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Visualizing Lattice Dynamics in 2D TMD Heterostructures Using Ultrafast Electron Diffraction

Carlström, Johan LU (2021) PHYM01 20201
Synchrotron Radiation Research
Department of Physics
Abstract
Atomically thin two-dimensional (2D) semiconductors like transition metal dichalcogenide (TMD) monolayers have garnered significant interest in the recent past due to their extraordinary opto-electronic properties, along with the possibility to seamlessly stack these layers into atomically precise van der Waals heterostructures. Type-II 2D semiconductor heterostructures show photoinduced interlayer charge separation on ultrafast timescales of 100 fs or less, despite a large ∼300 meV energy off-set between the materials’ valence- and conduction band extrema. On the other hand, these junctions are highly resistive for phonon-mediated thermal transport. Here, we study interlayer heat transport between single crystalline monolayers of WS2 and... (More)
Atomically thin two-dimensional (2D) semiconductors like transition metal dichalcogenide (TMD) monolayers have garnered significant interest in the recent past due to their extraordinary opto-electronic properties, along with the possibility to seamlessly stack these layers into atomically precise van der Waals heterostructures. Type-II 2D semiconductor heterostructures show photoinduced interlayer charge separation on ultrafast timescales of 100 fs or less, despite a large ∼300 meV energy off-set between the materials’ valence- and conduction band extrema. On the other hand, these junctions are highly resistive for phonon-mediated thermal transport. Here, we study interlayer heat transport between single crystalline monolayers of WS2 and WSe2 in these two transport regimes using MeV ultrafast electron diffraction. By developing a novel sample geometry, we are able to track transient changes in the phonon population in each material simultaneously. A baseline phonon-mediated boundary conductance is measured through sub-bandgap excitation and by monitoring the lattice dynamics as phonons traverse the sample stack. Following resonant excitation of monolayer WSe2 we observe a strong thermal coupling between the layers with near synchronous heating on ∼1 ps timescales. Using first-principles density functional perturbation theory, we calculate the momentum resolved electron-phonon scattering rates during interlayer charge transfer. A fast phonon-mediated scattering channel via electronically hybridized states is identified. Specifically, we see emission of primarily acoustic phonons on femtosecond timescales in both layers, consistent with the measured thermal synchronicity. We find that electron-phonon mediated heat transfer can increase the effective interfacial thermal conductance by approximately two orders of magnitude. (Less)
Popular Abstract
Imagine living in a world with only two dimensions. That’s what the English author Edwin Abbott did in his famous 1884 novel, Flatland: A Romance of Many Dimensions. Little did he know that 120 years later, such a world would exist, albeit with less Victorian drama.

Electronic components in modern computers and other devices are based on micro-structured silicon. This technology has now reached a component size of a few nanometers and is quickly approaching the physical limit of scaling down. For this reason, so-called two-dimensional (2D) materials have garnered significant research interest. 2D materials are atomically thin sheets consisting of a single layer of atoms. The most well-known 2D material is graphene, which is a single... (More)
Imagine living in a world with only two dimensions. That’s what the English author Edwin Abbott did in his famous 1884 novel, Flatland: A Romance of Many Dimensions. Little did he know that 120 years later, such a world would exist, albeit with less Victorian drama.

Electronic components in modern computers and other devices are based on micro-structured silicon. This technology has now reached a component size of a few nanometers and is quickly approaching the physical limit of scaling down. For this reason, so-called two-dimensional (2D) materials have garnered significant research interest. 2D materials are atomically thin sheets consisting of a single layer of atoms. The most well-known 2D material is graphene, which is a single isolated atomic layer of carbon atoms from graphite. In this 2D limit, graphene has completely unique properties compared to the thicker graphite crystal. Since the first discovery of graphene in 2004, many other 2D materials have been discovered with a range of different properties. Of particular interest for ultra-thin microelectronic, solar-cell and other opto-electronic applications are transition metal dichalcogenides (TMDs). TMDs, like silicon, are semiconductors and are thus suitable for a range of applications.

