Van der Waals (vdW) heterojunctions consisting of vertically-stacked individual or multiple layers of two-dimensional layered semiconductors, especially the transition metal dichalcogenides (TMDs), show novel optoelectronic functionalities due to the sensitivity of their electronic and optical properties to strong quantum confinement and interfacial interactions. Here, monolayers of n-type MoSe2 and p-type Mo1−x W x Se2 are grown by vapor transport methods, then transferred and stamped to form artificial vdW heterostructures with strong interlayer coupling as proven in photoluminescence and low-frequency Raman spectroscopy measurements. Remarkably, the heterojunctions exhibit an unprecedented photoconductivity effect that persists at room temperature for several days. This persistent photoconductivity is shown to be tunable by applying a gate bias that equilibrates the charge distribution. These measurements indicate that such ultrathin vdW heterojunctions can function as rewritable optoelectronic switches or memory elements under time-dependent photo-illumination, an effect which appears promising for new monolayer TMDs-based optoelectronic devices applications.

Purified single-walled nanotubes (SWNTs) were dispersed in an epoxy polymer and subjected to uniaxial compressive loading. The orientation and stress in the nanotubes were monitored in situ using polarized Raman microscopy. At strains less than 2%, the nanotubes reorient normal to the direction of compression, thereby minimizing the local strain energy. Above 2% strain, the Raman peak shift reaches a plateau. A new analytical model, which approximates the SWNT reorientation by varying the aspect ratio of a representative spheroid, predicted the rotation behavior of nanotubes under load. The results of this model suggest that the observed plateau of the Raman peak shift is caused by both polymer yielding and interfacial debonding at the ends of nanotubes.

Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.

Plasma enhanced chemical vapor deposition (PECVD) is a unique technique for growing vertically-aligned multiwall carbon nanotubes (VA-MWNTs) at controllable tube densities. This technique is of considerable importance for low temperature growth of VA-MWNTs at desired locations. However, the graphitic order of these MWNTs is inferior to those grown by laser ablation, arc discharge, and thermal CVD techniques. Previously, these VA-MWNTs were grown by a one-plasma approach (DC, microwave etc), either for gas decomposition or substrate biasing. Here, we describe a dual-RF plasma enhanced CVD (dual-RF-PECVD) technique that offers unique capability for controlling the graphitic order and diameters of VA-MWNTs.

The penetration of energetic pulsed ablation plumes through ambient gases is experimentally characterized to investigate a general phenomenon believed to be important to film growth by pulsed laser deposition (PLD). Under typical PLD conditions involving background gases, the ion flux in the ablation plume is observed to split into distinct fast and slow components over a limited range of distances1,2 the fast component is transmitted with near-initial velocities and high kinetic energies, potentially damaging to growing films at these distances. Formation of the second, significantly-slowed component correlates with the bright contact front3 formation observed1,4 in fast ICCD imaging studies. This general effect is explored in detail for the case of yttrium ablation into argon, a single-element target into an inert gas.5 Time-resolved optical absorption spectroscopy and optical emission spectroscopy are employed to simultaneously view the populations of both excited and ground states of Y and Y+ for comparison with quantitative intensified-CCD photography of the visible plume luminescence and ion flux measurements made with fast ion probes during this phenomenon. these measurements confirm that, in addition to the bright significantly-slowed front which has been described by shock or drag propagation models1, a fast-component of target material is transmitted to extended distances for some ambient pressures with near-initial velocities.

