Excitation-Dependent High-Lying Excitonic Exchange via Interlayer Energy Transfer from Lower-to-Higher Bandgap 2D Material

High light absorption (~15%) and strong photoluminescence (PL) emission in monolayer (1L) transition metal dichalcogenide (TMD) make it an ideal candidate for optoelectronic applications. Competing interlayer charge (CT) and energy transfer (ET) processes control the photocarrier relaxation pathways in TMD heterostructures (HSs). In TMDs, long-distance ET can survive up to several tens of nm, unlike the CT process. Our experiment shows that an efficient ET occurs from the 1Ls WSe2-to-MoS2 with an interlayer hBN, due to the resonant overlapping of the high-lying excitonic states between the two TMDs, resulting in enhanced HS MoS2 PL emission. This type of unconventional ET from the lower-to-higher optical bandgap material is not typical in the TMD HSs. With increasing temperature, the ET process becomes weaker due to the increased electron-phonon scattering, destroying the enhanced MoS2 emission. Our work provides new insight into the long-distance ET process and its effect on the photocarrier relaxation pathways.Comment: 5 figures and SI include

VLT/NACO Polarimetric Differential Imaging of HD100546 - Disk Structure and Dust Grain Properties between 10-140 AU

We present polarimetric differential imaging (PDI) data of the circumstellar disk around the Herbig Ae/Be star HD100546 obtained with VLT/NACO. We resolve the disk in polarized light in the H and Ks filter between ~0.1-1.4" (i.e., ~10-140 AU). The innermost disk regions are directly imaged for the first time and the mean apparent disk inclination and position angle are derived. The surface brightness along the disk major axis drops off roughly with S(r) ~ r^-3 but has a maximum around 0.15" suggesting a marginal detection of the main disk inner rim at ~15 AU. We find a significant brightness asymmetry along the disk minor axis in both filters with the far side of the disk appearing brighter than the near side. This enhanced backward scattering and a low total polarization degree of the scattered disk flux of 14%(+19%/-8%) suggests that the dust grains on the disk surface are larger than typical ISM grains. Empirical scattering functions reveal the backward scattering peak at the largest scattering angles and a second maximum for the smallest scattering angles. This indicates a second dust grain population preferably forward scattering and smaller in size. It shows that, relatively, in the inner disk regions (40-50 AU) a higher fraction of larger grains is found compared to the outer disk regions (100-110 AU). Finally, our images reveal distinct substructures between 25-35 AU physical separation from the star and we discuss the possible origin for the two features in the context of ongoing planet formation.Comment: Accepted for publication by Ap

Flexible, transparent single-walled carbon nanotube transistors with graphene electrodes

This paper reports a mechanically flexible, transparent thin film transistor that uses graphene as a conducting electrode and single-walled carbon nanotubes (SWNTs) as a semiconducting channel. These SWNTs and graphene films were printed on flexible plastic substrates using a printing method. The resulting devices exhibited a mobility of similar to 2 cm(2) V(-1) s(-1), On/Off ratio of similar to 10(2), transmittance of similar to 81% and excellent mechanical bendability

Dual Resonant Sum Frequency Generations from Two-Dimensional Materials

We propose dual resonant optical sum frequency generation (SFG), where the two most singular resonances could be selected, and report for the monolayer (1L-) WSe2 when one (ω1) of two excitation pulses is resonant to A exciton and their sum frequency (ω1 + ω2) to D exciton. The dual resonant SFG confirms that, under an irradiation of ω1 and ω2 pulses with the same fluence of ∼1.4 × 1010 W/m2, its signal intensity could be enhanced about 20 times higher than the resonant SHG (i.e., 2ω1 to the D excitonic absorption). Further, the dual resonant SFG intensity of 1L-WSe2 is found to be 1 order of magnitude higher than the single resonant SFG intensity of 1L-WS2 under the same condition of two-pulse irradiation. Finally, observations of the dual resonant SFG are thoroughly examined using real-time time-dependent density functional theory (rt-TDDFT), and the relevant nonlinear optical characteristics are scrutinized using the Greenwood-Kubo formalism. Copyright © 2020 American Chemical Society.1

High-Performance Flexible Graphene Field Effect Transistors with Ion Gel Gate Dielectrics

