Jianyi Chen

Chemical Vapor Deposition of Large‐Sized Hexagonal WSe2 Crystals on Dielectric Substrates

High-quality large-sized hexagoal WSe2 crystals can be grown on dielectric substrates using atmospheric chemical vapor deposition in the presence of hydrogen gas. These hexagonal crystals (lateral width >160 um) have a carrier mobility of 100 cm(2) V(-1) s(-1) and a photoresponsivity of ≈1100 mA W(-1), which is comparable to that of exfoliated flakes.

Engineering Bandgaps of Monolayer MoS2 and WS2 on Fluoropolymer Substrates by Electrostatically Tuned Many-Body Effects.

Intrinsic electrical and excitonic properties of monolayer transition-metal dichalcogenides can be revealed on CYTOP fluoropolymer substrates with greatly suppressed unintentional doping and dielectric screening. Ambipolar transport behavior is observed in monolayer WS2 by applying solid-state back gates. The excitonic properties of monolayer MoS2 and WS2 are determined by intricate interplays between the bandgap renormalization, Pauli blocking, and carrier screening against carrier doping.

Chemical Vapor Deposition of High‐Quality Large‐Sized MoS2 Crystals on Silicon Dioxide Substrates

Large‐sized MoS2 crystals can be grown on SiO2/Si substrates via a two‐stage chemical vapor deposition method. The maximum size of MoS2 crystals can be up to about 305 μm. The growth method can be used to grow other transition metal dichalcogenide crystals and lateral heterojunctions. The electron mobility of the MoS2 crystals can reach ≈30 cm2 V−1 s−1, which is comparable to those of exfoliated flakes.

In Situ Observation and Electrochemical Study of Encapsulated Sulfur Nanoparticles by MoS2 Flakes

Sulfur is an attractive cathode material for next-generation lithium batteries due to its high theoretical capacity and low cost. However, dissolution of its lithiated product (lithium polysulfides) into the electrolyte limits the practical application of lithium sulfur batteries. Here we demonstrate that sulfur particles can be hermetically encapsulated by leveraging on the unique properties of two-dimensional materials such as molybdenum disulfide (MoS2). The high flexibility and strong van der Waals force in MoS2 nanoflakes allows effective encapsulation of the sulfur particles and prevent its sublimation during in situ TEM studies. We observe that the lithium diffusivities in the encapsulated sulfur particles are in the order of 10-17 m2 s-1. Composite electrodes made from the MoS2-encapsulated sulfur spheres show outstanding electrochemical performance, with an initial capacity of 1660 mAh g-1 and long cycle life of more than 1000 cycles.

Direct Synthesis of Large-Area 2D Mo2C on In Situ Grown Graphene

As a new member of the MXene group, 2D Mo2 C has attracted considerable interest due to its potential application as electrodes for energy storage and catalysis. The large-area synthesis of Mo2 C film is needed for such applications. Here, the one-step direct synthesis of 2D Mo2 C-on-graphene film by molten copper-catalyzed chemical vapor deposition (CVD) is reported. High-quality and uniform Mo2 C film in the centimeter range can be grown on graphene using a Mo-Cu alloy catalyst. Within the vertical heterostructure, graphene acts as a diffusion barrier to the phase-segregated Mo and allows nanometer-thin Mo2 C to be grown. Graphene-templated growth of Mo2 C produces well-faceted, large-sized single crystals with low defect density, as confirmed by scanning transmission electron microscopy (STEM) measurements. Due to its more efficient graphene-mediated charge-transfer kinetics, the as-grown Mo2 C-on-graphene heterostructure shows a much lower onset voltage for hydrogen evolution reactions as compared to Mo2 C-only electrodes.

Defect dynamics and spectral observation of twinning in single crystalline LaAlO3 under subbandgap excitation

We have investigated the photoluminescence and ultrafast dynamics of LaAlO3 crystal. The photoluminescence consists of a broad spectrum and two sharp peaks, which arise from various defect levels within the bandgap. A doublet splitting of roughly 6 nm is seen in these two sharp peaks. An Al displacement of 0.09 A in a sublattice, which is possible because of twinning, is adequate to explain the spectral splitting. Femtosecond pump probe experiments reveal further that many of these defect levels have a few picosecond decay times while the lowest defect states have decay times longer than nanosecond to the valence band.

