New microscope can image an invisible 2D material

Key Aspects

• A fascinating material: Hexagonal boron nitride (hBN) is a very important material for the large and thriving field of two-dimensional (2D) materials research and emerging new devices.
• The challenge: Monolayers – only one atom thick – of hBN have been very difficult to use in 2D structures because they are very difficult to see.
• The novel approach: Sum-frequency microscopy resonantly driving an hBN lattice vibration makes monolayers of hBN light up.

Why study 2D materials?

2D materials are crystalline substances consisting of only a single layer of atoms and have very unusual properties that arise from their extremely thin nature. Materials science has been aware of thin layered materials for over a hundred years, but it was only in 2004 that the first 2D layers - namely graphene - were successfully produced. Since then, new 2D materials have been continuously developed and novel applications have emerged. The thin layers are in demand for applications in many future technologies, from electronics and energy systems to optical components. Thus, these fascinating materials are the subject of extensive research.

The challenge with hexagonal boron nitride

Boron nitride (BN) - also known as “white graphene” - is a layered material composed of the elements boron (B) and nitrogen (N), and can exist in various forms, including hexagonal boron nitride (hBN). Similar to graphene, hBN has a hexagonal lattice structure and its 2D layers are widely used in various applications, such as quantum optics or infrared nanophotonics, or simply as a substrate or encapsulating material. For such applications a precise characterization of the h-BN layers is critical. However, aside from the pronounced mid-infrared resonance, as a monolayer, hBN is transparent across the entire near-IR and visible spectral range. Thus, it cannot be studied using conventional optical microscopes.

This restriction has so far hindered its use in the development of new materials. For example, to identify possible distortions and grain boundaries in 2D layers, they must be accurately mapped. Moreover, researchers stack monolayers of 2D materials on top of each other to create “van der Waals structures” that can exhibit completely new and exciting properties. Ideally, this layering could be screened live under a microscope and the orientation of the individual layers visualized. The development of such capabilities for hBN are hence highly desired.

The sum-frequency microscope

The research team at the Fritz Haber Institute overcame this challenge through their recently developed microscope that uses a trick in nonlinear optics to make the otherwise invisible material hBN visible. Their method, called “phase-resolved sum-frequency microscopy”, mixes two laser beams, one in the mid-infrared and one in the visible range, to generate a sum-frequency signal in the sample that is measured. By resonantly driving an hBN lattice vibration, the measured sum-frequency signal becomes very intense, making it not only possible to image large 100 × 100 μm² sample areas in less than 1 s, but also even visualize the crystal orientation.

Thanks to their new microscope, the researchers revealed that the 2D layers of hBN that grow in triangular domains display nitrogen-terminated zigzag edges. Moreover, the observed high nonlinearity in the frequency range of the vibrational resonance highlights monolayer hBN as a promising material for frequency upconversion – from the infrared into the visible – in new optoelectronic devices.

Fruitful cooperation of two FHI Departments and international partners

This study is a testament to the highly collaborative research philosophy of the Fritz Haber Institute combining the expertise of multiple research groups from its different departments as well as its many international partners in joint projects to tackle major challenges in science. The hBN monolayer samples were synthesized at Vanderbilt University and then analyzed by sum-frequency microscopy at the Physical Chemistry Department at the Fritz Haber Institute. To support the advanced characterization of the samples, atomic force microscopy (AFM) images were recorded by collaboration partners in the Department of Physics at the Freie Universitat Berlin. Finally, incorporating the expertise of the Theory Department at the Fritz Haber Institute enabled the crystallographic details to be extracted.  

Prospects of the new experimental method

The newly developed microscope offers clear advantages over other existing methods. First and foremost, it can make optically transparent materials visible in an optical microscope. The resulting images have much higher contrast than traditional AFM images, and the use of the vibrational resonance enhancement enables “live imaging” of hBN, including online information on its crystal orientation. These advanced capabilities make a controlled fabrication of layered van der Waals structures feasible in future applications. Finally, the authors also anticipate an extension of this new imaging tool as non-invasive and label-free method to study a wider range of stacked 2D-materials as well as their combinations with anisotropic molecular assemblies.