Physical characteristics of the self-passivated single vacancy (SV). (a) High resolution STM image of the self-passivated SV showing that it adopts a butterfly-shaped feature spanning over two of its nearest neighbours. (b) Atomic structure of the self-passivated SV with labelled crystallographic directions and the corresponding side-view (bottom panel). The yellow (violet) atoms indicate the phosphorus (P) atoms at the top (bottom) sublayers. (c) Atom-resolved nc-AFM image of the self-passivated SV, clearly showing that one P atom has been removed. (d) Simulated nc-AFM image of the self-passivated SV showing agreement with experimental results. Credit: Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.128.176801
Black phosphorus (BP) is a two-dimensional (2D) semiconducting material that exhibits an electronic self-passivation phenomenon by rearranging its vacancy defects. NUS scientists discovered that BP exhibits this phenomenon. As a result, the material and its analogues may have a higher charge mobility.
Ultra-thin, high-speed, and energy-efficient electronic and optoelectronic devices require 2D semiconductors with high carrier mobility. Due to existing processes for 2D semiconductors, surface defects, particularly vacancies with dangling bonds, are inevitably introduced. For example, defects can act as undesirable sinks for charge carriers and nonradiative recombination centres for photoexcited electron-hole pairs, reducing device performance. High-mobility 2D semiconductor materials must therefore be effectively protected from surface vacancies in order to maintain their high-performance device characteristics. BP is a high-mobility 2D material with numerous applications in optoelectronics and photovoltaics. Its defect passivation behaviours are distinct from those of other 2D semiconductors made up of two or more elements because it only contains one element (for example, metal chalcogenides).
It was found that a single vacancy (SV) on surface of BP is negatively charged and ionised, leading to passivation of dangling bonds and making the SV electrically inactive. This was accomplished by using both scanning tunnelling microscopy and nc-AFM techniques, led by Associate Professor Jiong LU from National University of Singapore’s Department of Chemistry. Homo-elemental hypervalent bonding is a special type of chemical bond that forms at the defect site during self-passivation and can be triggered by thermal annealing or STM tip manipulation (Figure a-d) (see Figure b). The Yale-NUS College’s research group of Assistant Professor Aleksandr RODIN and the Institute of Physics of the Czech Academy of Sciences’ Professor Pavel Jelnek collaborate on this project.
This self-passivation effect of the SV was evaluated in a study published in Physical Review Letters by measuring a field-effect transistor (FET) device made of the semiconductor BP. Before and after self-passivation at the defect site, they compared the local electronic structure and scattering behaviour. FET device performance improved by up to 43 percent when the self-passivation mechanism was activated, according to the researchers. This is most likely because the dangling bonds at the defect site have been inactivated and the electronic states associated with them have been quenched.
Chemical functionalization and surface coating, both developed in the semiconductor industry, have been used to remove the harmful in-gap electronic states associated with surface vacancies in 2D semiconductors. Most passivation methods currently in use only improve the photoluminescence quantum yield, not the charge transport properties of semiconductors. Some go so far as to alter the molecular (van der Waals) structure that they degrade the electronic performance.
Professor Lu made the following statement: “The new passivation strategy reported may represent an ideal surface passivation strategy, which can selectively deactivate only the defect states without leaving a permanent crystal lattice change and degradation of the electronic performance, unlike conventional methods. An important aspect of our work is the discovery of a new route for electronic self-passivation of defects in BP and its analogues.”
Further information: Hanyan Fang et al, Electronic Self-Passivation of Single Vacancy in Black Phosphorus via Ionization, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.128.176801
Journal information: Physical Review Letters
Source: National University of Singapore