[Show abstract][Hide abstract] ABSTRACT: The formation of semiconductor heterojunctions and their high-density integration are foundations of modern electronics and optoelectronics. To enable two-dimensional crystalline semiconductors as building blocks in next-generation electronics, developing methods to deterministically form lateral heterojunctions is crucial. Here we demonstrate an approach for the formation of lithographically patterned arrays of lateral semiconducting heterojunctions within a single two-dimensional crystal. Electron beam lithography is used to pattern MoSe2 monolayer crystals with SiO2, and the exposed locations are selectively and totally converted to MoS2 using pulsed laser vaporization of sulfur to form MoSe2/MoS2 heterojunctions in predefined patterns. The junctions and conversion process are studied by Raman and photoluminescence spectroscopy, atomically resolved scanning transmission electron microscopy and device characterization. This demonstration of lateral heterojunction arrays within a monolayer crystal is an essential step for the integration of two-dimensional semiconductor building blocks with different electronic and optoelectronic properties for high-density, ultrathin devices.
[Show abstract][Hide abstract] ABSTRACT: Two-dimensional (2D) van der Waals (vdW) heterostructures are a family of artificially-structured materials that promise tunable optoelectronic properties for devices with enhanced functionalities. Compared to stamping, direct epitaxy of vdW heterostructures is ideal for clean interlayer interfaces and scalable device fabrication. Here, we explore the synthesis and preferred orientations of 2D GaSe atomic layers on graphene (Gr) by vdW epitaxy. Guided by the wrinkles and grain boundaries on graphene, GaSe nuclei form that share a predominant lattice orientation. Due to vdW epitaxial growth many nuclei grow as perfectly aligned crystals and coalesce to form large (tens of microns), single-crystal flakes. Through theoretical investigations of interlayer energetics, and measurements of preferred orientations by atomic-resolution STEM and electron diffraction, a 10.9° interlayer rotation of the GaSe lattice with respect to the underlying graphene is found to be the most energetically preferred vdW heterostructure with the largest binding energy and the longest-range ordering. These GaSe/Gr vdW heterostructures exhibit an enhanced Raman E21g band of monolayer GaSe along with highly-quenched photoluminescence due to strong charge transfer. Despite the very large lattice mismatch of GaSe/Gr through vdW epitaxy, the predominant orientation control and convergent formation of large single-crystal flakes demonstrated here is promising for the scalable synthesis of large-area vdW heterostructures for the development of new optical and optoelectronic devices.
[Show abstract][Hide abstract] ABSTRACT: Imperfections in organometal halide perovskite films such as grain boundaries (GBs), defects, and traps detrimentally cause significant non-radiative recombination energy loss and decrease power conversion efficiency (PCE) in solar cells. Here, a simple layer-by-layer fabrication process based on air-exposure followed by thermal annealing is reported to grow perovskite films with large, single-crystal grains and vertically-oriented GBs. The hole-transport medium Spiro-OMeTAD is then infiltrated into the GBs to form vertically-aligned bulk heterojunctions. Due to the space-charge-regions in the vicinity of GBs, the non-radiative re-combination in GBs is significantly suppressed. The GBs be-come active carrier collection channels. Thus, the internal quantum efficiencies of the devices approach 100% in the visible spectrum range. The optimized cells yield average PCE of 16.3 ± 0.9%, comparable to the best solution-processed perovskite devices, establishing them as important alternatives to growing ideal single crystal thin films in the pursuit toward theoretical maximum PCE with industrially realistic processing techniques.
Journal of the American Chemical Society 07/2015; DOI:10.1021/jacs.5b03144 · 11.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Realizing the commercialization of high-performance and robust perovskite solar cells urgently requires the development of economically scalable processing techniques. Here we report a high-throughput ultrasonic spray-coating (USC) process capable of fabricating perovskite film-based solar cells on glass substrates with a power conversion efficiency (PCE) as high as 13%. Perovskite films with high uniformity, crystallinity, and surface coverage are obtained in a single step. Moreover, we report USC processing on TiO2/
ITO-coated polyethylene terephthalate (PET) substrates to realize flexible perovskite solar cells with a PCE as high as 8.1% that are robust under mechanical stress. In this case, a photonic curing technique was used to achieve a highly conductive TiO2 layer on flexible PET substrates for the first time. The high device performance and reliability obtained by this combination of USC processing with optical curing appear very promising for roll-to-roll manufacturing of high-efficiency, flexible perovskite solar cells.
