New properties with old materials

The tunable direct bandgap, higher carrier mobility, and unique in-plane anisotropy of layered black phosphorous (BP) make it a promising candidate in design and optimization of electronics and optoelectronic devices with excellent performance. Although recent studies and perspectives aim at bringing this material to a level of maturity, the lack of wafer scale production and the surface reactivity of BP have hindered fully industrial processes. Here, we address the probable solutions to overcome these challenges.

In the past ten years after the discovery of graphene, as a result of its excellent electrical, mechanical, and optical properties, graphene have been proposed for use in various fields ranging from flexible electronics to energy conversion and storage [1] [2]. However, the absence of a natural energy bandgap hinders its application in electronic switches with high performance. This has led to an extensive search for other materials such as transition metal dichalcogenides (TMDCs), which can be isolated to two dimensions (2D) and have a sizeable bandgap [3]. The controled synthesis and utilization of these 2D TMDCs in photodetectors, solar cells, and light-emitting devices have demonstrated the potential to close the technological gap left by graphene [4]. However, the low charge carrier mobility of TMDCs restricts their performance. Scientists have never stopped seeking alternative materials that can bridging the gap between graphene and TMDCs. Until 2014, when a presentation at the APS March meeting and the following publication indicated that a new era in material science is coming – black phosphorus [5].

Black phosphorus (BP), which was discovered in 1914, is the most stable allotrope of phosphorus. Single layer BP consists of two-atomic layers and forms a puckered honeycomb lattice. Each phosphorus bonds to three neighboring atoms, leaving a lone pair which makes it reactive (see A and B in figure). The interlayer is weakly connected by van der Waals forces, while the in-plane P atoms are strongly covalently bonded. Similar to graphite, it can be mechanically exfoliated to atomically thin layers by adhesive tapes. Furthermore, besides graphene, phosphorene would be the second real elemental two-dimensional material stable in free-standing form.

BP is a semiconductor with a direct bandgap, the value of which increases with decreasing the layer thickness. The direct bandgap and layer dependent features provide the possibility to optimize the bandgap value by choosing the right thickness [6]. The bandgap of BP makes it a promising candidate for near and midinfrared optoelectronics (C), as well as thermal related applications such as imaging and power generation [7]. On the other hand, the carrier mobility of BP (1000 cm² V⁻¹ s⁻¹) also lies between graphene and TMDCs (D). The unique property of BP is its anisotropic band dispersion in the Brillouin zone, which originates from its in-plane structural anisotropy, and leads to an anisotropic effective mass. Consequently, the electric conductance and electron mobility exhibit anisotropy as well, which are responsible for the dramatically enhanced thermoelectric performance in phospherene [8].

However, two main challenges hinder the move of BP from lab to real life: (i) large scale, high quality, and thickness-controlled growth on arbitrary substrates; and (ii) effective surface protection to prevent BP from degradation under ambition conditions. Firstly, in the case of graphene, it took five years to develop an effective way of producing large-scale, single-layer graphene by using chemical vapor deposition (CVD). In the case of TMDCs, the development of the CVD method instead of the traditional mechanical isolation came two years later. Although the general scalable approach to synthesizing large-area BP requires high pressure [9], it is predictable that CVD growth of BP in the two-dimensional phase can be realized quite soon [10].

The precursors used for graphene growth are generally hydrocarbons, such as CH₄, C₂H₄, and C₂H₂. The growth of MoS₂ utilizes MoO₃ and S powder or H₂S as the precursors. However, in the case of borophene [11], silicene, germanene, and stanene, elementary substances are thermally evaporated onto substrates. Until now, silicene was only continuously grown on the Ag(111) surface [12], but borophene spreads in isolated patches on Ag(111) [13]. Germanene has been successfully synthesized on Au(111) [14], Pt(111) [15], and Al(111) [16]. However, there is only one report of the growth of stanene on topological insulators Bi₂Te₃ [17]. In the case of phosphorene, most phosphorous precursors are highly toxic, which is not suitable for CVD fabrication [18].

A recent study on the synthesis BP thin films on flexible polyester substrates utilized the ancient way of transferring red phosphorous to BP in a high-pressure multi-anvil cell at RT [9]. Analogous to the epitaxial growth of blue phosphorene by depositing of elemental phosphorous directly on the substrate followed by annealing [19], layer-controlled BP could be fabricated in a similar way, while suitable supporting substrates are necessary to ensure the precise growth because of the many structural varieties of phosphorus.

Secondly, various surface encapsulation/passivation methods have been developed using organic molecules, polymers, metal oxides, and boron nitride. The degradation mechanism of BP has been uncovered, indicating that oxygen plays the dominant role and that BP is stable in oxygen-free water [20]. Sandwiching BP between two hexagonal boron nitride layers can protect it and give rise to a clean interface, realizing a high mobility and on-off ratio [21]. However, this method strongly relies on the quality of the exfoliated samples, which may not be suitable for large scale production. Al₂O₃ capping [22] or hydrophobic fluoropolymer protection can dramatically improve the stability of BP in ambient conditions. Alternatively, the commercial success of organic molecular beam epitaxy has been broadly used in the fabrication of organic light-emitting diodes and photovoltaic devices. These developments have opened up the possibility of realizing a new family of electronics that can blend the chemical versatility of organic materials with well-defined functionalities [23]. Consequently, blocking the top surface of BP by depositing organic molecules that can form uniform layers would be a promising approach to realizing surface protection/passivation. Recently, the Hersam group at Northwestern University developed an effective surface passivation method that chemically modifies exfoliated BP using 4-nitrobenzene-diazonium (4-NBD) and 4-methoxybenzenediazonium (4-MBD) tetrafluoroborate salts to form surface covalent C-P bonds. This approach can significantly suppress chemical degradation and improve the field-effect transistor mobility and on/off current ratio as well [24].

Finally, integrating BP with well-established optoelectronic devices based on graphene and TMDCs paves the way to the commercial viability of two-dimensional materials with multi-functionality [25]. Furthermore, stacking these materials in a controlled sequence creates heterostructures, leveraging the desirable advantages of each component. This offers another promising approach to the design and fabricate of novel devices such as photodetectors and photovoltaic devices. However, the performance of these devices is significantly affected by the electrical contacts with external circuitry. As such, optimizing the geometry of the interface and selecting proper electrode materials, such as forming covalent bonds, can enable good electrical contacts that harness the superiority of two-dimensional materials [26].

Overall, the tunable direct bandgap, higher carrier mobility, and unique anisotropic properties make phosphorene a promising material for the realization of conceptually new devices. The proposed wafer-scale growth, surface-passivation methods, and development of novel devices based on heterostructures will make more breakthroughs in electronics and optoelectronics applications.

Acknowledgements

The author is from Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, 865 Changning Road, Shanghai 200050, China. Present address: Brookhaven National Laboratory, Upton, NY 11973, USA.

The author would like to thank the support from Natural Science Foundation of China under contract Nos.. 11227902, 21403282, and the Strategic Priority Research Program (B) of the Chinese Academy of Sciences, Grant No. XDB04040300. The author thanks Dr. Miao Zhou for helpful discussion.

This paper was originally published in Nano Today 12 (2017), doi: 10.1016/j.nantod.2016.08.013

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