Thermometry of Thin Materials, Including 2-D Structures

Robert L. Stevenson, Ph.D.

Thermometry of thick materials is well studied by bulk property techniques, including melting points, such as thermogravimetric analysis (TGA). However, when one has very thin materials such as graphene and transition metal dichalcogenides such as WS₂, bulk techniques are not suitable. The difference between bulk and 2-D properties is an important topic, since the unique, almost magical, properties of 2-D materials are expected to be a disruptive technology enabling spintronics, photovoltaics, and nanoelectronics for transistors.

One example: Fan Ye and colleagues at Case Western Reserve University (Cleveland, OH) have made a nano resonator with 2-D graphene of one, two, or three layers that operates in the range of 12–45 MHz up to 1200 ^oK (Ye, F.; Lee, J. et al. Nano Letts 2018, 18(3), 1678–85; doi: 10.1021/acs.nanolett.7b04685).

There are practical problems with 2-D technology, including heat dissipation and mismatch between the thermal expansion of the 2-D device and the substrate that it is packaged in or connected to. Thus, it is necessary to make temperature and structure measurements on the nanometer scale, which is a new challenge in metrology.

In “Mapping thermal expansion coefficients in freestanding 2-D materials at the nanometer scale” (Phys. Rev. Lett. Feb 2018, 120, 055902), Hu et al. describe combining scanning transmission electron microscopy (STEM) with electron energy-loss spectroscopy (EELS) to determine the energy shift of the plasmon resonance peak of 2-D materials as a function of sample temperature to measure the thermal expansion coefficients (TECs) of several 2-D materials, including graphene and WS₂.

With this technology, the authors were also able to profile the structure of monolayer particles with regions of several more layers (see table). The differences between the TECs of the bulk and monolayer are very large. Also, note that adding a second layer or more decreases the TEC significantly, making it much more rigid.

Comparison of in-plane TECs (10^-5 K^-1) obtained from plasmon energy shift measurements with reference data for bulk samples

The authors also recognized the need for high spatial resolution beyond the diffraction limit. They used the temperature-dependent plasmon energy shift to measure the thermal expansion coefficient. This was useful in measuring heat dissipation through grain boundaries.

Today, these technologies are not common. However, the driving forces for utilizing advanced materials are very strong, which will create demand for measurements and instruments to provide the data. Some of us will become involved, but most will watch in awe.

Robert L. Stevenson, Ph.D., is Editor Emeritus, American Laboratory/Labcompare; e-mail: [email protected]