[Show abstract][Hide abstract] ABSTRACT: Characterizing and controlling the interlayer orientations and stacking orders of two-dimensional (2D) bilayer crystals and van der Waals (vdW) heterostructures is crucial to optimize their electrical and optoelectronic properties. The four polymorphs of layered gallium selenide (GaSe) crystals that result from different layer stackings provide an ideal platform to study the stacking configurations in 2D bilayer crystals. Through a controllable vapor-phase deposition method, bilayer GaSe crystals were selectively grown and their two preferred 0° or 60° interlayer rotations were investigated. The commensurate stacking configurations (AA′ and AB stacking) in as-grown bilayer GaSe crystals are clearly observed at the atomic scale, and the Ga-terminated edge structure was identified using scanning transmission electron microscopy. Theoretical analysis reveals that the energies of the interlayer coupling are responsible for the preferred orientations among the bilayer GaSe crystals.
[Show abstract][Hide abstract] ABSTRACT: The slowing of Pt nanoparticles in argon background gas was characterized by Rayleigh scattering imaging using a plume of nanoparticles generated by femtosecond laser through thin film ablation of 20 nm-thick Pt films. The ablation was performed at threshold laser energy fluences for complete film removal to provide a well-defined plume consisting almost entirely of nanoparticles traveling with a narrow velocity distribution, providing a unique system to unambiguously characterize the slowing of nanoparticles during interaction with background gases. Nanoparticles of ∼200 nm diameter were found to decelerate in background Ar gas with pressures less than 50 Torr in good agreement with a linear drag model in the Epstein regime. Based on this model, the stopping distance of small nanoparticles in the plume was predicted and tested by particle collection in an off-axis geometry, and size distribution analysis by transmission electron microscopy. These results permit a basis to interpret nanoparticle propagation through background gases in laser ablation plumes that contain mixed components.
[Show abstract][Hide abstract] ABSTRACT: In situ optical diagnostics are used to reveal the isothermal nucleation and growth kinetics of graphene on Ni across a wide temperature range (560 °C < T < 840 °C) by chemical vapor deposition from single, sub-second pulses of acetylene. An abrupt, two-orders of magnitude change in growth times (∼100–1 s) is revealed at T = 680 °C. Above this temperature, sigmoidal kinetics are measured and attributed to autocatalytic nucleation and growth from carbon dissolved in the bulk of the Ni film. However, for T < 680 °C fast surface nucleation and growth occurring during the gas pulse appears responsible for the drastic alteration of the kinetics of subsequent dissolution-mediated growth. A simple and general kinetic model for isothermal graphene growth is developed that includes the nucleation phase and the effects of carbon solubility in metals, describes delayed nucleation, and allows the interpretation of the competition between surface- and bulk-nucleation and growth. The easily-implemented optical reflectivity diagnostics and the simple kinetic model described here allow a pathway to optimize the growth of graphene on metals with arbitrary carbon solubility.
[Show abstract][Hide abstract] ABSTRACT: Developing methods for the facile synthesis of two-dimensional (2D) metal chalcogenides and other layered materials is crucial for emerging applications in functional devices. Controlling the stoichiometry, number of the layers, crystallite size, growth location, and areal uniformity is challenging in conventional vapor-phase synthesis. Here, we demonstrate a method to control these parameters in the growth of metal chalcogenide (GaSe) and dichalcogenide (MoSe2) 2D crystals by precisely defining the mass and location of the source materials in a confined transfer growth system. A uniform and precise amount of stoichiometric nanoparticles are first synthesized and deposited onto a substrate by pulsed laser deposition (PLD) at room temperature. This source substrate is then covered with a receiver substrate to form a confined vapor transport growth (VTG) system. By simply heating the source substrate in an inert background gas, a natural temperature gradient is formed that evaporates the confined nanoparticles to grow large, crystalline 2D nanosheets on the cooler receiver substrate, the temperature of which is controlled by the background gas pressure. Large monolayer crystalline domains (∼100 μm lateral sizes) of GaSe and MoSe2 are demonstrated, as well as continuous monolayer films through the deposition of additional precursor materials. This PLD-VTG synthesis and processing method offers a unique approach for the controlled growth of large-area metal chalcogenides with a controlled number of layers in patterned growth locations for optoelectronics and energy related applications.
