(CH3)2和(NH2)2基团修饰的齐聚苯乙炔分子电子输运性质研究

采用密度泛函理论和非平衡格林函数相结合的方法研究了S原子作为单、双端基的(CH₃)₂-OPE (齐聚苯乙炔)和(NH₂)₂-OPE分子在金电极间的电子输运性质. 通过第一性原理优化计算获得分子部分稳定结构, 再置于Au电极之间构成两极系统, 然后再优化整个两极系统获得稳定结构. 另外, 通过非平衡格林函数方法计算了两极系统的电子输运性质. 计算结果表明, 不同的修饰基团和桥接方式可以导致两极系统的开关效应、负微分电阻行为和整流行为等不同的电子输运性质. 通过计算不同偏压下的分子体系投影轨道电子分布、透射谱、态密度, 对这些新异的电输运性质出现的机理进行了解释.

Electronic transport properties of oligo phenylene ethynylene molecule modified by the (CH3)2 and (NH2)2 groups

The modification effects of the groups (CH₃)₂ and (NH₂)₂ on the oligo phenylene ethynylene (OPE) molecules with single and double S atoms connected to the two electrodes are investigated by the density functional theory and non-equilibrium Green function. The modified OPE molecule is optimized and used to build a two-probe system with Au electrodes. Then the two-probe system is fully relaxed to obtain a stable structure. The electronic transport properties of the two-probe system are also calculated with the non-equilibrium Green function method. The calculation results show that both the modified groups and the bridge atoms can lead to the switch effect, the negative differential resistance behavior, and the rectifying behavior for the two-probe system. When molecules are connected with single S atom at one end, both the (NH₂)₂-OPE and the (CH₃)₂-OPE molecules show the rectifying behavior. However, the (NH₂)₂-OPE also shows a switch effect at larger voltage because there is current when the negative bias is over 1.3 V, while the (CH₃)₂-OPE molecule demonstrates a complete rectifying behavior because it is hardly conductive in the whole positive bias area. The current of OPE molecule without modification group always increases with the increase of voltage. Therefore, it is only a resistance. These results are different from that of NO₂-OPE-NH₂ molecule which shows a negative differential resistance behavior. For the case of the molecule connected with S atoms at both ends, the (NH₂)₂-OPE(S) and (CH₃)₂-OPE(S) models show negative differential resistance behaviors in the negative bias range. It is found that both (NH₂)₂-OPE and (CH₃)₂-OPE molecules demonstrate the negative differential resistance behaviors when they are connected with S atoms bridge at both ends. However, the current of the molecule with one S atom at one end is about two-order magnitude lower than that of the molecule with S atoms at both ends. It is shown that S atom acting as a bridge can significantly affect the characteristic of current-voltage. The mechanisms for the various characteristics of the electronic transport properties are explored by analyzing the projection orbit electron distribution, the transmission spectrum, and the density of states under the different bias voltages. For (NH₂)₂-OPE molecule with a single S atom at one end in the negative bias range, only the lowest unoccupied molecular orbital (LUMO) can transfer electron with low bias, but both highest occupied molecular orbital (HOMO) and LUMO can be conductive with high bias, which results in the switch effect. In the positive bias range, both HOMO and LUMO cannot be conductive with low bias, while LUMO can be conductive with high bias, which results in the switch behavior. For the case of (NH₂)₂-OPE molecule with one S atom at each end, only the HOMO can transfer electron in the low bias range, while the LUMO can be conductive at high positive bias, but both HOMO and LUMO cannot be conductive in high negative bias range, which leads to the non-symmetric negative differential resistance effect in the whole bias range. A similar analysis of the difference between HOMO and LUMO can be used to understand the characteristics of the current-voltage of (CH₃)₂-OPE. Considering the fact that the different modification groups can lead to various current-voltage properties of OPE molecule, the modified OPE molecule is a promising candidate for designing molecule device.