Accelerated Carrier Recombination by Grain Boundary/Edge Defects in MBE Grown Transition Metal Dichalcogenides

Defect-carrier interaction in transition metal dichalcogenides (TMDs) play important roles in carrier relaxation dynamics and carrier transport, which determines the performance of electronic devices. With femtosecond laser time-resolved spectroscopy, we investigated the effect of grain boundary/edge defects on the ultrafast dynamics of photoexcited carrier in MBE grown MoTe2 and MoSe2. We found that, comparing with exfoliated samples, carrier recombination rate in MBE grown samples accelerates by about 50 times. We attribute this striking difference to the existence of abundant grain boundary/edge defects in MBE grown samples, which can serve as effective recombination centers for the photoexcited carriers. We also observed coherent acoustic phonons in both exfoliated and MBE grown MoTe2, indicating strong electron-phonon coupling in this materials. Our measured sound velocity agrees well with previously reported result of theoretical calculation. Our findings provide useful reference for the fundamental parameters: carrier lifetime and sound velocity, reveal the undiscovered carrier recombination effect of grain boundary/edge defects, both of which will facilitate the defect engineering in TMD materials for high speed opto-electronics

Nine-Lump Kinetic Study of Catalytic Pyrolysis of Gas Oils Derived from Canadian Synthetic Crude Oil

Catalytic pyrolysis of gas oils derived from Canadian synthetic crude oil on a kind of zeolite catalyst was conducted in a confined fluidized bed reactor for the production of light olefins. The overall reactants and products were classified into nine species, and a nine-lump kinetic model was proposed to describe the reactions based on appropriate assumptions. This kinetic model had 24 rate constants and a catalyst deactivation constant. The kinetic constants at 620°C, 640°C, 660°C, and 680°C were estimated by means of nonlinear least-square regression method. Preexponential factors and apparent activation energies were then calculated according to the Arrhenius equation. The apparent activation energies of the three feed lumps were lower than those of the intermediate product lumps. The nine-lump kinetic model showed good calculation precision and the calculated yields were close to the experimental ones

Carrier Trapping by Oxygen Impurities in Molybdenum Diselenide

Understanding defect effect on carrier dynamics is essential for both fundamental physics and potential applications of transition metal dichalcogenides. Here, the phenomenon of oxygen impurities trapping photo-excited carriers has been studied with ultrafast pump-probe spectroscopy. Oxygen impurities are intentionally created in exfoliated multilayer MoSe2 with Ar+ plasma irradiation and air exposure. After plasma treatment, the signal of transient absorption first increases and then decreases, which is a signature of defect capturing carriers. With larger density of oxygen defects, the trapping effect becomes more prominent. The trapping defect densities are estimated from the transient absorption signal, and its increasing trend in the longer-irradiated sample agrees with the results from X-ray photoelectron spectroscopy. First principle calculations with density functional theory reveal that oxygen atoms occupying Mo vacancies create mid-gap defect states, which are responsible for the carrier trapping. Our findings shed light on the important role of oxygen defects as carrier trappers in transition metal dichalcogenides, and facilitates defect engineering in relevant material and device applications

Stacking Order Driven Optical Properties and Carrier Dynamics in ReS2

Two distinct stacking orders in ReS2 are identified without ambiguity and their influence on vibrational, optical properties and carrier dynamics are investigated. With atomic resolution scanning transmission electron microscopy (STEM), two stacking orders are determined as AA stacking with negligible displacement across layers, and AB stacking with about a one-unit cell displacement along the a axis. First-principle calculations confirm that these two stacking orders correspond to two local energy minima. Raman spectra inform a consistent difference of modes I & III, about 13 cm-1 for AA stacking, and 20 cm-1 for AB stacking, making a simple tool for determining the stacking orders in ReS2. Polarized photoluminescence (PL) reveals that AB stacking possesses blue-shifted PL peak positions, and broader peak widths, compared with AA stacking, indicating stronger interlayer interaction. Transient transmission measured with femtosecond pump probe spectroscopy suggests exciton dynamics being more anisotropic in AB stacking, where excited state absorption related to Exc. III mode disappears when probe polarization aligns perpendicular to b axis. Our findings underscore the stacking-order driven optical properties and carrier dynamics of ReS2, mediate many seemingly contradictory results in literature, and open up an opportunity to engineer electronic devices with new functionalities by manipulating the stacking order