[Show abstract][Hide abstract] ABSTRACT: Synthesis of functional metal chalcogenide (GaSe) nanosheet networks by stoichiometric transfer of laser-vaporized material from bulk GaSe targets is presented. Uniform coverage of interconnected, crystalline, and photoresponsive GaSe nanosheets in both in-plane and out-of-plane orientations are achieved under different ablation conditions. The propagation of the laser-vaporized material is characterized by in situ ICCD-imaging. High (1 Torr) Ar background gas pressure is found to be crucial for the stoichiometric growth of GaSe nanosheet networks. Individual 1–3 layer GaSe triangular nanosheets of ≈200 nm domain size are formed within 30 laser pulses, coalescing to form nanosheet networks in as few as 100 laser pulses. The thickness of the deposited networks increases linearly with pulse number, adding layers in a two-dimensional (2D) growth mode. GaSe nanosheet networks show p-type semiconducting characteristics with mobilities reaching as high as 0.1 cm2V−1s−1. Spectrally-resolved photoresponsivities and external quantum efficiencies range from 0.4 AW−1 and 100% at 700 nm, to 1.4 AW−1 and 600% at 240 nm, respectively. Pulsed laser deposition under these conditions appears to provide a versatile and rapid approach to stoichiometrically transfer and deposit functional networks of 2D nanosheets with digital thickness control and uniformity for a variety of applications.
[Show abstract][Hide abstract] ABSTRACT: We report sputtering measurements of anorthite-like material, taken to be representative of soils found in the lunar highlands, impacted by singly and multicharged ions representative of the solar wind. The ions investigated include protons, as well as singly and multicharged Ar ions (as proxies for the non-reactive heavy solar wind constituents), in the charge state range +1 to +9, at fixed solar-wind-relevant impact velocities of 165 and 310 km/s (0.25 keV/amu and 0.5 keV/amu). A quartz microbalance approach (QCM) for determination of total sputtering yields was used. The goal of the measurements was to determine the sputtering contribution of the heavy, multicharged minority solar wind constituents in comparison to that due to the dominant H+ fraction. The QCM results show a yield increase of a factor of about 80 for Ar+ vs H+ sputtering and an enhancement by a factor of 1.67 between Ar9+ and Ar+, which is a clear indication of a potential sputtering effect.
Journal of Geophysical Research: Space Physics 10/2014; 119(10). DOI:10.1002/2014JA020140 · 3.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Compared with their bulk counterparts, atomically thin two-dimensional (2D) crystals exhibit new physical properties, and have the potential to enable next-generation electronic and optoelectronic devices. However, controlled synthesis of large uniform monolayer and multi-layer 2D crystals is still challenging. Here, we report the controlled synthesis of 2D GaSe crystals on SiO2/Si substrates using a vapor phase deposition method. For the first time, uniform, large (up to ~60 μm in lateral size), single-crystalline, triangular monolayer GaSe crystals were obtained and their structure and orientation were characterized from atomic scale to micrometer scale. The size, density, shape, thickness, and uniformity of the 2D GaSe crystals were shown to be controllable by growth duration, growth region, growth temperature, and argon carrier gas flow rate. The theoretical modeling of the electronic structure and Raman spectroscopy demonstrate a direct-to-indirect bandgap transition and progressive confinement-induced bandgap shifts for 2D GaSe crystals. The 2D GaSe crystals show p-type semiconductor characteristics and high photoresponsivity (~1.7 A/W under white light illumination) comparable to exfoliated GaSe nanosheets. These 2D GaSe crystals are potentially useful for next-generation electronic and optoelectronic devices such as photodetectors and field-effect transistors.
[Show abstract][Hide abstract] ABSTRACT: We describe a two-step approach for suppressing nucleation of graphene on Cu using chemical vapor deposition. In the first step, as received Cu foils are oxidized in air at temperatures up to 500°C to remove surface impurities and to induce the regrowth of Cu grains during subsequent annealing in H2 flow at 1040°C prior to graphene growth. In the second step, transient reactant cooling is performed by using a brief Ar pulse at the onset of growth to induce collisional deactivation of the carbon growth species. The combination of these two steps results in a three orders of magnitude reduction in the graphene nucleation density, enabling the growth of millimeter-size single crystal graphene grains. A kinetic model shows that suppressing nucleation promotes a cooperative island growth mode that favors the formation of large area single crystal graphene, and it is accompanied by a roughly 3 orders of magnitude increase in the reactive sticking probability of methane compared to that in random nucleation growth.
