Ultra-clean high-mobility graphene on technologically relevant substrates.

Graphene grown via chemical vapour deposition (CVD) on copper foil has emerged as a high-quality, scalable material, that can be easily integrated on technologically relevant platforms to develop promising applications in the fields of optoelectronics and photonics. Most of these applications require low-contaminated high-mobility graphene (i.e., approaching 10 000 cm2 V-1 s-1 at room temperature) to reduce device losses and implement compact device design. To date, these mobility values are only obtained when suspending or encapsulating graphene. Here, we demonstrate a rapid, facile, and scalable cleaning process, that yields high-mobility graphene directly on the most common technologically relevant substrate: silicon dioxide on silicon (SiO2/Si). Atomic force microscopy (AFM) and spatially-resolved X-ray photoelectron spectroscopy (XPS) demonstrate that this approach is instrumental to rapidly eliminate most of the polymeric residues which remain on graphene after transfer and fabrication and that have adverse effects on its electrical properties. Raman measurements show a significant reduction of graphene doping and strain. Transport measurements of 50 Hall bars (HBs) yield hole mobility µh up to ∼9000 cm2 V-1 s-1 and electron mobility µe up to ∼8000 cm2 V-1 s-1, with average values µh ∼ 7500 cm2 V-1 s-1 and µe ∼ 6300 cm2 V-1 s-1. The carrier mobility of ultraclean graphene reaches values nearly double than those measured in graphene processed with acetone cleaning, which is the method widely adopted in the field. Notably, these mobility values are obtained over large-scale and without encapsulation, thus paving the way to the adoption of graphene in optoelectronics and photonics.

Formation of a Single-Crystal Aluminum-Based MOF Nanowire with Graphene Oxide Nanoscrolls as Structure-Directing Agents.

An innovative strategy is proposed to synthesize single-crystal nanowires (NWs) of the Al3+ dicarboxylate MIL-69(Al) MOF by using graphene oxide nanoscrolls as structure-directing agents. MIL-69(Al) NWs with an average diameter of 70±20 nm and lengths up to 2 µm were found to preferentially grow along the [001] crystallographic direction. Advanced characterization methods (electron diffraction, TEM, STEM-HAADF, SEM, XPS) and molecular modeling revealed the mechanism of formation of MIL-69(Al) NWs involving size-confinement and templating effects. The formation of MIL-69(Al) seeds and the self-scroll of GO sheets followed by the anisotropic growth of MIL-69(Al) crystals are mediated by specific GO sheets/MOF interactions. This study delivers an unprecedented approach to control the design of 1D MOF nanostructures and superstructures.

Transfer of Epitaxial SrTiO3 Nanothick Layers Using Water-Soluble Sacrificial Perovskite Oxides.

The integration of functional thin film materials with adaptable properties is essential for the development of new paradigms in information technology. Among them, complex oxides with perovskite structures have huge potential based on the particularly vast diversity of physical properties. Here, we demonstrate the possibility of transferring perovskite oxide materials like SrTiO3 onto a silicon substrate using an environmentally friendly process at the nanoscale by means of a water-soluble perovskite sacrificial layer, SrVO3. Based on in situ monitoring atomic force microscopy and photoemission studies, we reveal that the dissolution is initiated from a strontium-rich phase at the extreme surface of SrVO3. The nanothick SrTiO3-transferred layer onto silicon presents appropriate morphology and monocrystalline quality, providing a proof of concept for the integration and development of all-perovskite-oxide electronics or "oxitronics" onto any Si-based substrate.

Photon management properties of Yb-doped SnO2 nanoparticles synthesized by the sol-gel technique.

SnO2 is a transparent large band gap semiconductor, particularly interesting for optoelectronic and photovoltaic devices, mainly because its conduction can be easily tuned by doping or by modulating the amount of oxygen vacancies. Besides, rare earth doping was successfully exploited for up conversion properties. Here we report on the functionalization of SnO2 nanoparticles with optically active Yb3+ ions using the sol-gel method, which allows UV to NIR spectral (down) conversion. As starting solutions we used stable non-alkoxide metal-organic compounds, which is rather uncommon. Transmission electron microscopy analysis demonstrated the formation of small well-crystallized nanoparticles while X-ray photoelectron spectroscopy measurements have revealed that the Yb is well inserted in the host matrix and has a 3+ valence state. All nanoparticles present large absorption in the UV-visible range (250 to 550 nm) and a band gap that decreases down to 2.72 eV upon doping. The UV energy converted into NIR on the basis of efficient energy transfer from SnO2 to the Yb3+ ions ranges between 250 and 400 nm. Reference undoped SnO2 nanoparticles with a mean size of 20 nm allow converting UV light into broad visible emission centered at 650 nm. The incorporation of up to 3.5 at% of Yb3+ ions into the SnO2 host matrix results in a spectacular decrease of the nanoparticle size down to 6.6 nm. This allowed also the shift of the photoluminescence to NIR in the 970-1050 nm range. The energy level structure of Yb3+ in SnO2 was successfully determined from the deconvolution of the Yb emission. This emission is significantly enhanced by increasing the doping level. All optical measurements suggest that these nanoparticles can be efficiently used as down-shifting converters.

Material challenges for solar cells in the twenty-first century: directions in emerging technologies.

Photovoltaic generation has stepped up within the last decade from outsider status to one of the important contributors of the ongoing energy transition, with about 1.7% of world electricity provided by solar cells. Progress in materials and production processes has played an important part in this development. Yet, there are many challenges before photovoltaics could provide clean, abundant, and cheap energy. Here, we review this research direction, with a focus on the results obtained within a Japan-French cooperation program, NextPV, working on promising solar cell technologies. The cooperation was focused on efficient photovoltaic devices, such as multijunction, ultrathin, intermediate band, and hot-carrier solar cells, and on printable solar cell materials such as colloidal quantum dots.