Defect-modulated thermal transport behavior of BAs under high pressure

Boron arsenide (BAs) is a covalent semiconductor with a theoretical intrinsic thermal conductivity approaching 1300 W/m K. The existence of defects not only limits the thermal conductivity of BAs significantly but also changes its pressure-dependent thermal transport behavior. Using both picosecond transient thermoreflectance and femtosecond time-domain thermoreflectance techniques, we observed a non-monotonic dependence of thermal conductivity on pressure. This trend is not caused by the pressure-modulated phonon–phonon scattering, which was predicted to only change the thermal conductivity by 10%–20%, but a result of several competing effects, including defect–phonon scattering and modification of structural defects under high pressure. Our findings reveal the complexity of the defect-modulated thermal behavior under pressure.The authors are grateful for the support from the National Science Foundation (NASCENT, Grant No. EEC-1160494; Center for Dynamics and Control of Materials DMR-1720595; CBET- 2211660); F.T., Z.R., and L.S. were supported by the Office of Naval Research under Multidisciplinary University Research Initiative (Grant No. N00014-16-1-2436).Center for Dynamics and Control of Material

Strain tuning of thermal, electrical and optical properties of semiconductors

The discovery of graphene by mechanical exfoliation has opened a new realm of research. Compared to traditional 3D crystal structures, 2D materials are characterized by strong in-plane covalent bond and weak interlayer van der Waals force, giving them unique 2D crystal structure. In the past ten years, various 2D materials have been explored with very different electronic properties, ranging from wide band-gap insulators to conductors. Owing to the different bond strength, 2D materials behave differently in their electrical, thermal properties along the cross-plane and in-plane direction. In addition, the in-plane electrical, optical and thermal properties are also found to be anisotropic for some particular 2D materials due to the asymmetric crystal structure. Both the in-plane and cross-plane anisotropic properties of 2D materials give rise to a possibility to design the micro/nano devices in various applications. The intrinsic properties of TMDs can be further adjusted by external factors, such as electrical fields, temperature, magnetic field, et al. Among all the external stimulations, strain has been shown an effective method to control the electronic, thermal, optical properties of semiconductors. With the discovery of 2D materials, the application of strain tuning has been growing since the reduced dimensional structures can sustain much larger strains than bulk crystals. In this dissertation, in-plane anisotropic nonlinear optical nonlinearity is studies with an Intensity-scan spectroscopy at ambient conditions. Then a diamond anvil cell (DAC) device is employed to generate large strain on MoS₂. With our home-built pico-second Transient Thermoreflectance technique, ~7x enhancement in cross-plane is observed due to the pressure/strain modified interlayer interaction. Moreover, photoluminescence and Raman spectroscopy are used to probe the impurity levels in BAs crystal. Pressure/Strain modified impurity level change will also have significant effect on this high thermal conductivity material. Lastly, a modified pico-second Transient Thermoreflectance system is developed to achieve simultaneous measurement on thermal conductivity and specific heat of materials.Mechanical Engineerin

Nine-Lump Kinetic Study of Catalytic Pyrolysis of Gas Oils Derived from Canadian Synthetic Crude Oil

Catalytic pyrolysis of gas oils derived from Canadian synthetic crude oil on a kind of zeolite catalyst was conducted in a confined fluidized bed reactor for the production of light olefins. The overall reactants and products were classified into nine species, and a nine-lump kinetic model was proposed to describe the reactions based on appropriate assumptions. This kinetic model had 24 rate constants and a catalyst deactivation constant. The kinetic constants at 620 ∘ C, 640 ∘ C, 660 ∘ C, and 680 ∘ C were estimated by means of nonlinear least-square regression method. Preexponential factors and apparent activation energies were then calculated according to the Arrhenius equation. The apparent activation energies of the three feed lumps were lower than those of the intermediate product lumps. The nine-lump kinetic model showed good calculation precision and the calculated yields were close to the experimental ones