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Energy funneling effect of two-dimensional materials provides an important method for modulating carrier transfer. However, the formation of energy funneling and its influences on the carrier transfer are still relatively uncharacterized. In this work, the energy funneling induced by the layer number gradient effect in MoS2 is investigated through atomic-bond-relaxation approach and first-principles calculations. It is found that the bandgap of MoS2 monotonically increases with the decrease of the layer number, leading the conduction band minimum (valence band maximum) of thin layer MoS2 to be higher than (lower than) that of thick layer MoS2. Therefore, both dual thickness gradient and triple thickness gradient MoS2 can achieve the energy funneling effect. As a result, the carriers will be directionally transferred from the thin layer region to the thick layer region. According to Marcus theory, the carrier transfer rate is dependent on drive force caused by the energy level difference with different thicknesses of MoS2. For the dual thickness gradient MoS2, when the thickness difference between adjacent layers is the largest, the driving force is the highest, which is 1L/bulk. In addition, owing to the driving force smaller than the reorganization energy in dual thickness gradient MoS2, a large driving force corresponds to a high carrier transfer rate, resulting in a higher carrier transfer rate of 1L/bulk than those in other dual thickness gradient systems. For the triple thickness gradient MoS2, there are two consecutive interface energy differences that induce driving forces. However, the carrier transfer rate is exponentially correlated with the driving force. Therefore, the carrier transfer rate of dual thickness gradient MoS2 will be higher than that of the corresponding triple thickness gradient MoS2. Our results demonstrate that the energy funneling effect induced by thickness gradient can realize carrier accumulation in the thick layer region without the need for p-n junctions, which is of great benefit in collecting photogenerated carriers. The atomic force microscopy lithography and chemical vapor deposition will be used to engineer thickness-gradient two-dimensional materials with enhanced optoelectronic properties in future.
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Keywords:
- MoS2 /
- thickness gradient /
- energy funneling effect
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图 1 (a) 双厚度梯度MoS2结构示意图; (b) 双厚度梯度MoS2能带漏斗及其对电荷转移示意图; (c) 三厚度梯度MoS2结构示意图; (d) 三厚度梯度MoS2能带漏斗及其对电荷转移示意图
Figure 1. (a) Schematic diagrams of dual thickness gradient MoS2; (b) energy funneling and its effect on carrier transfer in dual thickness gradient MoS2; (c) schematic diagrams of triple thickness gradient MoS2; (d) energy funneling and its effect on carrier transfer in triple thickness gradient MoS2.
图 2 (a) 不同层数MoS2势函数与原子间距之间的关系; (b) MoS2层数依赖的带隙漂移, 插图是MoS2导带底和价带顶随层数的变化规律
Figure 2. (a) Relationship between potential and distance of MoS2 with different layer number; (b) layer number dependent bandgap of MoS2, the inset of (b) shows the conduction band minimum (CBM) and valence band maximum (VBM) of MoS2 as a function of layer number.
图 3 不同层数MoS2的能带结构
Figure 3. Band structures of MoS2 with different layer.
图 4 (a) 双厚度梯度MoS2界面能级差诱导的驱动力; (b) 双厚度梯度MoS2中的能带漏斗对电荷转移速率影响; (c) 三厚度梯度MoS2界面能级差诱导的驱动力; (d) 三厚度梯度MoS2中的能带漏斗对电荷转移速率影响
Figure 4. (a) Driving force and (b) transfer rate of electron and hole in dual thickness gradient MoS2; (c) the driving force and (d) transfer rate of electron and hole in triple thickness gradient MoS2.
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