Electronic junctions on edge Two-dimensional materials such as graphene are attractive materials for making smaller transistors because they are inherently nanoscale and can carry high currents. However, graphene has no band gap and the transistors are “leaky”; that is, they are hard to turn off. Related transition metal dichalcogenides (TMDCs) such as molybdenum sulfide have band gaps. Transistors based on these materials can have high ratios of “on” to “off” currents. However, it is often difficult to make a good voltage-biased (p-n) junction between different TMDC materials. Li et al. succeeded in making p-n heterojunctions between two of these materials, molybdenum sulfide and tungsten selenide. They did this not by stacking the layers, which make a weak junction, but by growing molybdenum sulfide on the edge of a triangle of tungsten selenide with an atomically sharp boundary Science , this issue p. 524

Studies of gene rearrangements and the consequent oncogenic fusion proteins have laid the foundation for targeted cancer therapy. To identify oncogenic fusions associated with glioma progression, we catalogued fusion transcripts by RNA-seq of 272 gliomas. Fusion transcripts were more frequently found in high-grade gliomas, in the classical subtype of gliomas, and in gliomas treated with radiation/temozolomide. Sixty-seven in-frame fusion transcripts were identified, including three recurrent fusion transcripts: FGFR3-TACC3, RNF213-SLC26A11, and PTPRZ1-MET (ZM). Interestingly, the ZM fusion was found only in grade III astrocytomas (1/13; 7.7%) or secondary GBMs (sGBMs, 3/20; 15.0%). In an independent cohort of sGBMs, the ZM fusion was found in three of 20 (15%) specimens. Genomic analysis revealed that the fusion arose from translocation events involving introns 3 or 8 of PTPRZ and intron 1 of MET. ZM fusion transcripts were found in GBMs irrespective of isocitrate dehydrogenase 1 (IDH1) mutation status. sGBMs harboring ZM fusion showed higher expression of genes required for PIK3CA signaling and lowered expression of genes that suppressed RB1 or TP53 function. Expression of the ZM fusion was mutually exclusive with EGFR overexpression in sGBMs. Exogenous expression of the ZM fusion in the U87MG glioblastoma line enhanced cell migration and invasion. Clinically, patients afflicted with ZM fusion harboring glioblastomas survived poorly relative to those afflicted with non-ZM-harboring sGBMs (P < 0.001). Our study profiles the shifting RNA landscape of gliomas during progression and reveled ZM as a novel, recurrent fusion transcript in sGBMs.

Stacking of MoS 2 and WSe 2 monolayers is conducted by transferring triangular MoS 2 monolayers on top of WSe 2 monolayers, all grown by chemical vapor deposition (CVD).Raman spectroscopy and photoluminescence (PL) studies reveal that these mechanically stacked monolayers are not closely coupled, but after a thermal treatment at 300 °C, it is possible to produce van der Waals solids consisting of two interacting transition metal dichalcogenide (TMD) monolayers. The layer-number sensitive Raman out-of-plane mode A 2 1g for WSe 2 (309 cm À1 ) is found sensitive to the coupling between two TMD monolayers. The presence of interlayer excitonic emissions and the changes in other intrinsic Raman modes such as E 00 for MoS 2 at 286 cm À1 and A 2 1g for MoS 2 at around 463 cm À1 confirm the enhancement of the interlayer coupling.

In crystals, energy band extrema in momentum space can be identified by a valley index. The internal quantum degree of freedom associated with valley pseudospin indices can act as a useful information carrier, analogous to electronic charge or spin [1][2][3][4] . Interest in valleytronics has been revived in recent years following the discovery of atomically thin materials such as graphene and transition metal dichalcogenides [5][6][7] . However, the valley coherence time-a crucial quantity for valley pseudospin manipulation-is di cult to directly probe. In this work, we use two-dimensional coherent spectroscopy to resonantly generate and detect valley coherence of excitons (Coulomb-bound electron-hole pairs) in monolayer WSe 2 (refs 8,9). The imposed valley coherence persists for approximately one hundred femtoseconds. We propose that the electron-hole exchange interaction provides an important decoherence mechanism in addition to exciton population recombination. This work provides critical insight into the requirements and strategies for optical manipulation of the valley pseudospin for future valleytronics applications.Group-VI transition metal dichalcogenides (TMDs) with 2H structure (for example, MX 2 , M = Mo, W; X = S, Se) are a particularly intriguing class of semiconductors when thinned down to monolayers 6,7 . The valence and conduction band extrema are located at both K and K points at the corners of the hexagonal Brillouin zone, as illustrated in Fig. 1a. The degenerate K and K points are related to each other by time reversal symmetry and give rise to the valley degree of freedom (DoF) of the band-edge electrons and holes. Strong Coulomb interactions lead to the formation of excitons with remarkably large binding energies due to the heavy effective mass and reduced dielectric screening in monolayer TMDs (refs 10-12). An exciton as a bound electron-hole pair inherits the valley DoF. Because of valley-dependent optical selection rules, they can be excited only by σ + (σ − ) circularly polarized light at the K(K ) valley. Owing to its close analogy to spin 4 , the valley DoF can be considered as a pseudospin represented by a vector S on the Bloch sphere (Fig. 1b). The out-of-plane component S z and in-plane component S x,y describe the valley polarization and the coherent superposition of exciton valley states, respectively. After optical initialization, valley depolarization and decoherence are manifested by a reduction in the magnitudes of S z and S x,y , respectively.The ability to coherently manipulate spins and pseudospins is at the heart of spintronics and valleytronics; however, previous investigations have focused mainly on the creation and relaxation of valley polarization using non-resonant photoluminescence (PL) or pump/probe spectroscopy techniques [13][14][15][16][17][18][19] . Optical excitation close to the lowest energy exciton resonance leads to nearly 100% valley polarization in monolayer TMDs such as MoS 2 (refs 13,15,20). Time-resolved PL spectroscopy has revealed a fewpicosecon...

