Highly Efficient and Anomalous Charge Transfer in van der Waals Trilayer Semiconductors

Two-dimensional materials, such as graphene and monolayer transition metal dichalcogenides, allow the fabrication of multilayer structures without lattice matching restriction. A central issue in developing such artificial materials is to understand and control the interlayer electron transfer process, which plays a key role in harnessing their emergent properties. Recent photoluminescence and transient absorption measurements revealed that the electron transfer in heterobilayers occurs on ultrafast time scales. However, there is still a lack of fundamental understanding on how this process can be so efficient at van der Waals interfaces. Here we show evidence suggesting the coherent nature of such interlayer electron transfer. In a trilayer of MoS₂–WS₂–MoSe₂, electrons excited in MoSe₂ transfer to MoS₂ in about one picosecond. Surprisingly, these electrons do not populate the middle WS₂ layer during this process. Calculations showed the coherent nature of the charge transfer and reproduced the measured electron transfer time. The hole transfer from MoS₂ to MoSe₂ is also found to be efficient and ultrafast. The separation of electrons and holes extends their lifetimes to more than one nanosecond, suggesting potential applications of such multilayer structures in optoelectronics

Ultrafast Interlayer Electron Transfer in Incommensurate Transition Metal Dichalcogenide Homobilayers

Two-dimensional materials, such as graphene, transition metal dichalcogenides, and phosphorene, can be used to construct van der Waals multilayer structures. This approach has shown potentials to produce new materials that combine novel properties of the participating individual layers. One key requirement for effectively harnessing emergent properties of these materials is electronic connection of the involved atomic layers through efficient interlayer charge or energy transfer. Recently, ultrafast charge transfer on a time scale shorter than 100 fs has been observed in several van der Waals bilayer heterostructures formed by two different materials. However, information on the transfer between two atomic layers of the same type is rare. Because these homobilayers are essential elements in constructing multilayer structures with desired optoelectronic properties, efficient interlayer transfer is highly desired. Here we show that electron transfer between two monolayers of MoSe₂ occurs on a picosecond time scale. Even faster transfer was observed in homobilayers of WS₂ and WSe₂. The samples were fabricated by manually stacking two exfoliated monolayer flakes. By adding a graphene layer as a fast carrier recombination channel for one of the two monolayers, the transfer of the photoexcited carriers from the populated to the drained monolayers was time-resolved by femtosecond transient absorption measurements. The observed efficient interlayer carrier transfer indicates that such homobilayers can be used in van der Waals multilayers to enhance their optical absorption without significantly compromising the interlayer transport performance. Our results also provide valuable information for understanding interlayer charge transfer in heterostructures

Charge Transfer Exciton and Spin Flipping at Organic–Transition-Metal Dichalcogenide Interfaces

Two-dimensional transition-metal dichalcogenides (TMD) can be combined with other materials such as organic small molecules to form hybrid van der Waals heterostructures. Because of different properties possessed by these two materials, the hybrid interface can exhibit properties that cannot be found in either of the materials. In this work, the zinc phthalocyanine (ZnPc)–molybdenum disulfide (MoS₂) interface is used as a model system to study the charge transfer at these interfaces. It is found that the optically excited singlet exciton in ZnPc transfers its electron to MoS₂ in 80 fs after photoexcitation to form a charge transfer exciton. However, back electron transfer occurs on the time scale of ∼1–100 ps, which results in the formation of a triplet exciton in the ZnPc layer. This relatively fast singlet–triplet transition is feasible because of the large singlet–triplet splitting in organic materials and the strong spin–orbit coupling in TMD crystals. The back electron transfer would reduce the yield of free carrier generation at the heterojunction if it is not avoided. On the other hand, the spin-selective back electron transfer could be used to manipulate electron spin in hybrid electronic devices

Photocarrier Transfer across Monolayer MoS₂–MoSe₂ Lateral Heterojunctions

In-plane heterojuctions formed from two monolayer semiconductors represent the finest control of electrons in condensed matter and have attracted significant interest. Various device studies have shown the effectiveness of such structures to control electronic processes, illustrating their potentials for electronic and optoelectronic applications. However, information about the physical mechanisms of charge carrier transfer across the junctions is still rare, mainly due to the lack of adequate experimental techniques. Here we show that transient absorption measurements with high spatial and temporal resolution can be used to directly monitor such transfer processes. We studied MoS₂–MoSe₂ in-plane heterostructures fabricated by chemical vapor deposition and lithographic patterning followed by laser-generated vapor sulfurization. Transient absorption measurements in reflection geometry revealed evidence of exciton transfer from MoS₂ to MoSe₂. By comparing the experimental data with a simulation, we extracted an exciton transfer velocity of 10⁴ m s^–1. These results provide valuable information for understanding and controlling in-plane carrier transfer in two-dimensional lateral heterostructures for their electronic and optoelectronic applications

Additional file 1 of iPSC-derived PSEN2 (N141I) astrocytes and microglia exhibit a primed inflammatory phenotype