In addition, different 2D materials can be combined into so-called heterostructures by stacking them on top of each other (nanoscale lego!) and the resulting structure can have radically different properties than the individual materials. This opens up many possibilities to engineer novel device characteristics using this family of materials. This thesis explores how heat propagates between two atomically thin TMDs. In addition, the microscopic details of atomic vibrations in TMD heterostructures are explored after excitation by a laser.

After excitation by light, excited electrons in a WSe2/WS2 TMD heterostructure jumps from one layer to another in less than 100 femtoseconds, or a millionth of a billionth of a second. This is about the time it takes light to travel half the thickness of a human hair. During this process, the electron releases energy and atoms start to vibrate. However, the details of this process is not well understood. We study this phenomenon by using an ultrafast ‘electron camera’, that can capture changes in atomic vibrations on 100 femtosecond timescales. The technique is called ultrafast electron diffraction (UED) and uses a pulsed laser to induce a change in the sample and then probes changes in atomic vibrations as a function of time by sending short pulses of electrons through the sample. How these electrons interact with the sample gives information about the vibrations and the temperature of the material. In our experiment, the laser selectively excites an electron in WSe2 and we measure the effects on vibrations and temperature of the two materials as the electron jumps to WS2.

We show that by developing a novel sample fabrication technique, it is possible to measure atomically thin TMD heterostructures using UED. To my knowledge, this has not previously been done. In addition, we see that upon photoexcitation, the two TMD layers heat up almost concurrently. This result is well-described by the microscopic processes of an electron jumping from one layer to another. On the other hand, we find purely thermal transport across the interface to be highly resistive. For applications using light, such as solar cells, understanding the thermal characteristics and light-induced processes is important to optimize device design and operating temperatures. These results provide insights to the microscopic details of interfacial thermal and electronic effects in TMD heterostructures and works as a proof-of-concept for measuring atomically thin stacks with UED. (Less)
Please use this url to cite or link to this publication:
author
Carlström, Johan LU
supervisor
organization
course
PHYM01 20201
year
type
H2 - Master's Degree (Two Years)
subject
keywords
Two dimensional material, transition metal dichalcogenides, TMD, heterostructure, ultrafast electron diffraction, thermal transport, charge transfer, electron-phonon coupling
language
English
id
9052792
date added to LUP
2021-06-15 15:55:54
date last changed
2021-06-15 15:55:54
@misc{9052792,
  abstract     = {{Atomically thin two-dimensional (2D) semiconductors like transition metal dichalcogenide (TMD) monolayers have garnered significant interest in the recent past due to their extraordinary opto-electronic properties, along with the possibility to seamlessly stack these layers into atomically precise van der Waals heterostructures. Type-II 2D semiconductor heterostructures show photoinduced interlayer charge separation on ultrafast timescales of 100 fs or less, despite a large ∼300 meV energy off-set between the materials’ valence- and conduction band extrema. On the other hand, these junctions are highly resistive for phonon-mediated thermal transport. Here, we study interlayer heat transport between single crystalline monolayers of WS2 and WSe2 in these two transport regimes using MeV ultrafast electron diffraction. By developing a novel sample geometry, we are able to track transient changes in the phonon population in each material simultaneously. A baseline phonon-mediated boundary conductance is measured through sub-bandgap excitation and by monitoring the lattice dynamics as phonons traverse the sample stack. Following resonant excitation of monolayer WSe2 we observe a strong thermal coupling between the layers with near synchronous heating on ∼1 ps timescales. Using first-principles density functional perturbation theory, we calculate the momentum resolved electron-phonon scattering rates during interlayer charge transfer. A fast phonon-mediated scattering channel via electronically hybridized states is identified. Specifically, we see emission of primarily acoustic phonons on femtosecond timescales in both layers, consistent with the measured thermal synchronicity. We find that electron-phonon mediated heat transfer can increase the effective interfacial thermal conductance by approximately two orders of magnitude.}},
  author       = {{Carlström, Johan}},
  language     = {{eng}},
  note         = {{Student Paper}},
  title        = {{Visualizing Lattice Dynamics in 2D TMD Heterostructures Using Ultrafast Electron Diffraction}},
  year         = {{2021}},
}