Laëtitia Marty

Homogeneous Optical and Electronic Properties of Graphene Due to the Suppression of Multilayer Patches During CVD on Copper Foils

By limiting the carbon segregation at the copper surface defects, a pulsed chemical vapor deposition method for single layer graphene growth is shown to inhibit the formation of few-layer regions, leading to a fully single-layered graphene homogeneous at the centimeter scale. Graphene field-effect devices obtained after transfer of pulsed grown graphene on oxidized silicon exhibit mobilities above 5000 cm^2.V^-1.s^-1.

A local optical probe for measuring motion and stress in a nanoelectromechanical system

Nanoelectromechanical systems can be operated as ultrasensitive mass sensors and ultrahigh-frequency resonators, and can also be used to explore fundamental physical phenomena such as nonlinear damping and quantum effects in macroscopic objects. Various dissipation mechanisms are known to limit the mechanical quality factors of nanoelectromechanical systems and to induce aging due to material degradation, so there is a need for methods that can probe the motion of these systems, and the stresses within them, at the nanoscale. Here, we report a non-invasive local optical probe for the quantitative measurement of motion and stress within a nanoelectromechanical system, based on Fizeau interferometry and Raman spectroscopy. The system consists of a multilayer graphene resonator that is clamped to a gold film on an oxidized silicon surface. The resonator and the surface both act as mirrors and therefore define an optical cavity. Fizeau interferometry provides a calibrated measurement of the motion of the resonator, while Raman spectroscopy can probe the strain within the system and allows a purely spectral detection of mechanical resonance at the nanoscale.Nanoelectromechanical systems (NEMSs) are emerging nanoscale elements at the crossroads between mechanics, optics and electronics, with significant potential for actuation and sensing applications. The reduction of dimensions compared to their micronic counterparts brings new effects including sensitivity to very low mass, resonant frequencies in the radiofrequency range, mechanical non-linearities and observation of quantum mechanical effects. An important issue of NEMS is the understanding of fundamental physical properties conditioning dissipation mechanisms, known to limit mechanical quality factors and to induce aging due to material degradation. There is a need for detection methods tailored for these systems which allow probing motion and stress at the nanometer scale. Here, we show a non-invasive local optical probe for the quantitative measurement of motion and stress within a multilayer graphene NEMS provided by a combination of Fizeau interferences, Raman spectroscopy and electrostatically actuated mirror. Interferometry provides a calibrated measurement of the motion, resulting from an actuation ranging from a quasi-static load up to the mechanical resonance while Raman spectroscopy allows a purely spectral detection of mechanical resonance at the nanoscale. Such spectroscopic detection reveals the coupling between a strained nano-resonator and the energy of an inelastically scattered photon, and thus offers a new approach for optomechanics.

Batch processing of nanometer-scale electrical circuitry based on in-situ grown single-walled carbon nanotubes

We present a fabrication method for a nanometer-scale conducting network made of self-assembled single-walled carbon nanotubes. The electrical connection of the suspended nanotubes to the metallic contacts is obtained during the nanotube synthesis itself, which involves the hot-filament CVD technique. We directly characterize, without any further processing, the electronic transport properties of samples with different pad geometries. At room temperature, all tested samples show ohmic behavior in the kΩ range, for both two-probe and four-probe geometries. At low temperature, non-linear transport is observed and a large discrepancy of resistance arises between two-probe and four-probe geometries, suggesting the dominant influence of the contact resistance.

Self-assembled single wall carbon nanotube field effect transistors

We report detailed characterization of in-situ wired single wall carbon nanotube (SWNT) field effect transistors (FETs). They were batch processed using a single step technique based on hot filament chemical vapor deposition. Raw samples show an ambipolar field effect. The temperature dependence of the gain confirms the presence of Schottky barriers at the nanotube/metal interface. Moreover the gate dependence exhibits hysteresis at any temperature due to extraction and trapping of charges. Below 30 K, Coulomb blockade occurs at low drain-source bias and partially washes out the influence of the Schottky barriers.

Integration of self-assembled carbon nanotube transistors: statistics and gate engineering at the wafer scale

We present a full process based on chemical vapour deposition that allows fabrication and integration at the wafer scale of carbon-nanotube-based field effect transistors. We make a statistical analysis of the integration yield that allows assessment of the parameter fluctuations of the titanium–nanotube contact obtained by self-assembly. This procedure is applied to raw devices without post-process. Statistics at the wafer scale reveal the respective role of semiconducting and metallic connected nanotubes and show that connection yields up to 86% can be reached. For large scale device integration, our process has to implement both wafer-scale self-assembly of the nanotubes and high transistor performances. In order to address this last issue, a gate engineering process has been investigated. We present the improvements obtained using low and high κ dielectrics for the gate oxide.

