Phosphorene

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Bulk black phosphorus consists of multiple phosphorene sheets

Phosphorene is a two-dimensional single-layer material consisting of a phosphorus allotrope – black phosphorus. It is a natural semiconductor where electron flow can be switched “on” and “off” due to a non-zero fundamental band gap. Its two-dimensional structure makes it conceptually similar to the carbon-based graphene, hence the name phosphorene. Phosphorene is predicted to be a strong competitor to graphene because of phosphorene’s semiconducting properties absent in graphene, and with silicene (also a semiconductor) as silicene tends to self-destruct upon peeling layers of it off while phosphorene does not. Phosphorene was first isolated in 2014 by mechanical exfoliation.

History

Phosphorus was first discovered in 1669 by Hennig Brand, however, it has not attracted significant attention for almost a decade because of its strong toxicity and structural instability. Subsequently, in 1960s black phosphorus, a layered allotrope of phosphorus, was produced, which exhibited a high carrier mobility. In 2014, Ye group introduces the single-layer phosphorene, a monolayer of black phosphorus. It has since then attracted renewed attention because of its potential in optoelectronics and electronics due to its bandgap property, which can be tuned via modifying its thickness, anisotropic photoelectronic properties and its high carrier mobility. Phosphorene was initially prepared using mechanical cleavage, a commonly used technique in graphene production that is difficult to scale up. However, liquid exfoliation is a promising method for scalable phosphorene production.

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Scotch-tape-based microcleavage synthesis of phosphorene

Synthesis

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Liquid exfoliation based synthesis of phosphorene

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Phosphorene structure: (a) tilted view, (b) side view, (c) top view. Red (blue) balls represent phosphorus atoms in the lower (upper) layer.

Synthesis of phosphorene is a significant challenge. Currently, there are two main ways of phosphorene production: scotch-tape-based microcleavage and liquid exfoliation, while several other methods are being developed as well. Phosphorene production from plasma etching is also reported.

In scotch-tape-based microcleavage, phosphorene is mechanically exfoliated from a bulk of black phosphorus crystal using scotch-tape. Phosphorene is then transferred on a Si/SiO₂ substrate, where it is then cleaned with acetone, isopropyl alcohol and methanol to remove any scotch tape residue. The sample is then heated to 180 °C to remove solvent residue.

In the liquid exfoliation method first reported by Brent et al. in 2014 and modified by others, bulk black phosphorus is first ground in a mortar and pestle and then sonicated in deoxygenated, anhydrous organic liquidssuch as NMP under inert atmosphere using low-power bath sonication. Suspensions are then centrifuged for 30 minutes to filter out the unexfoliated black phosphorus. Resulting 2D monolayer and few-layer phosphorene unoxidized and crystalline structure, while exposure to air oxidizes the phosphorene and produces acid.

Another variation of liquid exfoliation is “basic N-methyl-2-pyrrolidone (NMP) liquid exfoliation”. Bulk black phosphorene is added to a saturated NaOH/NMP solution, which is further sonicated for 4 hours to conduct liquid exfoliation. The solution is then centrifuged twice, first for 10 minutes to remove any unexfoliated black phosphorus and then for 20 minutes at higher speed to separate thick layers of phosphorene (5–12 layers) from NMP. The supernatant then is centrifuged again at higher speed for another 20 minutes to separate thinner layers of phosphorene (1–7 layers). The precipitate from centrifugation is then redispersed in water and washed several times by deionized water. Phosphorene/water solution is dropped onto silicon with a 280-nm SiO₂ surface, where it is further dried under vacuum. NMP liquid exfoliation method was shown to yield phosphorene with controllable size and layer number, excellent water stability and in high yield.

Properties

Structure

Phosphorene 2D materials are composed of individual layers held together by van der Waals forces in lieu of covalent or ionic bonds that are found in most materials. There are five electrons on 3p orbitals of phosphorus atom, thus, giving rise to sp³ hybridization of phosphorus atom within phosphorene structure. Monolayered phosphorene exhibits the structure of a quadrangular pyramid because three electrons of P atom bond with three other P atoms covalently at 2.18 A° leaving one lone pair. Two of the phosphorus atoms are in the plane of the layer at 99° from one another, and the third phosphorus is between the layers at 103°, yielding an average angle of 102°.

According to density functional theory (DFT) calculations, phosphorene forms in a honeycomb lattice structure with notable nonplanarity in the shape of structural ridges. It is predicted that crystal structure of black phosphorus can be discriminated under high pressure. This is mostly due to the anisotropic compressibility of black phosphorus because of the asymmetrical crystal structures. Subsequently, the van der Waals bond can be greatly compressed in the z-direction. However, there is a great variation in compressibility across the orthogonal x-y plane.

