Eric Prestat

Quality Heterostructures from Two-Dimensional Crystals Unstable in Air by Their Assembly in Inert Atmosphere

Many layered materials can be cleaved down to individual atomic planes, similar to graphene, but only a small minority of them are stable under ambient conditions. The rest react and decompose in air, which has severely hindered their investigation and potential applications. Here we introduce a remedial approach based on cleavage, transfer, alignment, and encapsulation of air-sensitive crystals, all inside a controlled inert atmosphere. To illustrate the technology, we choose two archetypal two-dimensional crystals that are of intense scientific interest but are unstable in air: black phosphorus and niobium diselenide. Our field-effect devices made from their monolayers are conductive and fully stable under ambient conditions, which is in contrast to the counterparts processed in air. NbSe2 remains superconducting down to the monolayer thickness. Starting with a trilayer, phosphorene devices reach sufficiently high mobilities to exhibit Landau quantization. The approach offers a venue to significantly expand the range of experimentally accessible two-dimensional crystals and their heterostructures.

Crossover from spin accumulation into interface states to spin injection in the germanium conduction band.

Electrical spin injection into semiconductors paves the way for exploring new phenomena in the area of spin physics and new generations of spintronic devices. However the exact role of interface states in the spin injection mechanism from a magnetic tunnel junction into a semiconductor is still under debate. In this Letter, we demonstrate a clear transition from spin accumulation into interface states to spin injection in the conduction band of n-Ge. We observe spin signal amplification at low temperature due to spin accumulation into interface states followed by a clear transition towards spin injection in the conduction band from 200 K up to room temperature. In this regime, the spin signal is reduced to a value compatible with the spin diffusion model. More interestingly, the observation in this regime of inverse spin Hall effect in germanium generated by spin pumping and the modulation of the spin signal by a gate voltage clearly demonstrate spin accumulation in the germanium conduction band.

Van der Waals pressure and its effect on trapped interlayer molecules.

Van der Waals assembly of two-dimensional crystals continue attract intense interest due to the prospect of designing novel materials with on-demand properties. One of the unique features of this technology is the possibility of trapping molecules between two-dimensional crystals. The trapped molecules are predicted to experience pressures as high as 1 GPa. Here we report measurements of this interfacial pressure by capturing pressure-sensitive molecules and studying their structural and conformational changes. Pressures of 1.2±0.3 GPa are found using Raman spectrometry for molecular layers of 1-nm in thickness. We further show that this pressure can induce chemical reactions, and several trapped salts are found to react with water at room temperature, leading to two-dimensional crystals of the corresponding oxides. This pressure and its effect should be taken into account in studies of van der Waals heterostructures and can also be exploited to modify materials confined at the atomic interfaces.

Electrical and thermal spin accumulation in germanium

In this letter, we first show electrical spin injection in the germanium conduction band at room temperature and modulate the spin signal by applying a gate voltage to the channel. The corresponding signal modulation agrees well with the predictions of spin diffusion models. Then, by setting a temperature gradient between germanium and the ferromagnet, we create a thermal spin accumulation in germanium without any charge current. We show that temperature gradients yield larger spin accumulations than electrical spin injection but, due to competing microscopic effects, the thermal spin accumulation remains surprisingly unchanged under the application of a gate voltage.

Self-catalytic membrane photo-reactor made of carbon nitride nanosheets†

Solar-driven photo-oxidation is a very attractive and efficient technique for chemical conversion of organic dyes in water into non-hazardous compounds, but it requires a catalyst in order to overcome the barrier of oxidation degradation. In this study we use a membrane photo-reactor (MPR) made of nanosheets of graphitic carbon nitride (g-C3N4), assembled by vacuum filtration. The membrane was characterized by X-Ray Diffraction (XRD), Nuclear Magnetic Resonance (NMR), electron microscopy and IR spectroscopy. Photo-degradation studies show that the membranes are very efficient in the degradation of Sudan orange G, rhodamine 110 and methylene blue. As the catalyst is a porous laminate, the reactant can flow through the pores of the membrane. Because the space between g-C3N4 nanosheets is comparable to the size of the dyes, the probability of the reactants to be close to the catalyst is enhanced, making the reaction very efficient.

A simple electrochemical route to metallic phase trilayer MoS2: evaluation as electrocatalysts and supercapacitors

