The optoelectronic properties of two-dimensional (2D) transition metal dichalcogenide (TMDC) monolayers such as WS 2 are largely dominated by excitons due to strong Coulomb interactions in these 2D confined monolayers, which lead to formation of Rydberg-like excitonic states below the free quasiparticle band gap. The precise knowledge of high order Rydberg excitonic states is of great importance for both fundamental understanding such as many-electron effects and device applications such as optical switching and quantum process information. Bright excitonic states could be probed by linear optical spectroscopy, while probing dark excitonic states generally requires nonlinear optical (NLO) spectroscopy. Conventional optical methods for probing high-order Rydberg excitonic states were generally performed at cryogenic temperatures to ensure enough signal-to-noise ratio (SNR) and narrow line width. Here we have designed a hybrid nanostructure of monolayer WS 2 integrated with a plasmonic cavity and investigated their NLO properties at the single particle level. Giant enhancement in NLO responses, stronger excitonic resonance effects, and narrowed line widths of NLO excitation spectra were observed when monolayer WS 2 was placed in our carefully designed plasmonic cavity. Optimum enhancement of 1000-, 3000-, and 3800-fold were achieved for two-photon photoluminescence (2PPL), second harmonic generation (SHG), and third-harmonic generation (THG), respectively, in the optimized cavity structure. The line width of SHG excitation spectra was reduced from 43 down to 15 meV. Plasmon enhanced NLO responses brought improved SNR and spectral resolution, which allowed us to distinguish discrete excitonic states with small energy differences at room temperature. By using three complementary NLO techniques in combination with linear optical spectroscopy, energies of Rydberg excitonic states of A (1s, 2s, 2p, 3s, 3p, 4s), B (1s), and C and D excitons of monolayer WS 2 have been accurately determined, which allow us to determine exciton binding energy and quasiparticle bandgap. It was interesting to find that the 2p lies 30 meV below 2s, which lends strong support to the theoretical prediction of nonlocal dielectric screening effects based on a non-hydrogenic model. Our results show that plasmon enhanced NLO spectroscopy could serve as a general method for probing high order Rydberg excitonic states of 2D materials.

Ferroelectricity in two-dimensional hybrid (2D) organic− inorganic perovskites (HOIPs) can be engineered by tuning the chemical composition of the organic or inorganic components to lower the structural symmetry and order−disorder phase change. Less efforts are made toward understanding how the direction of the polar axis is affected by the chemical structure, which directly impacts the anisotropic charge order and nonlinear optical response. To date, the reported ferroelectric 2D Dion−Jacobson (DJ) [PbI 4 ] 2− perovskites exhibit exclusively out-of-plane polarization. Here, we discover that the polar axis in ferroelectric 2D Dion−Jacobson (DJ) perovskites can be tuned from the out-of-plane (OOP) to the in-plane (IP) direction by substituting the iodide with bromide in the lead halide layer. The spatial symmetry of the nonlinear optical response in bromide and iodide DJ perovskites was probed by polarized second harmonic generation (SHG). Density functional theory calculations revealed that the switching of the polar axis, synonymous with the change in the orientation of the sum of the dipole moments (DMs) of organic cations, is caused by the conformation change of organic cations induced by halide substitution.

Shrinking the size of electronic components has been highly desirable in the semiconductor industry in recent years, and the next breakthrough in reducing the size of electronic devices may be via the bottom-up approach. Previous research showed that the deposition of metal atoms onto semiconductor surfaces could result in the self-assembly of atomically thin nanowires (NWs) extending for hundreds of nanometers. However, current understanding of the self-assembly mechanism of the NWs on semiconductor surfaces is still limited and unclear.Here, through a combination of scanning tunneling microscopy (STM) and density functional theory (DFT) calculation, the atomistic details of NWs growth on Pt-modified Ge(001) surfaces are uncovered. Directly observing the coexistence of various intermediate phases after Pt deposition enables us to explore the correlation among these phases, which provides important insights in deciphering the self-assembly mechanism of the NWs on semiconductor surfaces. Deposited Pt atoms are capable of chemically interacting with surface atoms on Ge(001) surface layer at elevated temperature (∼873 K) leading to the formation of various phases (e.g., α-terraces, β-terraces, and NWs). Among them, three-row-wide trench−plateau structures are clearly identified in this work, and an energetically favorable structure for this phase is unveiled using DFT calculations. It is found that this intermediate phase plays a critical role in the process of NWs growth, and the trenches on the edges are believed to be the cradle for NWs growth. The atomistic understanding of the self-assembly of nanostructure on Ge(001) surfaces could further help us design more complex nanostructures on semiconducting platforms, which is crucial for further reduction in the size of electronic devices.