absorption which can be tuned from the visible to the terahertz range. Therefore, they have been extensively considered as near IR emitters and as absorbers for low-cost IR detectors. However, the electroluminescence of HgTe remains poorly investigated despite its ability to go toward longer wavelengths compared to traditional lead sulfide (PbS). Here, we demonstrate a light-emitting diode (LED) based on an indium tin oxide (ITO)/zinc oxide (ZnO)/ZnO-HgTe/PbS/gold-stacked structure, where the emitting layer consists of a ZnO/HgTe bulk heterojunction which drives the charge balance in the system. This LED has low turn-on voltage, long lifetime, and high brightness. Finally, we conduct short wavelength infrared (SWIR) active imaging, where illumination is obtained from a HgTe NC-based LED, and demonstrate moisture detection.

Pushing Absorption of Perovskite Nanocrystals into the Infrared.

To date, defect-tolerance electronic structure of lead halide perovskite nanocrystals is limited to an optical feature in the visible range. Here, we demonstrate that IR sensitization of formamidinium lead iodine (FAPI) nanocrystal array can be obtained by its doping with PbS nanocrystals. In this hybrid array, absorption comes from the PbS nanocrystals while transport is driven by the perovskite which reduces the dark current compared to pristine PbS. In addition, we fabricate a field-effect transistor using a high capacitance ionic glass made of hybrid FAPI/PbS nanocrystal arrays. We show that the hybrid material has an n-type nature with an electron mobility of 2 × 10-3 cm2 V-1 s-1. However, the dark current reduction is mostly balanced by a loss of absorption. To overcome this limitation, we couple the FAPI/PbS hybrid to a guided mode resonator that can enhance the infrared light absorption.

Wave-Function Engineering in HgSe/HgTe Colloidal Heterostructures To Enhance Mid-infrared Photoconductive Properties.

The use of intraband transition is an interesting alternative path for the design of optically active complex colloidal materials in the mid-infrared range. However, so far, the performance obtained for photodetection based on intraband transition remains much smaller than the one relying on interband transition in narrow-band-gap materials operating at the same wavelength. New strategies have to be developed to make intraband materials more effective. Here, we propose growing a heterostructure of HgSe/HgTe as a means of achieving enhanced intraband-based photoconduction. We first tackle the synthetic challenge of growing a heterostructure on soft (Hg-based) material. The electronic spectrum of the grown heterostructure is then investigated using a combination of numerical simulation, infrared spectroscopy, transport measurement, and photoemission. We report a type-II band alignment with reduced doping compared with a core-only object and boosted hole conduction. Finally, we probe the photoconductive properties of the heterostructure while resonantly exciting the intraband transition by using a high-power-density quantum cascade laser. Compared to the previous generation of material based on core-only HgSe, the heterostructures have a lower dark current, stronger temperature dependence, faster photoresponse (with a time response below 50 µs), and detectivity increased by a factor of 30.

Charge Dynamics and Optolectronic Properties in HgTe Colloidal Quantum Wells.

We investigate the electronic and transport properties of HgTe 2D colloidal quantum wells. We demonstrate that the material can be made p- or n-type depending on the capping ligands. In addition to the control of majority carrier type, the surface chemistry also strongly affects the photoconductivity of the material. These transport measurements are correlated with the electronic structure determined by high resolution X-ray photoemission. We attribute the change of majority carriers to the strong hybridization of an n-doped HgS layer resulting from capping the HgTe nanoplatelets by S2- ions. We further investigate the gate and temperature dependence of the photoresponse and its dynamics. We show that the photocurrent rise and fall times can be tuned from 100 µs to 1 ms using the gate bias. Finally, we use time-resolved photoemission spectroscopy as a probe of the transport relaxation to determine if the observed dynamics are limited by a fundamental process such as trapping. These pump probe surface photovoltage measurements show an even faster relaxation in the 100-500 ns range, which suggests that the current performances are rather limited by geometrical factors.

