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Graphene phonons are excited by the local injection of electrons and holes from the tip of a scanning tunneling microscope. Despite the strong graphene-Ru(0001) hybridization, monolayer graphene unexpectedly exhibits pronounced phonon signatures in inelastic electron tunneling spectroscopy. Spatially resolved spectroscopy reveals that the strength of the phonon signal depends on the site of the moiré lattice with a substantial red-shift of phonon energies compared to those of free graphene. Bilayer graphene gives rise to more pronounced spectral signatures of vibrational quanta with energies nearly matching the free graphene phonon energies. Spectroscopy data of bilayer graphene indicate moreover the presence of a Dirac cone plasmon excitation.

The efficiency of hexagonal boron nitride and graphene to separate the hydrocarbon molecule C64H36 from Ru(0001) and Pt(111) surfaces is explored in low-temperature scanning tunneling microscopy an...

The adsorption of manganese-phthalocyanine molecules on Au(110) was investigated using a low-temperature scanning tunneling microscope. A rich variety of commensurate superstructures was observed upon increasing the molecule coverage from submonolayers to ultrathin films. All structures were associated with reconstructions of the Au(110) substrate. Molecules adsorbed in the second molecular layer exhibited negative differential conductance occurring symmetrically around zero bias voltage. A double-barrier tunneling model rationalized this observation in terms of a peaked molecular resonance at the Fermi energy together with a voltage drop across the molecular film.

Line shapes of molecular vibrational quanta in inelastic electron tunneling spectroscopy may indicate the strength of electron-vibration coupling, the hybridization of the molecule with its environment, and the degree of vibrational damping by electron-hole pair excitation. Bare as well as C60-terminated Pb tips of a scanning tunneling microscope and clean as well as C60-covered Pb(111) surfaces were used in low-temperature experiments. Depending on the overlap of orbital and vibrational spectral ranges different spectroscopic line shapes of molecular vibrational quanta were observed. The energy range covered by the molecular resonance was altered by modifying the adsorption configuration of the molecule terminating the tip apex. Concomitantly, the line shapes of different vibrational modes were affected. The reported observations represent an experimental proof to theoretical predictions on the contribution from resonant processes to inelastic electron tunneling.

Graphene on Ir(111) was intercalated by Li. The intercalant formed a variety of superstructures with increasing coverage. For all superstructures, the moire pattern of pristine graphene on Ir(111) was discernible. At low coverage, the intercalant exhibited a disperse pattern of elementary assemblies that served as building blocks for Li-induced structures at higher coverage. In contrast to previous reports on other intercalants, Li intercalation gave rise to isotropic assemblies in the low-coverage range. Pattern formation was qualitatively rationalized in terms of different energy costs and gains and a corresponding ranking of preferred intercalation regions. While for intermediate Li coverage an ordered intercalant superstructure was observed, higher coverage led to the decoration of step edges with adsorbed Li clusters.

Anisotropic magnetoresistance, that is, the sensitivity of the electrical resistance of magnetic materials on the magnetization direction, is expected to be strongly enhanced in ballistic transport through nanoscale junctions. However, unambiguous experimental evidence of this effect is difficult to achieve. We utilize single-atom junctions to measure this ballistic anisotropic magnetoresistance (AMR). Single Co and Ir atoms are deposited on domains and domain walls of ferromagnetic Fe layers on W(110) to control their magnetization directions. They are contacted with nonmagnetic tips in a low-temperature scanning tunneling microscope to measure the junction conductances. Large changes of the magnetoresistance occur from the tunneling to the ballistic regime due to the competition of localized and delocalized d-orbitals, which are differently affected by spin-orbit coupling. This work shows that engineering the AMR at the single atom level is feasible.

Porphyrin molecules are ubiquitous and play an important role in chemical and biological processes. Their wide panoply of functions may be controlled by modifying their peripheral substituents. Electron injection from the tip of a scanning tunneling microscope into single 5,10,15,20-tetrakis(4-bromophenyl)-porphyrin-cobalt molecules adsorbed to Ag(111) was used to controllably remove Br atoms from the molecule periphery. Spectroscopy of the differential conductance, together with density functional calculations, assign spectral features to the spectroscopic signatures of molecular frontier orbitals. Different molecular orbitals are visible in the spectra of intact and dehalogenated molecules. These findings combine single-molecule chemistry with a characterization of each product’s electronic structure.

A small-scale wastewater cleaning plant is described that includes the key physical pretreatment steps followed by the chemical treatment of mud by flocculation. Water, clay particles, and riverside deposits mimicked odorless wastewater. After a demonstration of the optimization step, the flocculation process was carried out with iron(III) chloride and a cationic polyelectrolyte and lasted less than 10 s for 7 L of wastewater. This experiment is primarily targeted toward high school students to give a real world demonstration of solution and colloid chemistry, but is also useful for a more general audience. The demonstration has already been successfully presented to 700 visitors.

