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Black Phosphorus Gets a Renaissance in Two Dimensions

 

An interview with a scientist at the forefront of black phosphorus research

 

In a little over 18 months, black phosphorus—sometimes referred to as phosphorene—has gone from relative obscurity, lurking in the back catalogues of chemical producers for a nearly a century, to taking much of the limelight away from graphene as the next big thing in two-dimensional materials. 

 

So, what is it all about? To give you some insight we have interviewed Philip Feng, an assistant professor of electrical engineering and computer science at Case Western Reserve University (CWRU), and a researcher in the field, whose team has demonstrated some of the first black phosphorus mechanical and electronic devices.

Feng and his colleagues draped atom-thick layers of black phosphorus and molybdenum disulfide over cavities to create a new type of resonator. See here and here. While others have made drumhead resonators, this marked the first time that anyone had done so with semiconducting 2D materials.

Stretching a semiconductor material alters its band gap, and since in this application the 2D materials are stretched significantly, the resonators operate across a wider range of frequencies than conventional crystal-based resonators. 

This work with resonators that have band gaps could all ultimately lead to way to couple mechanical vibrations to electrical and optical behavior.

 

  • When did research with black phosphorus really begin to take off? What was the trigger?

FENG: The excitement quickly got heated up since early 2014.  The first black phosphorus 2D field-effect transistors (FETs) were reported by a collaboration with leading groups in China. While this first paper brought a lot of attention to the field, several other teams – probably also have been exploring this new 2D crystal, in parallel and in competition – quickly reported competitive results, and several papers appeared on arXiv.org within weeks, and the growth went exponential afterwards.  

It should also be noted that as a semiconductor, bulk black phosphorus and its synthesis has been known for almost a century. The recent rapid growing interest has mainly focused on ultrathin or 2D form of black phosphorus, as now layers of this crystal can be isolated from the bulk, in the similar fashion graphene was extracted from graphite.  

 

  • What are the advantages of black phosphorus over other 2D materials? Since a natural band gap seems to be one of those advantages, could you discuss how much better its band gap is compared to other 2D materials (is it narrower, wider, does it make itself more usable for certain kind of electronics, etc.)?  

FENG: There are several main features and potential advantages.  First is its direct bandgap (~0.3 to 2.0 electronVolts (eV)) covers a regime otherwise unavailable from all other recently known 2D layered materials.  It bridges the gaps between the bandgap ranges of graphene (zero bandgap) and transition metal di-chalcogenides (TMDCs, ~1.5 to 2.5eV).  Not only does it bring infrared functions to 2D optoelectronic nanodevices, but also the direct bandgap is highly tunable with number of layers, greatly enhancing the possibilities for new devices.  

Second it is an elemental semiconductor with high electron field-effect mobility, much higher than those obtained in similar molybdenum disulfide field-effect transistor devices under general conditions at room temperature.  

Third is the material’s intrinsic, strong in-plane anisotropy that is unique, and not readily found in other 2D crystals derived from layered materials.  This can be manifested in the optical, electrical, thermal, and mechanical properties of the resulting devices, and has potential for a number of novel applications.  

 

  • Producing black phosphorus seems to be fairly problematic. What are the hurdles in ramping up bulk production? And where are we in the state-of-the-art of its production?  

FENG: Currently, black phosphorus crystal is synthesized from red phosphorus under high pressure (1gigapascal) and high temperature (1000 Celsius).  The resulting small crystals (mm sizes) are exfoliated into small flakes for making black phosphorus nanostructures and nanoscale devices.  In order to achieve large-scale, ideally roll-to-roll production, a method of uniformly growing/coating black phosphorus on substrates needs to be devised and validated first.  

 

  • At this point, what appears to be the most promising applications for black phosphorus (both inside out of electronics and outside of it) and what needs to be done to make those applications within reach?  

FENG: The exploration of black phosphorus device physics and engineering is still in its infancy, with a lot of unique properties to be understood and exploited in device-oriented nano/microstructures.  Currently, many researchers have been focusing on nanoelectronics and nanophotonics – on demonstrating basic devices and functions in these areas.  For example, infrared optoelectronics appears to be one obvious answer for applications; nonetheless extensive device engineering is required to make the device performance competitive (compared to exiting products) and ensure the black phosphorus crystal remains stable (against degradation), in addition to the challenges in fabrication mentioned above (i.e., how to produce black phosphorus of a given layer number uniformly over a large area in a scalable process).  

In addition to black phosphorus 2D nanoelectronics and nanophotonics, we believe that the unique mechanical and coupled electromechanical properties of black phosphorus can have strong potential for enabling novel mechanical devices and transducers, especially new 2D nanoelectromechanical systems (NEMS).  Recently we have demonstrated the first high-frequency black phosphorus NEMS resonators (see here).  Very interestingly, we have also found that the unique in-plane anisotropy of black phosphorus 2D crystals will lead to totally new elastic and frequency scaling capabilities in multimode black phosphorus NEMS resonators.  (see here.)

 

 

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