Tuning the Surface Chirality of Adsorbed Gly-Pro Dipeptide/Cu(110) by Changing Its Chemical Form via Electrospray Deposition.

By changing the ultrahigh vacuum (UHV) deposition method, classical sublimation versus electrospray ionization, one can tune the chemistry of a chiral dipeptide molecule (Gly-Pro, GP), when adsorbed on a Cu(110) surface, from anionic to zwitterionic. This chemical shift will influence the adsorption mode of the dipeptide, either in a three-point fashion in the case of anionic GP molecules with a strong interaction among the copper surface, both O atoms of the carboxylate moiety, and the nitrogen atoms, or in the case of zwitterions GP, the adsorption mode relies on the sole interaction of one carboxylate oxygen atom. These different anchoring modes strongly modify the expression of surface 2D chirality and the supramolecular assemblies with two very distinct unit cells.

Surface Chirality of Gly-Pro Dipeptide Adsorbed on a Cu(110) Surface.

The adsorption of chiral Gly-Pro dipeptide on Cu(110) has been characterized by combining in situ polarization modulation infrared reflection absorption spectroscopy (PM-RAIRS) and X-ray photoelectron spectroscopy (XPS). The chemical state of the dipeptide, and its anchoring points and adsorption geometry, were determined at various coverage values. Gly-Pro molecules are present on Cu(110) in their anionic form (NH2 /COO(-)) and adsorb under a 3-point binding via both oxygen atoms of the carboxylate group and via the nitrogen atom of the amine group. Low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM) have shown the presence of an extended 2D chiral array, sustained via intermolecular H-bonds interactions. Furthermore, due to the particular shape of the molecule, only one homochiral domain is formed, creating thus a truly chiral surface.

The Electronic Impact of Light-Induced Degradation in CsPbBr3 Perovskite Nanocrystals at Gold Interfaces.

The understanding of the interfacial properties in perovskite devices under irradiation is crucial for their engineering. In this study we show how the electronic structure of the interface between CsPbBr3 perovskite nanocrystals (PNCs) and Au is affected by irradiation of X-rays, near-infrared (NIR), and ultraviolet (UV) light. The effects of X-ray and light exposure could be differentiated by employing low-dose X-ray photoelectron spectroscopy (XPS). Apart from the common degradation product of metallic lead (Pb0), a new intermediate component (Pbint) was identified in the Pb 4f XPS spectra after exposure to high intensity X-rays or UV light. The Pbint component is determined to be monolayer metallic Pb on-top of the Au substrate from underpotential deposition (UPD) of Pb induced from the breaking of the perovskite structure allowing for migration of Pb2+.

How far the chemistry of self-assembled monolayers on gold surfaces affects their work function?

Self-assembled monolayers composed of various long-chain aliphatic molecules and different tail functional groups have been synthesized on the Au(111) surface and characterized by Kelvin probe force microscopy and ultraviolet photoelectron spectroscopy. Carboxy, amino, thio and methyl terminal groups have been considered in the design of self-assembled monolayers with different aliphatic chain lengths (from C6 to C16). Work function measurements by Kelvin probe force microscopy have been carried out under a controlled and room atmosphere. Remarkably, a reduction of the relative humidity from 40% to 3% has induced a work function shift of up to 0.3 eV. As expected, the changes of the chain length of the aliphatic moiety and of the tail group have a significant impact on the tuning of the measured work function (3.90 eV for dodecanethiol versus 4.57 eV for mercaptohexadecylamine). Surprisingly, the change of the net dipole moment of the tail group (sign and amplitude) does not dominate the work function variations. In contrast, the change of the chain length and the possibility of the tail group to form a complex hydrogen bond network between molecules lead to significant modulations of the work function. In order to interpret these original findings, density functional theory models of equivalent self-assembled monolayers adsorbed on the Au(111) surface have been developed at an unprecedented level of description with large supercells including simultaneously 27 co-adsorbed molecules and weak van der Waals interactions between them. Such large systems have allowed the theoretical modeling of complex hydrogen bond networks between molecules when possible (carboxy tail group). The comparison between computed and measured work functions shows a striking agreement, thus allowing the disentanglement of the previously mentioned competing effects. This consistency between experiment and theory will help in designing the electronic properties of self-assembled monolayers in the context of molecular electronics and organic transistors.