Etching and Chemical Control of the Silicon Nitride Surface.

Silicon nitride is used for many technological applications, but a quantitative knowledge of its surface chemistry is still lacking. Native oxynitride at the surface is generally removed using fluorinated etchants, but the chemical composition of surfaces still needs to be determined. In this work, the thinning (etching efficiency) of the layers after treatments in HF and NH4F solutions has been followed by using spectroscopic ellipsometry. A quantitative estimation of the chemical bonds found on the surface is obtained by a combination of infrared absorption spectroscopy in ATR mode, X-ray photoelectron spectroscopy, and colorimetry. Si-F bonds are the majority species present at the surface after silicon nitride etching; some Si-OH and a few Si-NHx bonds are also present. No Si-H bonds are present, an unfavorable feature for surface functionalization in view of the interest of such mildly reactive groups for achieving stable covalent grafting. Mechanisms are described to support the experimental results, and two methods are proposed for generating surface SiH species: enriching the material in silicon, or submitting the etched surface to a H2 plasma treatment.

Solution processable broadband transparent mixed metal oxide nanofilm optical coatings via substrate diffusion doping.

Devices composed of transparent materials, particularly those utilizing metal oxides, are of significant interest due to increased demand from industry for higher fidelity transparent thin film transistors, photovoltaics and a myriad of other optoelectronic devices and optics that require more cost-effective and simplified processing techniques for functional oxides and coatings. Here, we report a facile solution processed technique for the formation of a transparent thin film through an inter-diffusion process involving substrate dopant species at a range of low annealing temperatures compatible with processing conditions required by many state-of-the-art devices. The inter-diffusion process facilitates the movement of Si, Na and O species from the substrate into the as-deposited vanadium oxide thin film forming a composite fully transparent V0.0352O0.547Si0.4078Na0.01. Thin film X-ray diffraction and Raman scattering spectroscopy show the crystalline component of the structure to be α-NaVO3 within a glassy matrix. This optical coating exhibits high broadband transparency, exceeding 90-97% absolute transmission across the UV-to-NIR spectral range, while having low roughness and free of surface defects and pinholes. The production of transparent films for advanced optoelectronic devices, optical coatings, and low- or high-k oxides is important for planar or complex shaped optics or surfaces. It provides opportunities for doping metal oxides to ternary, quaternary or other mixed metal oxides on glass, encapsulants or other substrates that facilitate diffusional movement of dopant species.

Water adsorption on TiO2 surfaces probed by soft X-ray spectroscopies: bulk materials vs. isolated nanoparticles.

We describe an experimental method to probe the adsorption of water at the surface of isolated, substrate-free TiO2 nanoparticles (NPs) based on soft X-ray spectroscopy in the gas phase using synchrotron radiation. To understand the interfacial properties between water and TiO2 surface, a water shell was adsorbed at the surface of TiO2 NPs. We used two different ways to control the hydration level of the NPs: in the first scheme, initially solvated NPs were dried and in the second one, dry NPs generated thanks to a commercial aerosol generator were exposed to water vapor. XPS was used to identify the signature of the water layer shell on the surface of the free TiO2 NPs and made it possible to follow the evolution of their hydration state. The results obtained allow the establishment of a qualitative determination of isolated NPs' surface states, as well as to unravel water adsorption mechanisms. This method appears to be a unique approach to investigate the interface between an isolated nano-object and a solvent over-layer, paving the way towards new investigation methods in heterogeneous catalysis on nanomaterials.

Germanium oxide removal by citric acid and thiol passivation from citric acid-terminated Ge(100).

Many applications of germanium (Ge) are underpinned by effective oxide removal and surface passivation. This important surface treatment step often requires H-X (X = Cl, Br, I) or HF etchants. Here, we show that aqueous citric acid solutions are effective in the removal of GeOx. The stability of citric acid-treated Ge(100) is compared to HF and HCl treated surfaces and analyzed by X-ray photoelectron spectroscopy. Further Ge surface passivation was investigated by thiolation using alkane monothiols and dithiols. The organic passivation layers show good stability with no oxide regrowth observed after 3 days of ambient exposure.

X-ray Photoelectron Spectroscopy of Isolated Nanoparticles.

X-ray photoelectron spectroscopy (XPS) is a very efficient and still progressing surface analysis technique. However, when applied to nano-objects, this technique faces drawbacks due to interactions with the substrate and sample charging effects. We present a new experimental approach to XPS based on coupling soft X-ray synchrotron radiation with an in-vacuum beam of free nanoparticles, focused by an aerodynamic lens system. The structure of the Si/SiO2 interface was probed without any substrate interaction or charging effects for silicon nanocrystals previously oxidized in ambient air. Complete characterization of the surface was obtained. The Si 2p core level spectrum reveals a nonabrupt interface.

Nanopatterning Si(111) surfaces as a selective surface-chemistry route.

Using wet-chemical self-assembly, we demonstrate that standard surface reactions can be markedly altered. Although HF etching of Si surfaces is known to produce H-terminated surfaces, we show that up to approximately 30% of a monolayer of stable Si-F bonds can be formed on atomically smooth Si(111) surfaces on HF reaction, when chemically isolated Si atoms are the target of the reaction. Similarly, approximately 30% Si-OH termination can be achieved by immersion of the partially covered F-Si(111) surface in water without oxidation of the underlying Si substrate. Such reactions are possible when H-terminated (111)-oriented Si surfaces are initially uniformly patterned with methoxy groups. These findings are contrary to the knowledge built over the past twenty years and highlight the importance of steric interactions at surfaces and the possibility to stabilize products at surfaces that cannot be obtained on chemically homogeneous surfaces.