In addition to the research activities listed below, which have been specifically enabled by the seed resources awarded to the 2D Center, members have also published many research papers as part of their externally funded research programs in this field.
Facility for Exfoliation and Transfer
A centerpiece of the Center is the facility for exfoliation and transfer of 2D materials, intended for use by all Center members. A clean area has been constructed in Wean Hall 7327, and all equipment for the facility (transfer station, optical microscope, etc.) is in place in this clean area. This facility deals with relatively small “flakes” of 2D materials, including graphene (a monolayer of carbon atoms), hexagonal boron nitride (h-BN), or transition-metal dichalcogenides (TMDs) such as MoSe2. These flakes, typically 10 µm on a side, are formed by removing the atomic planes from a bulk crystal utilizing “exfoliation” with an adhesive material. With a “palette” of such flakes available, the transfer facility then allows formation of a stack (a “heterostructure”) by sequentially picking up one flake after another. Subsequent patterning (e-beam lithography, metal deposition, etc.) is employed to form gates (if needed) and contacts for a full electronic device.
Contact: Benjamin Hunt, Randall Feenstra
Boron Nitride Epitaxy
Hexagonal BN is an important material within the field of 2D electronic devices, since it is an insulator with relatively low fixed-charge density. It is therefore very useful for both dielectric layers and as a substrate on which to build 2D devices and circuits. An existing chemical vapor deposition (CVD) system has been retrofitted with new equipment needed to enable it to grow h-BN films. Most of the equipment aspects of this task have been completed, and tests of the system will commence shortly. It is anticipated that h-BN films will be deposited, and growth conditions optimized, during 2016.
Contact: Robert Davis
Optical Characterization of 2D Materials
A microscope-compatible cryogenic work station with infrared-transparent window was acquired, to allow optical measurements in both the visible and infrared, over temperatures of 4 – 500 K. This equipment is currently being employed to measure the performance of a graphene-based bolometer.
Contact: Sheng Shen
Thermoreflectance of 2D Materials
To study the phonon properties of 2D heterostructures using thermoreflectance experiments it is critical to eliminate the metal transducer that is typically used as a thermometer. The thermoreflectance of bare graphene itself must be relied upon for the measurement, but it is ~10 times less than that of a typical Au transducer. Hence, improved signal-to-noise ratio (SNR) is required, which has been achieved with a new lock-in amplifier and optical chopper purchased for this purpose. SNR was increased by a factor of ~6, enabling the measurements directly on graphene. This new capability opens the door to direct measurements of graphene heterostructures. Additionally, for these measurements on freestanding 2D materials, the geometry is different than in past work, in that the heat is transported radially (with cylindrical symmetry). A new solution of the Boltzmann transport equation was obtained for this case, and is expected to have impact for understanding nondiffusive thermal transport in a range of related materials and device configurations.
Contact: Jonathan Malen
Graphene / h-BN Thermal Conductance
Thermal conductance of graphene/h-BN interfaces is studied, based on the unique ability at CMU to fabricate and measure such samples. This measurement will be applied together with other fundamental thermal transport properties derived from thermoreflectance methods, to understand the operating temperature in real 2D heterostructure devices as measured by infra-red microscopy.
Contact: Jonathan Malen, Jeffrey Weldon