Andras Kis

Bio: Andras Kis is an academic researcher from École Polytechnique Fédérale de Lausanne. The author has contributed to research in topics: Monolayer & Semiconductor. The author has an hindex of 67, co-authored 165 publications receiving 53990 citations. Previous affiliations of Andras Kis include École Normale Supérieure & Lawrence Berkeley National Laboratory.

Thickness-modulated metal-to-semiconductor transformation in a transition metal dichalcogenide

Abstract: The possibility of tailoring physical properties by changing the number of layers in van der Waals crystals is one of the driving forces behind the emergence of two-dimensional materials. One example is bulk MoS2, which changes from an indirect gap semiconductor to a direct bandgap semiconductor in the monolayer form. Here, we show a much bigger tuning range with a complete switching from a metal to a semiconductor in atomically thin PtSe2 as its thickness is reduced. Crystals with a thickness of ~13 nm show metallic behavior with a contact resistance as low as 70 Ω·µm. As they are thinned down to 2.5 nm and below, we observe semiconducting behavior. In such thin crystals, we demonstrate ambipolar transport with a bandgap smaller than 2.2 eV and an on/off ratio of ~105. Our results demonstrate that PtSe2 possesses an unusual behavior among 2D materials, enabling novel applications in nano and optoelectronics.

Polarization switching and electrical control of interlayer excitons in two-dimensional van der Waals heterostructures.

Abstract: Long-lived interlayer excitons in van der Waals heterostructures based on transition metal dichalcogenides, together with unique spin-valley physics, make them promising for next-generation photonic and valleytronic devices. While the emission characteristics of interlayer excitons have been studied, efficient manipulation of their valley-state, a necessary requirement for information encoding, is still lacking. Here, we demonstrate comprehensive electrical control of interlayer excitons in a MoSe2/WSe2 heterostructure. Encapsulation of our well-aligned stack with hexagonal boron nitride (h-BN) allows us to resolve two separate narrow interlayer transitions with opposite helicities under circularly polarized excitation, either preserving or reversing the polarization of incoming light. By electrically controlling their relative intensities, we realize a polarization switch with tuneable emission intensity and wavelength. Finally, we demonstrate large Zeeman shifts of these two transitions upon application of an external magnetic field. These results are interpreted within the picture of moire-induced brightening of forbidden optical transitions. The ability to control the polarization of interlayer excitons is a step forward towards the manipulation of the valley degree-of-freedom in realistic device applications.

Small-signal amplifier based on single-layer MoS2

Abstract: In this letter we demonstrate the operation of an analog small-signal amplifier based on single-layer MoS2, a semiconducting analogue of graphene. Our device consists of two transistors integrated on the same piece of single-layer MoS2. The high intrinsic band gap of 1.8 eV allows MoS2-based amplifiers to operate with a room temperature gain of 4. The amplifier operation is demonstrated for the frequencies of input signal up to 2 kHz preserving the gain higher than 1. Our work shows that MoS2 can effectively amplify signals and that it could be used for advanced analog circuits based on two-dimensional materials.

Atomic Scale Microstructure and Properties of Se-Deficient Two-Dimensional MoSe2

Abstract: We study the atomic scale microstructure of nonstoichiometric two-dimensional (2D) transition metal dichalcogenide MoSe2–x by employing aberration-corrected high-resolution transmission electron microscopy. We show that a Se-deficit in single layers of MoSe2 grown by molecular beam epitaxy gives rise to a dense network of mirror-twin-boundaries (MTBs) decorating the 2D-grains. With the use of density functional theory calculations, we further demonstrate that MTBs are thermodynamically stable structures in Se-deficient sheets. These line defects host spatially localized states with energies close to the valence band minimum, thus giving rise to enhanced conductance along straight MTBs. However, electronic transport calculations show that the transmission of hole charge carriers across MTBs is strongly suppressed due to band bending effects. We further observe formation of MTBs during in situ removal of Se atoms by the electron beam of the microscope, thus confirming that MTBs appear due to Se-deficit, and...

Electrochemical Reaction in Single Layer MoS2: Nanopores Opened Atom by Atom

Abstract: Ultrathin nanopore membranes based on 2D materials have demonstrated ultimate resolution toward DNA sequencing. Among them, molybdenum disulfide (MoS2) shows long-term stability as well as superior sensitivity enabling high throughput performance. The traditional method of fabricating nanopores with nanometer precision is based on the use of focused electron beams in transmission electron microscope (TEM). This nanopore fabrication process is time-consuming, expensive, not scalable, and hard to control below 1 nm. Here, we exploited the electrochemical activity of MoS2 and developed a convenient and scalable method to controllably make nanopores in single-layer MoS2 with subnanometer precision using electrochemical reaction (ECR). The electrochemical reaction on the surface of single-layer MoS2 is initiated at the location of defects or single atom vacancy, followed by the successive removals of individual atoms or unit cells from single-layer MoS2 lattice and finally formation of a nanopore. Step-like features in the ionic current through the growing nanopore provide direct feedback on the nanopore size inferred from a widely used conductance vs pore size model. Furthermore, DNA translocations can be detected in situ when as-fabricated MoS2 nanopores are used. The atomic resolution and accessibility of this approach paves the way for mass production of nanopores in 2D membranes for potential solid-state nanopore sequencing.

Fast parallel algorithms for short-range molecular dynamics

Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

Abstract: 抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

Electronics and optoelectronics of two-dimensional transition metal dichalcogenides.

Abstract: Single-layer metal dichalcogenides are two-dimensional semiconductors that present strong potential for electronic and sensing applications complementary to that of graphene.