The electronic properties of FePc molecules adsorbed on hydrogen-passivated Si(111) surfaces are investigated by scanning tunneling microscopy at low temperatures. Spatially resolved spectroscopy reveals a significant variation of the electronic states at different positions above the molecule. The highest occupied ligand- and metal-based orbitals of FePc are determined by pronounced peaks in the tunneling spectra and voltage-dependent changes in the microscopic images. Comparison with density functional theory calculations indicates that the electronic ground state is an 3A1g state.

C60 molecules adsorbed to Cu(100) are contacted with the tip of a cryogenic scanning tunneling microscope. Images with submolecular resolution reveal distinct orientations of the molecules. We find that the orientation significantly affects the conductance of the contact despite the high symmetry of C60.

Advances in nanomechanics within recent years have demonstrated an always expanding range of devices, from top-down structures to appealing bottom-up MoS$_2$ and graphene membranes, used for both sensing and component-oriented applications. One of the main concerns in all of these devices is frequency noise, which ultimately limits their applicability. This issue has attracted a lot of attention recently, and the origin of this noise remains elusive up to date. In this Letter we present a very simple technique to measure frequency noise in nonlinear mechanical devices, based on the presence of bistability. It is illustrated on silicon-nitride high-stress doubly-clamped beams, in a cryogenic environment. We report on the same $T/f$ dependence of the frequency noise power spectra as reported in the literature. But we also find unexpected {\it damping fluctuations}, amplified in the vicinity of the bifurcation points; this effect is clearly distinct from already reported nonlinear dephasing, and poses a fundamental limit on the measurement of bifurcation frequencies. The technique is further applied to the measurement of frequency noise as a function of mode number, within the same device. The relative frequency noise for the fundamental flexure $\delta f/f_0$ lies in the range $0.5 - 0.01~$ppm (consistent with literature for cryogenic MHz devices), and decreases with mode number in the range studied. The technique can be applied to {\it any types} of nano-mechanical structures, enabling progresses towards the understanding of intrinsic sources of noise in these devices., Comment: Published 7 may 2018

Van der Waals magnetic materials are building blocks for novel kinds of spintronic devices and playgrounds for exploring collective magnetic phenomena down to the two-dimensional limit. Chromium-tellurium compounds are relevant in this perspective. In particular, the 1$T$ phase of CrTe$_2$ has been argued to have a Curie temperature above 300~K, a rare and desirable property in the class of lamellar materials, making it a candidate for practical applications. However, recent literature reveals a strong variability in the reported properties, including magnetic ones. Using electron microscopy, diffraction and spectroscopy techniques, together with local and macroscopic magnetometry approaches, our work sheds new light on the structural, chemical and magnetic properties of bulk 1$T$-CrTe$_2$ exfoliated in the form of flakes having a thickness ranging from few to several tens of nanometers. We unambiguously establish that 1$T$-CrTe$_2$ flakes are ferromagnetic above room temperature, have an in-plane easy axis of magnetization, low coercivity, and we confirm that their Raman spectroscopy signatures are two modes, $E_{2\text{g}}$ (103.5~cm$^{-1}$) and $A_{1\text{g}}$ (136.5~cm$^{-1}$). We also prove that thermal annealing causes a phase transformation to monoclinic Cr$_5$Te$_8$ and, to a lesser extent, to trigonal Cr$_5$Te$_8$. In sharp contrast with 1$T$-CrTe$_2$, none of these compounds have a Curie temperature above room temperature, and they both have perpendicular magnetic anisotropy. Our findings reconcile the apparently conflicting reports in the literature and open opportunities for phase-engineered magnetic properties.

International audience; The growth of ZnTe nanowires and ZnTe-CdTe nanowire heterostructures is studied by \emph{in situ} transmission electron microscopy. We describe the shape, and the change of shape, of the solid gold nanoparticle during vapor-solid-solid growth. We show the balance between one-monolayer and two-monolayer steps which characterizes the vapor-liquid-solid and vapor-solid-solid growth modes of ZnTe. We discuss the likely role of the mismatch strain and lattice coincidence between gold and ZnTe on the predominance of two-monolayer steps during vapor-solid-solid growth, and on the subsequent self-regulation of the step dynamics. Finally, the formation of an interface between CdTe and ZnTe is described.

