arXiv subjects

Georg S. Duesberg

Publications and source records attributed to Georg S. Duesberg.

51 records · Page 3Linked to original sources

arXiv2015

Mapping of Low-Frequency Raman Modes in CVD-Grown Transition Metal Dichalcogenides: Layer Number, Stacking Orientation and Resonant Effects

Layered inorganic materials, such as the transition metal dichalcogenides (TMDs), have attracted much attention due to their exceptional electronic and optical properties. Reliable synthesis and characterization of these materials must be developed if these properties are to be exploited. Herein, we present low-frequency Raman analysis of MoS2, MoSe2, WSe2 and WS2 grown by chemical vapour deposition (CVD). Raman spectra are acquired over large areas allowing changes in the position and intensity of the shear and layer-breathing modes to be visualized in maps. This allows detailed characterization of mono- and few-layered TMDs which is complementary to well-established (high-frequency) Raman and photoluminescence spectroscopy. This study presents a major stepping stone in fundamental understanding of layered materials as mapping the low-frequency modes allows the quality, symmetry, stacking configuration and layer number of 2D materials to be probed over large areas. In addition, we report on anomalous resonance effects in the low-frequency region of the WS2 Raman spectrum.

cond-mat.mtrl-sci↗

arXiv2015

Direct Observationof DegenerateTwo-Photon Absorption and Its Saturation in WS2 and MoS2 Monolayer and Few-Layer Films

The optical nonlinearity of WS2, MoS2 monolayer and few-layer films was investigated using the Z-scan technique with femtosecond pulses from the visible to the near infrared. The dependence of nonlinear absorption of the WS2 and MoS2 films on layer number and excitation wavelength was studied systematically. WS2 with 1~3 layers exhibits a giant two-photon absorption (TPA) coefficient. Saturation of TPA for WS2 with 1~3 layers and MoS2 with 25~27 layers was observed. The giant nonlinearity of WS2 and MoS2 is attributed to two dimensional confinement, a giant exciton effect and the band edge resonance of TPA.

physics.optics↗

arXiv2015

Spectral Sensitivity of Graphene/Silicon Heterojunction Photodetectors

We have studied the optical properties of two-dimensional (2D) Schottky photodiode heterojunctions made of chemical vapor deposited (CVD) graphene on n- and p-type Silicon (Si) substrates. Much better rectification behavior is observed from the diodes fabricated on n- Si substrates in comparison with the devices on p-Si substrates in dark condition. Also, graphene/n-Si photodiodes show a considerable responsivity of 270 mA/W within the silicon spectral range in DC reverse bias condition. The present results are furthermore compared with that of a molybdenum disulfide (MoS2)/p-type silicon photodiodes.

cond-mat.mes-hall↗

arXiv2015

Understanding and optimising the packing density of perylene bisimide layers on CVD-grown graphene

The non-covalent functionalisation of graphene is an attractive strategy to alter the surface chemistry of graphene without damaging its superior electrical and mechanical properties. Using the facile method of aqueous-phase functionalisation on large-scale CVD-grown graphene, we investigated the formation of different packing densities in self-assembled monolayers (SAMs) of perylene bisimide derivatives and related this to the amount of substrate contamination. We were able to directly observe wet-chemically deposited SAMs in scanning tunnelling microscopy (STM) on transferred CVD graphene and revealed that the densely packed perylene ad-layers adsorb with the conjugated π-system of the core perpendicular to the graphene substrate. This elucidation of the non-covalent functionalisation of graphene has major implications on controlling its surface chemistry and opens new pathways for adaptable functionalisation in ambient conditions and on the large scale.

cond-mat.mes-hall↗

arXiv2015

Low wavenumber Raman spectroscopy of highly crystalline MoSe2 grown by chemical vapor deposition

