Showing papers by "Andras Kis published in 2015"

Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems

Abstract: We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries. We also define an extensive list of acronyms in an effort to standardize the nomenclature in this emerging field.

Electrical contacts to two-dimensional semiconductors

Abstract: The performance of electronic and optoelectronic devices based on two-dimensional layered crystals, including graphene, semiconductors of the transition metal dichalcogenide family such as molybdenum disulphide (MoS2) and tungsten diselenide (WSe2), as well as other emerging two-dimensional semiconductors such as atomically thin black phosphorus, is significantly affected by the electrical contacts that connect these materials with external circuitry. Here, we present a comprehensive treatment of the physics of such interfaces at the contact region and discuss recent progress towards realizing optimal contacts for two-dimensional materials. We also discuss the requirements that must be fulfilled to realize efficient spin injection in transition metal dichalcogenides.

Optically active quantum dots in monolayer WSe2.

Abstract: Zero-dimensional photonic quantum emitters can be realized using defects in the two-dimensional dichalcogenides.

Valley Zeeman effect in elementary optical excitations of monolayer WSe2

Abstract: Charge carriers in transition metal dichalcogenides have an extra degree of freedom known as valley pseudospin, which is associated with the shape of the energy bands. Experiments show that this pseudospin can be manipulated using magnetic fields.

Large-Area Epitaxial Monolayer MoS2

Abstract: Two-dimensional semiconductors such as MoS2 are an emerging material family with wide-ranging potential applications in electronics, optoelectronics, and energy harvesting. Large-area growth methods are needed to open the way to applications. Control over lattice orientation during growth remains a challenge. This is needed to minimize or even avoid the formation of grain boundaries, detrimental to electrical, optical, and mechanical properties of MoS2 and other 2D semiconductors. Here, we report on the growth of high-quality monolayer MoS2 with control over lattice orientation. We show that the monolayer film is composed of coalescing single islands with limited numbers of lattice orientation due to an epitaxial growth mechanism. Optical absorbance spectra acquired over large areas show significant absorbance in the high-energy part of the spectrum, indicating that MoS2 could also be interesting for harvesting this region of the solar spectrum and fabrication of UV-sensitive photodetectors. Even though t...

Identification of single nucleotides in MoS2 nanopores

Abstract: Single nucleotides can be identified with atomically thin MoS2 nanopores by regulating molecular translocation speeds using a viscosity gradient system based on room-temperature ionic liquids.

Single-layer MoS2 electronics.

Abstract: ConspectusAtomic crystals of two-dimensional materials consisting of single sheets extracted from layered materials are gaining increasing attention. The most well-known material from this group is graphene, a single layer of graphite that can be extracted from the bulk material or grown on a suitable substrate. Its discovery has given rise to intense research effort culminating in the 2010 Nobel Prize in physics awarded to Andre Geim and Konstantin Novoselov. Graphene however represents only the proverbial tip of the iceberg, and increasing attention of researchers is now turning towards the veritable zoo of so-called “other 2D materials”. They have properties complementary to graphene, which in its pristine form lacks a bandgap: MoS2, for example, is a semiconductor, while NbSe2 is a superconductor. They could hold the key to important practical applications and new scientific discoveries in the two-dimensional limit. This family of materials has been studied since the 1960s, but most of the research fo...

Piezoresistivity and Strain-induced Band Gap Tuning in Atomically Thin MoS2.

Abstract: Continuous tuning of material properties is highly desirable for a wide range of applications, with strain engineering being an interesting way of achieving it. The tuning range, however, is limited in conventional bulk materials that can suffer from plasticity and low fracture limit due to the presence of defects and dislocations. Atomically thin membranes such as MoS2 on the other hand exhibit high Young’s modulus and fracture strength, which makes them viable candidates for modifying their properties via strain. The bandgap of MoS2 is highly strain-tunable, which results in the modulation of its electrical conductivity and manifests itself as the piezoresistive effect, whereas a piezoelectric effect was also observed in odd-layered MoS2 with broken inversion symmetry. This coupling between electrical and mechanical properties makes MoS2 a very promising material for nanoelectromechanical systems (NEMS). Here, we incorporate monolayer, bilayer, and trilayer MoS2 in a nanoelectromechanical membrane confi...

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.

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 disulphide (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 sub-nanometer 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.

Electronic properties of transition-metal dichalcogenides

Abstract: Graphene is not the only prominent example of two-dimensional (2D) materials. Due to their interesting combination of high mechanical strength and optical transparency, direct bandgap and atomic scale thickness transition-metal dichalcogenides (TMDCs) are an example of other materials that are now vying for the attention of the materials research community. In this article, the current state of quantum-theoretical calculations of the electronic and mechanical properties of semiconducting TMDC materials are presented. In particular, the intriguing interplay between external parameters (electric field, strain) and band structure, as well as the basic properties of heterostructures formed by vertical stacking of different 2D TMDCs are reviewed. Electrical measurements of MoS2, WS2, and WSe2 and their heterostructures, starting from simple field-effect transistors to more demanding logic circuits, high-frequency transistors, and memory devices, are also presented.