[Show abstract][Hide abstract] ABSTRACT: The synthesis of metal nanoparticles by ultrafast laser ablation of nanometers-thick metal films has been studied experimentally and computationally. Near-threshold backside laser ablation of 2–20 nm-thick Pt films deposited on fused silica substrates was found to produce nanoparticles with size distributions that were bimodal for the thicker films, but collapsed into a single mode distribution for the thinnest film. Time-resolved imaging of blackbody emission from the Pt nanoparticles was used to reveal the nanoparticle propagation dynamics and estimate their temperatures. The observed nanoparticle plume was compact and highly forward-directed with a well-defined collective velocity that permitted multiple rebounds with substrates to be revealed. Large-scale molecular dynamics simulations were used to understand the evolution of compressive and tensile stresses in the thicker melted liquid films that lead to their breakup and ejection of two groups of nanoparticles with different velocity and size distributions. Ultrafast laser irradiation of ultrathin (few nm) metal films avoids the splitting of the film and appears to be a method well-suited to cleanly synthesize and deposit nanoparticles from semitransparent thin film targets in highly directed beams.
[Show abstract][Hide abstract] ABSTRACT: We report total and mass resolved sputtering for H^+ and Ar^+q (q=1-9) ions incident on anorthite at 311 km/s, with enhanced O sputtering for Ar^+9 compared to Ar^+.
[Show abstract][Hide abstract] ABSTRACT: Simple kinetic models of carbon nanotube growth have been able to successfully link together many experimental parameters involved in the growth of carbon nanotubes for practical applications including the prediction of growth rates, terminal lengths, number of walls, activation energies, and their dependences on the growth environment. The implications of recent experiments utilizing in situ monitoring of carbon nanotube growth on our past kinetic model are first reviewed. Then, sub-second pulsed feedstock gas introduction is discussed to explore the nucleation and initial growth of carbon nanotubes in the context of the kinetic model. Moreover, kinetic effects in "pulsed CVD" - using repeated pulsed gas introduction to stop and restart nanotube growth - are explored to understand renucleation, the origin of alignment in nanotube arrays, and incremental growth. Time-resolved reflectivity of the surface is used to remotely understand the kinetics of nucleation and the coordinated growth of arrays. This approach demonstrates that continuous vertically aligned single wall carbon nanotubes can be grown incrementally by pulsed CVD, and that the first exposure of fresh catalyst to feedstock gas is critical to nanotubes site density required for coordinated growth. Aligned nanotube arrays (as short as 60 nm) are shown to nucleate and grow within single, sub-second gas pulses. The multiple-pulse growth experiments (> 100 pulses) show that a high fraction of nanotubes renucleate on subsequent gas pulses.
Proceedings of SPIE - The International Society for Optical Engineering 02/2014; DOI:10.1117/12.2045949 · 0.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The synthesis of metal nanoparticles by femtosecond laser vaporization of nm-thickness metal films is explored with the goal of comparing the salient features of femtosecond-based through thin film laser ablation (TTFA) to that of ns TTFA, and testing the feasibility of direct synthesis of clean nanoparticle alloys to explore the synthesis of carbon nanotubes by chemical vapor deposition. It is demonstrated that evaporated metal films are cleanly removed from quartz substrates using the technique, producing a highly forward-directed plume of nanoparticles (angle of divergence of ~2.5°) which were cleanly deposited onto different supports for analysis. TEM showed the nanoparticles were spherical with diameters that ranged from a few nm to hundreds of nm in a bimodal fashion. Unlike ns-TTFA, it was found that raising the pressure had no effect on the intensity of the smaller mode within the distribution, suggesting that nanoparticle formation by gas phase condensation was not at play under the present conditions. Close examination of size distributions from a 20 and 10nm Pt film revealed an 80nm downshift in the position of the large mode within the distribution, suggesting film thickness may provide a route to controlling the modal distribution of nanoparticles produced by this method. Lastly, particles sourced by a Fe/Mo bilayer film were found to be effective in growing single wall carbon nanotubes by atmospheric chemical vapor deposition, indicating sufficiently small and catalytically active particles were produced.