Transition metal dichalcogenides (TMDs) have recently attracted attention due to their interesting electronic and optical properties. Fabrication of these materials in a reliable and facile method is important for future applications, as are methods to characterize material quality. Here we present the chemical vapor deposition of MoSe2 monolayer and few layer crystals. These results show the practicality of using chemical vapor deposition to reliably fabricate these materials. Low frequency Raman spectra and mapping of shear and layer breathing modes of MoSe2 are presented for the first time. We correlate the behavior of these modes with layer number in the materials. The usefulness of low frequency Raman mapping to probe the symmetry, quality, and monolayer presence in CVD grown 2D materials is emphasized.

cond-mat.mtrl-sci↗

arXiv2015

Liquid exfoliation of solvent-stabilised black phosphorus: applications beyond electronics

Few layer black phosphorus is a new two-dimensional material which is of great interest for applications, mainly in electronics. However, its lack of stability severely limits our ability to synthesise and process this material. Here we demonstrate that high-quality, few-layer black phosphorus nanosheets can be produced in large quantities by liquid phase exfoliation in the solvent N-cyclohexyl-2-pyrrolidone (CHP). We can control nanosheet dimensions and have developed metrics to estimate both nanosheet size and thickness spectroscopically. When exfoliated in CHP, the nanosheets are remarkably stable unless water is intentionally introduced. Computational studies show the degradation to occur by reaction with water molecules only at the nanosheet edge, leading to the removal of phosphorus atoms and the formation of phosphine and phosphorous acid. We demonstrate that liquid exfoliated black phosphorus nanosheets are potentially useful in a range of applications from optical switches to gas sensors to fillers for composite reinforcement.

cond-mat.mes-hall↗

arXiv2014

Heterojunction Hybrid Devices from Vapor Phase Grown MoS$_{2}$

We investigate a vertically-stacked hybrid photodiode consisting of a thin n-type molybdenum disulfide (MoS$_{2}$) layer transferred onto p-type silicon. The fabrication is scalable as the MoS$_{2}$ is grown by a controlled and tunable vapor phase sulfurization process. The obtained large-scale p-n heterojunction diodes exhibit notable photoconductivity which can be tuned by modifying the thickness of the MoS$_{2}$ layer. The diodes have a broad spectral response due to direct and indirect band transitions of the nanoscale MoS$_{2}$. Further, we observe a blue-shift of the spectral response into the visible range. The results are a significant step towards scalable fabrication of vertical devices from two-dimensional materials and constitute a new paradigm for materials engineering.

cond-mat.mtrl-sci↗

arXiv2014

Biocompatibility of Pristine Graphene Monolayers, Nanosheets and Thin Films

There is an increasing interest to develop nanoscale biocompatible graphene structures due to their desirable physicochemical properties, unlimited application opportunities and scalable production. Here we report the preparation, characterization and biocompatibility assessment of novel graphene flakes and their enabled thin films suitable for a wide range of biomedical and electronic applications. Graphene flakes were synthesized by a chemical vapour deposition method or a liquid-phase exfoliation procedure and then thin films were prepared by transferring graphene onto glass coverslips. Raman spectroscopy and transmission electron microscopy confirmed a predominantly monolayer and a high crystalline quality formation of graphene. The biocompatibility assessment of graphene thin films and graphene flakes was performed using cultured human lung epithelial cell line A549 employing a multimodal approach incorporating automated imaging, high content screening, real-time impedance sensing in combination with biochemical assays. No detectable changes in the cellular morphology or attachment of A549 cells growing on graphene thin films or cells exposed to graphene flakes (0.1 to 5 ug/mL) for 4 to 72 h was observed. Graphene treatments caused a very low level of increase in cellular production of reactive oxygen species in A549 cells, but no detectable damage to the nuclei such as changes in morphology, condensation or fragmentation was observed. In contrast, carbon black proved to be significantly more toxic than the graphene. These data open up a promising view of using graphene enabled composites for a diverse scope of safer applications.

