Query: MoS 2 A New Direct-Gap Semiconductor

Candidate papers:

abstract: The electronic properties of ultrathin crystals of molybdenum disulfide consisting of N=1,2,,6 S Mo S monolayers have been investigated by optical spectroscopy. Through characterization by absorption, photoluminescence, and photoconductivity spectroscopy, we trace the effect of quantum confinement on the material's electronic structure. With decreasing thickness, the indirect band gap, which lies below the direct gap in the bulk material, shifts upwards in energy by more than 0.6 eV. This leads to a crossover to a direct gap material in the limit of the single monolayer. Unlike the bulk material, the MoS2 monolayer emits light strongly. The freestanding monolayer exhibits an increase in luminescence quantum efficiency by more than a factor of 104 compared with the bulk material.
author_names: Kin Fai Mak

Changgu Lee

James C Hone

Jie Shan

Tony F Heinz
title: Atomically thin MoS2: a new direct gap semiconductor.
venue: Physical review letters
year: 2010

abstract: Two dimensional (2D) layered materials, particularly semiconducting transition metal dichalcogenides (TMDs) such as MoS2 and WSe2, have already attracted tremendous research efforts, due not only to their atomically thin thickness, but also to many exciting new phenomena associated with new materials. Recent advances in heterostructures formed by vertically stacked or laterally stitched 2D semiconductors further show their promise for novel device applications. Here, I will present our recent endeavors on the material synthesis and fundamental study of 2D TMDs grown by chemical vapor deposition (CVD) In particular, I will present CVD grown noble metal dichalcogenides, such as PtSe2 and PdSe2, with strongly layer number dependent properties. This new type 2D material is promising for realizing metal semiconductor junctions by local engineering the semiconducting and metallic phases via layer control, providing a feasible scheme to solve the issue of high metal contact resistance commonly occurred in 2D semiconductors.
author_names: Wen-Hao Chang
title: 2D Layered Semiconductors beyond MoS2
venue: 2019 International Symposium on VLSI Technology, Systems and Application (VLSI TSA)
year: 2019

abstract: The advance in designing arrays of ultrathin two dimensional optical nanoresonators, known as metasurfaces, is currently enabling a large variety of novel flat optical components. The remarkable control over the electromagnetic fields offered by this technology can be further extended to the active regime in order to manipulate the light characteristics in real time. In this contribution, we couple the excitonic resonance of atomic thin MoS2 monolayers with gap surface plasmon (GSP) metasurfaces, and demonstrate selective enhancement of the exciton plasmon polariton emissions. We further demonstrate tunable emissions by controlling the charge density at the interface through electrically gating in the Metal Oxide Semiconductor (MOS) structure. Straddling two very active fields of research, this demonstration of electrically tunable light emitting metasurfaces enables real time manipulation of light matter interactions at the extreme subwavelength dimensions.
author_names: Pei-Nan Ni

Andres De Luna Bugallo

Victor M Arellano Arreola

Mario Flores Salazar

Elodie Strupiechonski

Virginie Brandli

Rajath Sawant

B Alloing

Patrice Genevet
title: Gate Tunable Emission of Exciton Plasmon Polaritons in Hybrid MoS2 Gap Mode Metasurfaces
venue: ACS Photonics
year: 2019

abstract: This chapter discusses the latest advances in the development of various two dimensional (2D) metal dichalcogenides as a new direction in modern science and technology. The chapter begins with a review of the physical chemical properties of transition metal dichalcogenides (TMDs) and how the tunable electronic structure of TMDs makes them attractive for a variety of applications. Most of the 2D TMDs are described in detail: molybdenum disulfide (MoS 2 titanium disulfide (TiS 2 tungsten disulfide (WS 2 molybdenum diselenide (MoSe 2 tungsten diselenide (WSe 2 and niobium diselenide (NbSe 2 Then, it is shown how to turn properties of 2D semiconductors through surface functionalization, and the new concept of UV photodetectors based on 2D semiconductors is investigated. The chapter concludes on the future impact of the combination of the 2D semiconductor nano crystals into 3D structures.
author_names: Serge Zhuiykov
title: New Directions in Science Technology Atomically Thin Metal Dichalcogenides
year: 2018

abstract: Graphene and phosphorene are two major types of atomically thin two dimensional materials under extensive investigation. However, the zero band gap of graphene and the instability of phosphorene greatly restrict their applications. Here, we make first principle unbiased structure search calculations to identify a new buckled graphene like PC6 monolayer with a number of desirable functional properties. The PC6 monolayer is a direct gap semiconductor with a band gap of 0.84 eV, and it has an extremely high intrinsic conductivity with anisotropic character (i.e. its electron mobility is 2.94 x 105 cm2 V 1 s 1 along the armchair direction, whereas the hole mobility reaches 1.64 x 105 cm2 V 1 s 1 along the zigzag direction) which is comparable to that of graphene. On the other hand, PC6 shows a high absorption coefficient (105 cm 1) in a broad band, from 300 to 2000 nm. Additionally, its direct band gap character can remain within a biaxial strain of 5% All these appealing properties make the predicted PC6 monolayer a promising candidate for applications in electronic and photovoltaic devices.
author_names: Tong Yu