The development of a simple, scalable and reproducible technique for the synthesis of two-dimensional MoS2 nanosheets is of paramount importance in the field of catalysis and energy storage devices. Current routes to produce MoS2 nanosheets in reasonable quantities involve either solution exfoliation of bulk MoS2 or intercalation of organo-lithium into bulk MoS2, which is then subsequently exfoliated by immersing it in water. The former process produces semiconducting 2H-MoS2 nanoplatelets with smaller lateral flake sizes whereas the latter process produces highly conducting metallic (1T) phase monolayer MoS2. 1T-MoS2 nanosheets have high catalytic activity for the hydrogen evolution reaction (HER) and are efficient electrode materials for supercapacitors when compared to the 2H phase. However, the feasibility of producing 1T-MoS2 by organolithium intercalation is undermined by the long reaction time (2–3 days) and by its pyrophoric nature. We report a simple, bench-top electrochemical process to produce exfoliated metallic phase MoS2 within two hours. By using an inert Li salt (LiClO4) as a source of lithium and a Pt counter electrode, an electrochemically lithium intercalated MoS2 electrode was obtained, which was subsequently exfoliated by immersing it in water. Characterization of the exfoliated product using a variety of methods confirmed the formation of the 1T phase. Remarkably, flake thickness measurement using atomic force microscopy and high-resolution transmission electron microscopy revealed that the majority of the nanosheets are trilayers. The 1T-MoS2 nanosheets showed enhanced electrocatalytic activity towards hydrogen evolution compared to 2H-MoS2 and are efficient materials for supercapacitor applications. Coin cells constructed from a 1T-MoS2–graphene composite achieved a volumetric capacitance of over 560 F cm−3 in an aqueous system and 250 F cm−3 in a non-aqueous electrolyte with capacity retention of over 90% after 5000 cycles. This process is readily scalable and should ultimately support the production of metallic MoS2 for various applications. It can also be extended to produce 2H-MoS2 nanosheets by heating the exfoliated 1T phase.

The application of in situ analytical transmission electron microscopy to the study of preferential intergranular oxidation in Alloy 600.

In situ analytical transmission electron microscopy (TEM) can provide a unique perspective on dynamic reactions in a variety of environments, including liquids and gases. In this study, in situ analytical TEM techniques have been applied to examine the localised oxidation reactions that occur in a Ni-Cr-Fe alloy, Alloy 600, using a gas environmental cell at elevated temperatures. The initial stages of preferential intergranular oxidation, shown to be an important precursor phenomenon for intergranular stress corrosion cracking in pressurized water reactors (PWRs), have been successfully identified using the in situ approach. Furthermore, the detailed observations correspond to the ex situ results obtained from bulk specimens tested in hydrogenated steam and in high temperature PWR primary water. The excellent agreement between the in situ and ex situ oxidation studies demonstrates that this approach can be used to investigate the initial stages of preferential intergranular oxidation relevant to nuclear power systems.

Desalination and Nanofiltration through Functionalized Laminar MoS2 Membranes

Laminar membranes of two-dimensional materials are excellent candidates for applications in water filtration due to the formation of nanocapillaries between individual crystals that can exhibit a molecular and ionic sieving effect, while allowing high water flux. This approach has been exemplified previously with graphene oxide, however these membranes suffer from swelling when exposed to liquid water, leading to low salt rejection and reducing their applicability for desalination applications. Here, we demonstrate that by producing thin (∼5 μm) laminar membranes of exfoliated molybdenum disulfide (MoS2) in a straightforward and scalable process, followed by a simple chemical functionalization step, we can efficiently reject ∼99% of the ions commonly found in seawater, while maintaining water fluxes significantly higher (∼5 times) than those reported for graphene oxide membranes. These functionalized MoS2 membranes exhibit excellent long-term stability with no swelling and consequent decrease in ion rejection, when immersed in water for periods exceeding 6 months. Similar stability is observed when exposed to organic solvents, indicating that they are ideal for a variety of technologically important filtration applications.

The effects of extensive glomerular filtration of thin graphene oxide sheets on kidney physiology

Understanding how two-dimensional (2D) nanomaterials interact with the biological milieu is fundamental for their development toward biomedical applications. When thin, individualized graphene oxide (GO) sheets were administered intravenously in mice, extensive urinary excretion was observed, indicating rapid transit across the glomerular filtration barrier (GFB). A detailed analysis of kidney function, histopathology, and ultrastructure was performed, along with the in vitro responses of two highly specialized GFB cells (glomerular endothelial cells and podocytes) following exposure to GO. We investigated whether these cells preserved their unique barrier function at doses 100 times greater than the dose expected to reach the GFB in vivo. Both serum and urine analyses revealed that there was no impairment of kidney function up to 1 month after injection of GO at escalating doses. Histological examination suggested no damage to the glomerular and tubular regions of the kidneys. Ultrastructural analysis by transmission electron microscopy showed absence of damage, with no change in the size of podocyte slits, endothelial cell fenestra, or the glomerular basement membrane width. The endothelial and podocyte cell cultures regained their full barrier function after >48 h of GO exposure, and cellular uptake was significant in both cell types after 24 h. This study provided a previously unreported understanding of the interaction between thin GO sheets with different components of the GFB in vitro and in vivo to highlight that the glomerular excretion of significant amounts of GO did not induce any signs of acute nephrotoxicity or glomerular barrier dysfunction.

Composition and morphology of self-organized Mn-rich nanocolumns embedded in Ge: Correlation with the magnetic properties

The morphology and composition of self organized manganese (Mn)-rich nanocolumns embedded in germanium (Ge) thin films were characterized at the atomic scale and in three dimensions with high resolution transmission electron microscopy and atom probe tomography. Experiments revealed Mn-enriched nano-columns of 3 nm in diameter with various morphologies. Their Mn-content was found smaller than that of the expected equilibrium phases and chemical fluctuations along the growth axis were additionally observed. By contrast, less than 0.05% of Mn was measured in the Ge-matrix. These results were correlated to the magnetic properties and allowed understanding the magnetic behavior of the nanocolumns.