A trend for future electronics is to utilize internal degrees of freedom of electron, in addition to its charge, for nonvolatile information processing. A paradigmatic example is spintronics utilizing the spin of electrons. 1,2 Degenerate valleys of energy bands well separated in momentum space constitute another discrete degrees of freedom for low-energy electrons with long relaxation time. This has led to the emergence of valleytronics, a conceptual electronics based on manipulating valley index, much in the same way as the spin index is used in spintronics applications. [3][4][5][6] As the first step, a controllable way to selectively fill or deplete valleys, thereby producing a valley polarization, is of crucial importance, and is the subject of growing theoretical and experimental efforts. 3-11 Here we report experimental evidences on selective occupation of the degenerate valleys by circularly polarized optical pumping in MoS 2 monolayer, an emerging multi-valley 2D semiconductor with remarkable optical and Generated by Foxit PDF Creator © Foxit Software

We report systematic optical studies of WS2 and WSe2 monolayers and multilayers. The efficiency of second harmonic generation shows a dramatic even-odd oscillation with the number of layers, consistent with the presence (absence) of inversion symmetry in even-layer (odd-layer). Photoluminescence (PL) measurements show the crossover from an indirect band gap semiconductor at multilayers to a direct-gap one at monolayers. A hot luminescence peak (B) is observed at ~0.4 eV above the prominent band edge peak (A) in all samples. The magnitude of A-B splitting is independent of the number of layers and coincides with the spin-valley coupling strength in monolayers. Ab initio calculations show that this thickness independent splitting pattern is a direct consequence of the giant spin-valley coupling which fully suppresses interlayer hopping at valence band edge near K points because of the sign change of the spin-valley coupling from layer to layer in the 2H stacking order.

Periodic stripe patterns are ubiquitous in living organisms, yet the underlying developmental processes are complex and difficult to disentangle. We describe a synthetic genetic circuit that couples cell density and motility. This system enabled programmed Escherichia coli cells to form periodic stripes of high and low cell densities sequentially and autonomously. Theoretical and experimental analyses reveal that the spatial structure arises from a recurrent aggregation process at the front of the continuously expanding cell population. The number of stripes formed could be tuned by modulating the basal expression of a single gene. The results establish motility control as a simple route to establishing recurrent structures without requiring an extrinsic pacemaker.

We report the observation of anomalously robust valley polarization and valley coherence in bilayer WS 2 . The polarization of the photoluminescence from bilayer WS 2 follows that of the excitation source with both circular and linear polarization, and remains even at room temperature. The near-unity circular polarization of the luminescence reveals the coupling of spin, layer, and valley degree of freedom in bilayer system, and the linearly polarized photoluminescence manifests quantum coherence between the two inequivalent band extrema in momentum space, namely, the valley quantum coherence in atomically thin bilayer WS 2 . This observation provides insight into quantum manipulation in atomically thin semiconductors.valleytronics | spin-valley coupling | valley quantum control T ungsten sulfide WS 2 , part of the family of group VI transition metal dichalcogenides (TMDCs), is a layered compound with buckled hexagonal lattice. As WS 2 thins to atomically thin layers, WS 2 films undergo a transition from indirect gap in bulk form to direct gap at monolayer level with the band edge located at energy-degenerate valleys (K, K′) at the corners of the Brillouin zone (1-3). Like the case of its sister compound, monolayer MoS 2 , the valley degree of freedom of monolayer WS 2 could be presumably addressed through nonzero but contrasting Berry curvatures and orbital magnetic moments that arise from the lack of spatial inversion symmetry at monolayers (3, 4). The valley polarization could be realized by the control of the polarization of optical field through valley-selective interband optical selection rules at K and K′ valleys as illustrated in Fig. 1A (4-6). In monolayer WS 2 , both the top of the valence bands and the bottom of the conduction bands are constructed primarily by the d orbits of tungsten atoms, which are remarkably shaped by spin-orbit coupling (SOC). The giant spin-orbit coupling splits the valence bands around the K (K′) valley by 0.4 eV, and the conduction band is nearly spin degenerated (7). As a result of time-reversal symmetry, the spin splitting has opposite signs at the K and K′ valleys. Namely, the Kramer's doublet jK↑i and jK′↓i is separated from the other doublet jK′↑i and jK↓i by the SOC splitting of 0.4 eV. The spin and valley are strongly coupled at K (K′) valleys, and this coupling significantly suppresses spin and valley relaxations as both spin and valley indices have to be changed simultaneously.In addition to the spin and valley degrees of freedom, in bilayer WS 2 there exists an extra index: layer polarization that indicates the carriers' location, either up-layer or down-layer. Bilayer WS 2 follows the Bernal packing order and the spatial inversion symmetry is recovered: each layer is 180°in plane rotation of the other with the tungsten atoms of a given layer sitting exactly on top of the S atoms of the other layer. The layer rotation symmetry switches K and K′ valleys, but leaves the spin unchanged, which results in a sign change for the spin-valley coupling from layer to layer (Fi...

