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Register a new userCorrelation of structural, nanomechanical and electrostatic properties of single and few-layers MoS2
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We have decoupled the intrinsic optical and electrostatic effects arising in monolayer and few-layer molybdenum disulphide from those influenced by the flake-substrate interaction.
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We present a photoluminescence study of freestanding and Si/SiO2 supported single- and few-layer MoS2. The single-layer exciton peak (A) is only observed in freestanding MoS2. The photoluminescence of supported single-layer MoS2 is instead originating from the A- (trion) peak as the MoS2 is n-type doped from the substrate. In bilayer MoS2, the van der Waals interaction with the substrate is decreasing the indirect band gap energy by up to ~ 80 meV. Furthermore, the photoluminescence spectra of suspended MoS2 can be influenced by interference effects.
Molybdenum disulfide (MoS2) of single and few-layer thickness was exfoliated on SiO2/Si substrate and characterized by Raman spectroscopy. The number of S-Mo-S layers of the samples was independently determined by contact-mode atomic-force microscopy. Two Raman modes, E12g and A1g, exhibited sensitive thickness dependence, with the frequency of the former decreasing and that of the latter increasing with thickness. The results provide a convenient and reliable means for determining layer thickness with atomic-level precision. The opposite direction of the frequency shifts, which cannot be explained solely by van der Waals interlayer coupling, is attributed to Coulombic interactions and possible stacking-induced changes of the intralayer bonding. This work exemplifies the evolution of structural parameters in layered materials in changing from the 3-dimensional to the 2-dimensional regime.
Single layers of MoS2 and MoSe2 were optically pumped with circularly polarized light and an appreciable polarization was initialized as the pump energy was varied. The circular polarization of the emitted photoluminescence was monitored as function of the difference between the excitation energy and the A-exciton emission at the K-point of the Brillouin zone. Our results show a threshold of twice the LA phonon energy, specific to the material, above which phonon-assisted intervalley scattering causes depolarization. In both materials this lead to almost complete depolarization within ~100 meV above the threshold energy. We identify the extra kinetic energy of the exciton (independent of whether it is neutral or charged) as the key parameter for presenting a unifying picture of the depolarization process.
Structural transformation between metallic (1T) and semiconducting (2H) phases of single-layered MoS2 was systematically investigated by an in situ STEM with atomic precision. The 1T/2H phase transition is comprised of S and/or Mo atomic-plane glides, and requires an intermediate phase ({alpha}-phase) as an indispensable precursor. Migration of two kinds of boundaries ({beta} and {gamma}-boundaries) is also found to be responsible for the growth of the second phase. The 1T phase can be intentionally introduced in the 2H matrix by using a high dose of incident electron beam during heating the MoS2 single-layers up to 400~700{deg}C in high vacuum and indeed controllable in size. This work may lead to the possible fabrication of composite nano-devices made of local domains with distinct electronic properties.
GeSe and SnSe monochalcogenide monolayers and bilayers undergo a two-dimensional phase transition from a rectangular unit cell to a square unit cell at a temperature $T_c$ well below the melting point. Its consequences on material properties are studied within the framework of Car-Parrinello molecular dynamics and density-functional theory. No in-gap states develop as the structural transition takes place, so that these phase-change materials remain semiconducting below and above $T_c$. As the in-plane lattice transforms from a rectangle onto a square at $T_c$, the electronic, spin, optical, and piezo-electric properties dramatically depart from earlier predictions. Indeed, the $Y-$ and $X-$points in the Brillouin zone become effectively equivalent at $T_c$, leading to a symmetric electronic structure. The spin polarization at the conduction valley edge vanishes, and the hole conductivity must display an anomalous thermal increase at $T_c$. The linear optical absorption band edge must change its polarization as well, making this structural and electronic evolution verifiable by optical means. Much excitement has been drawn by theoretical predictions of giant piezo-electricity and ferroelectricity in these materials, and we estimate a pyroelectric response of about $3times 10^{-12}$ $C/K m$ here. These results uncover the fundamental role of temperature as a control knob for the physical properties of few-layer group-IV monochalcogenides
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