The art depicts the transfer of electrons, driven by ultra-short laser pulses, through the interface between two atomically thin materials. This transfer is facilitated by an interlayer "bridge" state, into which electrons are able to enter due to lattice vibrations in both materials
(Photo credit: Gregory M. Stewart/SLAC.)
Researchers have found that electrons play a surprising role in heat transfer between semiconductor layers, which has important implications for the next generation of electronic devices.
As semiconductor devices become smaller and smaller, researchers are exploring the potential applications of two-dimensional (2D) materials in transistors and optoelectronics. Controlling the flow of electricity and heat through these materials is key to their function, but first we need to understand the details of these behaviors at the atomic scale.
Now, researchers have discovered that electrons play a surprising role in the energy transfer between layers of the 2D semiconductor material tungsten diselenide (WSe 2) and tungsten disulfide (WS2). The researchers found that while the layers were not tightly bound to each other, the electrons provided a bridge between them, facilitating rapid heat transfer.
"Our work shows that we need to go beyond the Lego analogy to understand stacks of different 2D materials, even if the layers are not firmly bound to each other," said Archana Raja, a scientist at the US Department of Energy's (DOE) Lawrence Berkeley National Laboratory (Berkeley Lab). "The fact that seemingly different layers communicate via a shared electronic path allows us to access and ultimately design properties that are greater than the sum of the parts."
The researchers report their findings in a paper in the journal, which combine insights from ultrafast-fast, atomic-level temperature measurements and extensive theoretical calculations.
"This experiment was motivated by fundamental questions about the motion of atoms in nanoscale junctions, but these findings have implications for energy dissipation in future electronic devices," said Aditya Sood, co-first author of the paper and currently a research scientist at Stanford University. "We were curious about how electrons and atomic vibrations are coupled to each other when heat flows between the two materials. By amplifying the interface with atomic precision, we discovered a surprisingly efficient mechanism.
The researchers studied devices consisting of stacked single layers of WSe 2 and WS2. The devices were built by Raja's group at Berkeley Lab's Molecular Foundry, who perfected the art of using scotch tape to remove crystalline monolayers of semiconductors, each less than 1nm thick. Using polymer seals aligned under a homemade stack microscope, the layers deposit on each other and are precisely placed on the microscopic window to allow electrons to be transported through the stack.
In experiments conducted at the U.S. Department of Energy's SLAC National Accelerator Laboratory, the team used a technique called ultrafart-electron diffraction (UED) to measure the temperature of individual layers in the stack while optically exciting electrons in layers 2 of the WSe. The UED acts as an "electronic camera," capturing the atomic positions within each layer. By varying the time interval between the excitation and detection pulses by trillionths of a second, the researchers can independently track the temperature changes in each layer, using theoretical simulations to convert the observed atomic motion into temperature.
"This UED method provides a new way to directly measure temperature within this complex heterogeneous structure," said Aaron Lindenberg, a co-author of the paper from Stanford University. "The layers are only a few angstroms apart, but we can selectively probe their response, and because of the temporal resolution, we can probe how energy is shared between these structures in new ways on fundamental timescales."
They found that, as expected, the excited WSe 2 layer heated up. But to their surprise, the WS2 layers also warmed up at the same time, indicating rapid heat transfer between the layers. In contrast, when they did not excite the electrons in WSe 2, but instead used a metallic contact layer to heat the heterogeneous structure, the interface transfer of heat between WSe 2 and WS2 was very poor, confirming previous reports.
"It was very surprising to see two layers heating up almost simultaneously after light excitation, which prompted us to have a deeper understanding of what was going on," Raja said.
To understand their observations, the team turned to theoretical calculations, using density-based functional theory methods to model how atoms and electrons behave in these systems. For this work, they were supported by the Center for Computational Research on Excited State Phenomena in Energy Materials (C2SEPEM), a Computational Materials Science Center at Berkeley Lab funded by the U.S. Department of Energy.
The researchers performed extensive calculations on the electronic structure of layered 2D WSe 2 / WS2 and the behavior of lattice vibrations within the layers. Just as a squirrel runs along paths defined by branches and occasionally jumps between them to cross a forest canopy, electrons in the material are limited to specific states and transitions (called scattering). Understanding this electronic structure can provide guidance for interpreting experimental results.
"Using computer simulations, we explored where electrons in a layer initially wanted to scatter due to lattice vibrations," said Jonah Haber, co-first author of the paper and now a postdoctoral researcher in Berkeley Lab's Materials Science Division. "We found that it wants to scatter into this mixed state - a kind of 'glue state' where electrons are suspended in both layers at the same time." We have a good idea of what these glue states look like now and what their characteristics are, which allows us to say with relative confidence that other 2D semiconductor heterostructures behave in the same way.
Large-scale molecular dynamics simulations confirmed that it takes longer for heat to move from one layer to another in the absence of a shared electron "gel state." These simulations were conducted primarily at the National Energy Research Scientific Computing Center (NERSC).
"The electrons here are doing something important: they're acting as a bridge to dissipate heat," said co-author Felipe de Jornada of Stanford University. If we can understand and control this, it provides a unique approach to the thermal management of semiconductor devices.
