MoS2 Substrate Research Articles

Ionic liquid-mediated epitaxy of high-quality C60 crystallites in a vacuum

Ionic liquid-mediated vacuum deposition was demonstrated as a novel growth technique for high-quality C60 crystallites. In this process the ionic liquid worked as a solvent, i.e. the gas phase C60 precursors were continuously fed into the ionic liquid on a substrate in a vacuum chamber, from which the nucleation and the subsequent growth of C60 crystallites proceeded. The impact of varying the substrate on the growth behaviour was intensively examined in terms of not only the lattice-mismatch between C60 and the substrates, but also the wetting behavior of the ionic liquid on the substrates. This led to a significant improvement of the C60 crystallinity and epitaxial growth was successfully found on certain substrates. In particular, molecularly smooth and thick C60 hexagonal-shaped crystallites epitaxially grew on a MoS2 substrate, the average size of which reached as large as 10 μm.

A procedure to determine the optimum imaging parameters for atomic/molecular resolution frequency modulation atomic force microscopy

We propose a general procedure to determine the optimum imaging parameters (spring constant and oscillation amplitude) to obtain the optimum resolution in frequency modulation atomic force microscopy. We calculated the effective signal-to-noise ratio for various spring constants and oscillation amplitudes, based on the measurement of frequency shift and energy dissipation versus tip-sample distance curves, to find the optimum. We applied this procedure for imaging a lead phthalocyanine (PbPc) thin film on a MoS(2)(0001) substrate, and found that the optimum parameters were about 5 N/m and 20 nm, respectively. An improved signal-to-noise ratio was attained in a preliminary experiment using parameters which were close to the calculated optimum.

Transferring metallic nano-island on hydrogen passivated silicon surface for nano-electronics

In a planar configuration, precise positioning of ultra-flat metallic nano-islands on semiconductor surface opens a way to construct nanostructures for atomic scale interconnects. Regular triangular Au nano-islands have been grown on atomically flat MoS2 substrates and manipulated by STM to form nanometer gap metal-pads connector for single molecule electronics study. The direct assembly of regular shaped metal nano-islands on H-Si(100) is not achievable. Here we present how to transfer Au triangle nano-islands from MoS2 onto H-Si(100) in a clean manner. In this experiment, clean MoS2 substrates are patterned as array of MoS2 pillars with height of 8 μm. The Au triangle nano-islands are grown on top of the pillars. Successful printing transfer of these Au nano-islands from the MoS2 pillars to the H-Si(100) is demonstrated.

Molecular-resolution imaging of lead phthalocyanine molecules by small amplitude frequency modulation atomic force microscopy using second flexural mode

Lead phthalocyanine molecules on MoS2(0001) substrates were imaged using an ultrahigh-vacuum AFM apparatus equipped with an optical beam deflection (OBD) sensor. The second flexural mode was employed to utilize its high effective spring constant in order to reduce the oscillation amplitude to 0.5 nm without oscillation instability. Submolecular-resolution images were obtained when a shorter cantilever, which had a very high resonance frequency and a low noise equivalent deflection in the OBD sensor, was used.

Atomic force microscopy manipulation with ultrasonic excitation

The exposure of nanostructures to ultrasound renders the opportunity to explore novel methods to control their assembly. In the presence of surface ultrasonic vibration, large Sb nanoparticles on MoS2 substrates can be laterally displaced using the tip of a compliant AFM cantilever by just increasing the ultrasonic excitation amplitude. Magnetic nanoparticles of about 10–2 nm in diameter are swept by the tip when scanning in contact mode in the absence of ultrasound, but remain undisturbed in the presence of low amplitude ultrasonic vibration. In Atomic Force Microscopy-assisted manipulation of nanoparticles, ultrasonic vibration affects both tip-particle and particle-surface frictional properties.

Epitaxial Growth of C60 Thin Films using Continuous-wave Laser Molecular Beam Epitaxy

ABSTRACTWe have fabricated C60 thin films on various substrates (mica, MoS2, HOPG, LiF, NaCl, KBr, KCl and CaF2) by using continuous-wave laser molecular beam epitaxy (CWL-MBE), which is very suitable technique to grow epitaxial organic thin films because of good controllability of evaporation as compared with Knudsen-cell method. The films were evaluated by reflection high-energy electron diffraction with micro channel imaging plate (MCP-RHEED) and atomic force microscopy (AFM). AFM images of the C60 films on mica, MoS2 and HOPG substrates show flat and homogeneous, morphology, and epitaxial growth of the films on mica and MoS2 substrates were observed by RHEED. This result shows mica, MoS2, HOPG substrates are good candidates for epitaxial growth of C60 thin films.

