Symmetry-dependent phonon renormalization in monolayer MoS2transistor

Abstract

Strong electron-phonon interaction which limits electronic mobility of semiconductors can also have significant effects on phonon frequencies. The latter is the key to the use of Raman spectroscopy for nondestructive characterization of doping in graphene-based devices. Using in-situ Raman scattering from single layer MoS2 electrochemically top-gated field effect transistor (FET), we show softening and broadening of A1g phonon with electron doping whereas the other Raman active E 1 2g mode remains essentially inert. Confirming these results with first-principles density functional theory based calculations, we use group theoretical arguments to explain why A1g mode specifically exhibits a strong sensitivity to electron doping. Our work opens up the use of Raman spectroscopy in probing the level of doping in single layer MoS2-based FETs, which have a high on-off ratio and are of enormous technological significance.PACS numbers: 78.30.-j Discovery of graphene 1 stimulated an intense research activity due to interesting fundamental phenomena it exhibits as well as the techonological promise it holds in a broad range of applications ranging from sensors to nanoelectronics. Vanishing bandgap of a single layer graphene is a sort of a limitation in developing a graphene-based field effect transistor with a high on/off ratio. This has spurred efforts to modify graphene to open up a gap and towards development of other two dimensional materials like MoS 2 , WS 2 and boron nitride (BN), both experimentally and theoretically. Avenues to open up gap through modification of graphene include quantum confinement in nanoribbons 2 , surface functionalization 3 , applying electric field in the bilayer 4,5 , deposition of graphene on other substrates like BN 6,7 , and B or N substitutional doping 8 , which require fine control over the procedure of synthesis.In contast to graphene, single layer MoS 2 consisting of a hexagonal planar lattice of Mo atoms sandwiched between two similar lattices of S atoms (S-Mo-S structure) with intralayer covalent bonding is a semiconductor with a direct band gap of ∼ 1.8 eV, and is quite promising for FET devices with a high on-off ratio. It has been shown that the luminescence quantum yield of monolayer MoS 2 is higher than its bulk counterpart 9,10 .Recently a monolayer MoS 2 transistor 11 has been shown to exhibit an on-off ratio of ∼10 8 and electron mobility of ∼200 cm 2 /V-sec. These values are comparable to silicon based devices and make MoS 2 based devices worth exploring further. It is known that in a field effect transistor, carrier mobility is limited by scattering from phonons and the maximum current is controlled by hot phonons. Both these issues in a FET depend on the electron-phonon coupling (EPC). Raman spectroscopy has been very effective to probe EPC for single 12-14 and bilayer graphene 15-17 transistors by investigating the renormalization of the G and 2D modes as a function of carrier density.Recent layer-dependent Raman studies of single and few layers of MoS 2 18 have shown th...

Gate‐Tunable Resonant Raman Spectroscopy of Bilayer MoS2

“…Figure 2a,b shows the V tg -dependent Raman spectra measured from a bilayer MoS 2 sample, with excitation laser of 532 and 633 nm, respectively (phonon assignment is shown in Figure S2 in the Supporting Information). The gate-dependent behavior of A 1g mode with 532 nm excitation in Figure 2a is in good agreement with previous reports: [15,23] The peak redshifts and broadens with increasing electron doping. On the other hand, E 2g 1 mode is less sensitive to electron doping due to the weaker electron-phonon coupling.…”

“…Theoretical calculations also indicate that the electron-phonon coupling for A 1g mode is much stronger than that of the E 2g 1 mode, which explains why E 2g 1 mode is insensitive to changes in electron concentration. [15] The dependence of Raman spectroscopy on the excitation energy may provide further information of the material properties, such as Davydov splitting and exciton-phonon coupling under resonant condition. [17][18][19][20] However, previous Raman spectroscopy studies in MoS 2 with electrostatic (2 of 6) 1701039 www.small-journal.com doping have not addressed the influence of excitation energy on the phonon behavior.…”

Spectroscopic Signatures for Interlayer Coupling in MoS2–WSe2 van der Waals Stacking

“…The Raman A'1 peak of MoS2 blue-shifts and its intensity increases (relative to E'), thus indicating that MoS2 is less n-doped (or a decrease in the electron concentration). 44,45 The blue-shift of the WSe2 Raman A'1 peak implies that the WSe2 may become less p-doped (Supporting Figure S3 shows that in our separate experiment, p-doping from AuCl4 -causes a red-shift of Raman A'1 peak in monolayer WSe2). These phenomena suggest that the electrons transfer from MoS2 to WSe2, which is expected for the PN-junction composed by the p-type WSe2 and n-type MoS2.…”

Band Gap Engineering with Ultralarge Biaxial Strains in Suspended Monolayer MoS2

“…2c) 39,40 . The position of the A 1g peak is known to vary with doping 41 , however as this is not the case with the more strain sensitive E 1 2g mode, its peak position can be used as a reliable way to measure the internal strain of monolayer MoS 2 .…”