In integrated photonics, specific wavelengths are preferred such as 1550 nm due to low-loss transmission and the availability of optical gain in this spectral region. For chip-based photodetectors, layered two-dimensional (2D) materials bear scientific and technologicallyrelevant properties such as electrostatic tunability and strong light-matter interactions. However, no efficient photodetector in the telecommunication C-band has been realized with 2D transition metal dichalcogenide (TMDCs) materials due to their large optical bandgap. Here, we demonstrate a MoTe2-based photodetector featuring strong photoresponse (responsivity = 0.5 A/W) operating at 1550 nm on silicon micro ring resonator enabled by strain engineering of the transition-metal-dichalcogenide film. We show that an induced tensile strain of ~4% reduces the bandgap of MoTe2, resulting in large photo-response in the telecommunication wavelength, in otherwise photo-inactive medium when unstrained. Unlike Graphene-based photodetectors relying on a gapless band structure, this semiconductor-2D material detector shows a ~100X improved dark current enabling an efficient noise-equivalent power of just 90 pW/Hz 0.5 . Such strain-engineered integrated photodetector provides new opportunities for integrated optoelectronic systems.

Silicon compatible wafer scale MoS2 heterojunctions are reported for the first time using colloidal quantum dots. Size dependent direct band gap emission of MoS2 dots are presented at room temperature. The temporal stability and decay dynamics of excited charge carriers in MoS2 quantum dots have been studied using time correlated single photon counting spectroscopy technique. Fabricated n-MoS2/p-Si 0D/3D heterojunctions exhibiting excellent rectification behavior have been studied for light emission in the forward bias and photodetection in the reverse bias. The electroluminescences with white light emission spectra in the range of 450–800 nm are found to be stable in the temperature range of 10–350 K. Size dependent spectral responsivity and detectivity of the heterojunction devices have been studied. The peak responsivity and detectivity of the fabricated heterojunction detector are estimated to be ~0.85 A/W and ~8 × 1011 Jones, respectively at an applied bias of −2 V for MoS2 QDs of 2 nm mean diameter. The above values are found to be superior to the reported results on large area photodetector devices fabricated using two dimensional materials.

The complex [Et4N]2[WVIO2(mnt)2] (1), [Et4N]2[WIVO(mnt)2] (2), and [Et4N]2[WVIO(S2)(mnt)2] (3) (mnt2- = 1,2-dicyanoethylenedithiolate) have been synthesized as possible models for the tungsten cofactor of inactive red tungsten protein (RTP) and the active aldehyde ferredoxin oxidoreductase (AOR) of the hyperthermophilic archaeon Pyrococcus furiosus. The [Ph4P]+ salt of the complex anion of 1·2H2O crystallizes in space group Pbcn, with a = 20.526(3) Å, b = 15.791(3) Å, c = 17.641(3) Å, and Z = 4. The WVIO2S4 core of [Ph4P]2[WVIO2(mnt)2]·2H2O has distorted octahedral geometry with cis dioxo groups. 2 crystallizes in space group P21212, with a = 14.78(3) Å, b = 30.08(2) Å, c = 7.37(4) Å, and Z = 4. The complex anion of 2 has a distorted square-pyramidal structure with an axial WO bond. 3 crystallizes in space group P21/a, with a = 12.238(3) Å, b = 18.873(2) Å, c = 15.026(2) Å, β = 102.84(2)°, and Z = 4. The anion of 3 with a terminal oxo group and a dihapto disulfido ligand in an adjacent position is the first example of a seven-coordinate W(VI) species with bis-dithiolene coordination. The complexes 1−3 have been characterized by IR, UV−visible, 13C NMR, negative ion FAB mass spectra, and electrochemical properties. Complex 1 reacts with H2S, PhSH, 1,4-dithiothreitol (DTT), or dithionite (S2O4 2-) to yield 2 with the oxidation of these reducing agents suggesting intramolecular electron transfer in the respective intermediates across the W(VI)−sulfur bond. Participation of this type of redox reaction, seemingly unrealistic from the point of view of real reduction potential values of 1 and of these reductants, is best explained by the formation of a precursor complex. This relates to the essential formation of a Michaelis (enzyme−substrate) complex wherein the individual chemical identity of the free enzyme and unbound substrate is lost. Subsequent atom transfer reaction embodies internal electron transfer between the two redox partners present in the enzyme−substrate complex. The terminal oxo group of 2 is readily protonated (pH < 4) to yield [WIV(mnt)3]2-. 2 responds to a metal exchange reaction with MoO4 2- to form [MoIVO(mnt)2]2- which is similar to in vitro reconstitution of the molybdenum cofactor by MoO4 2- in tungsten formate dehydrogenase (W-FDH). The model reaction between 2 and MoO4 2- involves a stepwise one-electron transfer reaction from W(IV) to Mo(VI) with the intermediate formation of EPR active W(V) species. Oxidative addition of elemental sulfur from 2 affords 3, which gives sulfur atom transfer reactions with several thiophiles. 3 reacts with Ph3P in a second-order process (A + 2B type) to yield 2 and Ph3PS with the observed rate constant k 2 = 4.3 (± 0.06) M-1 s-1 at 25 °C (ΔH ⧧ = 5.14 (± 0.46) kcal/mol, ΔS ⧧ = −38.35 (± 1.5) cal/(deg·mol)). A cyclic voltammetric study suggests the attack of Ph3P across the W−S bond in the WS2 moiety of 3. 2 catalyzes the reactions Ph3P + S → Ph3PS and H2 + S → H2S, demonstrating its sulfur reductase activity. No such reaction is observed in the absence of 2. Fo...

