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Surface optical rectification was observed from the layered semiconductor molybdenum disulfide (MoS) crystal via terahertz (THz) time-domain surface emission spectroscopy under linearly polarized femtosecond laser excitation. The radiated THz amplitude of MoS has a linear dependence on ever-increasing pump fluence and thus quadratic with the pump electric field, which discriminates from the surface Dember field induced THz radiation in InAs and the transient photocurrent-induced THz generation in graphite. Theoretical analysis based on space symmetry of MoS crystal suggests that the underlying mechanism of THz radiation is surface optical rectification under the reflection configuration. This is consistent with the experimental results according to the radiated THz amplitude dependences on azimuthal and incident polarization angles. We also demonstrated the damage threshold of MoS due to microscopic bond breaking under the femtosecond laser irradiation, which can be monitored via THz time-domain emission spectroscopy and Raman spectroscopy.

The elastic, electronic, and dielectric properties of layered transition metal dichalcogenides MX2 (M = Zr and Hf; X = S, Se) have been investigated using density functional theory (DFT) with van der Waals correction. The elastic modulus indicate that the structures belong to brittle materials. The band gaps of these materials through Heyd–Scuseria–Ernzerhof hybrid functional are in reasonable agreement with the experimental data. Partial density of state analysis suggests that the metallic atoms play a dominant role in the conduction band and the chalcogenide atoms have the main effect on the valence band. The presence of peaks in the dielectric constant spectra mainly result from the transition between first, second, third valence bands and the first conduction bands and the direction is from Γ to M, M to K, and K to Γ of the high symmetry k‐points for bulk and monolayer structures, respectively. What is more, parallel band effect has been observed in monolayer structures, which suggests strong light‐matter interactions in these materials. This work promotes the property understanding of these materials and holds potential for the development of optoelectronic devices based on these layered materials.

Polydepsipeptides (PDPs) are strictly alternating copolymers of α-hydroxy acids and α-amino acids produced via the ring-opening polymerization (ROP) of morpholino-2,5-dione derivatives (MDs). They have been used as promising biomaterials for their combined high thermal stability and good mechanical properties of polyamides as well as the inherent degradability of polyesters. ROP of MDs is usually carried out at high temperatures with metal catalysts or enzymes, with less control over the polymer molecular weights and dispersities. In this work, we developed a simple and efficient synthetic strategy of a new platform MD via the Passerini-type reaction between an isocyano derivative of the amino acid and an aldehyde, followed by intramolecular esterification. Nine new MDs were synthesized by using this method, and the organobase-catalyzed ROP of these MDs was investigated. When the ROPs of these MDs were catalyzed by either triazabicyclo[4.4.0]dec-5-ene (TBD) or diazabicyclo[5.4.0]undec-7-ene (DBU) in the presence of benzyl alcohol as an initiator, the polymerizations were uncontrolled with the formation of both linear PDPs and cyclic PDPs. By using binary catalytic systems of 1-(3,5-bis(trifluoromethyl)-phenyl-3-cyclohexyl-2-thiourea) (TU) with DBU or TBD ([TU]/[TBD] or [DBU] > 3), the polymerizations became well-controlled, allowing the synthesis of PDPs with controlled molecular weights, low dispersities, as well as block copolymers. Furthermore, cyclic PDPs were obtained when the ROP of these MDs was catalyzed with TBD in the absence of both TU and an initiator. Finally, we used two methods to recover the monomer precursors or pure MD monomers: the TBD-catalyzed alcoholysis of PDPs was very fast and generated the monomer precursors quantitatively, while the acid-catalyzed depolymerization of PDPs led to pure and quantitative monomer recovery.

The island growth mode of Pt was employed to guide the forma-tion of PdPt alloy nanodots on gold nanorods (Au@PdPt NRs). Well-defined alloy nanodots, with tunable Pd/Pt ratios from 0.2 to 5, distribute homogeneously on the surface of the Au NR. Formation of nanodots shell leads to the red-shift and broadening of the longitudinal surface plasmon resonance (LSPR) band of the Au NRs. The Au@PdPt alloy NRs exhibit catalytic activity toward oxidation of often-used chromogenic substrates by dissolved oxygen under mild conditions, suggesting a new type of oxidase mimics. Composition dependence catalytic activity is observed for the oxidation of ascorbic acid (AA) and 3,3',5,5'-tetramethylbenzidine (TMB) and for the reduction of p-nitrophenol. For AA and TMB, catalytic activity enhances quickly at lower Pd/Pt ratios and tends to saturate at higher Pd/Pt ratios. For p-nitrophenol reduction, catalytic activity shows a nice linear relationship with Pd/Pt ratio owing to much higher catalytic activity of Pd. In conclusion, proper alloying of Pd and Pt presents an effective route to tailor the catalytic activity. Interesting, alloy nanodots can also catalyze the oxidation of Fe (II) to Fe (III) by dissolved oxygen. Thus, based on the competitive oxidation of TMB and Fe (II), selective detection of the latter can be achieved.

Knowledge of band alignments and heterostructure formations is fundamental for a new generation of optoelectronics based on two-dimensional layered materials. Herein, band alignments and heterostructures of IVB-VIA monolayer MX (M = Zr, Hf; X = S, Se) and VIIB-VIA monolayer MX (M = Tc, Re; X = S, Se) are calculated by density functional theory with hybrid functionals. The results indicate that for monolayer MX, the valence bands mainly depend on the p state of the chalcogens and the conduction bands mainly depend on the d state of the transition metals. In contrast, for monolayer MX, both valence and conduction bands depend on the d state of the transition metals. This suggests that their work functions are obviously different. Meanwhile, the characteristics of the band alignments and the planar-averaged local density of states show that ZrS, HfS, TcSe and ReS could be favorable candidates for photocatalytic water splitting. ZrS, HfS and MX with the same structures are able to form type II heterostructures at their interfaces, which could be used for solar energy conversion. The power-conversion efficiency of an MX thin-film solar cell is approximately 16-18%, which is higher than those of MX thin-film solar cells. In addition, for heterostructures composed of MX, both of the two kinds of material (M and X) play an important role in every band formation. Meanwhile, for MX heterostructures, almost every band depends only on a single material. The charge density difference of the heterostructures demonstrates a higher charge accumulation at the interface of MX heterostructures than that of MX heterostructures. These phenomena show that type II heterostructures formed of MX are more stable than those of MX.

Flexible and anisotropic response of layered transition metal dichalcogenides MX 2 (M = Tc and Re; X = S, Se) is important for wearable and polarized optoelectronics. Herein, the elastic, electronic, and optical dielectric properties of these two-dimensional (2D) materials have been investigated by density functional theory (DFT) with different van der Waals correction and Heyd−Scuseria−Ernzerhof hybrid functional. The Young's modulus of these materials is low, which indicates that they are favorable for the flexible optoelectronic devices. The band gaps fall in between 1.70 and 2.12 eV with the d states of transition metal atoms playing an important role in conduction and valence bands. In addition, the appearance of band nesting implies that there are strong light−matter interactions in these materials, indicating they are suitable for photovoltaic and photocatalytic applications. Unlike the traditional 2D materials such as MoS 2 , the optical dielectric properties manifest highly in-plane anisotropic in the infrared and visible region, which is suitable for on-chip polarization manipulation with these materials. This work promotes the understanding of flexible and anisotropic response of these materials and their potential applications in new types of optoelectronic devices.