Research
A key part of the QPress project is to leverage the new capabilities in order to conduct frontier research in two-dimensional materials, heterostructures, and quantum material properties.
Understanding mechanical exfoliation
QPress represents a unique platform for understanding the materials physics associated with mechanical exfoliation of 2D materials, and their layering into heterostructures. The QPress exfoliator is designed to enable precise, systematic control of exfoliation processing parameters (pressure, temperature, pull-off angle, etc.), which in turn can allow systematic understanding of how these parameters influence exfoliation.
The QPress system also represents a unique platform for studying, and integrating, novel exfoliation methods. Prof. Davood Shahrjerdi’s research group at NYU has been developing a new exfoliation method called “PVA-assisted graphene exfoliation (PAGE)”, in which a soluble polymeric sacrificial layer is coated on silicon oxide wafer. This layer can be washed away (using water) to liberate attached 2D layers during exfoliation and/or stacking operations. This new method raises a host of fundamental and optimization challenges; for instance regarding residues and their influence on subsequent material properties. The QPress team is studying these topics. For instance, the QPress automatic stacker can be used to systematically investigate transfer conditions, in order to understand what conditions give rise to interfacial entrapment defects, and whether processing history can be used to improve upon the heterostructure quality.
Exfoliation of ionic materials
The QPress aims to allow users to study and synthesize heterostructures from a wide variety of layered materials. As part of this effort, the QPress team is studying methods to exfoliate a broad range of layered materials, beyond van der Waals solids. This correspondingly broadens the diversity of materials that can be included as ultrathin layers in multi-layered heterostructures.
Using chemical exfoliation methods, we are exfoliating ionic and intermetallic 2D materials such as perovskites, silicates, and MXenes; and adding these to the QPress library. The diversity of available nanosheets will enable scientific studies over a broad application space. For example, the CFN is using QPress to construct perovskite heterostructures for application in industrial catalysis. Here, multiple heterostructure architectures, such as vertically stacked and sandwich heterostructures, are used as supports that stabilize catalysts at elevated temperature and pressure conditions.
Novel substrates for ultra-thin materials
A key challenge in studies of 2D materials is characterization of the ultrathin layers. Ideally one would obtain complementary measurements assessing structural, chemical, optical, and electronic properties; for instance via optical spectroscopy, scanning probe methods, and electron spectro-microscopy. Conventional substrates are generally not suitable. For instance substrates optimized for flake detection via optical microscopy will have a thick oxide layer tuned to yield high-contrast optical contrast between flake and substrate; but such oxide layers are strongly insulating, with concomitant charging being problematic for electronic probes.
The QPress team has developed engineered substrates that allows for both optical and electron-based spectro-microscopy characterization of ultrathin flakes. The substrates are silicon wafers with a conventional SiO2 layer, and an engineered thin (3.5 nm) reduced titania film (grown by atomic layer deposition and annealed in forming gas to create an oxygen-vacancy-rich film). The total optical thickness is optimized for optical measurements, while the semiconducting TiOx layer provides necessary charge dissipation. This enables study of films using methods such as such low-energy electron microscopy (LEEM), low-energy electron diffraction (LEED), x-ray photoemission electron microscopy (XPEEM), angle-resolved photoemission (ARPES), and x-ray photoemission spectroscopy (XPS).
These unique substrates have been deployed in several research projects. We were able to optically map twisted-bilayer graphene (tBLG) films transferred onto these substrates, and subsequently perform electron spectro-microscopy and optical (Raman) measurements on selected, optically identified structures. We were able to show that the graphene bilayers, which are specifically twisted at 30° angle, have unique, previously undetected electronic structure, which potentially makes them very promising for novel opto-electronic devices. [Dai et al. Phys. Rev. Lett. 2021]