Biaxial Strain Transfer in Supported Graphene

Understanding the mechanism and limits of strain transfer between supported 2D systems and their substrate is a most needed step toward the development of strain engineering at the nanoscale. This includes applications in straintronics, nanoelectromechanical devices, or new nanocomposites. Here, we have studied the limits of biaxial compressive strain transfer among SiO2, diamond, and sapphire substrates and graphene. Using high pressure-which allows maximizing the adhesion between graphene and the substrate on which it is deposited-we show that the relevant parameter governing the graphene mechanical response is not the applied pressure but rather the strain that is transmitted from the substrate. Under these experimental conditions, we also show the existence of a critical biaxial stress beyond which strain transfer become partial and introduce a parameter, α, to characterize strain transfer efficiency. The critical stress and α appear to be dependent on the nature of the substrate. Under ideal biaxial strain transfer conditions, the phonon Raman G-band dependence with strain appears to be linear with a slope of -60 ± 3 cm-1/% down to biaxial strains of -0.9%. This evolution appears to be general for both biaxial compression and tension for different experimental setups, at least in the biaxial strain range -0.9% < ε < 1.8%, thus providing a criterion to validate total biaxial strain transfer hypotheses. These results invite us to cast a new look at mechanical strain experiments on deposited graphene as well as to other 2D layered materials.

Light control of charge transfer and excitonic transitions in a carbon nanotube/porphyrin hybrid

Carbon nanotube-chromophore hybrids are promising building blocks in order to obtain a controlled electro-optical transduction effect at the single nano-object level. In this work, a strong spectral selectivity of the electronic and the phononic response of a chromophore-coated single nanotube transistor is observed for which standard photogating cannot account. This paper investigates how light irradiation strongly modifies the coupling between molecules and nanotube within the hybrid by means of combined Raman diffusion and electron transport measurements. Moreover, a nonconventional Raman enhancement effect is observed when light irradiation is on the absorption range of the grafted molecule. Finally, this paper shows how the dynamics of single electron tunneling in the device at low temperature is strongly modified by molecular photoexcitation. Both effects will be discussed in terms of photoinduced excitons coupled to electronic levels.

Time- and space-modulated Raman signals in graphene-based optical cavities

We present fabrication and optical characterization of micro-cavities made of multilayer graphene (MLG) cantilevers clamped by metallic electrodes and suspended over Si/SiO2 substrates. Graphene cantilevers act as semi-transparent mirrors closing air wedge optical cavities. This simple geometry implements a standing-wave optical resonator along with a mechanical one. Equal thickness interference fringes are observed in both Raman and Rayleigh backscattered signals, with interfringe given by their specific wavelength. Chromatic dispersion within the cavity makes possible the spatial modulation of graphene Raman lines and selective rejection of the silicon background signal. Electrostatic actuation of the multilayer graphene cantilever by a gate voltage tunes the cavity length and induces space and time modulation of the backscattered light, including the Raman lines. We demonstrate the potential of these systems for high-sensitivity Raman measurements of generic molecular species grafted on a multilayer graphene surface. The Raman signal of the molecular layer can be modulated both in time and space in a similar fashion and shows enhancement with respect to a collapsed membrane.

Electronic transport in individual carbon nanotube bundles under pressure

Field-effect transistors based on individual carbon nanotubes with reduced Schottky barriers are studied up to pressures of 0.9 GPa and down to temperatures of less than 10 K. At ambient temperature and high pressure, complex effects are observed in a small bundle of tubes stemming from either the intrinsic modifications of the nanotubes at their ovalization, the evolution of barriers at the tube/electrodes contacts, or even both processes. Variations of the nanotube transport characteristics related to changes in the tube environment are most possibly also involved. Despite the highly complex pressure induced changes occurring in our device, low temperature measurements (<10 K) at high pressure (4.5 kbar) provide the first experimental evidence of Coulomb blockade at high pressure and the conservation of the ballistic behaviour of charges carriers in nanotubes under important stress-induced strain.

Weakly Trapped, Charged, and Free Excitons in Single-Layer MoS2 in the Presence of Defects, Strain, and Charged Impurities

Few- and single-layer MoS2 host substantial densities of defects. They are thought to influence the doping level, the crystal structure, and the binding of electron-hole pairs. We disentangle the concomitant spectroscopic expression of all three effects and identify to what extent they are intrinsic to the material or extrinsic to it, i.e., related to its local environment. We do so by using different sources of MoS2-a natural one and one prepared at high pressure and high temperature-and different substrates bringing varying amounts of charged impurities and by separating the contributions of internal strain and doping in Raman spectra. Photoluminescence unveils various optically active excitonic complexes. We discover a defect-bound state having a low binding energy of 20 meV that does not appear sensitive to strain and doping, unlike charged excitons. Conversely, the defect does not significantly dope or strain MoS2. Scanning tunneling microscopy and density functional theory simulations point to substitutional atoms, presumably individual nitrogen atoms at the sulfur site. Our work shows the way to a systematic understanding of the effect of external and internal fields on the optical properties of two-dimensional materials.