Molecular Orientation and Electronic Structure of Epitaxial Bucky Ferrocene (Fe(C60(CH3)5)C5H5) Thin Films

Single crystalline epitaxial films of bucky ferrocene (Fe(C60(CH3)5)C5H5), a hybrid molecule of C60 and ferrocene, were grown on MoS2(0001) substrates. By using reflection high-energy electron diffraction (RHEED) amplified by a microchannel plate (MCP), it was found that the lattice constant of the film decreased from 11.3 ± 0.2 A at 1−2 monolayer (ML) to 10.2 ± 0.2 A at 3 ML or thicker. Both of the lattice constants were incommensurate with that of the substrate, which indicates a new type of epitaxial growth. Valence electronic structures of the films at each thickness were measured with ultraviolet photoelectron spectroscopy (UPS) and the energy levels of molecular orbitals were identified in relation to the molecular orientation in the epitaxial film.

Bistable nematic and smectic anchoring in the liquid crystal octylcyanobiphenyl (8CB) adsorbed on a MoS2 single crystal.

We have studied the anchoring directions imposed on 4-n-octyl-4(')-cyanobiphenyl (8CB) smectic-A and nematic phases by a single crystal of molybdenum disulfide (MoS2). Combining optical microscopy and x-ray diffraction under grazing incidence we have demonstrated the occurrence of a bistable planar anchoring. A previous study of the two-dimensional (2D) network of adsorbed 8CB molecules under the liquid crystal film allows a direct connection to be made between the interface structure and the anchoring directions, demonstrating that bistability is induced by the presence of two dipolar groups in the skeleton of the 2D network. It is demonstrated that the Landau-de Gennes theory cannot account for the observed anchoring in the nematic phase. The Landau-de Gennes free energy has to be associated with a coupling with both the surface order and the MoS2 substrate to explain the experimental observations. The hypothesis of a nematic layer under the liquid crystal bulk is postulated in the smectic phase.

Low energy electron diffraction of the system In-[perylene-3,4,9, 10-tetracarboxylic dianhydride] on MoS2

The system In-[perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA)] on MoS2, prepared by the sequential evaporation of PTCDA and In on a cleaved MoS2 surface, was studied by low energy electron diffraction. The result indicates that reaction products form an ordered structure on the MoS2 surface. From the analysis of the diffraction pattern, the presence of six symmetry-equivalent domains of an oblique unit cell of In-PTCDA species results with the dimensions of 9.5 Å, 16.3 Å, and an enclosed angle of 80.2°. In addition, splitting in two domains by a mirror plane exists with the rotation angle R=±10.8° with respect to each of the three equivalent surface crystal axes of the MoS2 substrate. The new structure is explained by assuming that four In atoms are chemically bonded to the four carbonyl groups of the PTCDA molecules. Furthermore, it is concluded that the In4PTCDA species become tilted after a chemical reaction between the PTCDA molecules and the In atoms, which is in agreement with results previously obtained by angle-resolved ultraviolet photoemission experiments.

Microtribological properties of ultrathin C60 films grown by molecular beam epitaxy

Molecular beam epitaxy (MBE) has been applied to a condensation of thin C60 films with high crystallinity. In situ measurements with reflection high energy electron diffraction (RHEED) showed an epitaxial growth of C60 film with a lattice constant of 1.0 nm on a MoS2 substrate. Atomic force microscopy (AFM) and friction force microscopy (FFM) revealed a low friction coefficient of 0.012, the lowest value reported to date for C60.

Molecular beam epitaxy of GaN on a substrate of MoS 2 layered compound

The feasibility of MoS2 layered compound as a substrate for GaN growth was investigated. GaN films were successfully grown on MoS2 by plasma-enhanced molecular beam epitaxy and the crystal quality of GaN on MoS2 was compared with that on Al2O3. For GaN grown on MoS2 substrate, it was found that the surface flatness observed by atomic force microscopy, stress in the film measured by Raman spectroscopy, optical properties measured by photoluminescence spectroscopy, and threading dislocation density observed by transmission electron microscopy show superior properties compared with that grown on Al2O3. These results suggest the layered compound such as MoS2, which has no dangling bonds on the surface and has lattice mismatching of 0.9% to GaN, has high potential for a substrate of GaN growth.