Ziyuan Zhao

Yuanhui Sun

Aitor Bergara

Jianyan Lin

Shoutao Zhang

Haiyang Xu

Lijun Zhang

Guochun Yang

Yichun Liu
title: Two Dimensional PC6 with Direct Band Gap and Anisotropic Carrier Mobility.
venue: Journal of the American Chemical Society
year: 2019

abstract: Van der Waals heterostructures from atomically thin 2D materials have opened up new realms in modern semiconductor industry. Recently, 2D layered semiconductors such as MoS2 and SnSe2 have already demonstrated excellent electronic and optoelectronic properties due to their high electron mobility and unique band structures. Such combination of SnSe2 with MoS2 may provide a novel platform for the applications in electronics and optoelectronics. Thus, we constructed SnSe2/MoS2 based van der Waals heterostructures using MoS2 as templates, which may enrich the family of 2D van der Waals heterostructures. We demonstrate that the vdW heterostructures with high symmetry crystallographic directions show efficient interlayer charge transfer due to the strong coupling. This strong coupling is confirmed by theory calculations, low temperature photoluminescence (PL) spectra, and electrical transport properties. High performance photodetector based on the vdW heterostructure has been demonstrated with a high responsivity of up to 9.1 x 103 A W 1 which is higher by two orders of magnitude than those MoS2 only devices. The improved performance can be attributed to the efficient charge transfer from MoS2 to SnSe2 at the interface.
author_names: Xing Zhou

Nan Zhou

Chao Li

Hongyue Song

Qi Zhang

Xiaozong Hu

Lin Gan

Huiqiao Li

Jing-Tao Lu

Jun Luo

Jie Xiong

Tianyou Zhai
title: Vertical heterostructures based on SnSe 2 /MoS 2 for high performance photodetectors
year: 2017

abstract: The new 2D transitions metal dichalcogenide (TMDs) as MoS2 represents ideal material for multiple purposes. MoS2 is promising for electronic transistor and fundamentals phenomena such as superconductivity or valleytronic. It is a rich platform for optoelectronic; excitonic effects have high binding energy, strain engineering can induce a high tunability of the band gap itself. Moreover, MoS2 transit between different crystalline phase (2H 1T) making this material interesting for memristive devices and energy storage. Nanomechanical systems have been at the heart of recent physic discoveries of importance, from the detection of cosmic gravitational waves to the sensitivity record for detection of mass or force. It is a recent and almost universal probe of condensed matters issues and quantum mechanics. Since 2007, the emergence of suspended atomically thin materials, with the largest geometrical aspect ratio which can be obtained, brought new insight in nanomechanical resonators with very low mass and spring constant, high elongation resistance, high frequency tuning and especially strong mechanical non linearities. A high potential release in MoS2 for nanomechanics. It opens new experimental perspectives by measuring unique intrinsic properties when transduced into the mechanical motion. We propose to focus on unexpected electrical behaviors measured in our samples: a strong photodoping under illumination and a hysteretic loop in the I V curve corresponding to a memristive effect. We use the sensitivity of our mechanical MoS2 membrane as a non conventional probe to explore deeply these intriguing behaviors and we have seen a strong effect of softening due to the nonlinear charge of the devices. Figure 1 We explore these topics within unique sample geometry of a suspended single layer membrane of the MoS2 embedded in a nano opto electro mechanical system (NOEMS) 1. Chaste, J. et al. Intrinsic Properties of Suspended MoS2 on SiO2/Si Pillar Arrays for Nanomechanics and Optics. ACS Nano (2018) doi:10.1021/acsnano.7b07689 2. Chaste, J. et al. Nanostructures in suspended monoand bilayer epitaxial graphene. Carbon 125, 162 167 (2017)
author_names: Julien Chaste

