Switching of K-Q intervalley trions fine structure and their dynamics in n-doped monolayer WS_(2)

Monolayer group VI transition metal dichalcogenides(TMDs)have recently emerged as promising candidates for photonic and opto-valleytronic applications.The optoelectronic properties of these atomically-thin semiconducting crystals are strongly governed by the tightly bound electron-hole pairs such as excitons and trions(charged excitons).The anomalous spin and valley configurations at the conduction band edges in monolayer WS_(2)give rise to even more fascinating valley many-body complexes.Here we find that the indirect Q valley in the first Brillouin zone of monolayer WS_(2)plays a critical role in the formation of a new excitonic state,which has not been well studied.By employing a high-quality h-BN encapsulated WS_(2)field-effect transistor,we are able to switch the electron concentration within K-Q valleys at conduction band edges.Consequently,a distinct emission feature could be excited at the high electron doping region.Such feature has a competing population with the K valley trion,and experiences nonlinear power-law response and lifetime dynamics under doping.Our findings open up a new avenue for the study of valley many-body physics and quantum optics in semiconducting 2D materials,as well as provide a promising way of valley manipulation for next-generation entangled photonic devices.

High responsivity and near-infrared photodetector based on graphene/MoSe_2 heterostructure

Graphene has attracted great interest in optoelectronics, owing to its high carrier mobility and broadband absorption. However, a graphene photodetector exhibits low photoresponsivity because of its weak light absorption. In this work, we designed a graphene/MoSe_2 heterostructure photodetector, which exhibits photoresponse ranging from visible to near infrared and an ultrahigh photoresponsivity up to 1.3 × 104 A·W^(-1) at 550 nm. The electron–hole pairs are excited in a few-layered MoSe2 and separated by the built-in electric field. A large number of electrons shift to graphene, while the holes remain in the MoSe_2, which creates a photogating effect.

p-/n-Type modulation of 2D transition metal dichalcogenides for electronic and optoelectronic devices

Two-dimensional layered transition metal dichalcogenides(TMDCs)have demonstrated a huge potential in the broad fields of optoelectronic devices,logic electronics,electronic integration,as well as neural networks.To take full advantage of TMDC characteristics and efficiently design the device structures,one of the most key processes is to control their p-/n-type modulation.In this review,we summarize the p-/n-type modulation of TMDCs based on diverse strategies consisting of intrinsic defect tailoring,substitutional doping,surface charge transfer,chemical intercalation,electrostatic modulation,and dielectric interface engineering.The modulation mechanisms and comparisons of these strategies are analyzed together with a discussion of their corresponding device applications in electronics and optoelectronics.Finally,challenges and outlooks for p-/n-type modulation of TMDCs are presented to provide references for future studies.

Growth of centimeter scale Nb_(1−x)W_(x)Se_(2) monolayer film by promoter assisted liquid phase chemical vapor deposition

Two-dimensional(2D)transition-metal dichalcogenide materials(TMDs)alloys have a wide range of applications in the field of optoelectronics due to their capacity to achieve wide modulation of the band gap with fully tunable compositions.However,it is still a challenge for growing alloys with uniform components and large lateral size due to the random distribution of the crystal nucleus locations.Here,we applied a simple but effective promoter assisted liquid phase chemical vapor deposition(CVD)method,in which the quantity ratio of promoter to metal precursor can be controlled precisely,leading to tiny amounts of transition metal oxide precursors deposition onto the substrates in a highly uniform and reproducible manner,which can effectively control the uniform distribution of element components and nucleation sites.By this method,a series of monolayer Nb_(1−x)W_(x)Se_(2)alloy films with fully tunable compositions and centimeter scale have been successfully synthesized on sapphire substrates.This controllable approach opens a new way to produce large area and uniform 2D alloy film,which has the potential for the construction of optoelectronic devices with tailored spectral responses.

Carrier mobility tuning of MoS_(2) by strain engineering in CVD growth process

Strain engineering is proposed to be an effective technology to tune the properties of two-dimensional(2D)transition metal dichalcogenides(TMDCs).Conventional strain engineering techniques(e.g.,mechanical bending,heating)cannot conserve strain due to their dependence on external action,which thereby limits the application in electronics.In addition,the theoretically predicted strain-induced tuning of electrical performance of TMDCs has not been experimentally proved yet.Here,a facile but effective approach is proposed to retain and tune the biaxial tensile strain in monolayer MoS_(2) by adjusting the process of the chemical vapor deposition(CVD).To prove the feasibility of this method,the strain formation model of CVD grown MoS_(2) is proposed which is supported by the calculated strain dependence of band gap via the density functional theory(DFT).Next,the electrical properties tuning of strained monolayer MoS_(2) is demonstrated in experiment,where the carrier mobility of MoS_(2) was increased by two orders(~0.15 to~23 cm^(2)·V^(−1)·s^(−1)).The proposed pathway of strain preservation and regulation will open up the optics application of strain engineering and the fabrication of high performance electronic devices in 2D materials.