Structure of copper- and H2-phthalocyanine thin films on MoS2 studied by angle-resolved ultraviolet photoelectron spectroscopy and low energy electron diffraction

Angle-resolved ultraviolet photoelectron spectra (ARUPS) of copper phthalocyanine (CuPc) and metal-free phthalocyanine (H2Pc) films (thickness from monolayer to 50–80 Å) on cleaved MoS2 substrates were measured using monochromatic synchrotron radiation. Observed take-off angle (θ) and azimuthal angle (φ) dependencies of the top π band intensity were analyzed quantitatively by the single-scattering approximation theory combined with molecular orbital calculations. The analysis indicated that the molecules lie flat on the MoS2 surface in monolayer films of CuPc and H2Pc. The azimuthal orientation of the molecules (angle between molecular axis and surface crystal axis of MoS2), was found to be about −7°, −37°, or −67° for both monolayer films of CuPc and H2Pc. In the azimuthal orientation, the analyses indicated that there are only molecules with conterclockwise rotation, although both clockwise and counterclockwise rotations are expected. From the low energy electron diffraction, the two-dimensional lattice structure of the monolayer film was obtained. On the basis of these two kinds of experimental results, the full structure of the monolayer film, the two-dimensional lattice and the molecular orientation at the lattice points, was determined. Furthermore, for the thick films it is found from the analyses of ARUPS that CuPc and H2Pc molecules tilt about 10° from the surface plane.

Probing Single Nanometer-scale Particles with Scanning Tunneling Microscopy and Spectroscopies

Scanning tunneling microscopy can be used to isolate single particles on surfaces for further study. Local optical and electronic properties coupled with topographic information collected by the scanning tunneling microscope (STM) give insight into the intrinsic properties of the species under study. Since each spectroscopic measurement is done on a single particle, each sample is ‘monodisperse’, regardless of the degree of heterogeneity of the original preparation. We illustrate this with three example systems – a metal cluster of known atomic structure, metal nanoparticles dispersed from colloid suspensions, and metallocarbohedrenes (Met-Cars) deposited with other reaction products. Au and Ag nanoparticles were imaged using a photon emission STM. The threshold voltage, the lowest bias voltage at which photons are produced, was determined for Au nanoparticles. Electronic spectra of small clusters of Ni atoms on MoS2 were recorded. Preliminary images of Zr-based Met-Car-containing soot were obtained on Au and MoS2 substrates and partial electronic spectra were recorded of these possible Met-Car particles.

Initial states of epitaxial growth of vanadyl- and titanyl-phthalocyanines on MoS2 cleaved surfaces

The initial states of epitaxial growth of vanadyl-phthalocyanine (VOPc) and titanyl-phthalocyanine (TiOPc) on MoS2 cleaved surfaces have been investigated, using atomic force microscopy (AFM) and electron energy loss spectroscopy (EELS). In the case of VOPc, formation of the first layer at room temperature which consists of one molecular layer on a MoS2 surface was observed in the AFM image. In the case of TiOPc, in contrast, the first layer consisted of two molecular layers formed at room temperature, and the formation of one molecular layer occurred at the annealing at 420 K. These results indicate that the interaction between the VOPc molecule and the MoS2 surface is stronger than that between TiOPc and MoS2. This suggests that the overlap of orbitals of the transition metals at the center of the phthalocyanines with the orbitals of MoS2 substrate creates a large stabilization energy. The results of EELS measurement indicate that the molecular plane of VOPc is parallel to the MoS2 surface, while the molecular plane of TiOPc is inclined.

FIRST-PRINCIPLES STUDY OF STATIC NANOSCALE FRICTION BETWEEN MOO3 AND MOS2

The motion of a nanoscale MoO3 crystal on an MoS2 substrate is an ideal case for the study of the effects of microstructure on macroscopic phenomena like friction, because both components of this system can be prepared with atomically flat surfaces. We apply a recently developed real-space density functional theory method to investigate the energetics of sliding an MoO3 crystal on an MoS2 substrate. We then link the results to simple models based on continuum elastic theory, in order to study the mechanical behavior of the oxide crystals under loads that correspond to realistic situations. Based on these results, we extract estimates of the force which must be applied with an atomic force microscope in order to move the oxide crystal on the substrate, for different orientations of the two components.

