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The first η1-gallacyclopentadienyl complex 1 was synthesized in solution by reacting dichlorogallyliron complex [Cp*(OC)2FeGaCl2] (2, Cp*: η5-C5Me5) with zirconacyclopentadiene [Cp2Zr(C4Me4)] (3, Cp: η5-C5H5). It was isolated as its Lewis base-stabilized form [Cp*(OC)2Fe{GaC4Me4(DMAP)}] (4, DMAP: 4-(dimethylamino)pyridine) in 47% yield. X-ray crystal structure analysis for 4 revealed almost planar gallacyclopentadienyl core with C-C bond-length alternation. [ABSTRACT FROM AUTHOR]

A low-temperature growth method for crystalline thin films of transition metal dichalcogenides (TMDCs) is urgently required to use various materials as substrates. In this paper, we report the growth of crystalline tungsten disulfide (WS2) thin films at a low growth temperature of 400 °C by using organic liquid precursors: bis(tert-butylimido)bis(dimethylamido)tungsten(VI) [(t-BuN)2W(NMe2)2] and di-tert-butyl disulfide [(t-C4H9)2S2]. To achieve low-temperature growth, these precursors were introduced onto a substrate through pulse-controlled valves that are similar to the atomic layer deposition process. Furthermore, we determined that a crystalline WS2 film can be grown even at 350 °C on the entire surface of a SiO2/Si substrate by first depositing a catalytic metal film (W or Au) onto a limited area of the substrate, although almost no growth of the crystalline WS2 film was observed at 350 °C without pre-deposition of the metal film. [ABSTRACT FROM AUTHOR]

Understanding of the optical properties at a specific site in two-dimensional materials is important for their opto-electronic applications. Here, we investigated the site dependence of the relationship between the photoluminescence spectrum and an electric field in monolayer and bilayer molybdenum disulfide (MoS2) using a technique combining scanning near-field optical microscopy (SNOM) and a bias voltage application from the SNOM cantilever. We revealed the difference between two factors, doping level and PL quantum efficiency, at each site and their significant influence on the relationship between PL intensity and bias voltage, evaluating the specific influences of the two factors on the relationship. In addition, we clarified that carrier injection at a site in the bilayer region induces an increase in the A peak intensity and a decrease in the I peak, discussing the background in view of theoretical calculations. [ABSTRACT FROM AUTHOR]

Chemical mist deposition (CMD) of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) was investigated in terms of cavitation frequency f, solvent, flow rate of nitrogen, substrate temperature Ts, and substrate dc bias Vs as variables for efficient PEDOT:PSS/crystalline silicon (c-Si) heterojunction solar cells. The high-speed-camera and differential mobility analysis characterizations revealed that the average size and flux of PEDOT:PSS mist depend on f, type of solvent, and Vs. Film deposition occurred when positive Vs was applied to the c-Si substrate at Ts of 30–40 °C, whereas no deposition of films occurred with negative Vs, implying that the film is deposited mainly from negatively charged mist. The uniform deposition of PEDOT:PSS films occurred on textured c-Si(100) substrates by adjusting Ts and Vs. The adhesion of CMD PEDOT:PSS film to c-Si was greatly enhanced by applying substrate dc bias Vs compared with that of spin-coated film. The CMD PEDOT:PSS/c-Si heterojunction solar cell devices on textured c-Si(100) in 2 × 2 cm2 exhibited a power conversion efficiency η of 11.0% with better uniformity of the solar cell parameters. Furthermore, η was increased to 12.5% by adding an AR coating layer of molybdenum oxide MoOx formed by CMD. These findings suggest that CMD with negatively charged mist has great potential for the uniform deposition of organic and inorganic materials on textured c-Si substrates by suitably adjusting Ts and Vs. [ABSTRACT FROM AUTHOR]

Crystalline silicon double-heterojunction solar cells were fabricated using Si/organic and Si/Cs2CO3 heterojunctions. The front heterojunction is formed by spin-coating conductive polymer poly(3,4-ethyenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) on n-type Czochralski (CZ) (100) silicon, which separates the photogenerated carriers and blocks the electron dark current while allowing the photocurrent to pass through. The rear heterojunction, formed by spin-coating Cs2CO3 and polyethylenimine (PEI) dissolved in 2-ethoxyethanol and Al metal evaporation, functions as a back surface field that reduces the hole dark current while allowing the electron photocurrent to pass through. The double-heterojunction device showed a power conversion efficiency of 12.7% under AM1.5G simulated solar light exposure. [ABSTRACT FROM AUTHOR]