It is reported that controlling the centrifugal speed of production may aid in regulating the thickness of a material. For example, centrifuging at 18000 rpm during synthesis produced phosphorene with an average diameter of 210 nm and a thickness of 2.8 ± 1.5 nm (2–7 layers).

Band gap and conductivity properties

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Atomic force microscopy images of few-layer phosphorene sheets produced by ultrasonic exfoliation of black phosphorus in N-methyl-2-pyrrolidone and spin-coated onto a SiO₂/Si substrate.

Phosphorene has a thickness dependent direct band gap that changes to 1.88 eV in a monolayer from 0.3 eV in the bulk. Increase in band gap value in single-layer phosphorene is predicted to be caused by the absence of interlayer hybridization near the top of the valence and bottom of the conduction band. A pronounced peak centered at around 1.45 eV suggests the band gap structure in few- or single-layer phosphorene difference from bulk crystals.

Phosphorene can be tuned to superconductivity with electron doping. Superconductivity starts showing up in single layer phosphorene when electron doping concentration is above 1.3×10¹⁴ cm⁻². Higher doping concentrations raises the transition temperature, which saturates at 11.2 K when doping concentration reaches 7.2×10¹⁴ cm⁻².

Air stability

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Atomic force microscopy (AFM) 3D image of few-layer phosphorene sample continuously taken for 7 days. Phosphorene reacts with oxygen and water to develop liquid phase bubbles.

One major disadvantage of phosphorene is its air-stability. Composed of hygroscopic phosphorus and with extremely high surface-to-volume ratio, phosphorene reacts with water vapor and oxygen assisted by visible light to degrade within the scope of hours. Through the degradation process, phosphorene (solid) reacts with oxygen/water to develop liquid phase acid 'bubbles' on the surface, and finally evaporate (vapor) to fully vanish (S-B-V degradation) and severely reducing overall quality.

Applications

Transistor

Researchers have fabricated transistors of phosphorene to examine its performance in actual devices. Phosphorene-based transistor consists of a channel of 1.0 μm and uses few layered phosphorene with a thickness varying from 2.1 to over 20 nm. Reduction of the total resistance with decreasing gate voltage is observed, indicating the p-type characteristic of phosphorene. Linear I-V relationship of transistor at low drain bias suggests good contact properties at the phosphorene/metal interface. Good current saturation at high drain bias values was observed. However, it was seen that the mobility is reduced in few-layer phosphorene when compared to bulk black phosphorus. Field-effect mobility of phosphorene-based transistor shows a strong thickness dependence, peaking at around 5 nm and decrease steadily with further increase of crystal thickness.

Atomic layer deposition (ALD) dielectric layer and/or hydrophobic polymer is used as encapsulation layers in order to prevent device degradation and failure. Phosphorene devices are reported to maintain their function for weeks with encapsulation layer, whereas experience device failure within a week when exposed to ambient condition.

Inverter

Researchers have also constructed the CMOS inverter (logic circuit) by combining a phosphorene PMOS transistor with a MoS₂ NMOS transistor, achieving high heterogeneous integration of semiconducting phosphorene crystals as a new channel material for potential electronic applications. In the inverter, the power supply voltage is set to be 1 V. The output voltage shows a clear transition from VDD to 0 within the input voltage range from −10 to −2 V. A maximum gain of ~1.4 is attained.

Solar-cell donor material (optoelectronics)

The potential applications of mixed bilayer phosphorene in solar-cell material was examined as well. The predicted power conversion efficiency for a monolayer MoS₂/AA-stacked bilayer phosphorene and MoS₂/AB-stacked bilayer phosphorene can get as high as ~18% and 16%, respectively. Results suggest that trilayer MoS₂ phosphorene is a promising candidate in flexible optoelectronic devices.

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Illustration of the bottom gated flexible few-layer phosphorene transistors with the hydrophobic dielectric encapsulation.

Flexible circuits

Phosphorene is a promising candidate for flexible nano systems due to its ultra-thin nature with ideal electrostatic control and superior mechanical flexibility. Researchers have demonstrated the flexible transistors, circuits and AM demodulator based on few-layer phosphorus, showing enhanced am bipolar transport with high room temperature carrier mobility as high as ~310 cm²/Vs and strong current saturation. Fundamental circuit units including digital inverter, voltage amplifier and frequency doubler have been realized.

See also

• Borophene
• Germanene
• Graphene
• Silicene
• Stanene