Imen Hnid

Ali Madouri

Alan Durnez

Xavier Lafosse

- Meng

Qiang Zhao

A T Charlie Johnson

Remy Braive

Abdelkarim Ouerghi
title: Strong nanomechanical softening signature induced by memristive charge accumulation in suspended monolayer MoS 2
year: 2018

abstract: Two dimensional (2D) atomic crystals and van der Waals heterostructures constitute an emerging platform for developing new functional ultra thin electronic and optoelectronic materials for novel energy efficient devices. However, in most thin film optical applications, there is a long existing trade off between the effectiveness of light matter interactions and the thickness of semiconductor materials, especially when the materials are scaled down to atom thick dimensions. Consequently, enhancement strategies can introduce significant advances to these atomically thick materials and devices. Here we demonstrate enhanced absorption and photoluminescence generation from MoS2 monolayers coupled with a planar nanocavity. This nanocavity consists of an alumina nanolayer spacer sandwiched between monolayer MoS2 and an aluminum reflector, and can strongly enhance the light matter interaction within the MoS2, increasing the exclusive absorption of monolayer MoS2 to nearly 70% at a wavelength of 450 nm. The nanocavity also modifies the spontaneous emission rate, providing an additional design freedom to control the interaction between light and 2D materials.
author_names: Corey Janisch

Haomin Song

Chanjing Zhou

Zhong Lin

Ana Laura Elias

Dengxin Ji

Mauricio Terrones

Qiaoqiang Gan

Zhiwen Liu
title: MoS 2 monolayers on nanocavities: enhancement in light matter interaction
year: 2016

abstract: Quantification of lattice thermal conductivity of two dimensional semiconductors like MoS2 is necessary for the design of electronic and thermoelectric devices, but direct experimental measurements on free standing samples is challenging. Molecular dynamics simulations, with appropriate corrections, can provide a reference value for thermal conductivity for these material systems. Here, we construct a new empirical forcefield of the Stillinger Weber form, parameterized to phonon dispersion relations, lattice constants and elastic moduli and we use it to compute a material intrinsic thermal conductivity of 38.1 W/m K at room temperature and estimate a maximum thermal conductivity of 85.4 W/m K at T 200 K. We also identify that phonon scattering by the large isotopic mass distribution of Mo and S contributes a significant correction >45% to the thermal conductivity at low temperatures. (c) 2019 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http:/creativecommons.org/licenses/by/4.0/ https:/doi.org/10.1063/1.5085336 In plane thermal transport is an important consideration for the design of nanoelectronic and thermoelectric devices made from layered and two dimensional (2D) materials which possess some of the lowest out of plane thermal boundary conductance for solidsolid interfaces.1 Lattice thermal conductivity also determines the thermal profile of growing two dimensional crystals in exothermic synthesis techniques like chemical vapor deposition and is an important metric for the fabrication of energy harvesting devices based on the thermoelectric effect.2 4 Despite this importance, definitive experimental measurements of fundamental thermal properties of these materials like lattice thermal conductivity have not been conclusive. The primary experimental challenges, as detailed in recent reviews,2,5 stem from difficulties in accurately measuring temperature gradients on atomic length scales in suspended monolayer samples. Further, thermal conductivity of suspended monolayers and sub nm thin materials is greatly quenched (by up to 85% when these monolayers are supported on a substrate or are investigated using contact probes.6,7 Even state of the art noncontact techniques for thermal conductivity measurement like optothermal Raman are undermined by several implicit assumptions about optical absorption and equilibrium of phonon modes8 and are affected by heat losses to the environment9 leading to large errors in reported values of thermal conductivity.10 12 Further, such techniques involving localized laser heating can only be used to investigate room temperature and high temperature thermal conductivity and cannot readily access low temperature regimes.13 In contrast, atomistic MD simulations are an attractive method for estimating temperature dependent thermal conductivity of suspended monolayer systems directly from nm scale temperature gradients. Specifically, classical non equilibrium molecular dynamics (NEMD) provides a computationally inexpensive way of computing lattice thermal conductivity for monolayer materials, which includes the full anharmonicity of interatomic forces within a steady state (i.e. dynamic and non equilibrium) simulation. Classical NEMD simulations offers several advantages over the calculation of lattice thermal conductivity by ab initio atomistic simulations. Solutions to the phonon Boltzmann transport equation from ab initio simulations involve strong assumptions about phonon scattering rates and lifetimes as well as boundary conditions for phonon scattering.14 19 These assumptions are reflected in the large scatter in the AIP Advances 9, 035042 (2019) doi: 10.1063/1.5085336 9, 035042 1
author_names: Aravind Krishnamoorthy