Proceedings of SPIE - The International Society for Optical Engineering 02/2014; DOI:10.1117/12.2045951 · 0.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The attractive optoelectronic properties of conducting polymers depend sensitively upon intra- and inter-polymer chain interactions, and therefore new methods to manipulate these interactions are continually being pursued. Here, we report a study of the isotopic effects of deuterium substitution on the structure, morphology and optoelectronic properties of regioregular poly(3-hexylthiophene)s with an approach that combines the synthesis of deuterated materials, optoelectronic properties measurements, theoretical simulation and neutron scattering. Selective substitutions of deuterium on the backbone or side-chains of poly(3-hexylthiophene)s result in distinct optoelectronic responses in poly(3-hexylthiophene)/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) photovoltaics. Specifically, the weak non-covalent intermolecular interactions induced by the main-chain deuteration are shown to change the film crystallinity and morphology of the active layer, consequently reducing the short-circuit current. However, side-chain deuteration does not significantly modify the film morphology but causes a decreased electronic coupling, the formation of a charge transfer state, and increased electron-phonon coupling, leading to a remarkable reduction in the open circuit voltage.
[Show abstract][Hide abstract] ABSTRACT: We demonstrate a facile technique to electrophoretically deposit homogenous assemblies of carbon nanohorns (CNHs) from common solvents such as acetone and water, onto nearly any substrate including insulators, dielectrics, and three-dimensional metal foams; in many cases without the aid of surfactants. This enables the generation of pristine film-coatings formed on timescales as short as a few seconds and on three-dimensional templates that enable the formation of freestanding polymer-CNH supported materials. As electrophoretic deposition is usually only practical on conductive electrodes, we emphasize our observation of efficient deposition on nearly any material, such as PTFE tape and dielectrics. The one-step versatility of deposition on these materials provides the capability to directly assemble CNH materials onto functional surfaces for a broad range of applications. In this manner, we utilized as-deposited CNH films as conductometric gas sensors exhibiting better sensitivity in comparison to equivalent single-walled carbon nanotube sensors. This gives a route toward scalable and inexpensive solution-based processing routes to manufacture functional nanocarbon materials for catalysis, energy, and sensing applications among others.
[Show abstract][Hide abstract] ABSTRACT: Co3O4 exhibits intriguing physical, chemical and catalytic properties and has demonstrated great potential for next-generation renewable energy applications. These interesting properties and promising applications are underpinned by its electronic structure and optical properties, which are unfortunately poorly understood and the subject of considerable debate over many years. Here, we unveil a consistent electronic structural description of Co3O4 by synergetic infrared optical and in situ photoemission spectroscopy as well as standard density functional theory calculations. In contrast to previous assumptions, we demonstrate a much smaller fundamental band gap, which is directly related to its efficient electro-/photo-activity. The present results may help to advance the fundamental understanding and provide guidance for the use of oxide materials in photocatalysis and solar applications.
[Show abstract][Hide abstract] ABSTRACT: The kinetics and mechanisms of graphene growth on Ni films at 720-880 °C have been measured using fast pulses of acetylene and real-time optical diagnostics. In situ UV-Raman spectroscopy was used to unambiguously detect isothermal graphene growth at high temperatures, measure the growth kinetics with ∼1 s temporal resolution, and estimate the fractional precipitation upon cooldown. Optical reflectivity and videography provided much faster temporal resolution. Both the growth kinetics and the fractional isothermal precipitation were found to be governed by the C2H2 partial pressure in the CVD pulse for a given film thickness and temperature, with up to ∼94% of graphene growth occurring isothermally within 1 second at 800 °C at high partial pressures. At lower partial pressures, isothermal graphene growth is shown to continue 10 seconds after the gas pulse. These flux-dependent growth kinetics are described in the context of a dissolution/precipitation model, where carbon rapidly dissolves into the Ni film and later precipitates driven by gradients in the chemical potential. The combination of pulsed-CVD and real-time optical diagnostics opens new opportunities to understand and control the fast, sub-second growth of graphene on various substrates at high temperatures.