Browsing by Author "Kriegel, Ilka"

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Light-Driven Permanent Charge Separation across a Hybrid Zero-Dimensional/Two-Dimensional Interface
(2020-04) Kriegel, Ilka; Ghini, Michele; Bellani, Sepastiano; Zhang, Kehao; Jansons, Adam W.; Crockett, Brandon M.; Koskela, Kristopher M.; Barnard, Edward S.; Penzo, Erika; Hutchison, James E.; Robinson, Joshua A.; Manna, Liberato; Borys, Nicholas J.; Schuck, P. James
We report the first demonstration of light-driven permanent charge separation across an ultrathin solid-state zero-dimensional (0D)/2D hybrid interface by coupling photoactive Sn-doped In2O3 nanocrystals with monolayer MoS2, the latter serving as a hole collector. We demonstrate that the nanocrystals in this device-ready architecture act as local light-controlled charge sources by quasi-permanently donating ∼5 holes per nanocrystal to the monolayer MoS2. The amount of photoinduced contactless charge transfer to the monolayer MoS2 competes with what is reached in electrostatically gated devices. Thus, we have constructed a hybrid bilayer structure in which the electrons and holes are separated into two different solid-state materials. The temporal evolution of the local doping levels of the monolayer MoS2 follows a capacitive charging model with effective total capacitances in the femtofarad regime and areal capacitances in the μF cm–2 range. This analysis indicates that the 0D/2D hybrid system may be able to store light energy at densities of at least μJ cm–2, presenting new potential foundational building blocks for next-generation nanodevices that can remotely control local charge density, power miniaturized circuitry, and harvest and store optical energy.
Light-Driven Permanent Charge Separation across a Hybrid Zero-Dimensional/Two-Dimensional Interface
(2020-03) Kriegel, Ilka; Ghini, Michele; Bellani, Sebastiano; Zhang, Kehao; Jansons, Adam W.; Crockett, Brandon M.; Koskela, Kristopher M.; Barnard, Edward S.; Penzo, Erika; Hutchison, James E.; Robinson, Joshua A.; Manna, Liberato; Borys, Nicholas J.; Schuck, P. James
We report the first demonstration of light-driven permanent charge separation across an ultrathin solid-state zero-dimensional (0D)/2D hybrid interface by coupling photoactive Sn-doped In2O3 nanocrystals with monolayer MoS2, the latter serving as a hole collector. We demonstrate that the nanocrystals in this device-ready architecture act as local light-controlled charge sources by quasi-permanently donating ∼5 holes per nanocrystal to the monolayer MoS2. The amount of photoinduced contactless charge transfer to the monolayer MoS2 competes with what is reached in electrostatically gated devices. Thus, we have constructed a hybrid bilayer structure in which the electrons and holes are separated into two different solid-state materials. The temporal evolution of the local doping levels of the monolayer MoS2 follows a capacitive charging model with effective total capacitances in the femtofarad regime and areal capacitances in the μF cm–2 range. This analysis indicates that the 0D/2D hybrid system may be able to store light energy at densities of at least μJ cm–2, presenting new potential foundational building blocks for next-generation nanodevices that can remotely control local charge density, power miniaturized circuitry, and harvest and store optical energy.
The ultrafast onset of exciton formation in 2D semiconductors
(Springer Science and Business Media LLC, 2020-10) Trovatello, Chiara; Katsch, Florian; Borys, Nicholas J.; Selig, Malte; Yao, Kaiyuan; Borrego-Varillas, Rocio; Scotognella, Francesco; Kriegel, Ilka; Yan, Aiming; Zettl, Alex; Schuck, P. James; Knorr, Andreas; Cerullo, Giulio; Dal Conte, Stefano
The equilibrium and non-equilibrium optical properties of single-layer transition metal dichalcogenides (TMDs) are determined by strongly bound excitons. Exciton relaxation dynamics in TMDs have been extensively studied by time-domain optical spectroscopies. However, the formation dynamics of excitons following non-resonant photoexcitation of free electron-hole pairs have been challenging to directly probe because of their inherently fast timescales. Here, we use extremely short optical pulses to non-resonantly excite an electron-hole plasma and show the formation of two-dimensional excitons in single-layer MoS2 on the timescale of 30 fs via the induced changes to photo-absorption. These formation dynamics are significantly faster than in conventional 2D quantum wells and are attributed to the intense Coulombic interactions present in 2D TMDs. A theoretical model of a coherent polarization that dephases and relaxes to an incoherent exciton population reproduces the experimental dynamics on the sub-100-fs timescale and sheds light into the underlying mechanism of how the lowest-energy excitons, which are the most important for optoelectronic applications, form from higher-energy excitations. Importantly, a phonon-mediated exciton cascade from higher energy states to the ground excitonic state is found to be the rate-limiting process. These results set an ultimate timescale of the exciton formation in TMDs and elucidate the exceptionally fast physical mechanism behind this process.