Nanoscale room-temperature ferroelectricity is ideal for developing advanced non-volatile high-density memories. However, reaching the thin film limit in conventional ferroelectrics is a long-standing challenge due to the presence of the critical thickness effect. van der Waals materials, thanks to their stable layered structure, saturated interfacial bonding and weak interlayer couplings, are promising for exploring ultra-thin two-dimensional (2D) ferroelectrics and device applications. Here, we demonstrate a switchable room-temperature ferroelectric diode built upon a 2D ferroelectric α-In2Se3 layer as thin as 5 nm in the form of a graphene/α-In2Se3 heterojunction. The intrinsic out-of-plane ferroelectricity of the α-In2Se3 thin layers is evidenced by the observation of reversible spontaneous electric polarization with a relatively low coercive electric field of ∼2 × 105 V cm-1 and a typical ferroelectric domain size of around tens μm2. Owing to the out-of-plane ferroelectricity of the α-In2Se3 layer, the Schottky barrier at the graphene/α-In2Se3 interface can be effectively tuned by switching the electric polarization with an applied voltage, leading to a pronounced switchable double diode effect with an on/off ratio of ∼105. Our results offer a new way for developing novel nanoelectronic devices based on 2D ferroelectrics.

Titanium oxide (TiO2) has been commonly used as an electron transport layer (ETL) of regular‐structure perovskite solar cells (PSCs), and so far the reported PSC devices with power conversion efficiencies (PCEs) over 21% are mostly based on mesoporous structures containing an indispensable mesoporous TiO2 layer. However, a high temperature annealing (over 450 °C) treatment is mandatory, which is incompatible with low‐cost fabrication and flexible devices. Herein, a facile one‐step, low‐temperature, nonhydrolytic approach to in situ synthesizing amino‐functionalized TiO2 nanoparticles (abbreviated as NH2‐TiO2 NPs) is developed by chemical bonding of amino (‐NH2) groups, via TiN bonds, onto the surface of TiO2 NPs. NH2‐TiO2 NPs are then incorporated as an efficient ETL in n‐i‐p planar heterojunction (PHJ) PSCs, affording PCE over 21%. Cs0.05FA0.83MA0.12PbI2.55Br0.45 (abbreviated as CsFAMA) PHJ PSC devices based on NH2‐TiO2 ETL exhibit the best PCE of 21.33%, which is significantly higher than that of the devices based on the pristine TiO2 ETL (19.82%) and is close to the record PCE for devices with similar structures and fabrication procedures. Besides, due to the passivation of the surface trap states of perovskite film, the hysteresis of current–voltage response is significantly suppressed, and the ambient stability of devices is improved upon amino functionalization.

Atomically thin MoS2 crystals have been recognized as a quasi-2D semiconductor with remarkable physical properties. This letter reports our Raman scattering measurements on multilayer and monolayer MoS2, especially in the low-frequency range (<50 cm −1 ). We find two low-frequency Raman modes with contrasting thickness dependence. With increasing the number of MoS2 layers, one shows a significant increase in frequency while the other decreases following a 1/N (N denotes the number of unit layers) trend. With the aid of first-principles calculations we assign the former as the shear mode E 2 2g . The latter is distinguished as the compression vibrational mode, similar to the surface vibration of other epitaxial thin films. The opposite evolution of the two modes with thickness demonstrates novel vibrational modes in atomically thin crystal as well as a new and more precise way to characterize thickness of atomically thin MoS2 films. In addition, we observe a broad feature around 38 cm −1 ( 5 meV) which is visible only under near-resonance excitation and pinned at the fixed energy independent of thickness. We interpret the feature as an electronic Raman scattering associated with the spin-orbit coupling induced splitting in conduction band at K points in their Brillouin zone.

The ultimate goal of making atomically thin electronic devices stimulates intensive research on layered materials, in particular the group-VI transition metal dichalcogenides (TMDs).