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.
Abstract: A variety of two-dimensional materials have been reported in recent years, yet single-element systems such as graphene and black phosphorus have remained rare. Boron analogues have been predicted, as boron atoms possess a short covalent radius and the flexibility to adopt sp2 hybridization, features that favour the formation of two-dimensional allotropes, and one example of such a borophene material has been reported recently. Here, we present a parallel experimental work showing that two-dimensional boron sheets can be grown epitaxially on a Ag(111) substrate. Two types of boron sheet, a β12 sheet and a χ3 sheet, both exhibiting a triangular lattice but with different arrangements of periodic holes, are observed by scanning tunnelling microscopy. Density functional theory simulations agree well with experiments, and indicate that both sheets are planar without obvious vertical undulations. The boron sheets are quite inert to oxidization and interact only weakly with their substrate. We envisage that such boron sheets may find applications in electronic devices in the future.
Pub.: 28 Mar '16, Pinned: 20 Aug '17
Abstract: At the atomic-cluster scale, pure boron is markedly similar to carbon, forming simple planar molecules and cage-like fullerenes. Theoretical studies predict that two-dimensional (2D) boron sheets will adopt an atomic configuration similar to that of boron atomic clusters. We synthesized atomically thin, crystalline 2D boron sheets (i.e., borophene) on silver surfaces under ultrahigh-vacuum conditions. Atomic-scale characterization, supported by theoretical calculations, revealed structures reminiscent of fused boron clusters with multiple scales of anisotropic, out-of-plane buckling. Unlike bulk boron allotropes, borophene shows metallic characteristics that are consistent with predictions of a highly anisotropic, 2D metal.
Pub.: 19 Dec '15, Pinned: 20 Aug '17
Abstract: Two-dimensional boron sheets (that is, borophene) have recently been realized experimentally and found to have promising electronic properties. Because electronic devices and systems require the integration of multiple materials with well-defined interfaces, it is of high interest to identify chemical methods for forming atomically abrupt heterostructures between borophene and electronically distinct materials. Toward this end, we demonstrate the self-assembly of lateral heterostructures between borophene and perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA). These lateral heterostructures spontaneously form upon deposition of PTCDA onto submonolayer borophene on Ag(111) substrates as a result of the higher adsorption enthalpy of PTCDA on Ag(111) and lateral hydrogen bonding among PTCDA molecules, as demonstrated by molecular dynamics simulations. In situ x-ray photoelectron spectroscopy confirms the weak chemical interaction between borophene and PTCDA, while molecular-resolution ultrahigh-vacuum scanning tunneling microscopy and spectroscopy reveal an electronically abrupt interface at the borophene/PTCDA lateral heterostructure interface. As the first demonstration of a borophene-based heterostructure, this work will inform emerging efforts to integrate borophene into nanoelectronic applications.
Pub.: 07 Mar '17, Pinned: 20 Aug '17
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