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Hetero-Interfaces in 2D-Based Semiconductor Devices
Published in Mohammad Karbalaei Akbari, Serge Zhuiykov, Ultrathin Two-Dimensional Semiconductors for Novel Electronic Applications, 2020
Mohammad Karbalaei Akbari, Serge Zhuiykov
In addition to neutral excitons, the formation of charged trions at 2D heterostructures can influence or even dominate the photoluminescence spectra when free charges are present. Thus, the trions can be charged positively or negatively. The evidence of existence of trions at 2D hetero-interfaces is theoretically confirmed. Different kinds of combination of trions in MoS2/WS2 heterostructure are presented in Figure 4.4. The existence of bound interlayer trions below the neutral interlayer was predicted at the MoS2/WS2 heterostructure, which was governed by the neutral excitons and charged trions. It was found that the binding energies were 18 and 28 meV for the positive and negative interlayer trions, respectively, when both electron/hole pairs reside on the same layer (Figure 4.4). On the other hand, it was found that the electron/hole binding energy for interacting with interlayer exciton of the other layer is negligible [50].
Electronic and Optical Properties of Phosphorene
Published in Yongqing Cai, Gang Zhang, Yong-Wei Zhang, Phosphorene, 2019
Yongqing Cai, Gang Zhang, Yong-Wei Zhang
In contrast to a neutral exciton consisting of an electron–hole pair, a trion is a charged exciton which consists of a neutral exciton coupled with an additional electron or hole similar to H− or H2+ Therefore, the density of trions can be controlled by applying the gate voltage, allowing tunable optoelectronic properties. Moreover, due to this additional charge with nonzero spin, characteristics of trions can be highly important for spin manipulation. For potential applications, a large binding energy of trions is highly desired to overcome thermal fluctuations. The dynamics of excitons and trions in monolayer phosphorene were studied by controlling the photocarrier injection.66 It was found that the exciton binding energy is only 0.3 eV for phosphorene supported by the SiO2/Si substrate, greatly smaller than that of the free-standing layer, as shown before. The lifetime of the triggered exciton was found to be around 220 ps, comparable to that of MoS2. The binding energy of trion was shown to be 0.1 eV, which is huge and around 5 times larger than that of MoS2.
Perspective
Published in Qiaoliang Bao, Hui Ying Hoh, Yupeng Zhang, Graphene Photonics, Optoelectronics, and Plasmonics, 2017
One class of material that renders attention is TMDCs, which have been studied for decades and re-visited recently [11,12]. TMDCs offer promising properties, such as direct bandgaps of around 1–2 eV in molybdenum disulfide (MoS2) and tungsten diselenide (WSe2), encouraging light-emitting properties in the near-infrared range [17]. More interestingly, TMDCs demonstrated unique physics due to the lack of an inversion center in the crystal structure [14], leading to valley-contrasting orbital magnetic moment and circular dichroism [18–20]. Polarized LEDs in TMDC monolayer p-n junctions have been reported [21–23]. Strong and long-lived excitons in TMDC monolayers are also attractive for LEDs and related applications. In fact, trions have been observed in doped TMDC monolayers [24,25]. Trions are quasiparticles consisting of two electrons and a hole or two holes and an electron. Since trions are charged, they may be manipulated by electric fields, allowing efficient charge transport and collection of photo-generated current in photodetectors and solar cells [26,27].
Electroluminescence from a phthalocyanine monolayer encapsulated in a van der Waals tunnel diode
Published in Molecular Physics, 2023
Tyler James, Jonathan Bradford, James Kerfoot, Vladimir V. Korolkov, Manal Alkhamisi, Takashi Taniguchi, Kenji Watanabe, Anton S. Nizovtsev, Elisa Antolín, Elena Besley, Simon A. Svatek, Peter H. Beton
Moreover, it has recently been suggested that the assignment of emission from a triplet in a related STML study [39] is actually due to trion emission from a charged molecule. Although not directly relevant to the results discussed here, it is interesting to consider whether trions might play a role in the EL of molecular/van der Waals tunnel devices. Trion emission occurs when singly-charged molecular anions are excited into a doublet state and subsequently relax through the emission of a photon; this transition conserves spin and is optically-allowed. We would expect that both trion emission, and the presence of long-lived charged molecules in our devices would be detected in changes in the voltage-dependent PL of our device structures. In fact, PL measurements reveal a complete absence of any voltage dependence. These data were previously reported for the PTCDI variation of the device (Ref 5 SI Figure S6) and similar measurements for the current study on H2Pc are shown in SI (Figure S6); for both molecules the PL peak positions are close to those previously reported for the same molecules on thick hBN flakes [4,34] and confirm that the molecules remain neutral over the measured voltage range. These results show charged molecules cannot account for the long-lived intermediary excited states which are necessary for up-conversion and which have been previously attributed to the presence of triplets [5,24]. However, it is not possible to rule out (or in) a possible pathway in which a long-lived triplet acquires a charge via electron capture followed by rapid relaxation via trion emission. To determine whether this process might be relevant would require the identification of the spectral position for trion emission for PTCDI which would, in turn, require a means of forming an ensemble of charged molecules for investigation using PL. Interestingly this may be possible using an asymmetric variation of our device in which the upper and lower barrier thicknesses are significantly different and we plan to undertake such studies in the near future.