International audience; This study introduces a new nanocrystallization method assisted by ultrasounds that produces needles or nanocubes of CMONS, a stilbene dye, with an excellent control over the polymorphism, and with a narrow size distribution. Owing to the production of radicals from dissolved dioxygen by high-intensity ultrasounds, trans-to-cis isomerization was observed in the absence of nitrogen bubbling, with the formation of two distinct crystalline phases for the different diastereomers. The crystallinity of CMONS needles was probed by various techniques, including X-ray and electron diffractions, fluorescence spectroscopy, and dynamic nuclear polarization (DNP) enhanced solid-state nuclear magnetic resonance. The latter was used to hyperpolarize 1 H nuclei and to record 1 H-13 C and 1 H-15 N CPMAS NMR spectra at natural isotopic abundance with very high signal-to-noise ratio. With such sensitivity, one can easily discriminate between cis and trans-I forms of CMONS, detect the presence of multiple polymorphic phases (even with minor contributions) and check the absence of amorphous phase. Finally, the mechanism involved in the formation of CMONS needles was ascertained after stabilizing intermediate nanocubes against Ostwald ripening and ordered aggregation mechanisms using the CTAB surfactant.

5 pages, 5 figures; International audience; We report on a two-dimensional (2D) V1–xPtxSe2 alloy that exhibits ferromagnetic order and Rashba spin–orbit coupling. Although ferromagnetism is absent in 1T-VSe2 because of the competition with the charge density wave phase, we demonstrate theoretically and experimentally that the substitution of vanadium by platinum in VSe2 (10–50%) to form a homogeneous 2D alloy restores ferromagnetic order down to one monolayer of V0.65Pt0.35Se2. Moreover, the presence of platinum atoms gives rise to Rashba spin–orbit coupling in (V,Pt)Se2, providing an original platform to study the interplay between ferromagnetism and spin–orbit coupling in the 2D limit.

International audience; We report on high-dimensional quantum dynamical simulations of electron−hole separation in self-assembled mesomorphic nanostructures composed of donor−acceptor conjugated co-oligomers. The latter are based on perylene diimide (PDI) acceptor units combined with fluorene-thiophene-benzothiadiazole donor units, which form highly ordered, stacked structural motifs upon self-assembly. Simulations are shown for a first-principles parametrized model lattice of 25 stacked PDI units under the effects of an applied external field and temperature. The simulations are carried out with the multilayer multiconfiguration timedependent Hartree (ML-MCTDH) method with nearly 900 vibrational degrees of freedom and 25 electronic states. Temperature effects are included using the thermofield dynamics approach. A transition between a short-time coherent dynamics and a kinetic regime is highlighted. From a flux-overpopulation analysis, electron−hole dissociation rates are obtained in the range of 5−20 ns −1 in the absence of static disorder, exhibiting a moderate field and temperature dependence. These results for electron−hole separation rates can be employed as a benchmark to calibrate the parametrization of kinetic Monte Carlo simulations applied to much larger lattice sizes.

The functional diversification and adaptability of the elementary switching units of computational circuits are disruptive approaches for advancing electronics beyond the static capabilities of conventional complementary metal-oxide-semiconductor-based architectures. Thereto, in this work the one-dimensional nature of monocrystalline and monolithic Al-Ge-based nanowire heterostructures is exploited to deliver charge carrier polarity control and furthermore to enable distinct programmable negative differential resistance at runtime. The fusion of electron and hole conduction together with negative differential resistance in a universal adaptive transistor may enable energy-efficient reconfigurable circuits with multivalued operability that are inherent components of emerging artificial intelligence electronics.

International audience; Developing nanoscale electrical characterization techniques adapted to three-dimensional (3D) geometry is essential for optimization of the epitaxial structure and doping process of nano- and microwires. In this paper, we demonstrate the assessment of the depletion width as well as the doping profile at the nanoscale of individual microwire core–shell light-emitting devices by capacitance–voltage measurements. A statistical study carried out on single wires shows the consistency of the doping profile values measured for individual microwires compared to assemblies of hundreds of wires processed on the same sample. The robustness of this method is then demonstrated on four epitaxial structures with different growth and doping conditions. Finally, electron-beam-induced current and secondary electron profiles are used to validate the depletion region width and the position in the core–shell structure.

International audience; CsPbBr3 has received wide attention due to its superior emission yield and better thermal stability compared to other organic–inorganic lead halide perovskites. In this study, through an interplay of theory and experiments, we investigate the molecular origin of the asymmetric low-temperature photoluminescence spectra of CsPbBr3. We conclude that the origin of this phenomenon lies in a local dipole moment (and the induced Stark effect) due to the preferential localization of Cs+ in either of two off-center positions of the empty space between the surrounding PbBr6 octahedra. With increasing temperature, Cs+ ions are gradually occupying positions closer and closer to the center of the cavities. The gradual loss of ordering in the Cs+ position with increasing temperature is the driving force for the formation of tetragonal-like arrangements within the orthorhombic lattice

