Strain-engineering the Schottky barrier and electrical transport on MoS2 (original) (raw)

Strain-engineering the Schottky barrier and electrical transport on MoS2

Nanotechnology

Strain provides an effective means to tune the electrical properties while retaining the native chemical composition of the material. Unlike three-dimensional solids, two-dimensional materials withstand higher levels of elastic strain making it easier to tune various electrical properties to suit the technology needs. In this work we explore the effect of uniaxial tensilestrain on the electrical transport properties of bi-and few-layered MoS2, a promising 2D semiconductor. Raman shifts corresponding to the in-plane vibrational modes show a redshift with strain indicating a softening of the in-plane phonon modes. Photoluminescence measurements reveal a redshift in the direct and the indirect emission peaks signaling a reduction in the material bandgap. Transport measurements show a substantial enhancement in the electrical conductivity with a high piezoresistive gauge factor of ~ 321 superior to that for Silicon for our bi-layered device. The simulations conducted over the experimental findings reveal a substantial reduction of the Schottky barrier height at the electrical contacts in addition to the resistance of MoS2. Our studies reveal that strain is an important and versatile ingredient to tune the electrical properties of 2D materials and also can be used to engineer high-efficiency electrical contacts for future device engineering.

Grain-Boundary-Induced Strain and Distortion in Epitaxial Bilayer MoS2 Lattice

The Journal of Physical Chemistry C, 2020

Grain boundaries between 60° rotated and twinned crystals constitute the dominant type of extended line defects in two-dimensional transition metal dichalcogenides (2D MX2) when grown 2 on a single crystalline template through van der Waals epitaxy. The two most common 60°grain boundaries in MX2 layers, i.e., -and -boundaries, introduce distinct distortion and strain into the 2D lattice. They impart a localized tensile or compressive strain on the subsequent layer, respectively, due to van der Waals coupling in bilayer MX2 as determined by combining atomic resolution electron microscopy, geometric phase analysis and density functional theory. Based on these observations, an alternate route to strain engineering through controlling intrinsic van der Waals forces in homo-bilayer MX2 is proposed. In contrast to commonly used external means, this approach enables localized application of strain to tune electronic properties of the 2D semiconducting channel in ultra-scaled nanoelectronic applications. Owing to their unique optical and electronic properties, two-dimensional (2D) transition metal dichalcogenides (MX2, with M a transition metal, and X a chalcogen) have garnered significant interest in the past decade for applications in next-generation nanoelectronics. Molybdenum disulfide (MoS2) is the most investigated member of the MX2 family. In its bulk form, MoS2 has a layered structure in which individual Mo atoms are covalently bound to S atoms within a single layer; while the individual layers are coupled by a weak van der Waals interaction. MoS2 is an ntype semiconductor with a layer-dependent bandgap ranging from 1.2 to 1.9 eV. The band gap decreases with the number of layers and evolves to a direct band-gap in a single layer due to 2D-confinement1-3. Apart from this layer thickness dependence, 2D MX2 optoelectronic properties are strongly influenced by changes to the atomic arrangement, such as polymorphism,4,5 stacking differences,6-9 presence of grain-boundaries10-14 and strain.15-21 Introducing lattice strain in atomically thin materials emerges as a promising route to modify a wide range of their properties including electrical,15,16 optical,17-19 magnetic20 and catalytic.21

Two-Dimensional Metallic/Semiconducting MoS2 under Biaxial Strain

ACS Applied Nano Materials

In this work, we present biaxial strain-induced modification in the structural and electronic properties of a MoS 2 hybrid structure made of a metallic (1T) ribbon embedded in the semiconducting (2H) phase. The results are based on density-functional theory. Biaxial strain is gradually applied on the hybrid structure, and the structural modifications are monitored. The MoS 2 hybrid material was found to be stable up to 6% (extension) and −4% (compression) strain. The onset of bending and breaking of the 2D material was identified and correlated to its electronic behavior. The alteration of the density of states with biaxial strain was also investigated and revealed the enhancement of either the metallic or the semiconducting character of the hybrid depending on the amount and direction of strain. There is also a clear mapping of the structural asymmetry of the interfaces in the material to the anisotropy in its electronic features. This anisotropy becomes more pronounced as the strain on the material increases. Our results shed light on the relevance of the morphology and electronic properties and allow us to tailor these properties through straining. In the end we discuss the relevance of this material in realizing novel nanoelectronic devices with tunable properties related to sensing, nanopore materials for sequencing, etc.

Moderate strain induced indirect bandgap and conduction electrons in MoS2 single layers

npj 2D Materials and Applications, 2019

MoS2 single layers are valued for their sizeable direct bandgap at the heart of the envisaged electronic and optoelectronic applications. Here we experimentally demonstrate that moderate strain values (~2%) can already trigger an indirect bandgap transition and induce a finite charge carrier density in 2D MoS2 layers. A conclusive proof of the direct-to-indirect bandgap transition is provided by directly comparing the electronic and optical bandgaps of strained MoS2 single layers obtained from tunneling spectroscopy and photoluminescence measurements of MoS2 nanobubbles. Upon 2% biaxial tensile strain, the electronic gap becomes significantly smaller (1.45 ± 0.15 eV) than the optical direct gap (1.73 ± 0.1 eV), clearly evidencing a strain-induced direct to indirect bandgap transition. Moreover, the Fermi level can shift inside the conduction band already in moderately strained (~2%) MoS2 single layers conferring them a metallic character.

