PN heterojunctions comprising layered van der Waals (vdW) semiconductors have been used to demonstrate current rectifiers, photodetectors, and photovoltaic devices. However, a direct or neardirect bandgap at the heterointerface that can significantly enhance optical generation, for high light absorbing few/multi-layer vdW materials, has not yet been shown. In this work, for the first time, few-layer group-6 transition metal dichalcogenide (TMD) WSe2 is shown to form a sizeable (0.7 eV) near-direct bandgap with type-II band alignment at its interface with the group-7 TMD ReS2 through density functional theory calculations. Further, the type-II alignment and photogeneration across the interlayer bandgap have been experimentally confirmed through micro-photoluminescence and IR 2 photodetection measurements, respectively. High optical absorption in few-layer flakes, large conduction and valence band offsets for efficient electron-hole separation and stacking of light facing, direct bandgap ReS2 on top of gate tunable WSe2 are shown to result in excellent and tunable photodetection as well as photovoltaic performance through flake thickness dependent optoelectronic measurements. Few-layer flakes demonstrate ultrafast response time (5 µs) at high responsivity (3 A/W) and large photocurrent generation and responsivity enhancement at the heterostructure overlap region (10-100×) for 532 nm laser illumination. Large open circuit voltage of 0.64 V and short circuit current of 2.6 µA enables high output electrical power. Finally, long term air-stability and a facile single contact metal fabrication process makes the multi-functional few-layer WSe2/ReS2 heterostructure diode technologically promising for next-generation optoelectronic applications.The semiconducting group-6 TMD WSe2, generally found in trigonal prismatic phase, 17 is an indirect bandgap material in its bulk form. 18,19 However, group-7 TMD e.g. 1T phase of ReS2 is distorted octahedral in structure 17,20 and exhibits direct (or, near-direct) bandgap at (or, close to) the Γ point of the Brillouin zone (BZ). Interestingly, unlike the group-6 TMDs, ReS2 exhibits a unique property owing to its distorted structure and weak interlayer coupling-the direct or near-direct nature of its bandgap remains unchanged from monolayer to bulk. [20][21][22] Closer examination of the bandstructure of group-7 TMDs reveals that the conduction band minimum of ReS2 remains at the Γ point, irrespective of the number of layers. 20,23 But for group-6 TMDs (such as WSe2) the valence band maximum relocates from K to Γ point of the BZ with increasing number of layers. 18,24 It is important to note that the valence band maximum at the Γ point differs in energy only slightly from that at the K point. 18 This gives rise to an increased probability of direct as well as indirect transitions from the Γ and the K valence maxima of WSe2 to the Γ conduction minimum of ReS2 respectively, for a predicted type-II band alignment. The possibility of a direct bandgap transition is not observed in a heter...

Organic–inorganic hybrid crystals with lead halides as the inorganic moiety are remarkable, in that they show low-dimensional behavior despite their three-dimensional structures. Here, we report on a unique structure, imidazolium lead iodide C3N2H5PbI3 (ImPI), and show that its electronic ground state represents a one-dimensional (1D) solid with carrier motion confined along nanometer-thin corrugated chains formed by face-sharing PbI6 4– octahedrons. ImPI has a very broad photoluminescence (PL) spectrum peaking around 688 nm (1.8 eV) with a width of 173 nm at high temperatures. This PL emission shows a significant red shift of 1.3 eV relative to the excitonic band gap of ImPI at 400 nm (3.1 eV) estimated from PL excitation spectroscopy. These results are compared with measurements on PbI2. Calculations based on the effective-mass theory show that the increased band gap of ImPI originates from quantum confinement, with the PbI6 4– chains acting as 1D quantum wires. Time-resolved and pump intensity-dependent PL studies provide evidence that PL emission arises from self-trapped exciton recombination, a characteristic of 1D systems with easily deformable lattice. This study establishes ImPI as a 1D hybrid semiconductor with a broad emission spectrum with promise for opto-electronic device applications.

 Users may download and print one copy of any publication from the public portal for the purpose of private study or research.  You may not further distribute the material or use it for any profit-making activity or commercial gain  You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.

No abstract

An optical arrangement and procedure for photolithography on microscopic flakes of two-dimensional materials with an arbitrary shape/size is described. The technique combines projection of demagnified images of simple geometry macroscopic masks with writing. Only a few masks, such as vertical/horizontal slit and square hole, are sufficient to generate most of the required patterns. The setup allows for initially locating the photoresist coated flake on a substrate by imaging it. Thereafter, the automated precise sample stage motion followed by projection of the demagnified mask image is repeated several times to expose the photoresist in the shape of the required pattern. Appropriate light wavelength regimes for imaging and for exposure are chosen through automated optical filter switching. Programming steps for the process are described. The setup allows for direct lithography in one round on microscopic samples without requiring sample shape/size specific masks or predefined position markers. Making of electrode lines of width down to 3 μm, at desired locations on tiny flakes of MoS2, is demonstrated.