Molybdenum ditelluride (MoTe2) is known to have two different crystal phases: α-MoTe2 and β-MoTe2. α-MoTe2 is the polytype stable below 815 °C, and β-MoTe2 is stable above 900 °C. Here, we report changes in the structure and composition of each single crystal during heating in low and high vacuum conditions. Contrary to the results of a recently published paper, we did not observe α-MoTe2 transforming to β-MoTe2 at elevated temperatures. In a low vacuum, the α-MoTe2 sample was mainly oxidized, forming MoO2. In a high vacuum, it partially decomposed with surface oxidation. The recently reported “reversible phase transition” of α-MoTe2 is the misinterpretation of X-ray diffraction data. [ABSTRACT FROM AUTHOR]

We report an enhancement in the efficiency of poly(3,4-ethylenedioxythiophene):poly(stylene sulfonate) (PEDOT:PSS)/n-Si hybrid solar cell by incorporating silver nanoparticles (AgNPs) with PEDOT:PSS. AgNPs were prepared by reducing silver nitrate in green-tea solution, which showed characteristic absorption peak due to the surface plasmonic resonance effect. AgNPs incorporated PEDOT:PSS/n-Si hybrid device shows power conversion efficiency (η) of 10.21%, which is comparatively higher to the performance of pristine device without AgNPs. Here, we noticed that incorporation of AgNPs decreases sheet resistance and enlarged surface roughness of PEDOT:PSS film for the efficient collection of charges, rather than plasmonic effect. [ABSTRACT FROM AUTHOR]

We demonstrate the potential of barium hydroxide, Ba(OH)2, as a hole blocking layer on the photovoltaic performance of front- and back-organic/n-type crystalline silicon (n-Si) heterojunction solar cells with poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)(PEDOT:PSS). The power conversion efficiency (PCE) of the front-PEDOT:PSS/n-Si heterojunction solar cell was increased from 12.8% for pristine to 13.6% with a 2-nm-thick Ba(OH)2 interlayer at the rear n-Si and aluminum (Al) cathode interface due to the enhanced hole blocking as well as electron injection capability to the Al cathode in the infrared region. PCE was further increased to 14.3% with a short-circuit density JSC of 30.27mA/cm2, an open-circuit voltage VOC of 0.632V, and a fill factor FF of 0.75 using a 20- nm-thick 4,4'-Cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine] as an antireflection layer. PCE of the back-PEDOT:PSS/n-Si heterojunction solar cells was also increased from 4.4% for pristine to 8.1% with a JSC of 33.40mA/cm2, a VOC of 0.573V, and an FF of 0.423 by inserting a 2-nm-thick Ba(OH)2 layer at the front-Al and isotropically textured n-Si interface. These findings imply that Ba(OH)2 has great potential as an efficient hole-blocking layer for both front- and back- PEDOT:PSS/n-Si heterojunction solar cells. [ABSTRACT FROM AUTHOR]

We investigated the relationship between the fine structure of spin-coated conductive polymer poly(3,4-ethylenedioxythiphene):poly(styrene sulfonate) (PEDOT:PSS) films and the photovoltaic performance of PEDOT:PSS crystalline-Si (PEDOT:PSS/c-Si) heterojunction solar cells. Real-time spectroscopic ellipsometry revealed that there were two different time constants for the formation of the PEDOT:PSS network. Upon removal of the polar solvent, the PEDOT:PSS film became optically anisotropic, indicating a conformational change in the PEDOT and PSS chain. Polarized Fourier transform infrared attenuated total reflection absorption spectroscopy and Raman spectroscopy measurements also indicated that thermal annealing promoted an in-plane π-conjugated Cα=Cβ configuration attributed to a thiophene ring in PEDOT and an out-of-plane configuration of -SO3 groups in the PSS chain with increasing composition ratio of oxidized (benzoid) to neutral (quinoid) PEDOT, Iqui/Iben. The highest power conversion efficiency for the spin-coated PEDOT:PSS/c-Si heterojunction solar cells was 13.3% for Iqui/Iben=9-10 without employing any light harvesting methods. [ABSTRACT FROM AUTHOR]

