A pinboard by
Xiaolong Liu

Graduate Student, Northwestern University


By using ultrahigh vacuum scanning tunneling microscopy, I examine borophene at the atomic level

My current research primarily involves ultra-high vacuum (UHV) scanning tunneling microscopy/spectroscopy (STM/STS) of novel two-dimensional (2D) nanomaterials, which are atomically thin films. The overall goal is to realize applications in a new generation of electronics and optoelectronics. Specifically, my objectives include the creation of functionalized 2D heterostructures composed of different 2D materials followed by atomic-level characterization by UHV STM/STS, which is a powerful technique based on quantum tunneling effect.

2D boron, known as borophene, was recently synthesized and determined to be metallic and highly anisotropic, which distinguishes it from other 2D materials. Due to its high chemical reactivity, current development necessitates in situ UHV growth. By tuning the growth conditions, I have been able to control both the surface coverage and the atomic structure by targeting different borophene polymorphs. I have been working on chemical functionalizations of borophene and creating borophene-based heterostructures via depositing organics in UHV. In situ characterization at the atomic scale reveals the resulting interfacial structures and effects of structural defects. The associated electronic effects at the heterojunction interface including band-bending are readily revealed by STS. Meanwhile, in situ X-ray photoelectron spectroscopy (XPS) provides complementary chemical information at each stage of sample preparation.

To illustrate the aforementioned methodology, I will delineate one project in more detail. I have been able to fabricate lateral heterostructures composed of borophene and the n-type organic semiconductor perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA). After evaporation of PTCDA onto submonolayer borophene on silver substrates, PTCDA preferentially self-assembles into highly ordered domains on silver, resulting in a lateral heterostructure with borophene. STS reveals the formation of electronically abrupt heterojunction interfaces with a transition distance of ~1 nm (i.e., ~the size of a PTCDA molecule). The realization of a borophene heterostructure is a step forward toward realistic applications of borophene. However, direct transport measurements entail development of passivation and transfer methods that allow the removal of the substrates. Current research efforts include additional covalent functionalization strategies that can alter the chemical reactivity of borophene.