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Urban population growth has significantly complicated the management of mobility systems, demanding innovative tools for planning. Generative Crowd-Flow  (GCF) models, which leverage machine learning to simulate urban movement patterns, offer a promising solution but lack sufficient evaluation of their fairness–a critical factor for equitable urban planning. We present an approach to measure and benchmark the fairness of GCF  models by developing a first-of-its-kind set of fairness metrics specifically tailored for this purpose. Using observed flow data, we employ a stochastic biased sampling approach to generate multiple permutations of Origin-Destination  datasets, each demonstrating intentional bias. Our proposed framework allows for the comparison of multiple GCF  models to evaluate how models introduce bias in outputs. Preliminary results indicate a tradeoff between model accuracy and fairness, underscoring the need for careful consideration in the deployment of these technologies. To this end, this study bridges the gap between human mobility literature and fairness in machine learning, with potential to help urban planners and policymakers leverage GCF  models for more equitable urban infrastructure development. 

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 MoS 2 /WS 2 /MoSe 2 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. 

We fabricated a van der Waals heterostructure of WS 2 –ReSe 2 and studied its charge-transfer properties. Monolayers of WS 2 and ReSe 2 were obtained by mechanical exfoliation and chemical vapor deposition, respectively. The heterostructure sample was fabricated by transferring the WS 2 monolayer on top of ReSe 2 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 WS 2 to ReSe 2 on an ultrafast timescale. Meanwhile, electron transfer from ReSe 2 to WS 2 was also observed. The charge-transfer properties show that monolayers of ReSe 2 and WS 2 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 ReSe 2 for novel electronic and optoelectronic applications and introduce ReSe 2 to the family of two-dimensional materials to construct van der Waals heterostructures. 

Understanding excitonic dynamics in two-dimensional semiconducting transition metal dichalcogenides is important for developing their optoelectronic applications. Recently, transient absorption techniques based on resonant excitonic absorption have been used to study various aspects of excitonic dynamics in these materials. The transient absorption in such measurements originates from phase-space state filling, bandgap renormalization, or screening effects. Here we report a new method to probe excitonic dynamics based on exciton intraband absorption. In this Drude-like process, probe photons are absorbed by excitons in their intraband excitation to higher energy states, causing a transient absorption signal. Although the magnitude of the transient absorption is lower than that of the resonant techniques, the new method is less restrictive on the selection of probe wavelength, has a larger linear range, and can provide complementary information on photocarrier dynamics. Using the WS 2 monolayer and bulk samples as examples, we show that the new method can probe exciton–exciton annihilation at high densities and reveal exciton formation processes. We also found that the exciton intraband absorption cross section of the WS 2 monolayer is on the order of 10 −18 cm 2 . 

The electron dynamics in heterostructures formed by multilayer graphite and monolayer or bulk MoS 2 were studied by femtosecond transient absorption measurements. Samples of monolayer MoS 2 -multilayer graphite and bulk MoS 2 -multilayer graphite were fabricated by exfoliation and dry transfer techniques. Ultrafast laser pulses were used to inject electron–hole pairs into monolayer or bulk MoS 2 . The transfer of these photocarriers to the adjacent multilayer graphite was time resolved by measuring the differential reflection of a probe pulse. We found that photocarriers injected into monolayer MoS 2 transfer to graphite on an ultrafast time scale shorter than 400 fs. Such an efficient charge transfer is key to the development of high performance optoelectronic devices with MoS 2 as the light absorbing layer and graphite as electrodes. The absorption coefficient of monolayer MoS 2 can be controlled by the carriers in graphite. This process can be used for interlayer coupling and control. In a bulk MoS 2 -graphite heterostructure, the photocarrier transfer time is about 220 ps, due to the inefficient interlayer charge transport in bulk MoS 2 . These results provide useful information for developing optoelectronic devices based on MoS 2 -graphite heterostructures.