Anharmonicity of monolayer MoS2, MoSe2, and WSe2: A Raman study under high pressure and elevated temperature

Yang, Mei; Cheng, Xuerui; Li, Yuanyuan; Ren, Yufen; Liu, Miao; Qi, Zeming

doi:10.1063/1.4977877

Cited by 89 publications

(64 citation statements)

References 44 publications

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“…The measured Raman spectra of bare (orange) and AlO x -capped (blue) 1L MoSe 2 are displayed in Figure 1 c. The A 1 ′ Raman active mode (out-of-plane vibration of Se atoms) is observed at 240.2 cm –1 for 1L MoSe 2 , which is consistent with previous reports on the MoSe 2 1L. 42 − 45 We note that A 1 ′ notation of this Raman mode is valid for 1L and odd number of (few) layers, whereas it is labeled A 1g for bulk and even number of layers. 7 , 46 , 47 After capping the 1L MoSe 2 by AlO x , a blue shift of ∼1.15 cm –1 and broadening in the A 1 ′ Raman mode are observed, demonstrating strong doping dependence, as previously reported for 1L MoS 2 .…”

Section: Results and Discussionsupporting

confidence: 88%

Improved Current Density and Contact Resistance in Bilayer MoSe₂ Field Effect Transistors by AlO_x Capping

Somvanshi

Ber

Bailey

et al. 2020

ACS Appl. Mater. Interfaces

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Atomically thin semiconductors are of interest for future electronics applications, and much attention has been given to monolayer (1L) sulfides, such as MoS 2 , grown by chemical vapor deposition (CVD). However, reports on the electrical properties of CVD-grown selenides, and MoSe 2 in particular, are scarce. Here, we compare the electrical properties of 1L and bilayer (2L) MoSe 2 grown by CVD and capped by sub-stoichiometric AlO x . The 2L channels exhibit ∼20× lower contact resistance ( R C ) and ∼30× larger current density compared with 1L channels. R C is further reduced by >5× with AlO x capping, which enables improved transistor current density. Overall, 2L AlO x -capped MoSe 2 transistors (with ∼500 nm channel length) achieve improved current density (∼65 μA/μm at V DS = 4 V), a good I on / I off ratio of >10 6 , and an R C of ∼60 kΩ·μm. The weaker performance of 1L devices is due to their sensitivity to processing and ambient. Our results suggest that 2L (or few layers) is preferable to 1L for improved electronic properties in applications that do not require a direct band gap, which is a key finding for future two-dimensional electronics.

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Section: Results and Discussionsupporting

confidence: 88%

Improved Current Density and Contact Resistance in Bilayer MoSe₂ Field Effect Transistors by AlO_x Capping

Somvanshi

Ber

Bailey

et al. 2020

ACS Appl. Mater. Interfaces

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“…The vibrational frequencies from the rigid-ion model are in good agreement with the experimental data, as shown in Table 1. The calculated results from the DFT frame are about 9% lower than the experimental results, which is a well-known underestimation of the phonon frequency by using the PBE functional [43,44,45]. To study the behavior of the phonon structure in greater detail, we analyzed the pressure dependence of the phonon frequency at center point Γ.…”

Section: Resultsmentioning

confidence: 98%

Pressure Effect of the Vibrational and Thermodynamic Properties of Chalcopyrite-Type Compound AgGaS2: A First-Principles Investigation

