Charge-transfer excitons are formed by photoexcited electrons and holes following charge transfer across a heterojunction. They are important quasiparticles for optoelectronic applications of semiconducting heterostructures. The newly developed two-dimensional heterostructures provide a new platform to study these excitons. We report spatially and temporally resolved transient absorption measurements on the dynamics of charge-transfer excitons in a MoS2/WS2/MoSe2 trilayer heterostructure. We observed a non-classical lateral diffusion process of charge-transfer excitons with a decreasing diffusion coefficient. This feature suggests that hot charge-transfer excitons with large kinetic energies are formed and their cooling process persists for about 100 ps. The long energy relaxation time of excitons in the trilayer compared to its monolayer components is attributed to the reduced carrier and phonon scattering due to the dielectric screening effect in the trilayer. Our results help develop an in-depth understanding of the dynamics of charge-transfer excitons in two-dimensional heterostructures.

自2004年发现石墨烯以来,在许多学科中,二维材料独特的电学、化学、热学和光学性质引起了相当大的关注,在此后的十余年间开创了一个全新的领域。目前过渡金属硫化物中的二硒化钼相对其他材料来说是研究最少的,探索二硒化钼及其范德瓦尔斯异质结的动力学过程及其潜在应用,对于丰富该研究领域内容、拓展学科发展具有重要意义。本文采用机械剥离技术成功设计并制备二硒化钼单层及其范德瓦尔斯异质结,研究了二硒化钼单层及其范德瓦尔斯异质结构的动力学和发光过程。本文中利用六方氮化硼薄层部分覆盖二硒化钼单层来制备样品,并利用其改变局部介电环境来控制二维半导体中的激子输运。这为激子器件室温操作和激子输运控制提供了新的解决办法。在对样品的测试中,利用光致发光测量、时空分辨差分反射测量等研究了二硒化钼中的激子动力学过程。利用matlab构建刚性位移模型,提出一种理论模拟方法来描述激子的输运过程,计算出激子在50皮秒内移动约200纳米的距离,精确地确定了激子输运速度的数量级。这些结果表明,范德瓦尔斯介电屏蔽可以有效地控制二硒化钼单层半导体中激子的输运,这有助于理解介电环境对二维半导体电子和光学性质的影响。合理地设计了由单层二硒化钼-二硫化钨-二硫化钼构成的三层范德瓦尔斯异质结,首次观察到在该异质结构中的上转换光致发光现象。以该三层异质结为研究对象,用670纳米的低能激光激发样品,获得了620纳米的光致发光。该结果源于电子和空穴分别从二硒化钼和二硫化钼层转移到二硫化钨中间层并在那里重新复合。该研究结果首次揭示了范德瓦尔斯异质结上转换光致发光的物理机制,为其在光电子器件上的应用提供了一定基础。

Non-reciprocal electronic transport in a material occurs if both time reversal and inversion symmetries are broken. The superconducting diode effect (SDE) is an exotic manifestation of this type of behavior where the critical current for positive and negative currents are mismatched, as recently observed in some non-centrosymmetric superconductors with a magnetic field. Here, we demonstrate a SDE in non-magnetic Nb/Ru/Sr2RuO4 Josephson junctions without applying an external magnetic field. The cooling history dependence of the SDE suggests that time-reversal symmetry is intrinsically broken by the superconducting phase of Sr2RuO4. Applied magnetic fields modify the SDE dynamically by randomly changing the sign of the non-reciprocity. We propose a model for such a topological junction with a conventional superconductor surrounded by a chiral superconductor with broken time reversal symmetry. © 2023, Springer Nature Limited.

We report three-pulse photodope-pump-probe measurements on photocarrier dynamics in semiconducting transition metal dichalcogenide monolayers of MoS2, WS2, MoSe2, and WSe2. The samples are fabricated by metal-organic chemical vapor deposition and mechanical exfoliation techniques and characterized by photoluminescence spectroscopy. In the time-resolved measurement, the samples are first photodoped by a prepulse, which injects background photocarriers of various densities. A pump pulse then injects photocarriers, whose dynamics is monitored by measuring a differential reflection of a time-delayed probe pulse. We found that the ultrafast decay component of the differential reflection signal, which has been widely reported before, shows minimal dependence on the background exciton density. This observation shows that a previously suggested carrier-trapping model cannot account for this component. The results thus further support an exciton-formation model that was previously proposed based on spectroscopic evidence.

In this paper, we report a transient absorption microscopy study of photocarrier dynamics in CuS crystals at room temperature. The photocarriers are injected into the sample by interband absorption of a 410 nm pump pulse and are detected by measuring the differential reflection of a 660 nm probe pulse as a function of both the time delay and the spatial position of the probe pulse. By analyzing the spatiotemporal dynamics of these carriers, we obtained a diffusion coefficient of 4.4 +/- 0.5 cm(2) s(-1), which corresponds to a mobility of 163 cm(2) V-1 s(-1). The moderate photocarrier transport performance indicates potential optoelectronic applications of CuS as a semiconducting material. Our results also illustrate the prospects of fabricating two-dimensional materials and heterostructures with CuS.

