Black Phosphorus: Pristine and doped surface investigations using Scanning Tunneling Microscopy

Black Phosphorus (bP) is the most stable allotrope of phosphorus, first synthesized in 1914 by Bridgman. It was investigated along with other layered materials like GaS, GaSe, GaTe, graphite, boron nitride and transition metal dichalcogenides for nearly a century. One important characteristic of these layered materials is that they are composed of twodimensional (2D) sheets of covalently bonded atoms that are kept together by van der Waals forces. Therefore, they were perceived as interesting materials with an ambition to achieve thinner and thinner materials, up to a single monolayer, called 2D materials. In 2004, Andre Geim and Konstantin Novoselov demonstrated the first successful preparation of one-atomic-thin carbon films, called graphene. They used the technique of scotch tape exfoliation and studied the marvelous properties of graphene, which got them the Nobel Prize in physics in 2010. With the discovery of interesting properties of graphene, a search for other 2D materials started. A range of such materials have been realized since then, like hexagonal boron nitride (h-BN), silicene, germanene, stanene, and transition metal dichalcogenides (TMDCs). Black Phosphorus (bP) is an important part of this class of 2D materials, from which phosphorene is exfoliated. Phosphorene, first exfoliated in 2014, has emerged as an important material. With its band gap tunable with thickness (from 2.0 eV for the single layer to 0.3 eV for the bulk), it occupies a special position between zero band gap graphene and high band gap TMDCs. Anisotropy is another important aspect discovered in its properties like effective mass, mobility, thermal conductivity, and plasmon resonance. This opens a gate for potential applications in electronics, photonics, thermoelectrics, and for gas sensing devices. Surface studies of bP are quite limited so far. There have been some works reported showing bP atomic resolution and tunneling spectra on bP surfaces. However, most of them have been performed on cleaved bulk bP crystals. From a 2D application point of view, however, it is important to study thin exfoliated bP flakes. The majority of work reported until now on thin bP flakes concerns electrical transport or optical experiments, performed on flakes encapsulated in a protective layer. The reason for this is to protect the material from oxidation upon exposure to air, since bP is known to be highly reactive. One STM study on a few nanometer-thin bP flake shows surface atomic resolution, but mainly focuses on the spectroscopic properties of bP. Since surface plays an important role in nanomaterials due to the high surface-to-volume ratio, it is very important to understand the surface behavior of such materials. Here, we have prepared samples on which surface studies on exfoliated bP flakes could be done. We have prepared samples by exfoliating bP in a glove bag continuously flushed with nitrogen, which provides an inert atmosphere and protects the reactive bP surface from oxidation. We have used graphene on silicon carbide (SiC) as a substrate, which provides large atomically flat terraces of SiC and a conducting graphene sheet on top – fulfilling major requirements for scanning tunneling microscopy (STM) experiments. We mount the sample inside the glove bag and transfer it under inert atmosphere, which helps in preserving the highly reactive bP surface from oxidation. This is confirmed by flat, clean, and oxide-free bP surfaces as seen in STM. We initially started with the investigation of surface behavior with temperature. We found that 200 ◦C to 300 ◦C is a proper temperature range for cleaning the surface. We saw defects on the clean bP surface, reported earlier to be the reason for the intrinsic p-type doping of bP. At 375 ◦C to 400 ◦C, eye-shaped craters started to develop on the surface due to phosphorus desorption. Meanwhile, we solved an existing debate in the literature regarding the orientation of the long axis of these elongated craters: along the crystallographic armchair direction due to an atomic phosphorus desorption mechanism, or along the crystallographic zigzag direction due to a molecular P2 desorption mechanism, both investigated by electron microscopy and diffraction studies. Armed with the power of atomic resolution imaging enabled by the STM technique, we were able to resolve smaller craters, which are the seeds of the larger craters reported in previous studies. With a statistical analysis, we confirmed the specific directionality of the orientation of these anisotropic craters. Furthermore, with the help of atomic resolution provided by STM, we could directly compare the crater alignment with respect to atomic arrangement, and found that the long axis of the craters is aligned with the zigzag direction – thus solving the existing debate in the literature. bP is intrinsically p-doped. Some works were performed to obtain an n-type behavior by doping, as it would allow p-type and n-type behavior in the same material, very useful for basic diode applications. In one paper focusing on devices, copper adatom doping has been shown to yield n-type behavior, using transport measurements. Here, we study copper growth morphology on bP. We observed the preference of copper atoms to occupy atomic vacancies of the bP surface. We also observed an alignment of copper islands along the crystallographic armchair direction of bP, and a step decoration of copper islands at bP step edges. With scanning tunneling spectroscopy, we studied the transfer doping of bP by copper at the local atomic level and found a shift of the Fermi level in bP from p-type behavior to n-type behavior. We also observed an increase in the band gap value measured on doped bP, consistent with DFT calculations.