Pankaj Rajak

Payam Norouzzadeh

David J Singh

Rajiv K Kalia

Aiichiro Nakano

Priya Vashishta
title: Thermal conductivity of MoS 2 monolayers from molecular dynamics simulations
year: 2019

abstract: We present photosensitivity in large area physical vapour deposited mono and bi layer MoS2 films. Photo voltaic effect was observed in single layer MoS2 without any apparent rectifying junctions, making device fabrication straightforward. For bi layers, no such effect was present, suggesting strong size effect in light matter interaction. The photo voltaic effect was observed to highly direction dependent in the film plane, which suggests that the oblique deposition configuration plays a key role in developing the rectifying potential gradient. To the best of our knowledge, this is the first report of any large area and transfer free MoS2 photo device with performance comparable to their exfoliated counterparts. 1 Correspondence/Findings Atomically thin semiconductor materials with large band gaps are touted for aggressively scaled electronic and optoelectronic applications [1] Even though graphene does not have a band gap, its advent has galvanized the research on two dimensional (2D) materials, notably the transition metal di chalcogenides (TMD) [2] The best example is molybdenum di sulfide (MoS2) which has received significant attention in the scientific community due to high carrier mobility, large current on/off ratio and excellent interface quality with the gate dielectrics [3] Bulk MoS2 has an indirect band gap of 1.2 eV which changes to a direct band gap of 1.83 eV for a monolayer MoS2 [4,5] This leads to unprecedented light matter interaction (high absorption coefficient and efficient electron hole pair generation under photo excitation) that has been studied in the form of photoluminescence, photoreponsivity, photoconductivity and photo voltaics [1,6 10] While the existing literature on mono or fewlayer MoS2 opto electronics promise revolutionary capabilities of next generation devices, all these studies are performed on exfoliated flakes. Since exfoliation is not a sustainable path beyond basic science, there is a critical need for large area growth of mono layer MoS2 and its opto electronic characterization. This provides the Correspondence: mah37@psu.edu Mechanical Nuclear Engineering, the Pennsylvania State University, University Park, PA 16802, USA Full list of author information is available at the end of the article (c) 2014 Wang et al. licensee Springer. This is a Attribution License (http:/creativecommons.or in any medium, provided the original work is p 1 Approved for public releas motivation for the present study, where we deposit large area cm) mono and bi layer MoS2 through magnetron sputtering and then study the effect of layer number on the interaction of matter and light. Since the deposition is directly performed on the desired substrate, our fabrication processes are performed directly on the specimen. Specimen transfer to another sample is not needed and contamination is avoided. To the best of our knowledge, this is the first report of any large area, transfer free monolayer MoS2 photo voltaic or photodetector device with performance comparable to their exfoliated counterparts. We employed physical vapour deposition with base pressure below 5x10 Torr for atomically sharp and clean interfaces. The mono and bi layer specimens were grown on 100 nm thick thermal oxide coated silicon wafer pieces via magnetron sputtering using a solid 3.3 cm diameter MoS2 target of 99.95% purity. The substrates were ultra sonically cleaned prior to introduction via a vacuum load lock and mounted on an electrically grounded fixture with heating and rotation features. The substrates were then heated to 350degC while ultra high purity argon gas was introduced at a constant flow rate of 25 sccm to a pressure of 15 mTorr. Figure 1a shows the deposition setup schematically. Figure 1b shows the cross sectional transmission electron microscopy of the specimens using a FEI Titan microscope for thickness measurements and calibration. The red dashed lines are superimposed on the figure to reveal the parallel atomic layers with likely turbo stratic structure, where all basal n Open Access article distributed under the terms of the Creative Commons g/licenses/by/4.0) which permits unrestricted use, distribution, and reproduction roperly credited. e; distribution unlimited. Figure 1 Experimental settings and characterization of 2D MoS2 films directly grown by sputtering. (a) schematic of the pulsed dc magnetron sputtering configuration (b) cross sectional TEM (dashed lines are added for highlighting the MoS2 layers, (c) x ray photoelectron spectroscopy data revealing the structure (d) 1 cmx 1 cm MoS2 specimen packaged for van der Pauw measurements showing the large active area. Table 1 Dark dc resistivity of the mono and bi layer MoS2 specimens Specimen type Specimen 1 Specimen 2 Mono layer 1.87 x 10 ohm cm 1.88 x 10 ohm cm Bi layer 2.98 x 10 ohm cm 2.93 x 10 ohm cm Wang et al. Nano Convergence 2014, 1:22 Page 2 of 5 http:/www.nanoconvergencejournal.com/content/1/1/22 planes are parallel and coplanar, but