Large-area MoS2 grown using H2S as the sulphur source

Abstract: We report on the growth of molybdenum disulphide (MoS2) using H2S as a gas-phase sulfur precursor that allows controlling the domain growth direction of domains in both vertical (perpendicular to the substrate plane) and horizontal (within the substrate plane), depending on the H2S:H2 ratio in the reaction gas mixture and temperature at which they are introduced during growth. Optical and atomic force microscopy measurements on horizontal MoS2 demonstrate the formation of monolayer triangular-shape domains that merge into a continuous film. Scanning transmission electron microscopy of monolayer MoS2 shows a regular atomic structure with a hexagonal symmetry. Raman and photoluminescence spectra confirm the monolayer thickness of the material. Field-effect transistors fabricated on MoS2 domains that are transferred onto Si/SiO2 substrates show a mobility similar to previously reported exfoliated and chemical vapor deposition-grown materials.

Thickness-dependent mobility in two-dimensional MoS₂ transistors.

Abstract: Two-dimensional (2D) semiconductors such as mono and few-layer molybdenum disulphide (MoS2) are very promising for integration in future electronics as they represent the ultimate miniaturization limit in the vertical direction. While monolayer MoS2 attracted considerable attention due to its broken inversion symmetry, spin/valley coupling and the presence of a direct band gap, few-layer MoS2 remains a viable option for technological application where its higher mobility and lower contact resistance are believed to offer an advantage. However, it remains unclear whether multilayers are intrinsically superior or if they are less affected by environmental effects. Here, we report the first systematic comparison of the field-effect mobilities in mono-, bi- and trilayer MoS2 transistors after thorough in situ annealing in vacuum. We show that the mobility of field-effect transistors (FETs) based on monolayer MoS2 is significantly higher than that of FETs based on two or three layers. We demonstrate that it is important to remove the influence of gaseous adsorbates and water before comparing mobilities, as monolayers exhibit the highest sensitivity to ambient air exposure. In addition, we study the influence of the substrate roughness and show that this parameter does not affect FET mobilities.

Electromechanical oscillations in bilayer graphene

Abstract: Nanoelectromechanical systems constitute a class of devices lying at the interface between fundamental research and technological applications. Realizing nanoelectromechanical devices based on novel materials such as graphene allows studying their mechanical and electromechanical characteristics at the nanoscale and addressing fundamental questions such as electron-phonon interaction and bandgap engineering. In this work, we realize electromechanical devices using single and bilayer graphene and probe the interplay between their mechanical and electrical properties. We show that the deflection of monolayer graphene nanoribbons results in a linear increase in their electrical resistance. Surprisingly, we observe oscillations in the electromechanical response of bilayer graphene. The proposed theoretical model suggests that these oscillations arise from quantum mechanical interference in the transition region induced by sliding of individual graphene layers with respect to each other. Our work shows that bilayer graphene conceals unexpectedly rich and novel physics with promising potential in applications based on nanoelectromechanical systems.

Direct fabrication of thin layer MoS2 field-effect nanoscale transistors by oxidation scanning probe lithography

Abstract: Thin layer MoS2-based field effect transistors (FET) are emerging candidates to fabricate very fast and sensitive devices. Here, we demonstrate a method to fabricate very narrow transistor channel widths on a single layer MoS2 flake connected to gold electrodes. Oxidation scanning probe lithography is applied to pattern insulating barriers on the flake. The process narrows the electron path to about 200 nm. The output and transfer characteristics of the fabricated FET show a behavior that is consistent with the minimum channel width of the device. The method relies on the direct and local chemical modification of MoS2. The straightforward character and the lack of specific requirements envisage the controlled patterning of sub-100 nm electron channels in MoS2 FETs.

High-frequency, scaled MoS2 transistors

Abstract: The interest in MoS2 for radio-frequency (RF) application has recently increased1'2. However, little is known on the scaling behavior of transistors made from MoS2 for RF applications, which is important for establishing performance limits for electronic circuits based on 2D semiconductors on flexible and rigid substrates. Here, we present a systematic study of top-gated trilayer MoS2 RF transistors with gate lengths scaled down to 70 and 40 nm. In addition, by introducing "edge-contacted" injection of electrons3 in trilayer MoS2 devices, we decrease the contact resistance and as a result obtain the highest cutoff frequency of 6 GHz before the de-embedding procedure and 25 GHz after the de-embedding procedure.