Two dimensional (2D) transition-metal dichalcogenide (TMD) based semiconductors have generated intense recent interest due to their novel optical and electronic properties, and potential for applications. In this work, we characterize the atomic and electronic nature of intrinsic point defects found in single crystals of these materials synthesized by two different methods-chemical vapor transport and self-flux growth. Using a combination of scanning tunneling microscopy (STM) and scanning transmission electron microscopy (STEM), we show that the two major intrinsic defects in these materials are metal vacancies and chalcogen antisites. We show that by control of the synthetic conditions, we can reduce the defect concentration from above 10 13 /cm 2 to below 10 11 /cm 2. Because these point defects act as centers for non-radiative recombination of excitons, this improvement in material quality leads to a hundred-fold increase in the radiative recombination efficiency.

Materials with high second harmonic generation (SHG) efficiency and reduced dimensions are favorable for integrated photonics and novel nonlinear optical applications. Here, we fabricate MoS 2 nanoscrolls with different chiralities and study their SHG performances. As a 1D material, MoS 2 nanoscroll shows reduced symmetry and strong chirality dependency in the polarizationresolved SHG characterizations. This SHG performance can be well explained by the coherent superposition theory of second harmonic field of the nanoscroll walls. MoS 2 nanoscrolls with certain chiralities in our experiment can have SHG intensity up to 98 times stronger than that of monolayer MoS 2 , and the full potential can still be further exploited. The same chiralitydependent SHG can be expected for nanoscrolls or nanotubes composed of other noncentrosymmetric 2D materials, such as WS 2 , WSe 2 , and hBN. The characterization and analysis results presented here can also serve as a non-destructive technique to determine the chiralities of these nanoscrolls and nanotubes.' $%& (() (nanoscroll) = ∫ ' $%& (() :-1,3 )*++ ,1 )*++ ; (< 4566 /2>

Infrared laser systems are vital for applications in spectroscopy, communications, and biomedical devices, where infrared nonlinear optical (NLO) crystals are required for broadband frequency down‐conversion. Such crystals need to have high non‐resonant NLO coefficients, a large bandgap, low absorption coefficient, and phase‐matchability among other competing demands; for example, a larger bandgap leads to smaller NLO coefficients. Here, the successful growth of single crystals of γ ‐NaAsSe2 that exhibit a giant second harmonic generation (SHG) susceptibility of d11 = 590 pm V−1 at 2 µm wavelength is reported; this is ~18 times larger than that of commercial AgGaSe2 while retaining a similar bandgap of ~1.87 eV, making it an outstanding candidate for quasi‐phase‐matched devices utilizing d11. In addition, γ ‐NaAsSe2 is both Type I and Type II phase‐matchable, and has a transparency range up to 16 µm wavelength. Thus, γ ‐NaAsSe2 is a promising bulk NLO crystal for infrared laser applications.

High-power infrared laser systems with broadband tunability are of great importance due to their wide range of applications in spectroscopy and free-space communications. These systems require nonlinear optical (NLO) crystals for wavelength up/down conversion using sum/difference frequency generation, respectively. NLO crystals need to satisfy many competing criteria, including large nonlinear optical susceptibility, large laser induced damage threshold (LIDT), wide transparency range and phase-matchability. Here, we report bulk single crystals of SnP2S6 with a large non-resonant SHG coefficient of d33 = 53 pm V −1 at 1550nm and a large LIDT of 350 GW cm -2 for femtosecond laser pulses. It also exhibits a broad transparency range from 0.54μm to 8.5μm (bandgap of ~2.3 eV) and can be both Type I and Type II phase-matched. The complete linear and SHG tensors are measured as well as predicted by first principles calculations, and they are in excellent agreement. A proximate double-resonance condition in the electronic band structure for both the fundamental and the SHG light is shown to enhance the non-resonant SHG response. Therefore, SnP2S6 is an outstanding candidate for infrared laser applications.