Exceptional Tunability of Band Energy in a Compressively Strained Trilayer MoS2 Sheet

ACS Nano, 2013

Tuning band energies of semiconductors through strain engineering can significantly enhance their electronic, photonic, and spintronic performances. Although low-dimensional nanostructures are relatively flexible, the reported tunability of the band gap is within 100 meV per 1% strain. It is also challenging to control strains in atomically thin semiconductors precisely and monitor the optical and phonon properties simultaneously. Here, we developed an electromechanical device that can apply biaxial compressive strain to trilayer MoS 2 supported by a piezoelectric substrate and covered by a transparent graphene electrode. Photoluminescence and Raman characterizations show that the direct band gap can be blue-shifted for ∼300 meV per 1% strain. First-principles investigations confirm the blue-shift of the direct band gap and reveal a higher tunability of the indirect band gap than the direct one. The exceptionally high strain tunability of the electronic structure in MoS 2 promising a wide range of applications in functional nanodevices and the developed methodology should be generally applicable for two-dimensional semiconductors.

Strain-shear coupling in bilayer MoS2

Nature communications, 2017

Layered materials such as graphite and transition metal dichalcogenides have extremely anisotropic mechanical properties owing to orders of magnitude difference between in-plane and out-of-plane interatomic interaction strengths. Although effects of mechanical perturbations on either intralayer or interlayer interactions have been extensively investigated, mutual correlations between them have rarely been addressed. Here, we show that layered materials have an inevitable coupling between in-plane uniaxial strain and interlayer shear. Because of this, the uniaxial in-plane strain induces an anomalous splitting of the degenerate interlayer shear phonon modes such that the split shear mode along the tensile strain is not softened but hardened contrary to the case of intralayer phonon modes. We confirm the effect by measuring Raman shifts of shear modes of bilayer MoS2 under strain. Moreover, by analyzing the splitting, we obtain an unexplored off-diagonal elastic constant, demonstratin...

Strain tuning of optical emission energy and polarization in monolayer and bilayer MoS2

Physical Review B, 2013

We use micro-Raman and photoluminescence (PL) spectroscopy at 300K to investigate the influence of uniaxial tensile strain on the vibrational and optoelectronic properties of monolayer and bilayer MoS2 on a flexible substrate. The initially degenerate E 1 2g Raman mode is split into a doublet as a direct consequence of the strain applied to MoS2 through Van der Waals coupling at the sample-substrate interface. We observe a strong shift of the direct band gap of 48meV/(% of strain) for the monolayer and 46meV/% for the bilayer, whose indirect gap shifts by 86meV/%. We find a strong decrease of the PL polarization linked to optical valley initialization for both monolayer and bilayer samples, indicating that scattering to the spin-degenerate Γ valley plays a key role.

Piezoresistivity and Strain-induced Band Gap Tuning in Atomically Thin MoS 2

Nano Letters, 2015

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 is however limited in conventional bulk materials which can suffer from plasticity and low fracture limit due to the presence of defects and dislocations. Atomically thin membranes such as MoS 2 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 MoS 2 is highly strain-tunable which results in the modulation of its electrical conductivity and manifests itself as the piezoresistive effect while a piezoelectric effect was also observed in odd-layered MoS 2 with broken inversion symmetry. This coupling between electrical and mechanical properties makes MoS 2 a very promising material for nanoelectromechanical systems (NEMS). Here we incorporate monolayer, bilayer and trilayer MoS 2 in a nanoelectromechanical membrane configuration. We detect strain-induced band gap tuning via electrical conductivity measurements and demonstrate the emergence of the piezoresistive effect in MoS 2. Finite element method (FEM) simulations are used to quantify the band gap change and to obtain a comprehensive picture of the spatially varying bandgap profile on the membrane. The piezoresistive gauge factor is calculated to be −148 ± 19, −224 ± 19 and −43.5 ± 11 for monolayer, bilayer and trilayer MoS 2 respectively which is comparable to state-of-the-art silicon strain sensors and two orders of magnitude higher than in strain sensors based on suspended graphene. Controllable modulation of resistivity in 2D nanomaterials using strain-induced bandgap tuning offers a novel approach for implementing an important class of NEMS transducers, flexible and wearable electronics, tuneable photovoltaics and photodetection.

Effect of Tensile Strain on Performance Parameters of Different Structures of MoS2 Monolayer

2021

The present work is based on the computational study of MoS2 monolayer and effect of tensile strain on its atomic level structure. The bandgap for MoS2 monolayer, defected MoS2 monolayer and Silicon-doped monolayer are 1.82 eV (direct bandgap), 0.04 (indirect bandgap) and 1.25 eV (indirect bandgap), respectively. The impact of tensile strain (0-0.7 %) on the bandgap and effective mass of charge carriers of these MoS2 structures has been investigated. The bandgap decrease of 5.76 %, 31.86 % and 6.03 % has been observed in the three structures for biaxial strain while the impact of uniaxial strain is quite low. The impact of higher temperature on the bandgap under biaxial tensile strain has been also analyzed in this paper. These observations are extremely important for 2D material-based research for electronic applications.