Highly luminescent MoS 2 nanocrystals (NCs) with controlled size distribution have been achieved using a simple yet inexpensive and impurity free sono-chemical exfoliation method followed by gradient centrifugation. The size of nanocrystals could be varied within the diameter range of ∼4 to 70 nm. Typical MoS 2 nanocrystal has exhibited high crystalline quality with 0.25 nm lattice fringe spacing for (002) planes for 2-H phase of MoS 2 . Raman spectra has revealed that both out-of-plane and in-plane vibrational modes are stiffen due to the edge effect of MoS 2 NCs. The size tunable optical properties of MoS 2 NCs have been investigated by optical absorption and photoluminescence spectroscopy. The coexistence of direct band gap emission from 2D MoS 2 nanosheets and quantum confined nanocrystals has been achieved. A strong and tunable photoluminescence (560−518 nm) emission due to the quantum size effect of tiny NCs below a critical dimension is reported for the first time. The photocurrent measurement of the Au/MoS 2 −NCs/Au junction has been performed at room temperature to investigate the optical responsivity and switching characteristics, demonstrating the potential of MoS 2 nanocrystals for next generation photonic devices.

Graphene has extraordinary electro-optic properties and is therefore a promising candidate for monolithic photonic devices such as photodetectors. However, the integration of this atom-thin layer material with bulky photonic components usually results in a weak light-graphene interaction leading to large device lengths limiting electro-optic performance.In contrast, here we demonstrate a plasmonic slot graphene photodetector on silicon-oninsulator platform with high-responsivity given the 5 µm-short device length. We observe that the maximum photocurrent, and hence the highest responsivity, scales inversely with the slot gap width. Using a dual-lithography step, we realize 15 nm narrow slots that show a 15-times higher responsivity per unit device-length compared to photonic graphene photodetectors. Furthermore, we reveal that the back-gated electrostatics is overshadowed by channel-doping contributions induced by the contacts of this ultra-short channel graphene photodetector. This leads to quasi charge neutrality, which explains both the previously-unseen offset between the maximum photovoltaic-based photocurrent relative to graphene's Dirac point and the observed non-ambipolar transport. Such micrometer compact and absorption-efficient photodetectors allow for short-carrier pathways in nextgeneration photonic components, while being an ideal testbed to study short-channel carrier physics in graphene optoelectronics. Introduction.Graphene has become a complementary platform for electronics and optoelectronics because of its remarkable properties and versatility(1). A variety of applications exploit graphene's peculiar features to include modulators(2), plasmonic optoelectronics(3-6), photovoltaic devices (7), ultrafast lasers (8), and photo-detection(9, 10). For photo conversion applications the linear and gap-less band structure of graphene results in wavelength-independent absorption (11,12).Moreover, graphene's carrier can be tuned via electrostatically doping, thus modulating light absorption. Due to its superb carrier mobility (13,14), graphene-based absorption enables ultrafast conversion of photons or plasmons to electrical currents or voltages. However, the light-graphene interaction, and consequently the responsivity of graphene-based devices, is usually rather weak due to the geometrical mismatch between graphene's atom-thin thickness and the diffractionlimited optical mode area of photonic components.The first-generation of graphene-based free-space photodetectors (PDs) uses metal-graphenemetal structures(14); choosing different work-functions for the source-and drain contacts results in an asymmetric band structure, thus enabling non-biased band-bending for charge polarity separation, leading to near-zero dark current. Interdigitated metallic contacts, are typically adopted Corresponding AuthorVolker Sorger, sorger@gwu.edu Funding SourcesVS is funded by AFOSR (FA9550-17-1-0377) and ARO (W911NF-16-2-0194).