Uncovering the Electronic State Interplay at Metal Oxide Electron Transport Layer/Nonfullerene Acceptor Interfaces in Stable Organic Photovoltaic Devices.

The implementation of sputter-deposited TiOx as an electron transport layer in nonfullerene acceptor-based organic photovoltaics has been shown to significantly increase the long-term stability of devices compared to conventional solution-processed ZnO due to a decreased photocatalytic activity of the sputtered TiOx. In this work, we utilize synchrotron-based photoemission and absorption spectroscopies to investigate the interface between the electron transport layer, TiOx prepared by magnetron sputtering, and the nonfullerene acceptor, ITIC, prepared in situ by spray deposition to study the electronic state interplay and defect states at this interface. This is used to unveil the mechanisms behind the decreased photocatalytic activity of the sputter-deposited TiOx and thus also the increased stability of the organic solar cell devices. The results have been compared to similar measurements on anatase TiOx since anatase TiOx is known to have a strong photocatalytic activity. We show that the deposition of ITIC on top of the sputter-deposited TiOx results in an oxidation of Ti3+ species in the TiOx and leads to the emergence of a new O 1s peak that can be attributed to the oxygen in ITIC. In addition, increasing the thickness of ITIC on TiOx leads to a shift in the O 1s and C 1s core levels toward higher binding energies, which is consistent with electron transfer at the interface. Resonant photoemission at the Ti L-edge shows that oxygen vacancies in sputtered TiOx lie mostly in the surface region, which contrasts the anatase TiOx where an equal distribution between surface and subsurface oxygen vacancies is observed. Furthermore, it is shown that the subsurface oxygen vacancies in sputtered TiOx are strongly reduced after ITIC deposition, which can reduce the photocatalytic activity of the oxide, while the oxygen vacancies in model anatase TiOx are not affected upon ITIC deposition. This difference can explain the inferior photocatalytic activity of the sputter-deposited TiOx and thus also the increased stability of devices with sputter-deposited TiOx used as an electron transport layer.

The Ground State of Epitaxial Germanene on Ag(111).

Two-dimensional (2D) honeycomb lattices beyond graphene, such as germanene, appear very promising due to their outstanding electronic properties, such as the quantum spin Hall effects. While there have been many claims of germanene monolayers up to now, no experimental evidence of a honeycomb structure has been provided up to now for these grown monolayers. Using scanning tunneling microscopy (STM), surface X-ray diffraction (SXRD), and density functional theory, we have elucidated the Ge-induced (109×109)R±24.5° reconstruction on Ag(111). We demonstrate that a powerful algorithm combining SXRD with STM allows us to solve a giant surface reconstruction with more than a hundred atoms per unit cell. Its extensive unit cell indeed consists of 98 2-fold or 3-fold coordinated Ge atoms, forming a periodic arrangement of pentagons, hexagons, and heptagons, with the inclusion of six dispersed Ag atoms. By analogy, we show that the (77×77)R±19.1° reconstruction obtained by segregation of Ge through an epitaxial Ag/Ge(111) film possesses a similar structure, i.e., Ge pentagons/hexagons/heptagons with a few Ag atoms. Such an organization is more stable than that of pure Ge monolayers and can be assigned to the ground state of epitaxial germanene.

Deciphering Electron Interplay at the Fullerene/Sputtered TiOx Interface: A Barrier-Free Electron Extraction for Organic Solar Cells.

