Background:Nineteen years after graphene was first discovered, 2D materials remain the focus of research in order to overcome the challenges of nanoelectronics and explore new physics. This is mainly due to the two major properties of 2D materials: uniform thickness at the atomic scale (meaning tunability) and atomic planes without hanging bonds (which ensure the physical and chemical integrity of the material), which give them obvious advantages in the construction of various electronic optoelectronic devices. With a rare combination of tunable bandgap and high mobility, black phosphorus stands out among 2D materials as the most promising candidate for high-performance digital applications and beyond. In particular, black phosphorus nanoribbons exhibit excellent electrostatic gate control and can suppress the short-channel effect in nanoscale transistors. However, the controlled synthesis of black phosphorus nanoribbons is still an open question.
Research ResultsRecently,Yuanbo Zhang, Changlin Zheng, Yichen Song from Fudan University, Liping Ding from Shenzhen Advanced Research Institute, and Xianhui Chen from University of Science and Technology of China jointly reported a growth strategy that can directly grow black phosphorus nanoribbons in large areas on insulating substrates. The researchers used black phosphorus nanoparticles for chemical vapor transport (CVT) growth, and obtained uniform single-crystal black phosphorus nanoribbons with a thickness as low as three atomic layers and a width as low as 140 nm。Through structural calculations, it was found that the self-passivation of the zigzag edge is the key to preferential one-dimensional growth. A single nanoribbon FET exhibits an on-off ratio of up to 104, confirming the good semiconductor properties of the nanoribbons. This study demonstrates the potential of black phosphorus nanoribbons in nanoelectronic devices and also provides a platform for studying strange physical phenomena in black phosphorus.
The related research work was published in Nature Materials, a top international journal, with the title of "seeded growth of single-crystal black phosphorus nanoribbons".
What to studyFigure 1a shows a schematic diagram of the setup. Nanoparticle seeds allow black phosphorus to grow directly without nucleation at relatively low temperatures. Researchers can grow nanoribbons at a temperature of 470°C. Because growth takes place in slightly supersaturated red phosphorus steam, the growth pressure is primarily determined by the growth temperature. Therefore, during the growing process, the lower temperature will greatly reduce the pressure to 40 bar or less. The moderate temperature and pressure at which black phosphorus CVT is successfully grown makes it possible to produce black phosphorus nanoribbons on a large scale, potentially compatible with existing semiconductor technologies.
Figure 1CVT growth of black phosphorus nanoribbons inoculated by black phosphorus nanoparticles.
The seed CVT growth method is able to produce nanoribbons under the limitation of a few layers. Figure 2a shows thicknesses as low as 3A representative number of nanoribbons at 6nm (equivalent to about seven layers). So far, the thinnest nanoribbons identified by the researchers are about three layers. The width of these ribbons is about 100nm。These sizes are at least an order of magnitude higher than the best results in CVT growth in the previous black phosphorus zone.
Figure 1D shows a general view of a typical grow-state nanoribbon on a SiO2 Si substrate. The characteristic A1G, B2G, and A2G peaks of the Raman spectrum confirm that the nanoribbons are indeed black phosphorus (Figure 1C). AFM images of three typical nanoribbons are shown in the 2C-E figure, where the bands are uniform over their entire length, a feature that is particularly important for large-scale fabrication of integrated electronic optoelectronic circuits. AFM survey of all 374 nanoribbons in the labeled region in Figure 1D with an average thickness and width of 150±0.2nm and 510 10 nm (Figure 1e, f). The average aspect ratio of the nanoribbon is about 102, reaching a maximum of 103 (Figure 3a).
Figure 2AFM characterization of individual black phosphorus nanoribbons.
Figure 3A shows a complete view of a typical nanoribbon recorded in brightfield TEM. The nanoribbons are marked by three circles, about 40 nm wide and more than 30 m in length. Two-dimensional lattice fringes appear upon magnification, as shown in Figure 3b. The inset confirms that the nanoribbon has a black phosphorus orthorhombic crystal structure; The diffraction pattern also shows the growth axis along the [100] direction (Figure 3b). To further investigate the crystallinity of the entire nanoribbon, SAED was performed. Figure 3C-E shows three representative SAED patterns. All patterns show the same crystal orientation, except for the out-of-plane inclination of a few milliarc measurements. The near-perfect arrangement indicates that the entire nanoribbon is a single crystal. Figure 3F,G shows Haadf-STEM images of another typical color band taken with the [110] and [010] region axes, respectively. The pairs of phosphorus atoms are clearly resolved in the two projections (the relative inclination of the two projections is 17.).9°）。More than 33 nanobands were examined with TEM, all of which exhibited black phosphorus orthogonal structures along the [100] direction.
Figure 3TEM characterization of black phosphorus nanoribbons.
To further elucidate the growth mechanism, the researchers used flake seeds of non-black phosphorus nanoparticles for the growth of nanoribbons for CVT growth. As shown in Figure 4a, the nanoribbons do grow out of the edges of the lamellae. Surprisingly, however, growth occurs only at specific active sites at the edges, resulting in nanoribbons arranged parallel to each other. Similarly, no catalyst droplets were found at the free end of the nanoribbons. Figure 4d shows the extinction of the same black phosphorus lamellae shown in Figure 4a, at 0It is obtained as a function of the angle of polarization at the incident photon energy of 35 ev. Extinction does follow the CO2 relationship, which is expected in bisymmetric crystals such as black phosphorus (Figure 4D, red line). The lattice orientation of the lamellae is shown by the red arrows in Figure 4d, which exactly matches the orientation of the nanoribbons in Figure 4a; Measurements of all other lamellae seeds have come to the same conclusion. Apparently, nanoribbons grow uniformly in the [100] direction from thin slices; The active growth site may be the exposed edge of the (100) crystal plane.
Figure 4Mechanism of black phosphorus nanoribbon growth.
Single-crystal black phosphorus nanoribbons exhibit excellent electrical performance in transistor operation. FETs were fabricated directly on a single nanoribbon on a SiO2 Si substrate for growth, with degraded-doped Si used as a backgate. The length of the channel is 40A typical nanoband FET device at 0nm is shown in Figure 5d. When the applied gate voltage VG conducts the device on the hole side, the I-V characteristic exhibits linear behavior, indicating ohmic contact (Figure 4c). The on-off ratio of the FET is about 104 (Figure 5A), which is one of the highest ever reported values in black phosphorus FETs.
Figure 5Black phosphorus nanoribbon fets
Conclusions and prospectsIn conclusion, the researchers have achieved large-area growth of single-crystal z-shaped black phosphorus nanoribbons. Seed-vs growth occurs on the edge of the activated armchair and produces long nanoribbons suitable for the manufacture of high-performance FETs. The results of this study open the door to large-scale growth of high-density, arranged black phosphorus nanoribbon arrays for electro-optoelectronic applications. Once the jagged edges are properly activated, the study also increases the likelihood of growing 2D single crystal black phosphorus.
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