We have measured circularly polarized photoluminescence in monolayer MoSe2 under perpendicular magnetic fields up to 10 T. At low doping densities, the neutral and charged excitons shift linearly with field strength at a rate of ∓0.12  meV/T for emission arising, respectively, from the K and K' valleys. The opposite sign for emission from different valleys demonstrates lifting of the valley degeneracy. The magnitude of the Zeeman shift agrees with predicted magnetic moments for carriers in the conduction and valence bands. The relative intensity of neutral and charged exciton emission is modified by the magnetic field, reflecting the creation of field-induced valley polarization. At high doping levels, the Zeeman shift of the charged exciton increases to ∓0.18  meV/T. This enhancement is attributed to many-body effects on the binding energy of the charged excitons.

We have measured circularly polarized photoluminescence in monolayer MoSe 2 under perpendicular magnetic fields up to 10 T. At low doping densities, the neutral and charged excitons shift linearly with field strength at a rate of ∓ 0.12 meV/T for emission arising, respectively, from the K and K' valleys. The opposite sign for emission from different valleys demonstrates lifting of the valley degeneracy. The magnitude of the Zeeman shift agrees with predicted magnetic moments for carriers in the conduction and valence bands. The relative intensity of neutral and charged exciton emission is modified by the magnetic field, reflecting the creation of field-induced valley polarization. At high doping levels, the Zeeman shift of the charged exciton increases to ∓ 0.18 meV/T. This enhancement is attributed to many-body effects on the binding energy of the charged excitons.PACS numbers: 75.70. Ak, 78.20.Ls, 73.20.Mf, 73.22Monolayer MoSe 2 features two inequivalent valleys in the Brillouin zone of its electronic structure. The broken inversion symmetry of the monolayer allows this valley degree of freedom to be selectively accessed by optical helicity, providing a unique platform to probe and manipulate the charge carriers in the two valleys. [1][2][3][4][5][6][7][8] Since the valleys are linked by timereversal symmetry, they are energetically degenerate, while the magnetic moments of the corresponding valley states are of the same magnitude, but have opposite sign [1,9,10]. Coupling to the valley magnetic moments by a magnetic field thus provides an attractive, but as yet unexplored method of breaking the valley degeneracy [11,12]. This presents new opportunities for the study of the fundamental physical properties of the valley electronic states, as well as for the development of new approaches to valleytronic control.In this work, we experimentally investigate the ability of a perpendicular magnetic field to tune the valley energies in monolayer MoSe 2 by valley-resolved magneto-photoluminescence (magneto-PL) spectroscopy. Lifting of the valley degeneracy is demonstrated through the opposite energy shifts induced in the excitonic transitions in the two valleys by the magnetic field. The magnitude of this Zeeman shift, 0.12 meV/T, agrees with the predicted magnetic moments of the valley states. In the presence of a magnetic field, with split K and K' valleys, we create an equilibrium valley polarization, i.e., an imbalance in the charge distribution in the two valleys, by doping the sample. This behavior is revealed by the variation of the relative emission intensity of the charged and neutral excitons. Further, by comparing the direction of the energy shift of the conduction band and the relative intensity of the negatively charged exciton, we are able to clarify the valley configuration of these bright trion states. In addition, the doping dependent trion Zeeman shift reveals the modification to the many-body binding energy by the creation of valley polarization. 3MoSe 2 monolayers were prepared by mechanic...

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Ultrafast electrically driven nanoscale light sources are critical components in nanophotonics. Compound semiconductor-based light sources for the nanophotonic platforms have been extensively investigated over the past decades. However, monolithic ultrafast light sources with a small footprint remain a challenge. Here, we demonstrate electrically driven ultrafast graphene light emitters that achieve light pulse generation with up to 10 GHz bandwidth across a broad spectral range from the visible to the near-infrared. The fast response results from ultrafast charge-carrier dynamics in graphene and weak electron-acoustic phonon-mediated coupling between the electronic and lattice degrees of freedom. We also find that encapsulating graphene with hexagonal boron nitride (hBN) layers strongly modifies the emission spectrum by changing the local optical density of states, thus providing up to 460% enhancement compared to the gray-body thermal radiation for a broad peak centered at 720 nm. Furthermore, the hBN encapsulation layers permit stable and bright visible thermal radiation with electronic temperatures up to 2000 K under ambient conditions as well as efficient ultrafast electronic cooling via near-field coupling to hybrid polaritonic modes under electrical excitation. These high-speed graphene light emitters provide a promising path for on-chip light sources for optical communications and other optoelectronic applications.