We fabricated a van der Waals heterostructure of WS2-ReSe2 and studied its charge-transfer properties. Monolayers of WS2 and ReSe2 were obtained by mechanical exfoliation and chemical vapor deposition, respectively. The heterostructure sample was fabricated by transferring the WS2 monolayer on top of ReSe2 by a dry transfer process. Photoluminescence quenching was observed in the heterostructure, indicating efficient interlayer charge transfer. Transient absorption measurements show that holes can efficiently transfer from WS2 to ReSe2 on an ultrafast timescale. Meanwhile, electron transfer from ReSe2 to WS2 was also observed. The charge-transfer properties show that monolayers of ReSe2 and WS2 form a type-II band alignment, instead of type-I as predicted by theory. The type-II alignment is further confirmed by the observation of extended photocarrier lifetimes in the heterostructure. These results provide useful information for developing van der Waals heterostructure involving ReSe2 for novel electronic and optoelectronic applications and introduce ReSe2 to the family of two-dimensional materials to construct van der Waals heterostructures.

A comprehensive experimental study on optical properties and photocarrier dynamics in Bi₂O₂Se monolayers and nanoplates is presented. Large and uniform Bi₂O₂Se nanoplates with various thicknesses down to the monolayer limit are fabricated. In nanoplates, a direct optical transition near 720 nm is identified by optical transmission, photoluminescence, and transient absorption spectroscopic measurements and is attributed to the transition between the valence and conduction bands in the Γ valley. Time-resolved differential reflection measurements reveal ultrafast carrier thermalization and energy relaxation processes and a photocarrier recombination lifetime of about 200 ps in nanoplates. Furthermore, by spatially resolving the differential reflection signal, a photocarrier diffusion coefficient of about 4.8 cm² s⁻¹ is obtained, corresponding to a mobility of about 180 cm² V⁻¹ s⁻¹. A similar direct transition is also observed in monolayer Bi₂O₂Se, suggesting that the states in the Γ valley do not change significantly with the thickness. The temporal dynamics of the excitons in the monolayer is quite different from the nanoplates, with a strong saturation effect and fast exciton–exciton annihilation at high densities. Spatially and temporally resolved measurements yield an exciton diffusion coefficient of about 20 cm² s⁻¹.
© 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We introduce getchellite as a new layered material for fabrication of two-dimensional van der Waals materials and heterostructures. Nanofilms of AsSbS3 were fabricated by mechanical exfoliation. Transient absorption spectroscopy measurements identified a direct bandgap at about 710 nm, which is close to the ideal single-junction photovoltaic bandgap. Transient absorption microscopy measurements with high spatial and temporal resolution were performed to reveal the spatiotemporal dynamics of photocarriers in AsSbS3. We obtained a photocarrier lifetime of about 200 ps, a diffusion coefficient of about 5 cm(2) s(-1), a diffusion length of about 320 nm, and a carrier mobility of about 200 cm(2) V-1 s(-1). These results establish AsSbS3 as a promising two-dimensional semiconductor for optoelectronic applications as an individual material or in heterostructures.

Higher-rank generalizations of electrodynamics have recently attracted considerable attention because of their ability to host "fracton" excitations, with connections to both fracton topological order and gravity. However, the search for higher-rank gauge theories in experiment has been greatly hindered by the lack of materially relevant microscopic models. Here we show how a spin liquid described by rank-2 U(1) gauge theory can arise in a magnet on the breathing pyrochlore lattice. We identify Yb-based breathing pyrochlores as candidate systems, and make explicit predictions for how the rank-2 U(1) spin liquid would manifest itself in experiment. © 2020 American Physical Society.

We fabricated a van der Waals heterostructure of WS2-ReSe2 and studied its charge-transfer properties. Monolayers of WS2 and ReSe2 were obtained by mechanical exfoliation and chemical vapor deposition, respectively. The heterostructure sample was fabricated by transferring the WS2 monolayer on top of ReSe2 by a dry transfer process. Photoluminescence quenching was observed in the heterostructure, indicating efficient interlayer charge transfer. Transient absorption measurements show that holes can efficiently transfer from WS2 to ReSe2 on an ultrafast timescale. Meanwhile, electron transfer from ReSe2 to WS2 was also observed. The charge-transfer properties show that monolayers of ReSe2 and WS2 form a type-II band alignment, instead of type-I as predicted by theory. The type-II alignment is further confirmed by the observation of extended photocarrier lifetimes in the heterostructure. These results provide useful information for developing van der Waals heterostructure involving ReSe2 for novel electronic and optoelectronic applications and introduce ReSe2 to the family of two-dimensional materials to construct van der Waals heterostructures. © 2020 Materials Research Society.