Extraordinarily Bound Quasi-One-Dimensional Trions in Two-Dimensional Phosphorene Atomic Semiconductors

Abstract

The anisotropic nature of the new two-dimensional (2D) material phosphorene 1-9 , in contrast to other 2D materials such as graphene 10 A neutral exciton is a bound quasi-particle state between one electron and one hole through a Coulomb interaction, similar to a neutral hydrogen atom. A trion is a charged exciton composed of two electrons and one hole (or two holes and one electron), analogous to H -(or H2 + ) 20 . Trions have been of considerable interest for the fundamental studies of many-body interactions, such as carrier multiplication and Wigner crystallization 21 . In contrast to the exciton, a trion has an extra charge with nonzero spin, which can be used for spin manipulation 22,23 . More importantly, the density of trions can be electrically tuned by the gate voltage, enabling remarkable optoelectronic applications [18][19][20]24,25 . For these purposes, a large trion binding energy is critical in order to overcome the room-temperature thermal fluctuations as well as to widen the spectral tuning range. The dimensional confinement is the dominating factor that determines the binding energy of trions. In quasi-2D quantum wells, the trion binding energy is only 1-5 meV, and trions are highly unstable, except at cryogenic temperatures 16,17 . The complete separation of the exciton and trion emission peaks was observed at room temperature 16,17 . However, the application of 1D carbon nanotubes for practical optoelectronic devices is intrinsically limited by their small cross-sections. The overall optical responses of such 1D lines are extremely weak. The diverse distribution of the chirality in carbon nanotubes also makes it impossible to assemble a large-size film with uniform optoelectronic responses.While the reduced dimensionality leads to far more attractive exciton and trion properties, the trade-off between the cross-section and the dimensional confinement has hindered the development of useful excitonic optoelectronic devices.Here, we show that phosphorene presents an intriguing platform to overcome the aforementioned trade-off. We observed quasi-1D trions with ultra-high binding energies up to This new type of material, few-layer phosphorene, is unstable and does not survive well in many standard nanofabrication processes. To overcome the challenge of the instability, we designed special fabrication and characterization techniques. We used mechanical exfoliation to drily transfer 26 a phosphorene flake onto a SiO2/Si substrate (275 nm thermal oxide on n + -doped silicon). The phosphorene was placed near a gold electrode that was pre-patterned on the substrate. Another thick graphite flake was similarly transferred to electrically bridge the phosphorene flake and the gold electrode, forming a MOS device (Figure 1). This fabrication procedure kept the phosphorenes free from chemical contaminations by minimizing the postprocesses after the phosphorene flake was transferred. In the measurement, the gold electrode is grounded, and the n + -doped Si substrate functions as a back gate providing uniform...

Defect Engineering in Few‐Layer Phosphorene

“…The 3L phosphorene sample on thermal oxide showed a PL peak at ≈1250 nm (designated as A peak, Figure b), which is consistent with our previous reports . In contrast, the 3L phosphorene sample showed two clearly distinctive peaks at 1250 and 1430 nm, assigned as A and X peak, respectively.…”

“…To further explore the nature of the localized excitons, we performed incident polarization dependent PL measurements on these peaks by changing the incident angle Ɵ . Interestingly, both the A and X peaks exhibited anisotropic behavior and showed the same polarization direction along the armchair direction of the phosphorene crystal (Figure S2a,b, Supporting Information) . This further suggests that the localized states are caused by point‐defects, like O adatoms absorbed on the phosphorene PECVD interface, since point‐defects would not change the polarization of the exciton emissions …”

“…Phosphorene serves as a perfect alternative material to bridge the gap between zero‐bandgap graphene and large‐bandgap transition metal dichalcogenide (TMD) semiconductors for suitable applications in the infrared wavelength range. Owing to its puckered lattice configuration, phosphorene possesses quasi‐1D excitons and trions with highly enhanced binding energies, which is in contrast to other TMD 2D semiconductors and will enable several promising novel optoelectronic and excitonic devices. Defect engineering has been demonstrated to be an important technique to modulate the properties of semiconductors for various applications .…”

Black phosphorus: a two-dimensional reductant for in situ nanofabrication

“…18,19 BP has strong in-plane bonds in conjunction with weak van der Waals interlayer interactions thereby enabling exfoliation into few-layer BP sheets or phosphorene (single-layer BP). [20][21][22][23][24] Since 2014, BP has attracted tremendous research interest [25][26][27][28][29][30][31][32][33][34][35][36][37][38] because of its unique properties such as layerdependent direct bandgap, 28,39 anisotropy, 25,26,31,32,34,40 hingelike structure, 41 and quasi-one-dimensional excitonicnature, 42 all of which contrast markedly with those of other types of 2D materials. Nevertheless, BP degrades under ambient conditions as a result of the reaction with oxygen (P→P x O y ) and subsequent transformation to PO 4 3-in the presence of water.…”

Excitons and optical spectra of phosphorene nanoribbons

“…Furthermore, the phosphorene unique anisotropic structure makes the particular behavior of the layerdependent photoluminescence [9] and quasi-one dimensional excitons [8,10,11]. This leads to a unique 2D platform for investigation of the dynamics of excitons in nanoribbon structure together with the many-body impacts.…”