Static Frictional Forces at Crystalline Interfaces

A statistical thermodynamic description of the atomic force microscope is developed and used to compute the force of static friction for a single-atom tip on a hexagonal close-packed substrate surface under constant load in vacuum. The substrate atoms are taken to be independent isotropic harmonic oscillators, and the tip−substrate interaction is taken to be Lennard-Jones (12,6). Movement of the tip is treated as a quasistatic (reversible) process. The force of static friction (i.e., the maximum of the component of the force parallel to the direction of movement of the tip) is computed by the Monte Carlo technique for several crystallographic directions (paths) and found to be strongly anisotropic. The frictional force is minimum along a particular path where rows of substrate atoms form a “groove”; it is up to 2 orders of magnitude greater for the path perpendicular to the groove. The dependence of the frictional force on the hardness of the substrate (as measured by the force constant of the substrate harmonic potential) and on temperature was examined for these two extreme paths. For hard substrates the frictional force is nearly linear with load. As the substrate gets softer, or as the temperature increases at fixed hardness, the frictional force declines. The results of the computations are correlated with recent experimental observations on the sliding of nanocrystals of MoO3 over an MoS2 substrate.

Nanostructure fabrication by selective growth of molecular crystals on layered material substrates

Nanostructures consisting of C60 molecules were fabricated on a GaSe/MoS2 heterostructure. A submonolayer film of a lamellar compound semiconductor GaSe was grown on a MoS2 substrate to form nanoscale holes or grooves surrounded by monolayer steps. Atomic force microscope (AFM) observation indicates that C60 molecules grow only in the bare MoS2 nanoregions at a substrate temperature of 180 °C. C60 molecules fill up those holes and grooves, and nanoscale C60 domains with polygonal shapes can be formed. This selective growth method can be combined with nanoscale patterning made by a scanning tunneling microscope or AFM to produce nanostructures of molecular crystals with designed shapes.

Electrochemical Heteroepitaxial Growth of Molecular Films on Ordered Substrates

ABSTRACTElectrocrystallization of the organic superconductor bis(ethylenedithio)tetrathiafulvalene triiodide, (ET)2I3, on a highly ordered pyrolytic graphite (HOPG) substrate has been visualized using in situ atomic force microscopy (AFM). Previous studies have revealed the formation of a coincident epitaxial monolayer with a structure identical to that of a (001) layer of the superconducting beta phase of this material prior to bulk crystal growth. However, the symmetry of the HOPG substrate leads to domain boundary defects during self assembly of the separately growing domains. The number of defects is significantly reduced after an electrochemical annealing process in which the potential is cycled about the monolayer deposition potential. Annealing of these films is important if they are to be used in electronic devices as the defects may serve as barriers to electron transport in the two-dimensional layers.In addition to (ET)2I3, monolayer growth also has been visualized during electrocrystallization of (ET)2ReO4 on HOPG. The role of coincident epitaxy with HOPG in the monolayer formation and molecular orientation will be discussed.Structurally modified substrates have also been investigated. Studies of the electrocrystallization of (ET)2I3 on HOPG, thermally treated to produce well-defined monolayer depth pit structures, have demonstrated that (ET)2I3 monolayer growth can occur inside the pit structures. The presence of monolayer domain boundaries within large pits indicates that multiple, independent nucleation events can occur in the pits, providing an opportunity to determine critical nucleation sizes by varying the pit dimensions. Recently we have discovered that MoS2 substrates can be electrochemically etched, giving rise to monolayer deep triangular pits.

Observation of Outermost Surface Layers of Phthalocyanine Ultra-Thin Films by Penning Ionization Electron Spectroscopy: Chloroaluminium Phthalocyanine on MoS2

Abstract Using Penning ionization electron spectroscopy, which provides information on the local distribution of the individual orbitals exposed outside the outermost surface layer, ultra-thin films (1 to several monolayers) of chloroaluminium phthalocyanine were characterized during layer-by-layer vacuum deposition onto an MoS2 substrate and the change in the molecular orientation was sensitively detected.

Photoelectron Angular Distribution of Thin Films of Copper Phthalocyanine on MoS2 Surfaces: Quantitative Determination of Molecular Orientation

Abstract The angle-resolved ultraviolet photoelectron spectra of ultrathin films of copper phthalocyanine (CuPc) on an MoS2 substrate were measured using synchrotron radiation. The azimuthal angle dependence of the photoelectron intensity was analyzed using independent-atomic-center approximation combined with molecular orbital calculation. The results indicate that CuPc molecules lie flat on the substrate and that the film grows epitaxially with respect to the three equivalent directions of the MoS2 surface.