Yang

Fan

et al. 2018

Materials

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To explore the structural, vibrational, and thermodynamic properties of the chalcopyrite-type compound AgGaS2 under pressure, we applied hydrostatic pressure to the relaxed compound based on the first principles calculation and quasi-harmonic approximation. The structural parameters, including lattice constants and bond lengths decrease monotonically with the increasing pressure. The phonon dispersion curves under various pressures reveal the structural phase transition of chalcopyrite-type compound AgGaS2 at about 4 GPa. The intrinsic mechanism of thermal conductivity for the chalcopyrite-type compound AgGaS2 has been shown with phonon anharmonicity. The frequencies of the optical phonons at the center point Γ of the first Brillouin zone were calculated with the longitudinal optical–transverse optical (LO–TO) splitting mode. The dependence of the frequencies of the optical phonons on the pressure provides the information for the Raman spectroscopic study under high pressure. The pressure dependence of the Grüneisen parameters indicates that the instability of chalcopyrite-type compound AgGaS2 is associated with the softening of the acoustic phonon modes at around the center point Γ. The thermal conductivity for chalcopyrite-type compound AgGaS2 could be reduced by applying external pressure. The various thermodynamic properties, such as the Helmholtz free energy, entropy, and heat capacity, at different temperatures and pressures were discussed and analyzed based on the phonon properties.

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“…Including both the laser intensity distribution I ( z ) and the induced temperature response T ( z ) in the Lorentzian Raman profile function in Equation leads to the following line shape function:

I ((),, ω, T) = \int_{- c}^{c} \frac{I_{o} e^{- {(z / b)}^{2}}}{{[ω - ω_{o} (T (z))]}^{2} + {[Γ (T (z)) / z]}^{2}} italicdz .

The integral in z is defined by the laser spot size and c = 1/2 b . There are few reports on the inhomogeneous heating effects in TMDs, 73,74 although some of the reported phonon line shapes of the layered effect in TMDs exhibit asymmetric broadening 23,75–77 characteristic of an inhomogeneous laser heating effect. Huang et al 78 reported homogenous temperature‐dependent evolution of the Raman spectra of a chemical vapor deposition (CVD)‐grown monolayer WS 2 ; they showed the dependence of the Raman shifts of the modes on the three‐phonon process and the coefficient of thermal expansion.…”

Section: Laser Flux‐induced Effect: Inhomogeneous Heatingmentioning

confidence: 99%

Investigating the phonon line shapes of TMDs: An analytical approach

Zhou

Adu

2020

J Raman Spectroscopy

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Several phenomena can affect the phonon bands of low-dimensional systems, and their proper assignment and interpretation are essential in elucidating their effect on electronic, optical, and optoelectronic properties. Using an analytical approach, we investigate the similarities and differences of the layered effect, the phonon confinement effect, the Breit-Wigner-Fano effect, the inhomogeneous heating effect, and disorder-induced effects in the phonon line shapes of WS 2 and MoS 2. The subtle differences and similarities are critical in making proper assignments and interpretations of the phonon line shapes and are useful as guidance in the behavior of phonon bands, particularly in TMDs, and more generally in low-dimensional systems.

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Anharmonicity of monolayer MoS2, MoSe2, and WSe2: A Raman study under high pressure and elevated temperature

Cited by 89 publications

References 44 publications

Improved Current Density and Contact Resistance in Bilayer MoSe₂ Field Effect Transistors by AlO_x Capping

Improved Current Density and Contact Resistance in Bilayer MoSe₂ Field Effect Transistors by AlO_x Capping

Pressure Effect of the Vibrational and Thermodynamic Properties of Chalcopyrite-Type Compound AgGaS2: A First-Principles Investigation

Investigating the phonon line shapes of TMDs: An analytical approach

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Anharmonicity of monolayer MoS2, MoSe2, and WSe2: A Raman study under high pressure and elevated temperature

Cited by 89 publications

References 44 publications

Improved Current Density and Contact Resistance in Bilayer MoSe2 Field Effect Transistors by AlOx Capping

Improved Current Density and Contact Resistance in Bilayer MoSe2 Field Effect Transistors by AlOx Capping

Pressure Effect of the Vibrational and Thermodynamic Properties of Chalcopyrite-Type Compound AgGaS2: A First-Principles Investigation

Investigating the phonon line shapes of TMDs: An analytical approach

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Improved Current Density and Contact Resistance in Bilayer MoSe₂ Field Effect Transistors by AlO_x Capping

Improved Current Density and Contact Resistance in Bilayer MoSe₂ Field Effect Transistors by AlO_x Capping