1wileyonlinelibrary.com gap around 1-3 eV and display advantageous optoelectronic properties. [ 9,10 ] For example, the fi eld-effect transistors based on monolayer or few-layer MoS 2 have been reported to exhibit an excellent on/ off ratio (∼10 8 ) and room-temperature mobility of >200 cm 2 /Vs, and the layered WS 2 has also been reported to exhibit 10 5 room temperature modulation and bipolar behavior. [ 11,12 ] For photodetection, the layered TMDs based photodetectors have been demonstrated with very high responsivity and fast photoresponse. [13][14][15] In parallel with the study of the single layer graphene-like materials, Van der Waals heterostructures and the corresponding devices fabricated by stacking different 2D crystals on top of each other have also been gaining much attention recently. These systems have revealed some novel properties and new phenomena that could trigger new revolution in architecture design of heterostructures for integrated optoelectronic applications. For example, the graphene heterostructures with 2D h-BN or MoS 2 layers acting as vertical transport barrier are fabricated and exhibit room-temperature switching ratio of 50 and 10 000, respectively. [ 16 ] Following the success, the vertical integration of multi-heterostructures of graphene and MoS 2 are reported for the fabrication of a new generation of vertical fi eld-effect transistors [ 17 ] (VFETs) with an on-off ratio >10 3 and a high current density of up to 5000 A/cm 2 . The vertically stacked heterostructures of graphene and MoS 2 are also demonstrated to display remarkable multiple optoelectronic functionality, including highly sensitive photodetection and gate-tunable persistent photoconductivity, [ 18 ] gate-tunable photocurrent generation with a maximum internal quantum effi ciency up to 85%, and huge capacity of information storage with a factor of 10 4 difference between memory states and erase states. [ 19,20 ] Moreover, the VFETs based on the graphene-WS 2 heterostructures are also fabricated with unprecedented current modulation exceeding 10 6 at room temperature. [ 21 ] These results suggest that the stacked multi-heterostructures of layered materials could open up new opportunities in future nanoelectronic devices with large-scale integration. Indeed, there are large number of 2D materials that can be exfoliated with micromechanical cleavage method and combined together to create various and tailored heterostructures. Novel and Enhanced Optoelectronic Performances of Multilayer MoS 2 -WS

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The origin of ferromagnetism in d;{0} semiconductors is studied using first-principles methods with ZnO as a prototype material. We show that the presence of spontaneous magnetization in nitrides and oxides with sufficient holes is an intrinsic property of these first-row d;{0} semiconductors and can be attributed to the localized nature of the 2p states of O and N. We find that acceptor doping, especially doping at the anion site, can enhance the ferromagnetism with much smaller threshold hole concentrations. The quantum confinement effect also reduces the critical hole concentration to induce ferromagnetism in ZnO nanowires. The characteristic nonmonotonic spin couplings in these systems are explained in terms of the band coupling model.

Here, we propose general strategies for the rational design of semiconductors to simultaneously meet all of the requirements for a high-efficiency, solar-driven photoelectrochemical ͑PEC͒ water-splitting device. As a case study, we apply our strategies for engineering the popular semiconductor, anatase TiO 2. Previous attempts to modify known semiconductors such as TiO 2 have often focused on a particular individual criterion such as band gap, neglecting the possible detrimental consequence to other important criteria. Density-functional theory calculations reveal that with appropriate donor-acceptor coincorporation alloys with anatase TiO 2 hold great potential to satisfy all of the criteria for a viable PEC device. We predict that ͑Mo, 2N͒ and ͑W, 2N͒ are the best donor-acceptor combinations in the low-alloy concentration regime whereas ͑Nb, N͒ and ͑Ta, N͒ are the best choice of donor-acceptor pairs in the high-alloy concentration regime.

Hybrid halide perovskites such as methylammonium lead iodide (CH 3 NH 3 PbI 3 ) exhibit unusually low free-carrier concentrations despite being processed at low-temperatures from solution. We demonstrate, through quantum mechanical calculations, that an origin of this phenomenon is a prevalence of ionic over electronic disorder in stoichiometric materials. Schottky defect formation provides a mechanism to selfregulate the concentration of charge carriers through ionic compensation of charged point defects. The equilibrium charged vacancy concentration is predicted to exceed 0.4 % at room temperature. This behavior, which goes against established defect conventions for inorganic semiconductors, has implications for photovoltaic performance.

Van der Waals (vdW) p-n heterojunctions consisting of various 2D layer compounds are fascinating new artificial materials that can possess novel physics and functionalities enabling the next-generation of electronics and optoelectronics devices. Here, it is reported that the WSe2/WS2 p-n heterojunctions perform novel electrical transport properties such as distinct rectifying, ambipolar, and hysteresis characteristics. Intriguingly, the novel tunable polarity transition along a route of n-"anti-bipolar"-p-ambipolar is observed in the WSe2/WS2 heterojunctions owing to the successive work of conducting channels of junctions, p-WSe2 and n-WS2 on the electrical transport of the whole systems. The type-II band alignment obtained from first principle calculations and built-in potential in this vdW heterojunction can also facilitate the efficient electron-hole separation, thus enabling the significant photovoltaic effect and a much enhanced self-driven photoswitching response in this system.

Although many oxide semiconductors possess wide bandgaps in the ultraviolet (UV) regime, currently the majority of them cannot efficiently emit UV light because the band-edge optical transition is forbidden in a perfect lattice as a result of the symmetry of the band-edge states. This quantum mechanical rule severely constrains the optical applications of wide-bandgap oxides, which is also the reason why so few oxides enjoy the success of ZnO. Here, using SnO 2 as an example, we demonstrate both theoretically and experimentally that UV photoluminescence and electroluminescence can be recovered and enhanced in wide-bandgap oxide thin films with 'forbidden' energy gaps by engineering their nanocrystalline structures. In our experiments, the tailored low-temperature annealing process results in a hybrid structure containing SnO 2 nanocrystals in an amorphous matrix, and UV emission is observed in such hybrid SnO 2 thin films, indicating that the quantum mechanical dipole-forbidden rule has been effectively overcome. Using this approach, we demonstrate the first prototypical electrically pumped UV-lightemitting diode based on nanostructured SnO 2 thin films.

Nanocrystals (NCs) with identical components and sizes but different crystal structures could not be distinguished by conventional absorption and emission spectra. Herein, we find that circular dichroism (CD) spectroscopy can easily distinguish the CdSe nanoplatelets (NPLs) with different crystal structures of wurtzite (WZ) and zincblende (ZB) with the help of chiral L-or D-cysteine ligands. In particular, the CD signs of the first excitonic transitions in WZ and ZB NPLs capped by the same chiral cysteine are opposite. Theoretic calculation supports the viewpoint of different crystal structures and surfaces arrangements between WZ and ZB NPLs contributing to this significant phenomenon. The CD peaks appearing at the first excitonic transition band of WZ or ZB CdSe NPLs are clearly assigned to the different transition polarizations along 4p (x,y,z),Se → 5s Cd or 4p (x,y),Se → 5s Cd. This work not only provides a deep insight into the origin of the optical activity inside chiral semiconductor nanomaterials but also proposes the design principle of chiral semiconductor nanocrystals with high optic activity.