5 research outputs found
Long-Lived Charge Separation Following Pump-Energy Dependent Ultrafast Charge Transfer in Graphene/WS Heterostructures
Van der Waals heterostructures consisting of graphene and transition metal
dichalcogenides (TMDCs) have recently shown great promise for high-performance
optoelectronic applications. However, an in-depth understanding of the critical
processes for device operation, namely interfacial charge transfer (CT) and
recombination, has so far remained elusive. Here, we investigate these
processes in graphene-WS heterostructures, by complementarily probing the
ultrafast terahertz photoconductivity in graphene and the transient absorption
dynamics in WS following photoexcitation. We find that CT across
graphene-WS interfaces occurs via photo-thermionic emission for
sub-A-exciton excitation, and direct hole transfer from WS to the valence
band of graphene for above-A-exciton excitation. Remarkably, we observe that
separated charges in the heterostructure following CT live extremely long:
beyond 1 ns, in contrast to ~1 ps charge separation reported in previous
studies. This leads to efficient photogating of graphene. These findings
provide relevant insights to optimize further the performance of optoelectronic
devices, in particular photodetection
Long-lived charge separation following pump-wavelength-dependent ultrafast charge transfer in graphene/WS2 heterostructures
Van der Waals heterostructures consisting of graphene and transition metal dichalcogenides have shown great promise for optoelectronic applications. However, an in-depth understanding of the critical processes for device operation, namely, interfacial charge transfer (CT) and recombination, has so far remained elusive. Here, we investigate these processes in graphene-WS2 heterostructures by complementarily probing the ultrafast terahertz photoconductivity in graphene and the transient absorption dynamics in WS2 following photoexcitation. We observe that separated charges in the heterostructure following CT live extremely long: beyond 1 ns, in contrast to ~1 ps charge separation reported in previous studies. This leads to efficient photogating of graphene. Furthermore, for the CT process across graphene-WS2 interfaces, we find that it occurs via photo- thermionic emission for sub-A-exciton excitations and direct hole transfer from WS2 to the valence band of graphene for above-A-exciton excitations. These findings provide insights to further optimize the perform ance of optoelectronic devices, in particular photodetection
Harvesting the photoexcited holes on a photocatalytic proton reduction metal–organic framework
The highly porous titanium based metal–organic framework NH2-MIL-125(Ti) has recently attracted significant attention in the field of photocatalysis as a promising material for H+ reduction. This work reveals charge transfer upon visible light illumination from this MOF to two different charge acceptors, as an alternative to sacrificial electron donors. Charge transfer is demonstrated through a combined spectroscopic study between this MOF and: (1) 2-(1H-pyrazol-3-yl)phenol, a molecule that functionally mimics the tyrosine–histidine pair, responsible for shuttling the holes to the oxygen evolving centre in natural photosynthesis, and (2) TEMPO, a well known and stable radical. Charge transfer of the holes from the MOF to these occluded molecules takes place on the picosecond time scale. This work suggests that, by coupling a stable and recyclable charge acceptor to the photogenerated holes, the charges can be utilised for oxidation reactions and, thus, link the reduction to the oxidation reactions in water splitting.ChemE/Catalysis EngineeringChemE/Opto-electronic Material
Finding and Fixing Traps in II-VI and III-V Colloidal Quantum Dots: The Importance of Z-Type Ligand Passivation
Energy levels in the band gap arising from surface states can dominate the optical and electronic properties of semiconductor nanocrystal quantum dots (QDs). Recent theoretical work has predicted that such trap states in II-VI and III-V QDs arise only from two-coordinated anions on the QD surface, offering the hypothesis that Lewis acid (Z-type) ligands should be able to completely passivate these anionic trap states. In this work, we provide experimental support for this hypothesis by demonstrating that Z-type ligation is the primary cause of PL QY increase when passivating undercoordinated CdTe QDs with various metal salts. Optimized treatments with InCl3 or CdCl2 afford a near-unity (>90%) photoluminescence quantum yield (PL QY), whereas other metal halogen or carboxylate salts provide a smaller increase in PL QY as a result of weaker binding or steric repulsion. The addition of non-Lewis acidic ligands (amines, alkylammonium chlorides) systematically gives a much smaller but non-negligible increase in the PL QY. We discuss possible reasons for this result, which points toward a more complex and dynamic QD surface. Finally we show that Z-type metal halide ligand treatments also lead to a strong increase in the PL QY of CdSe, CdS, and InP QDs and can increase the efficiency of sintered CdTe solar cells. These results show that surface anions are the dominant source of trap states in II-VI and III-V QDs and that passivation with Lewis acidic Z-type ligands is a general strategy to fix those traps. Our work also provides a method to tune the PL QY of QD samples from nearly zero up to near-unity values, without the need to grow epitaxial shells.ChemE/Opto-electronic Material