We have fabricated bismuth (Bi) ultrathin films on a charge-density-wave (CDW) compound 1T-TaS2 and elucidated electronic states by angle-resolved photoemission spectroscopy and first-principles band-structure calculations. We found that the Bi film on 1T-TaS2 undergoes a structural transition from (111) to (110) upon reducing the film thickness, accompanied by a drastic change in the energy band structure. We also revealed that while two-bilayer-thick Bi(110) film on Si(111) is characterized by a dispersive band touching the Fermi level (EF), the energy band of the same film on 1T-TaS2 exhibits holelike dispersion with a finite energy gap at EF. We discuss the origin of such intriguing differences in terms of the CDW proximity effect. [ABSTRACT FROM AUTHOR]

The controlled synthesis of large-area, atomically thin molybdenum ditelluride (MoTe2) crystals is crucial for its various applications based on the attractive properties of this emerging material. In this work, we developed a chemical vapor deposition synthesis to produce large-area, uniform, and highly crystalline few-layer 2H and 1T' MoTe2 films. It was found that these two different phases of MoTe2 can be grown depending on the choice of Mo precursor. Because of the highly crystalline structure, the as-grown few-layer 2H MoTe2 films display electronic properties that are comparable to those of mechanically exfoliated MoTe2 flakes. Our growth method paves the way for the large-scale application of MoTe2 in high-performance nanoelectronics and optoelectronics. [ABSTRACT FROM AUTHOR]

Growth of a uniform oxide film with a tunable thickness on two-dimensional transition metal dichalcogenides is of great importance for electronic and optoelectronic applications. Here we demonstrate homogeneous surface oxidation of atomically thin WSe2 with a self-limiting thickness from single- to trilayers. Exposure to ozone (O3) below 100 °C leads to the lateral growth of tungsten oxide selectively along selenium zigzag-edge orientations on WSe2. With further O3 exposure, the oxide regions coalesce and oxidation terminates leaving a uniform thickness oxide film on top of unoxidized WSe2. At higher temperatures, oxidation evolves in the layer-by-layer regime up to trilayers. The oxide films formed on WSe2 are nearly atomically flat. Using photoluminescence and Raman spectroscopy, we find that the underlying single-layer WSe2 is decoupled from the top oxide but hole-doped. Our findings offer a new strategy for creating atomically thin heterostructures of semiconductors and insulating oxides with potential for applications in electronic devices. [ABSTRACT FROM AUTHOR]

Reaction of (silyl)(silylene)tungsten complex Cp*(OC)2W(SiMe3)(SiMes2) (1) with sulfur reagents, S8and ethylene sulfide 4, afforded cyclic carbene tungsten complex Cp*(S)W{C(SiMe3)C(O)OSi(Mes)2S} (3) via CO bond fission of a carbonyl ligand and 1,2-migration of the SiMe3group from W to the resulting carbene carbon. In contrast to the tungsten complex, reaction of (silyl)(silylene)molybdenum complex Cp*(OC)2Mo(SiMe3)(SiMes2) (2) with 4gave η2-thiocarbonyl complex Cp*(S)Mo[{η2-SC(SiMe3)}C(O)OSi(Mes)2S] (5), which would be formed by addition of a sulfur atom to the MoC bond of a molybdenum analogue of 3. [ABSTRACT FROM AUTHOR]

In van der Waals (vdW) heterostructures formed by stacking two monolayers of transition metal dichalcogenides, multiple exciton resonances with highly tunable properties are formed and subject to both vertical and lateral confinement. We investigate how a unique control knob, the twist angle between the two monolayers, can be used to control the exciton dynamics. We observe that the interlayer exciton lifetimes in MoSe2/WSe2 twisted bilayers (TBLs) change by one order of magnitude when the twist angle is varied from 1° to 3.5°. Using a low-energy continuum model, we theoretically separate two leading mechanisms that influence interlayer exciton radiative lifetimes. The shift to indirect transitions in the momentum space with an increasing twist angle and the energy modulation from the moiré potential both have a significant impact on interlayer exciton lifetimes. We further predict distinct temperature dependence of interlayer exciton lifetimes in TBLs with different twist angles, which is partially validated by experiments. While many recent studies have highlighted how the twist angle in a vdW TBL can be used to engineer the ground states and quantum phases due to many-body interaction, our studies explore its role in controlling the dynamics of optically excited states, thus, expanding the conceptual applications of "twistronics". [ABSTRACT FROM AUTHOR]