PEC cells demands the development of stable, economical, and efficient photoanodes. Stable, earth-abundant metal oxides such as TiO 2 , WO 3 , Fe 2 O 3 , and BiVO 4 are popular photoanode candidates. [2,3] However, these metal oxide photoanodes exhibit poor efficiency because they cannot achieve simultaneously high light absorption, charge separation, and charge transfer efficiencies. [4][5][6] One common strategy for improving these metal oxide photoanodes is to decorate them with various plasmonic metals, such as metal nanoparticles or nanorods, to introduce nearfield localized surface plasmon resonance (LSPR) and/or surface plasmon polaritons (SPP). [7][8][9][10][11][12][13][14][15][16][17][18] Many studies on these plasmonic metal nanostructures have focused on the light absorption enhancement effect from LSPR and SPP. [9,[19][20][21][22][23][24][25][26] In addition, LSPR can improve the performance of metal oxide photoanodes through plasmonic energy transfer through two mechanisms: direct electron transfer (DET) and plasmon-induced resonant energy transfer (PIRET). [9,21] DET refers to the hot-electrons injection from plasmonic metal nanoparticles to the conduction band of neighboring metal oxides, and it requires direct contact between metal and metal oxides. [9] PIRET was recently proposed by several pioneering studies. [9,21] PIRET utilizes the nonradiative dipole-dipole coupling between metals and metal oxides to Plasmonic metal nanostructures have been extensively investigated to improve the performance of metal oxide photoanodes for photoelectrochemical (PEC) solar water splitting cells. Most of these studies have focused on the effects of those metal nanostructures on enhancing light absorption and enabling direct energy transfer via hot electrons. However, several recent studies have shown that plasmonic metal nanostructures can improve the PEC performance of metal oxide photoanodes via another mechanism known as plasmon-induced resonant energy transfer (PIRET). However, this PIRET effect has not yet been tested for the molybdenum-doped bismuth vanadium oxide (Mo:BiVO 4 ), regarded as one of the best metal oxide photoanode candidates. Here, this study constructs a hybrid Au nanosphere/Mo:BiVO 4 photoanode interwoven in a hexagonal pattern to investigate the PIRET effect on the PEC performance of Mo:BiVO 4 . This study finds that the Au nanosphere array not only increases light absorption of the photoanode as expected, but also improves both its charge transport and charge transfer efficiencies via PIRET, as confirmed by time-correlated single photon counting and tran-

To further increase the open‐circuit voltage (V oc) of perovskite solar cells (PSCs), many efforts have been devoted to doping the TiO2 electron transport/selective layers by using metal dopants with higher electronegativity than Ti. However, those dopants can introduce undesired charge traps that hinder charge transport through TiO2, so the improvement in the V oc is often accompanied by an undesired photocurrent density–voltage (J–V) hysteresis problem. Herein, it is demonstrated that the use of a rapid flame doping process (40 s) to introduce cobalt dopant into TiO2 not only solves the J–V hysteresis problem but also increases the V oc and power conversion efficiency of both mesoscopic and planar PSCs. The reasons for the simultaneous improvements are two fold. First, the flame‐doped Co‐TiO2 film forms Co‐Ov (cobalt dopant‐oxygen vacancy) pairs and hence reduces the number density of Ti3+ trap states. Second, Co doping upshifts the band structure of TiO2, facilitating efficient charge extraction. As a result, for planar PSCs, the flame doping of Co increases the efficiency from 17.1% to 18.0% while reducing the hysteresis from 16.0% to 1.7%. Similarly, for mesoscopic PSCs, the flame doping of Co increases the efficiency from 18.5% to 20.0% while reducing the hysteresis from 7.0% to 0.1%.

We investigate the valley Hall effect (VHE) in monolayer WSe2 field-effect transistors using optical Kerr rotation measurements at 20 K. While studies of the VHE have so far focused on n-doped MoS2, we observe the VHE in WSe2 in both the n- and p-doping regimes. Hole doping enables access to the large spin-splitting of the valence band of this material. The Kerr rotation measurements probe the spatial distribution of the valley carrier imbalance induced by the VHE. Under current flow, we observe distinct spin-valley polarization along the edges of the transistor channel. From analysis of the magnitude of the Kerr rotation, we infer a spin-valley density of 44 spins/μm, integrated over the edge region in the p-doped regime. Assuming a spin diffusion length less than 0.1 μm, this corresponds to a spin-valley polarization of the holes exceeding 1%.

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