Static and ultrafast dynamics of defects of SrTiO3 in LaAlO3/SrTiO3 heterostructures

A detailed defect energy level map was investigated for heterostructures of 26 unit cells of LaAlO3 on SrTiO3 prepared at a low oxygen partial pressure of 10−6 mbar. The origin is attributed to the presence of dominating oxygen defects in SrTiO3 substrate. Using femtosecond laser spectroscopy, the transient absorption and relaxation times for various transitions were determined. An ultrafast relaxation process of 2–3 ps from the conduction band to the closest defect level and a slower process of 70–92 ps from conduction band to intraband defect level were observed. The results are discussed on the basis of the proposed defect-band diagram.

Two-Dimensional Polymer Synthesized via Solid-State Polymerization for High-Performance Supercapacitors

Two-dimensional (2-D) polymer has properties that are attractive for energy storage applications because of its combination of heteroatoms, porosities and layered structure, which provides redox chemistry and ion diffusion routes through the 2-D planes and 1-D channels. Here, conjugated aromatic polymers (CAPs) were synthesized in quantitative yield via solid-state polymerization of phenazine-based precursor crystals. By choosing flat molecules (2-TBTBP and 3-TBQP) with different positions of bromine substituents on a phenazine-derived scaffold, C-C cross coupling was induced following thermal debromination. CAP-2 is polymerized from monomers that have been prepacked into layered structure (3-TBQP). It can be mechanically exfoliated into micrometer-sized ultrathin sheets that show sharp Raman peaks which reflect conformational ordering. CAP-2 has a dominant pore size of ∼0.8 nm; when applied as an asymmetric supercapacitor, it delivers a specific capacitance of 233 F g-1 at a current density of 1.0 A g-1, and shows outstanding cycle performance.

Homoepitaxial Growth of Large‐Scale Highly Organized Transition Metal Dichalcogenide Patterns

Controllable growth of highly crystalline transition metal dichalcogenide (TMD) patterns with regular morphology and unique edge structure is highly desired and important for fundamental research and potential applications. Here, single-crystalline MoS2 flakes are reported with regular trigonal symmetric patterns that can be homoepitaxially grown on MoS2 monolayer via chemical vapor deposition. The highly organized MoS2 patterns are rhombohedral (3R)-stacked with the underlying MoS2 monolayer, and their boundaries are predominantly terminated by zigzag Mo edge structure. The epitaxial MoS2 crystals can be tailored from compact triangles to fractal flakes, and the pattern formation can be explained by the anisotropic growth rates of the S and Mo edges under low sulfur chemical potential. The 3R-stacked MoS2 pattern demonstrates strong second and third-harmonic-generation signals, which exceed those reported for monolayer MoS2 by a factor of 6 and 4, correspondingly. This homoepitaxial growth approach for making highly organized TMD patterns is also demonstrated for WS2 .

Effect of Nb and Ta substitution on donor electron transport and ultrafast carrier dynamics in anatase TiO2 thin films

Ta and Nb substituted TiO2 are important transparent conducting oxides that have potential for applications in photovoltaics, photocatalysis, and water splitting/CO2 sequestration. In addition to donating electrons, what are the effects of Nb and Ta substitution? Here we observe strong experimental evidence that Ta and Nb substitution induces large and small polarons in anatase TiO2 epitaxial thin films. The degenerate donor electrons (from both Nb and Ta) show a high temperature T3 dependence on electrical resistivity, which confirms the presence of large polarons, along with room temperature metallic transport. This is further confirmed by the enhancement in the electron effective mass, which was estimated from thermopower measurements. Femtosecond transient absorption (fs-TA) reveals the life time of the Ti-t2g and eg levels and the separation of these levels are consistent with the X-ray absorption spectroscopy (XAS) measurement. In addition, fs-TA reveals the presence of small polarons with a life time substantially >1 ns, which arises from defect levels and is a consequence of Ta and Nb substitution. X-ray photoelectron spectroscopy (XPS) provides evidence of Ti3+, which may be identified as the defects responsible for the small polarons. These long-lived small polarons may provide a way to minimize recombination dynamics in TiO2-based electrodes for photo-excited devices.