Art depicts electron transfer driven by ultra short laser pulses, passing through the interface between two atomically thin materials. This transfer is facilitated by the interlayer "bridge" state, where electrons can enter due to lattice vibrations in both materials
(Image source: Gregory M. Stewart/SLAC.)
Researchers have found that electrons play a surprising role in heat transfer between semiconductor layers, which is of great significance for the next generation of electronic devices.
As semiconductor devices become 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 functionality, but first we need to understand the details of these behaviors at the atomic scale.
Now, researchers have found that electrons play a surprising role in energy transfer between the 2D semiconductor material tungsten diselenide (WSe2) and tungsten disulfide (WS2) layers. Researchers have found that although these layers are not tightly bonded to each other, electrons provide a bridge between them, promoting rapid heat transfer.
Our work suggests that we need to go beyond the analogy of LEGO bricks to understand stacks of different 2D materials, even if these layers are not firmly bonded to each other, "said Archana Raja, a scientist at the Lawrence Berkeley National Laboratory (Berkeley Lab) of the US Department of Energy (DOE). In fact, seemingly different layers communicate through shared electronic paths, allowing us to access and ultimately design properties greater than the sum of the parts.
Researchers reported their findings in a paper in the journal, which combine insights from ultrafast, atomic level temperature measurements and extensive theoretical calculations.
The motivation for this experiment is about fundamental issues of atomic motion 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 are curious about how electron and atomic vibrations couple with each other when heat flows between two materials. By amplifying the interface with atomic precision, we have discovered a surprisingly effective mechanism.
Researchers studied a device composed of stacked monolayers of WSe 2 and WS2. These devices were manufactured by Raja's team at the Berkeley Laboratory Molecular Foundry, who perfected the art of using transparent tape to remove crystal monolayers of semiconductors, with each semiconductor having a thickness of less than 1nm. Using polymer stamps aligned under a homemade stacking microscope, these layers are deposited onto each other and precisely placed on the microscopic window to allow electron transport through the stack.
In experiments conducted at the US Department of Energy's SLAC National Accelerator Laboratory, the team used a technique called ultrafast electron diffraction (UED) to measure the temperature of each layer in the stack while optically exciting electrons in the WSe 2 layer. UED acts as an "electronic camera" to capture the atomic positions within each layer. By changing the time interval between excitation and detection pulses by trillionths of a second, researchers can independently track temperature changes in each layer and use theoretical simulations to convert observed atomic motion into temperature.
This UED method provides a new way to directly measure the temperature inside this complex heterostructure, "said Aaron Lindenberg, co-author of the paper at Stanford University. These layers are only a few angstroms apart, but we can selectively detect their responses, and due to temporal resolution, we can explore how these structures share energy in new ways on a basic time scale.
They found that, as expected, the excited WSe 2 layer heated up. But to their surprise, the WS2 layer also heated up simultaneously, indicating rapid heat transfer between the layers. In contrast, when they do not excite electrons in WSe2 and instead use a metal contact layer to heat the heterostructure, the interface heat transfer between WSe2 and WS2 is very poor, confirming previous reports.
It is very surprising to see two layers heating up almost simultaneously after photoexcitation, which prompts us to have a deeper understanding of what is happening, "Raja said.
In order to understand their observations, the team employed theoretical calculations, using density functional theory based methods to simulate the behavior of atoms and electrons in these systems. For this work, they received support from the Center for Computing Excited State Phenomena in Energy Materials (C2SEPEM), which is the Berkeley Laboratory Center for Computational Materials Science funded by the US Department of Energy.
Researchers have conducted extensive calculations on the electronic structure and behavior of lattice vibrations within layered 2D WSe 2/WS2. Just like squirrels running along paths defined by tree branches and occasionally jumping between them to traverse forest canopies, electrons in materials are limited to specific states and transitions (known as scattering). Understanding this electronic structure can provide guidance for interpreting experimental results.
Using computer simulations, we explored where electrons in a layer initially want to scatter due to lattice vibrations, "said Jonah Haber, co first author of the paper and now a postdoctoral researcher in the Materials Science Department at Berkeley Lab. We found that it wants to scatter into this mixed state - a 'glue state' where electrons are suspended in two layers simultaneously. We have a good understanding of what these glue states look like now and their characteristics, which gives us relative confidence to say that the behavior of other 2D semiconductor heterostructures is the same.
Large scale molecular dynamics simulations have confirmed that it takes longer for heat to move from one layer to another without a shared electron "gel state". These simulations are mainly conducted at the National Energy Research Scientific Computing Center (NERSC).
The electrons here are doing something important: they are acting as a bridge for heat dissipation, "said co-author Felipe de Jornada from Stanford University. If we can understand and control this, it provides a unique method for thermal management of semiconductor devices.
