Chinh Tam Le

Near bandgap second-order nonlinear optical characteristics of MoS2 monolayer transferred on transparent substrates

We have investigated the second-order nonlinear optical (NLO) properties of CVD-grown MoS2 monolayer (ML) transferred onto transparent substrates such as fused silica and polyethylene terephthalate. The physical properties of the transferred MLs were characterized by optical and NLO methods. We measured the second-order susceptibility χ(2) in the spectral range of λ= 1064–1600 nm in which the corresponding second harmonic radiation resonates with the exciton levels. It was found that χ(2) is strongly enhanced by up to a factor of 5 near the A- and B-exciton levels due to two-photon resonance. The absolute χ(2) values of our samples determined by both reflection and transmission geometry are on par with that of as-grown MLs. Our results imply that the cavity-confinement scheme can be employed for maximizing the nonlinear optical efficiency of atomically thin transition metal dichalcogenides for transparent/flexible optoelectronics applications, especially when oriented stacking of transferred MLs are contr...

Pulsed laser deposition assisted grown continuous monolayer MoSe2

As compared with sulfur, the low reactivity of selenium makes it more challenging to synthesize high quality continuous single layer selenides required for practical applications. Here, we present the synthesis of monolayer MoSe2 by selenization of a pulsed laser deposited MoO3 film on SiO2/Si and sapphire substrates. The laser energy was carefully varied to optimize the growth of highly uniform, continuous monolayer films. Morphological characterization including optical microscopy, field emission scanning electron microscopy and atomic force microscopy results clearly demonstrate that the synthesized film is monolayer, continuous and homogeneous. The Raman and photoluminescence maps of the continuous film exhibit uniform brightness, indicating the uniform and homogeneous nature of the film. Moreover, the full width at half maximum values of A1g and A exciton peaks were found to be 3.7 cm−1 and 24 nm, respectively, showing the excellent optical quality of the grown film.

Temperature dependence of the critical points of monolayer MoS2 by ellipsometry

ABSTRACT We report the complex pseudodielectric function of molybdenum disulfide (MoS2) monolayers for energies from 1.40 to 6.42 eV and temperatures from 35 to 350 K. MoS2 was grown as a continuous monolayer on a SiO2/Si substrate in a two zone hot-wall furnace using a catalyst-free chemical vapor growth process. The monolayer was then transferred onto a sapphire substrate. We investigated the optical properties of MoS2 using a dual-rotating-compensator ellipsometer with the sample in ultrahigh vacuum to prevent degradation and to minimize condensation artifacts in the data at low sample temperatures. Critical-point (CP) energies were determined using numerically calculated second energy derivatives of the spectra. At low temperature, we observed a splitting of A-excitonic peak. This identifies the bound excitonic states, including negatively charged excitons, trion states in monolayer MoS2. Splitting of the B-excitonic peak is also indicated. Blue shifts of the CP energies and enhanced structures with reduced electron–phonon interaction are observed as the temperature is lowered. The temperature dependences of the CP energies were fit to a phenomenological expression that contains the Bose–Einstein statistical factor and the temperature coefficient.

Friction characteristics of mechanically exfoliated and CVD-grown single-layer MoS2

In this work, the friction characteristics of single-layer MoS2 prepared with chemical vapor deposition (CVD) at three different temperatures were quantitatively investigated and compared to those of single-layer MoS2 prepared using mechanical exfoliation. The surface and crystalline qualities of the MoS2 specimens were characterized using an optical microscope, atomic force microscope (AFM), and Raman spectroscopy. The surfaces of the MoS2 specimens were generally flat and smooth. However, the Raman data showed that the crystalline qualities of CVD-grown single-layer MoS2 at 800 °C and 850 °C were relatively similar to those of mechanically exfoliated MoS2 whereas the crystalline quality of the CVD-grown single-layer MoS2 at 900 °C was lower. The CVD-grown single-layer MoS2 exhibited higher friction than mechanically exfoliated single-layer MoS2, which might be related to the crystalline imperfections in the CVD-grown MoS2. In addition, the friction of CVD-grown single-layer MoS2 increased as the CVD growth temperature increased. In terms of tribological properties, 800 °C was the optimal temperature for the CVD process used in this work. Furthermore, it was observed that the friction at the grain boundary was significantly larger than that at the grain, potentially due to defects at the grain boundary. This result indicates that the temperature used during CVD should be optimized considering the grain size to achieve low friction characteristics. The outcomes of this work will be useful for understanding the intrinsic friction characteristics of single-layer MoS2 and elucidating the feasibility of single-layer MoS2 as protective or lubricant layers for micro- and nano-devices.