Organic photovoltaics (OPVs) technology now offers power conversion efficiency (PCE) of over 18% and is one of the main emerging photovoltaic technologies. In such devices, titanium dioxide (TiOx) has been vastly used as an electron extraction layer, typically showing unwanted charge-extraction barriers and the need for light-soaking. In the present work, using advanced photoemission spectroscopies, we investigate the electronic interplay at the interface between low-temperature-sputtered TiOx and C70 acceptor fullerene molecules. We show that defect states in the band gap of TiOx are quenched by C70 while an interfacial state appears. This new interfacial state is expected to support the favorable energy band alignment observed, showing a perfect match of transport levels, and thus barrier-free extraction of charges, making low-temperature-sputtered TiOx a good candidate for the next generation of organic solar cells.

Impact of dimensionality and confinement on the electronic properties of mercury chalcogenide nanocrystals.

We demonstrate the growth of 2D nanoplatelets (NPLs) made of a HgTe/CdS heterostructure, with an optical absorption reaching the shortwave infrared range. The material is an interesting platform to investigate the effect of dimensionality (0D vs. 2D) and confinement on the electronic spectrum and carrier dynamics in colloidal materials. We bring consistent evidence for the p-type nature of this material from transport and photoemission measurements. The majority carrier dynamics probed using pump-probe photoemission is found to be mostly dependent on the presence of a confinement barrier at the surface rather than on the material dimensionality. The minority carrier, on the other hand, is strongly affected by the material shape showing a longer lived minority carrier in 2D NPLs compared to their 0D equivalent with a similar band gap. Finally, we test the potential of this material for photodetection in the short-wave infrared range (SWIR) and show that fast photoresponse and detectivity reaching 109 Jones at room temperature can be achieved.

Gas-induced selective re-orientation of Au-Cu nanoparticles on TiO2 (110).

Au-Cu bimetallic nanoparticles (NPs) grown on TiO2(110) have been followed in situ using grazing incidence X-ray diffraction and X-ray photoemission spectroscopy from their synthesis to their exposure to a CO/O2 mixture at low pressure (P < 10-5 mbar) and at different temperatures (300 K-470 K). As-prepared samples are composed of two types of alloyed NPs: randomly oriented and epitaxial NPs. Whereas the introduction of CO has no effect on the structure of the NPs, an O2 introduction triggers a Cu surface segregation phenomenon resulting in the formation of a Cu2O shell reducible by annealing the sample over 430 K. A selective re-orientation of the nanoparticles, induced by the exposure to a CO/O2 mixture, is observed where the randomly oriented NPs take advantage of the mobility induced by the Cu segregation to re-orient their Au-rich core relatively to the TiO2(110) substrate following specifically the orientation ((111)NPs//(110)TiO2) when others epitaxial relationships were observed on the as-prepared sample.

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.

Crystalline Molybdenum Oxide Thin-Films for Application as Interfacial Layers in Optoelectronic Devices.

The ability to control the interfacial properties in metal-oxide thin films through surface defect engineering is vital to fine-tune their optoelectronic properties and thus their integration in novel optoelectronic devices. This is exemplified in photovoltaic devices based on organic, inorganic or hybrid technologies, where precise control of the charge transport properties through the interfacial layer is highly important for improving device performance. In this work, we study the effects of in situ annealing in nearly stoichiometric MoOx (x ∼ 3.0) thin-films deposited by reactive sputtering. We report on a work function increase of almost 2 eV after inducing in situ crystallization of the films at 500 °C, resulting in the formation of a single crystalline α-MoO3 overlaid by substoichiometric and highly disordered nanoaggregates. The surface nanoaggregates possess various electronic properties, such as a work function ranging from 5.5 eV up to 6.2 eV, as determined from low-energy electron microscopy studies. The crystalline underlayer possesses a work function greater than 6.3 eV, up to 6.9 eV, characteristic of a very clean and nearly defect-free MoO3. By combining electronic spectroscopies together with structural characterizations, this work addresses a novel method for tuning, and correlating, the optoelectronic properties and microstructure of device-relevant MoOx layers.

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.