A high-performance low-voltage graphene field-effect transistor (FED array was fabricated on a flexible polymer substrate using solution-processable, high-capacitance ion gel gate dielectrics. The high capacitance of the ion gel, which originated from the formation of an electric double layer under the application of a gate voltage, yielded a high on-current and low voltage operation below 3 V. The graphene FETs fabricated on the plastic substrates showed a hole and electron mobility of 203 +/- 57 and 91 +/- 50 cm(2)/(V.s), respectively, at a drain bias of - I V. Moreover, ion gel gated graphene FETs on the plastic substrates exhibited remarkably good mechanical flexibility. This method represents a significant step in the application of graphene to flexible and stretchable electronics

Stretchable Graphene Transistors with Printed Dielectrics and Gate Electrodes

With the emergence of human interface technology, the development of new applications based on stretchable electronics such as conformal biosensors and rollable displays are required. However, the difficulty in developing semiconducting materials with high stretchability required for such applications has restricted the range of applications of stretchable electronics. Here, we present stretchable, printable, and transparent transistors composed of monolithically patterned graphene films. This material offers excellent mechanical, electrical, and optical properties, capable of use as semiconducting channels as well as the source/drain electrodes. Such monolithic graphene transistors show hole and electron mobilities of 1188 +/- 136 and 422 +/- 52 cm(2)/(Vs), respectively, with stable operation at stretching up to 5% even after 1000 or more cycles

Observation of the Inverse Giant Piezoresistance Effect in Silicon Nanomembranes Probed by Ultrafast Terahertz Spectroscopy

The anomalous piezoresistance (a-PZR) effects, including giant PZR (GPZR) with large magnitude and inverse PZR of opposite, have exciting technological potentials for their integration into novel nanoelectromechanical systems. However, the nature of a-PZR effect and the associated kinetics have not been clearly determined yet. Even further, there are intense research debates whether the a-PZR effect actually exists or not; although numerous investigations have been conducted, the origin of the effect has not been clearly understood. This paper shows the existence of a-PZR and provides direct experimental evidence through the performance of well-established electrical measurements and terahertz spectroscopy on silicon nanomembranes (Si NMs). The clear inverse PZR behavior was observed in the Si NMs when the thickness was less than 40 nm and the magnitude of the PZR response linearly increased with the decreasing thickness. Observations combined with electrical and optical measurements strongly corroborate that the a-PZR effect originates from the carrier concentration changes via charge carrier trapping into strain-induced defect states

Large-scale pattern growth of graphene films for stretchable transparent electrodes

Problems associated with large- scale pattern growth of graphene constitute one of the main obstacles to using this material in device applications(1). Recently, macroscopic- scale graphene films were prepared by two- dimensional assembly of graphene sheets chemically derived from graphite crystals and graphene oxides(2,3). However, the sheet resistance of these films was found to be much larger than theoretically expected values. Here we report the direct synthesis of large- scale graphene films using chemical vapour deposition on thin nickel layers, and present two different methods of patterning the films and transferring them to arbitrary substrates. The transferred graphene films show very low sheet resistance of similar to 280 Omega per square, with 80 per cent optical transparency. At low temperatures, the monolayers transferred to silicon dioxide substrates show electron mobility greater than 3,700 cm(2) V-1 s(-1) and exhibit the half- integer quantum Hall effect(4,5), implying that the quality of graphene grown by chemical vapour deposition is as high as mechanically cleaved graphene(6). Employing the outstanding mechanical properties of graphene(7), we also demonstrate the macroscopic use of these highly conducting and transparent electrodes in flexible, stretchable, foldable electronics(8,9)

Local Strain Induced Band Gap Modulation and Photoluminescence Enhancement of Multilayer Transition Metal Dichalcogenides

The photocarrier relaxation between direct and indirect band gaps along the high symmetry K−Γ line in the Brillion zone reveals interesting electronic properties of the transition metal dichalcogenides (TMDs) multilayer films. In this study, we reported on the local strain engineering and tuning of an electronic band structure of TMDs multilayer films along the K−Γ line by artificially creating one-dimensional wrinkle structures. Significant photoluminescence (PL) intensity enhancement in conjunction with continuously tuned optical energy gaps was recorded at the high strain regions. A direct optical band gap along K–K points and an indirect optical gap along Γ–K points measured from the PL spectra of multilayer samples monotonically decreased as the strain increased, while the indirect band gap along Λ–Γ was unaffected owing to the same level of local strain in the range of 0%–2%. The experimental results of band gap tuning were in agreement with the density functional theory calculation results. Local strain modified the band structure in which K-conduction band valley (CBV) was aligned below the Λ-CBV, and this explained the observed local PL enhancement that made the material indirect via the K−Γ transition. The study also reported experimental evidence for the funneling of photogenerated excitons toward regions of a higher strain at the top of the wrinkle geometry