Chih Chiang Shen

Optical properties of monolayer transition metal dichalcogenides probed by spectroscopic ellipsometry

Spectroscopic ellipsometry was used to characterize the complex refractive index of chemical-vapor-deposited monolayer transition metal dichalcogenides (TMDs). The extraordinary large value of the refractive index in the visible frequency range is obtained. The absorption response shows a strong correlation between the magnitude of the exciton binding energy and band gap energy. Together with the observed giant spin-orbit splitting, these findings advance the fundamental understanding of their novel electronic structures and the development of monolayer TMDs-based optoelectronic and spintronic devices.

Charge Dynamics and Electronic Structures of Monolayer MoS2 Films Grown by Chemical Vapor Deposition

THz absorption and spectroscopic ellipsometry were used to investigate the charge dynamics and electronic structures of chemical-vapor-deposited monolayer MoS2 films. THz conductivity displays a coherent response of itinerant charge carriers at zero frequency. Drude plasma frequency (~7 THz) decreases with decreasing temperature while carrier relaxation time (~26 fs) is almost temperature independent. The absorption spectrum of monolayer MoS2 shows a direct 1.95 eV band gap and charge transfer excitations that are ~0.2 eV higher than those of the bulk counterpart. The ground-state exciton binding energy is found to be about 0.48 eV.

Gap opening and orbital modification of superconducting FeSe above the structural distortion.

We utilize steady-state and transient optical spectroscopies to examine the responses of nonthermal quasiparticles with respect to orbital modifications in normal-state iron-chalcogenide superconductors. The dynamics shows the emergence of gaplike quasiparticles (associated with a ~36 meV energy gap) with a coincident transfer of the optical spectral weight in the visible range, at temperatures above the structural distortion. Our observations suggest that opening of the high-temperature gap and the lattice symmetry breaking are possibly driven by short-range orbital and/or charge orders, implicating a close correlation between electronic nematicity and precursor order in iron-based superconductors.

Charge dynamics and electronic structures of monolayer graphene with molecular doping

THz absorption and spectroscopic ellipsometry were used to investigate the charge dynamics and electronic structures of monolayer graphene with doping by triazine. The absorption spectrum of the triazine-doped film shows an optical excitation near 5.1 eV that is blue shifted compared with that of undoped analog. THz conductivity displays a coherent response of itinerant charge carriers at zero frequency and a disorder-induced finite frequency peak around 155 cm−1. Drude plasma frequency (∼4400 cm−1) decreases with decreasing temperature while carrier relaxation time (∼13 fs) is almost temperature independent, implying the semiconducting behavior with thermal activation energy of 3 meV.

Semiconducting Coordination Polymers Based on the Predesigned Ternary Te-Fe-Cu Carbonyl Cluster and Conjugation-Interrupted Dipyridyl Linkers

A series of semiconducting cluster-incorporated Cu-based coordination polymers, namely, 1D zigzag polymers [{TeFe3 (CO)9 Cu2 }(L)]n (L=1,2-bis(4-pyridyl)ethane (bpea), 1; L=1,2-bis(4-pyridyl)ethylene (bpee), 5), 2D honeycomb-like polymers [{TeFe3 (CO)9 Cu}Cu(L)2.5 ]n (L=bpea, 2; L=bpee, 6), and 2D wave-like cation-anion polymer [{Cu2 (L)4 }({TeFe3 (CO)9 Cu}2 (L))]n (L=1,3-bis(4-pyridyl)propane (bpp), 4), as well as the macrocycle [{TeFe3 (CO)9 Cu2 }2 (bpp)2 ] (3) have been quantitatively synthesized via the liquid-assisted grinding from the pre-designed cluster [TeFe3 (CO)9 Cu2 (MeCN)2 ] with conjugated or conjugation-interrupted dipyridyl linkers. Notably, the most conjugation-interrupted bpp-bridged polymer 4 exhibited extraordinary semiconducting characteristics with an ultra-narrow bandgap of 1.43 eV and a DC conductivity of 1.5×10-2 Ω-1  cm-1 , which violates our knowledge, mainly attributed to the through-space electron transport via non-classical C-H⋅⋅⋅O(carbonyl) hydrogen bonds and aromatic C-H⋅⋅⋅π interactions. The incorporated Te-Fe-CO anions can not only provide numerous possibilities for secondary interactions within these Cu-based polymers but also serve as a redox-active coordination ligand to promote their conductivities. The intriguing structure-property relationships were studied by X-ray and DFT analyses and further demonstrated by significant change in the oxidation state of Cu atoms by XPS and Cu K-edge XANES.

Liquid crystal assisted replica molding method to align uniaxial molecules in patterned polymer at ambient temperature

In this paper, we demonstrate an easy approach – liquid crystal assisted replica molding method – to align uniaxial molecules in a patterned polymer at ambient temperature. During the replica molding process, the orientation of liquid crystals was parallel to the groove of the grating and the uniaxial molecules were aligned by liquid crystals. In order to confirm our observations, we had measured photoluminescence (PL) and polarized absorption spectra to quantify the degree of alignment. This approach to align uniaxial molecules in a polymer at ambient temperature opens up a simple way to manufacture optoelectronic and electronic devices.

Using Liquid Crystal Assisted Replica Molding Method to Align Uniaxial Molecules in Patterned Polymers at Ambient Temperature

In this paper, we demonstrate an easy approach – liquid crystal assisted replica molding method – to align uniaxial molecules in a patterned polymer at ambient temperature. During the process, the orientation of liquid crystals is parallel to the groove of the grating mold and the uniaxial molecules are aligned by liquid crystals. Many research groups in the field of optics and organic optoelectronics have paid huge attention to the polymer-chromospher system. Here, in order to quantify the degree of alignment, we measure both polarized absorption and photoluminescence (PL) spectra. According to our results, our approach to align molecules in a polymer can truly open up an easy way to manufacture optoelectronic and electronic devices.