rotated about the crystals' z axes. Or in other words, the specimens are not single crystal layers, but rather a large ensemble of small MoS2 grains with uniform thickness and excellent lateral coherence. Specific processing conditions in terms of power modulation to the sputtering cathode were selected based on prior work [11,12] to obtain atomic scale thickness control and very high crystallinity. Both Raman and x ray photoelectron spectroscopy were then employed to characterize the specimens for thickness and stoichiometry. Figure 1c shows high resolution Mo 3d spectra with Mo peak shift to 229.0 eV indicating the +4 oxidation state for Mo, the hallmark of covalent bonding with sulfur in MoS2 [13] Figure 1c also shows a peak identified at 232.2 eV, which is the 3d3/2 peak for Mo and its shift from 231.4 eV also indicates bonding to S. To verify the continuity and integrity of the deposited films, we prepared specimens with 1 cm x 1 cm active area, whereas the conventional practice is to pattern the semiconducting material as micro to nanoscale strips with metallic contacts as source and drain for a transistor configuration. A physical shadow mask was used to protect the specimen while exposing the four corners, on which 5 nm thick titanium and 100 nm thick gold were evaporated to make the electrical contacts. The chip is then mounted on another wafer piece with large gold plated electrodes using a thermal tape. The four corners of the specimen are then wire bonded to the larger pads. Figure 1c shows a specimen prepared for van der Pauw characterization [14] of the electrical resistivity under both dark and illuminated conditions. The van der Pauw setup was calibrated with a calibration specimen to achieve 5% error limit. Table 1 shows the dark dc conductivity results for the specimens. Such high values of resistivity are due to the fact that ultra high vacuum condition results in cleaner MoS2 surfaces and domain structures. More importantly, the device fabrication process did not require the film to be transferred to a different substrate, so the associated contamination is avoided. 2 Approved for public releas After dark dc characterization, we illuminated the devices with a 30 W light bulb located at about 10 cm distance. The measured luminous flux per unit area was measured to be about 24 kLux using a light meter. Figure 2a and b show the experimental results for the mono and bi layer specimens. For the mono layer, we observed the highest open circuit photo induced voltage of about 9.25 mV (without any applied bias) This value is significantly reduced to about 0.75 mV for the bi layer specimen. These experiments were performed for negative bias to exhibit consistent behaviour. For even thicker specimens, we did not observe any appreciable photovoltaic effects. We also observe that the effect is extremely direction dependent. For example, Figure 2c and d shows the photo voltage measurement on the terminals perpendicular to that shown in Figure 2a and b. Such change in direction resulted in dramatic changes in the voltage obtained from the monolayer specimens (Figure 2c) and completely eliminated any such voltage in the bilayer specimens (Figure 2d) Note that in Figure 2d, the dark and illuminated behaviour is almost the same. A remarkable feature of this study is the apparent ohmic nature of the electrical contacts, as shown in Figure 2. Almost all existing photo detector devices in the literature [6 9,15,16] present Schottky type contact and the choice of electrode work function is very critical in 2D devices to obtain ohmic contacts. Desired features of the contact are (i) small lattice mismatch at the interface (ii) maximized overlap between the density of states (DOS) at both sides of the interface, (iii) large DOS at the Fermi level throughout the interface region to form delocalized states with low effective electron mass in order to e; distribution unlimited. Figure 2 I V characteristics under dark and 1.2 milli Watts illumination for (a) mono layer and (b) bi layer MoS2 devices. The corresponding values, when measured with electrodes at perpendicular direction in (c) monolayer and (d) bilayer devices exhibit remarkable anisotropy in photo sensitivity. Predominant nature of ohmic contact is seen for all the devices. Wang et al. Nano Convergence 2014, 1:22 Page 3 of 5 http:/www.nanoconvergencejournal.com/content/1/1/22 efficiently transfer electrons between the metal and the semiconductor, (iv) a minimized potential barrier at the interface to maximize current injection [17] Our choice of metals for the electrical contacts therefore appears to be satisfying these conditions. While it is expected that the direct band gap will enhance the photon absorption, formation of an electrical bias through predominantly ohmic contact as seen in Figure 2 is unprecedented. One way to view the observed phenomenon could be the negative photoconductivity (increase in resistivity under photo excit
author_names: Baoming Wang

Christopher Muratore

Andrey A Voevodin

Md Amanul Haque
title: AFRL RX WP TP 2014 0146 PHOTOSENSITIVITY OF LARGE AREA PHYSICAL VAPOR DEPOSITED MONO AND BILAYER MOS 2 POSTPRINT
year: 2014