q-bio.CB↗

arXiv2013

Origami-based spintronics in graphene

We show that periodically folded graphene sheets with enhanced spin-orbit interaction due to curvature effects can carry spin polarized currents and have gaps in the electronic spectrum in the presence of weak magnetic fields. Our results indicate that such origami-like structures can be used efficiently in spintronic applications.

cond-mat.mes-hall↗

arXiv2006

Silicon Nanowires, Catalytic Growth and Electrical Characterization

Nominally undoped silicon nanowires (NW) were grown by catalytic chemical vapor deposition. The growth process was optimized to control the NWs diameters by using different Au catalyst thicknesses on amorphous SiO2, Si3N4, or crystalline-Si substrates. For SiO2 substrates an Ar plasma treatment was used to homogenize the catalyst coalescence, and thus the NWs diameter. Furthermore, planar field effect transistors (FETs) were fabricated by implementing 10 to 30 nm thin nominally undoped Si-NWs as the active region. Various silicides were investigated as Schottky-barrier source and drain contacts for the active region. For CoSi, NiSi and PdSi contacts, the FETs transfer characteristics showed p-type behavior. A FET consisting of a single Si-NW with 20 nanometers diameter and 2.5 micrometer gate-length delivers as much as 0.15 microA on-current at 1 volt bias voltage and has an on/off current ratio of 10^7. This is in contrast to recent reports of low conductance in undoped Si.

cond-mat.mtrl-sci↗

arXiv2005

Electron diffraction analysis of individual single-walled carbon nanotubes

We present a detailed electron diffraction study of individual single-walled carbon nanotubes. A novel sample preparation procedure provides well-separated, long and straight individual single-shell nanotubes. Diffraction experiments are carried out at 60kV, below the threshold for knock-on damage in carbon nanotubes. We describe experimental parameters that allow single-tube electron diffraction experiments with widely available thermal emission transmission electron microscopes. Further, we review the simulation of diffraction patterns for these objects.

cond-mat.mtrl-sci↗

arXiv2005

Raman-modes of index-identified free-standing single-walled carbon nanotubes

Using electron diffraction on free-standing single-walled carbon nanotubes we have determined the structural indices (n,m) of tubes in the diameter range from 1.4 to 3nm. On the same free-standing tubes we have recorded Raman spectra of the tangential modes and the radial breathing mode. For the smaller diameters (1.4-1.7nm) these measurements confirm previously established radial breathing mode frequency versus diameter relations, and would be consistent with the theoretically predicted proportionality to the inverse diameter. However, for extending the relation to larger diameters, either a yet unexplained environmental constant has to be assumed, or the linear relation has to be abandoned.

cond-mat.other↗

arXiv2005

Lattice structure and vibrational properties of the same nano-object

We present a procedure for determining independently the lattice structure and the vibrational properties of the same individual nano-object. For the example of an individual single-walled carbon nanotube we demonstrate the determination of the structural indices (n,m) of the nanotube by electron diffraction and of the frequencies of vibrational modes by micro-Raman spectroscopy. The precise and independent determination of both structure and mode frequencies allows for direct and unambiguous verification of molecular dynamical calculations and of conclusions drawn from Raman-only experiments.

cond-mat.mtrl-sci↗

arXiv2004

Carbon Nanotubes for Interconnect Applications

We briefly review the status of the application of carbon nanotubes (CNTs) for future interconnects and present results concerning possible integration schemes. Growth of single nanotubes at lithographically defined locations (vias) has been achieved which is a prerequisite for the use of CNTs as future interconnects. For the 20 nm node, a current density of 5 10^8 A/cm^2 and a resistance of 7.8 kOhm could be achieved for a single multi-walled CNT vertical interconnect.

cond-mat.mtrl-sci↗

arXiv2004

Microelectronic interconnects based on carbon nanotubes

Carbon nanotubes have emerged as a possible new material for electronic applications. They show promising characteristics for transistors as well as for interconnects. Here we review their basic properties and focus on the status of nanotubes with respect to their application as interconnects and discuss the challenges facing their integration.

cond-mat.mtrl-sci↗