Chirality reveals symmetry breaking of the fundamental interaction of elementary particles. In condensed matter, for example, the chirality of electrons governs many unconventional transport phenomena such as the quantum Hall effect. Here we show that phonons can exhibit intrinsic chirality in monolayer tungsten diselenide. The broken inversion symmetry of the lattice lifts the degeneracy of clockwise and counterclockwise phonon modes at the corners of the Brillouin zone. We identified the phonons by the intervalley transfer of holes through hole-phonon interactions during the indirect infrared absorption, and we confirmed their chirality by the infrared circular dichroism arising from pseudoangular momentum conservation. The chiral phonons are important for electron-phonon coupling in solids, phonon-driven topological states, and energy-efficient information processing.

Experimental determinations of the electronic band structures in TMDs are quite non-trivial.Optical spectroscopies [1][2][3][4][5] are unsuitable to measure the quasi-particle band structures due to the existence of large exciton binding energies. Using angle resolved photoemission (ARPES), it is difficult to probe the conduction band structures 6,7 . In principle, scanning tunneling spectroscopy (STS) would be an ideal probe to determine both the valence and conduction band structures.However, the reported results have been controversial thus far, even for the determination of the quasi-particle band gaps [8][9][10] . As we will show, this is due primarily to the intriguing influence of the lateral momentum in the tunneling process, making certain critical points difficult to access in the conventional scanning tunneling spectroscopy acquired at a constant tip-to-sampledistance (Z). By using a comprehensive approach combining the constant Z and variable Z spectroscopies, as well as state-resolved tunneling decay constant measurements, we have shown 3 that detailed electronic structures, including quasi-particle gaps, critical point energy locations and their origins in the BZs in TMDs can be revealed.The TMD samples are grown using chemical vapor deposition (CVD) for WSe 2 11 or molecular beam epitaxy (MBE) for MoSe 2 on highly-oriented-pyrolytic-graphite (HOPG)substrates. In addition, MoSe 2 has also been grown on epitaxial bi-layer graphene to investigate the environmental influences on the quasi-particle band structures. Figures 1a and b show scanning tunneling microscopy (STM) images of MoSe 2 and WSe 2 , respectively. Due to a nearly 4:3 lattice match with the graphite, the TMD samples also show Moiré patterns with a periodicity of ~ 1nm. An example is shown as an inset in Fig. 1b, similar to those reported earlier 9 . In Fig. 1c we show a generic electronic structure for SL-TMD materials. We first discuss the result of MoSe 2 due to the availability of experimentally determined E vs. k dispersion in the valence band which can be used to cross-check with our results. 4Such a large difference in the decay constant is responsible for the difficulty in detecting the states near the VBM. The lack of the sensitivity in the constant-Z STS can be overcome by acquiring spectra at variable Z as described before 14 . Here we adopt a form of variable Z spectroscopy by performing STS at constant current. In this mode, as the sample bias is scanned across different thresholds, Z (the dependent variable) will respond automatically in order to keep the current constant. The differential conductivity (I/V) I is measured by using a lock-in amplifier (Fig. 2b). In the meantime, the acquired Z-value is used to deduce (Z/V) I which can be used to identify individual thresholds (Fig. 2c).For the valence band (left panel), the state at the  point also appears as a prominent peak in the (I/V) I spectrum. Moreover, spectroscopic features above  are observed due to the significant enhancement of the sensitivity. A shoul...

Vertical integration of two-dimensional van der Waals materials is predicted to lead to novel electronic and optical properties not found in the constituent layers. Here, we present the direct synthesis of two unique, atomically thin, multi-junction heterostructures by combining graphene with the monolayer transition-metal dichalcogenides: molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2) and tungsten diselenide (WSe2). The realization of MoS2–WSe2–graphene and WSe2–MoS2–graphene heterostructures leads to resonant tunnelling in an atomically thin stack with spectrally narrow, room temperature negative differential resistance characteristics.

Spectroscopic ellipsometry was used to characterize the complex refractive index of chemical-vapordeposited monolayer transition metal dichalcogenides (TMDs). The extraordinary large value of the refractive index in the visible frequency range is obtained. The absorption response shows a strong correlation between the magnitude of the exciton binding energy and band gap energy. Together with the observed giant spin-orbit splitting, these findings advance the fundamental understanding of their novel electronic structures and the development of monolayer TMDs-based optoelectronic and spintronic devices. V