arXiv subjects

Julian Picker

Publications and source records attributed to Julian Picker.

5 recordsLinked to original sources

arXiv2026

Spatially-resolved multiphoton photoemission from a lateral transition metal dichalcogenide heterostructure

Transition metal dichalcogenides (TMDs) in their monolayer form offer a premier platform for next-generation optoelectronics, particularly through the local manipulation of their robust excitonic states using nanoscale electric fields. These localized states can be dynamically controlled through spatial structuring as well as through ultrafast field modulations driven by tailored optical pulses. Characterizing the resulting rapid, nanoscale charge carrier dynamics requires a technique with exceptional spatial and temporal resolution. Here, we report on the spatio-temporally resolved investigation of ground state and excited state photoemission from a lateral heterostructure built of monolayers of WSe$_2$ and MoSe$_2$ using few-cycle light pulses with a photon energy of 0.62 eV. We utilize photoemission electron microscopy to spatially resolve the highly nonlinear photoemission from the monolayer structure with few tens of nanometer resolution. By varying the laser pulse energy, we extract the nonlinearity of the photoemission process and thus the dynamic binding energy of the photoelectrons before and after optical excitation with high spatial and temporal resolution.

physics.optics↗

arXiv2026

Sonochemically Boosted Hydrogen Evolution Activity of Janus TMD Monolayers

2D electrocatalysts that enable hydrogen evolution at low overpotentials offer an attractive alternative to expensive platinum-based systems. Here, we report the growth of Janus transition metal dichalcogenide (TMD) monolayers (MLs), SeMoS and SeWS, on Au foils using chemical vapor deposition, and systematically compare their catalytic properties in the context of hydrogen evolution reaction (HER) with those of their parent TMDs. The Janus MLs exhibited significantly enhanced catalytic performance relative to the parent TMDs. Furthermore, these MLs on Au foils were subjected to sonochemical treatment in polar and non-polar solvents, in which the treatment with polar solvents led to a substantial improvement in the HER activity of Janus MLs. In particular, SeMoS MLs treated with water showed a low overpotential of ~63 mV, a Tafel slope of ~42 mV/dec, and an exchange current density of ~10$^{-3}$ mA cm$^{- 2}$, approaching that of platinum. Analyses indicate that enhanced electrocatalytic activity is associated with tensile strain induced by Au surface restructuring and the formation of defects in Janus MLs, as shown by experimental observations and by density functional theory calculations. The enhancement in catalytic performance due to sonochemical treatment emphasizes the importance of our results for developing novel catalytic systems for HER based on Janus 2D materials.

cond-mat.mtrl-sci↗

arXiv2026

CVD Grown Hybrid MoSe$_2$-WSe$_2$ Lateral/Vertical Heterostructures with Strong Interlayer Exciton Emission

Lateral heterostructures of 2D transition metal dichalcogenide offer a powerful platform to investigate photonic and electronic phenomena at atomically sharp interfaces. However, their controlled engineering, including tuning lateral domain size and integration into vertical van der Waals heterostructures with other 2D materials, remains challenging. Here, we present a facile route for the synthesis of two types of heterostructures consisting of monolayers of MoSe$_2$ and WSe$_2$ - purely lateral (HS I) and hybrid lateral/vertical (HS II) - using liquid precursors of transition metal salts and chemical vapor deposition (CVD). Depending on the growth parameters, the heterostructure type and their lateral dimensions can be adjusted. We characterized properties of the HS I and HS II by complementary spectroscopic and microscopic techniques including Raman and photoluminescence spectroscopy, and optical and atomic force microscopy, and scanning electron and transmission electron microscopy. The photoluminescence measurements reveal strong interlayer exciton emission in the MoSe$_2$/WSe$_2$ region of HS II, which dominates the spectrum at 4 K and persisting up to room temperature. These results demonstrate high optical quality of the grown heterostructures which in combination with scalability of the developed approach paves the way for fundamental studies and device applications based on these unique 2D quantum materials.

cond-mat.mtrl-sci↗

arXiv2025

Impact of charge transfer excitons on unidirectional exciton transport in lateral TMD heterostructures

Lateral heterostructures built of monolayers of transition metal dichalcogenides (TMDs) are characterized by a thin 1D interface exhibiting a large energy offset. Recently, the formation of spatially separated charge-transfer (CT) excitons at the interface has been demonstrated. The technologically important exciton propagation across the interface and the impact of CT excitons has remained in the dark so far. In this work, we microscopically investigate the spatiotemporal exciton dynamics in the exemplary hBN-encapsulated WSe$_2$-MoSe$_2$ lateral heterostructure and reveal a highly interesting interplay of energy offset-driven unidirectional exciton drift across the interface and efficient capture into energetically lower CT excitons at the interface. This interplay triggers a counterintuitive thermal control of exciton transport with a less efficient propagation at lower temperatures - opposite to the behavior in conventional semiconductors. We predict clear signatures of this intriguing exciton propagation both in far- and near-field photoluminescence experiments. Our results present an important step toward a microscopic understanding of the technologically relevant unidirectional exciton transport in lateral heterostructures.

cond-mat.mes-hall↗

arXiv2022

One pot chemical vapor deposition of high optical quality large area monolayer Janus transition metal dichalcogenides

We report one-pot chemical vapor deposition (CVD) growth of large-area Janus SeMoS monolayers, with the asymmetric top (Se) and bottom (S) chalcogen atomic planes with respect to the central transition metal (Mo) atoms. The formation of these two-dimensional semiconductor monolayers takes place upon the thermodynamic equilibrium-driven exchange of the bottom Se atoms of the initially grown MoSe2 single crystals on gold foils with S atoms. The growth process is characterized by complementary experimental techniques including Raman and X-ray photoelectron spectroscopy and the growth mechanisms are rationalized by first principle calculations. The remarkably high optical quality of the synthesized Janus monolayers is demonstrated by optical and magneto-optical measurements which reveal the strong exciton-phonon coupling and enable to obtain the exciton g-factor of -3.3.

cond-mat.mtrl-sci↗