Additional file 1: Supplementary Methods. Table S1. Primary antibodies used for immunofluorescence. Table S2. Secondary antibodies used for immunofluorescence. Fig. S1. Sanger sequencing chromatograms showing APOE genotyping of codon 112 (rs429358) and codon 158 (rs7412) for all iPSC lines. Yellow highlight indicates the position of the single nucleotide polymorphism. Fig. S2. Immunofluorescence images of iPSCs from three healthy control lines (Ctrl-06, Ctrl-71, Ctrl-88) and three familial AD lines harbouring a PSEN2 (N141I) mutation (fAD-08, fAD-948, fAD-950). The cells were stained for pluripotency markers Nanog (red), Oct 3 (green) and all nuclei were counterstained with DAPI (blue). Scale bars = 50 μm. Fig. S3. Immunofluorescence images of iPSC-derived NPCs from three healthy control lines (Ctrl-06, Ctrl-71, Ctrl-88) and three familial AD lines harbouring a PSEN2 (N141I) mutation (fAD-08, fAD-948, fAD-950). The cells were stained for A the neural progenitor markers Pax-6 (red) and Nestin (green), B a pluripotency marker Oct3 (green) and all nuclei were counterstained with DAPI (blue). Scale bars = 50 μm. Fig. S4. Immunofluorescence images of iPSC-derived astrocytes from three healthy control lines (Ctrl-06, Ctrl-71, Ctrl-88) and three familial AD lines harbouring a PSEN2 (N141I) mutation (fAD-08, fAD-948, fAD-950). The cells were stained for A astrocyte markers GFAP (red) and S100β (green), B the NPC marker nestin (green). All nuclei were counterstained with DAPI (blue). Scale bars = 50 μm. Fig. S5. Transcriptomic analysis of iPSC-derived cell types and primary human fetal astrocytes. A Principal component analysis and B cluster analysis of iPSC-derived astrocytes (black) from healthy control lines (lines 06, 71 & 88) generated in our study and commercially-available primary astrocytes grown in our lab (green) combined with a datasets from Tcw et al (23), including primary astrocytes (purple), iPSC-derived NPCs (light blue), astrocytes (dark blue) and neurons (yellow). Contrast matrix of differential gene expression between cell types comparing datasets C our iPSC-derived astrocytes vs. Tcw et al’s primary astrocyte and iPSC-derived neurons datasets, D Tcw et al’s dataset alone, E our data alone. Fig. S6 Immunofluorescence images of iPSC-derived microglia-like cells from three healthy control lines (Ctrl-06, Ctrl-71, Ctrl-88) and three familial AD lines harbouring a PSEN2 (N141I) mutation (fAD-08, fAD-948, fAD-950). Images show cells stained for A the microglial markers IBA1 (red), TREM2 (green), B CX3CR1 (red) and all nuclei were counterstained with DAPI (blue). Scale bars = 50 μm. Fig. S7. Multi-cytokine array of Alzheimer’s or healthy iPSC-derived astrocytes A basally and B after 24 h exposure to 10 μM Aβ42 and iPSC-derived microglia-like cells C basally and D after 24 h exposure to 10 μM Aβ42. The figure displays the mean ± SD of three cell lines with the average of two experimental duplicates per line. Multiple unpaired, non-parametric Mann-Whitney t-tests adjusting for a 0.05 false discovery rate were used to test whether there were statistically significant differences between mean cytokine/chemokine release of AD-derived and healthy control astrocytes and microglia-like cells (*p < 0.05, **p < 0.01). Cytokines that yielded an average intensity value less than 10% of the maximum (represented by the dotted line) were considered background and not included in the statistical analysis. Fig. S8. Multi-cytokine array of APOE ε3/ε3 and APOE ε3/ε4 iPSC-derived astrocytes A basally and B after 24 h exposure to 10 μM Aβ42 and iPSC-derived microglia-like cells C basally and D after 24 h exposure to 10 μM Aβ42. The figure displays the mean ± SD of three cell lines with the average of two experimental duplicates per line. Multiple unpaired, non-parametric Mann–Whitney t-tests adjusting for a 0.05 false discovery rate were used to test whether there were statistically significant differences between mean cytokine/chemokine release of APOE ε3/ε3 and APOE ε3/ε4 astrocytes and microglia-like cells (*p < 0.05). Cytokines that yielded an average intensity value less than 10% of the maximum (represented by the dotted line) were considered background and not included in the statistical analysis. Fig. S9. Concentrations of A Aβ42, B Aβ40, C the ratio of Aβ42:40 and D total Aβ protein quantified from iPSC-derived astrocyte supernatants 72 h after plating. Secreted Aβ concentrations were measured using a highly sensitive ELISA and normalised to total protein concentration determined by BCA. The figure displays the mean ± SD of three cell lines with n ≥ 2 independent experiments per line. A post hoc unpaired t-test was used to test whether there were statistically significant differences between mean of APOE ε3/ε3 and APOE ε3/ε4 astrocytes