Gated photon counting spectroscopy and species-resolved ICCD photography have been applied to study the weak plasma luminescence which occurs following the propagation of the initial ablation plume in vacuum and during the ‘rebound’ of the plume with a substrate during pulsed laser deposition of amorphous diamond. These time- and spatially-resolved spectroscopic techniques were required in order to investigate notable differences between amorphous diamond-like carbon films formed by pulsed laser deposition from ArF (193 nm) and KrF (248 nm) irradiation of pyrolytic graphite in vacuum. Three principal regions of plume emission have been characterized: (1) a bright luminescent ball (v ∼3-5 cm/(μ.s) displaying nearly entirely C+ emission which appears to result from laser interaction with the initial ejecta, (2) a spherical ball of emission (v ∼1 cm/μs) displaying neutral carbon atomic emission lines and, at early times, jets of excited C2, and (3) a well-defined region of broadband emission (v ∼ 0.3 cm/μs) near the target surface first containing emission bands from C2, then weak, continuum emission thought to result from C3 and higher clusters and/or blackbody emission from hot clusters or nanoparticles. For both lasers, the measurements reveal an explosive interaction within the plume which results in a variety of new gas dynamic observations in vacuum:. These include (a) generation of instabilities or jets, (b) confinement of a residual part of the plume near the pellet surface, (c) cluster formation in the collisional, confined regions of the plume, and (d) reflection of the confined region backward to splash and redeposit on the pellet surface. Evidence for gas-phase formation of these clusters in vacuum is indicated from the dynamic evolution of the same cluster bands observed during the collision of the plume with the substrate surface during film growth. Addition of background gases strongly enhances the third (cluster) component, in accordance with plume-splitting phenomena. The combination of sensitive imaging and photon-counting diagnostic techniques permit an understanding of the importance of gas dynamic effects, such as clustering, on the time-of-flight distributions of species arriving during the deposition of thin films in both vacuum and background gases.

Fast intensified CCD photography and gated photon counting following KrF-laser irradiation of YBCO and BN targets reveal the first observations of very weak emission from slow-moving ejecta up to 2 cm from the target and times extending to ∼1.5 ms. Time-of-flight velocities inferred from the emission measurements indicate velocities (v ˜ (0.45−1.2) x 104 cm s−1) comparable to those measured for the large particles which often accompany the pulsed laser deposition process. Gated photon counting is employed to obtain temporally resolved spectra of this weak emission. The spectral shape is characteristic of blackbody emission, which shifts to longer wavelengths as the particles cool during flight in vacuum. Estimates of the temperature of the particles are made based on the emissivity of a perfect blackbody and range from 2200 K to 3200 K for both BN and YBCO when irradiated at Φ248 = 3.5 J cm−2 and 1.5 J cm−2, respectively. The temperature decrease of the particles in vacuum is compared to a radiative cooling model which gives estimates of the initial surface temperature and radii of the particles.

Spatial and temporal measurements of the optical absorption, optical emission and ion probe response in the ablation plume formed following pulsed 248 nm irradiation of Y1Ba2Cu3Ox are reported over laser energy densities from near threshold into the film growth regime. Time of flight absorbance-velocity profiles in vacuum indicate the formation and acceleration of a plasma front, with ions leading neutrals on the edge of the expanding plume. Ion probe screening measurements show that the laser plume is a well-shielded plasma with Debye lengths <10 μm at film deposition distances. Velocity distributions and estimates of ground state Ba+, Ba, Y+, and Y densities indicate that the populations of the ions outnumber those of the neutrals at high energy densities in vacuum. Measurements of the slowing of the plasma front and attenuation of the total charge reaching the substrate are reported for laser ablation in background pressures of oxygen. Absorption by ground state YO and BaO in the region close to the pellet indicates oxide densities ˜5 × 1013 cm−3 close to the pellet.

Optical absorption in the plume produced by excimer laser ablation of Y1Ba2Cu3O7‐x under film growth conditions has been observed for the first time and used to characterize the transport of ground state Y, Ba, and Cu neutrals as well as Y+ and Ba+ ions. Spatially and temporally resolved absorption measurements (0.6 mm, 20 ns resolution) indicate significant ground state number densities at times following the laser pulse that are up to an order of magnitude longer than the duration of fluorescence from excited states. Time‐of‐flight absorbance profiles result in velocity distributions that are broadened significantly toward lower velocities and reveal a low velocity component (<10 5 cm s_1) to the ablation process which is not observed using emission spectroscopy. Electric ion probe measurements of the time dependence of Y+ ions confirm the existence of the new, slow velocity component. Electron densities >1016 cm‐3 in the white plasma close to the pellet have been estimated using spectrally broadened emission lines. The effects of oxygen ambient pressures and the detection of YO, BaO, and CuO also are reported. This technique is applicable as an situ monitor of the kinetic energy of ablated species during low temperature deposition of epitaxial 1:2:3 superconducting thin films.