Black-Phosphorus: Understanding New Quantum Materials

Title: Bright Mid-Infrared Photoluminescence from Thin-Film Black Phosphorus
Authors: Chen Chen, and Fengnian Xia et al.
Publication Info: Nanolett., 2019, Article ASAP, DOI: 10.1021/acs.nanolett.8b04041

To keep advancing modern electronics, scientists continue to explore nanoscale materials. One of the new exciting materials in the field of optoelectronics is black phosphorous. Black phosphorus is an allotrope (one type) of phosphorus, but it is unique since it has strong luminescence (the “glow” of a material) coupled with electrical conductivity. These properties make black phosphorus potentially useful for light-emitting diodes (LEDS) as well as lasers.

To enable these applications, more fundamental work needs to be done to understand black phosphorus. One of the most important features of black phosphorus is that the luminescence is in the mid-infrared wavelength range, which is not common.  This paper steps back from applications and investigates the fundamental optical properties of black phosphorous, focusing on this mid-infrared emission.


Figure 1. Image of black phosphorus. This file is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.

Nanoscale properties

Nanoscale thin films of black phosphorus, with thicknesses ranging from 4.5 to 46 nm, were studied in this report to investigate size dependence. To give an idea of these scales, an atom is roughly 0.1 nm, while a piece of paper is 100,000 nm thick. This is a very thin material, where the layers of phosphorous atoms range from ~10 to ~90 layers.

At these size scales, materials begin to interact with light in new ways. This is exhibited in black phosphorus by the change in color of the luminescence at the nanoscale. The effect of size dependent luminescence is due to quantum confinement. The quantum confinement effect is observed as a material becomes smaller and smaller into the nanoscale. Because of quantum confinement, the wavelengths/color of a materials luminescence shifts to higher energy (called a blue-shift) as the material gets smaller.

This size dependence of luminescence is observed in black phosphorus, as demonstrated in Figure 2. When the black phosphorus is extremely thin, the band gap, which corresponds to the wavelength of the luminescence, significantly increases in energy, typical of a quantum confined material.

Figure 2. (Left) the size dependent emission of black phosphorous, where the thinner materials have higher energy emission. (Right) A curve showing the relationship of the number of layers with band gap.

How does it glow?

To understand more about black phosphorus, the strength of its luminescence was measured relative to a standard, well-understood compound, indium arsenide. It was about 7X weaker then indium arsenide, which is still very strong (Figure 3, Left), indicating desirable properties for LEDs and lasers.

In addition to standard luminescence, black phosphorus has an interesting property of its directionality. Black phosphorus has a layered crystal structure, where the atoms pack in layer like sheets (Figure 3). This layering leads to a phenomenon called anisotropy, which means that its properties are direction-dependent.

Figure 3. (Left) Luminescence of Black phosphorous relative to Indium Arsenide. (Middle) The crystal structure of black phosphorous, showing its layered orientation. (Right) The yellow line (y) shows decreased luminescence relative to the red line (x), due to the orientation of a polarizing filter

The anisotropy of black phosphorus manifests itself in something called light polarization. The light that black phosphorus emits is all orientated in one direction. This effect is called linear polarization. To better think about this, remember light has wavelike properties. You can think of unpolarized light as a busy swimming pool, where all the splashing makes waves in every direction. Polarized light is more like a wave pool, where all the waves go in one direction.

Due to the anisotropy of black phosphorus, the light it emits is linearly polarized. This is observed by using a polar filter and orientating it in various positions to test the emission intensity. In the correct orientation (deemed armchair or (x) in this material) the emission intensity is strong (Figure 3), where when the filter and orientation don’t match there is low intensity emission (deemed zigzag or y).

Black Phosphorus Future

Now you know a little bit about black phosphorous. It has strong luminescence and can be made into quantum confined materials. It also exhibits linearly polarized light emission, in part due to its layered crystal structure. These properties of black phosphorous can be useful then for lots of light emission applications including sought after mid-infrared lasers.

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