International audience; Although numerous papyri from ancient Egypt have been collected and preserved over the centuries, the recipe used to prepare black inks was only reported in manuscripts from the late Greco-Roman period. Black inks were mostly obtained after mixing carbon black with a binder agent and water. In previous studies performed on black inks apposed on papyri from ancient Egypt, additional chemical elements such as lead, iron, or copper were also identified, and the resulting chemical contrast with the papyrus support was used to virtually decrypt highly degraded or rolled papyri. Combining a series of synchrotron-based techniques with Raman spectroscopy and scanning electron microscopy, we investigated 10 papyri fragments from J.-F. Champollion's private collection. For each fragment, the carbon-black pigment found in the ink is identified as flame carbon (lampblack or soot). Using Xray diffraction computed tomography, we show that the diffraction signal of the carbon-based pigment itself can be isolated. As a result, a contrast with the papyrus support is obtained, even in the absence of a specific chemical element in the ink. This is opening up new opportunities to decipher words written millennia ago, as part of our Cultural Heritage.

International audience; While reversibility is a fundamental concept in thermodynamics, most reactions are not readily reversible, especially in solid state physics. For example, thermal diffusion is a widely known concept, used among others to inject dopant atoms into the substitutional positions in the matrix and improve the device properties. Typically, such a diffusion process will create a concentration gradient extending over increasingly large regions, without possibility to reverse this effect. On the other hand, while the bottom up growth of semiconducting nanowires is interesting, it can still be difficult to fabricate axial heterostructures with high control. In this paper, we report a reversible thermal diffusion process occurring in the solid-state exchange reaction between an Al metal pad and a SixGe1-x alloy nanowire observed by in-situ transmission electron microscopy. The thermally assisted reaction results in the creation of a Si-rich region sandwiched between the reacted Al and unreacted SixGe1-x part, forming an axial Al/Si/SixGe1-x heterostructure. Upon heating or (slow) cooling, the Al metal can repeatably move in and out of the SixGe1-x alloy nanowire while maintaining the rod-like geometry and crystallinity, allowing to fabricate and contact nanowire heterostructures in a reversible way in a single process step, compatible with current Si based technology. This interesting system is promising for various applications, such as phase change memories in an all crystalline system with integrated contacts, as well as Si/SixGe1-x/Si heterostructures for near-infrared sensing applications.

International audience; To understand the importance of the particle size on the stabilization of metastable tetragonal ZrO2, ultrafine ZrO2 nanocrystals were synthesized via (i) the precipitation method in supercritical water using nitrate precursors, (ii) the sol–gel method in a supercritical ethanol–water mixture, and (iii) the borderline nonhydrolytic sol–gel route in supercritical ethanol using propoxide precursors. The obtained nanocrystals displayed a variation of the monoclinic versus tetragonal molar fractions from 100 wt % down to ≈10 wt % of monoclinic phase. This variation was concomitant with an overall size decrease of the nanocrystals, ranging from 7 to 2 nm depending on the synthesis procedures. Phase contents were quantified by refinement analysis of X-ray scattering data sets and crosschecked with Raman spectroscopy. Our results suggest that an upper limit of ≈90 wt % of the tetragonal ZrO2 phase is possible, even for ultrafine nanoparticles (2 nm). These findings thus question the existence of any critical size limit below which stabilization of pure t-ZrO2 is attainable at low temperatures.

International audience; The clear description of the morphology and location, with respect to the support, of metallic sub-nanometric particles remains a current experimental strenuous challenge in numerous catalytic applications. High resolution-HAADF-STEM coupled with in situ and tomographic analyses are undertaken on platinum (Pt) active phase supported on chlorinated alumina (γ-Al 2 O 3) with 0.3 and 1% w/w Pt loadings highlighting the formation of flat nanoparticles (NPs) of 0.9 nm diameter and Pt single atoms (SAs) in the reduced state. While SAs and weakly cohesive clusters are predominantly observed in the oxide state, with a coordination sphere of Pt composed of O and Cl as revealed by EXAFS, the ratio between SAs and Pt NPs in the reduced state is found to be about 2.8. This ratio is not affected by metal loading which increases both the total number of NPs and SA. Electron tomography reveals that the vast majority of NPs are located on the edges or defects (steps, kinks) of the alumina support crystallites. Density functional theory calculations further highlight the optimized structures of NPs located at the γ-Al 2 O 3 (110)(100) edge and near-edge with a stability competing with NPs located either on the (110) or on the (100) γ-Al 2 O 3 facet. A mathematical analysis of the segmented volumes shows that the average geodesic distances between NPs is linked to Pt loading: 9 nm for 1% w/w Pt, and 16 nm for 0.3% w/w Pt. Evaluation of support tortuosity descriptors using the nanoparticles positions confirms a uniform distribution on the support. A square network geometric model compatible with the geodesic distances between NPs reveals that 1 to 5 NPs can be present at the same time on each alumina crystallite depending on Pt loading.