Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 11664010, 11264013), the Hunan Provincial Natural Science Foundation of China (Grant Nos. 2017JJ2217, 12JJ4003), the Scientific Research Fund of Hunan Provincial Education Department of China (Grant No. 18A293), and the Research Program of Jishou University, China (Grant Nos. JGY201851, Jdy1849, Jdy19039)
Received Date:27 July 2019
Accepted Date:23 September 2019
Available Online:26 November 2019
Published Online:05 December 2019
Abstract:Since graphene was successfully obtained in the end of 2004, the research on graphene and relevant devices has attracted extensive attention. The armchair- and zigzag-edge graphene nanoribbons, as the building blocks, are often used to design the graphene-based molecular electronic devices. Quinoline, an important intermediate between metallurgical dyes and polymers, is an organic conjugated small molecule which is simple in structure and easy to synthesize and modify the chemical structure, and quinoline has become one of the research focuses in the field of molecular electronic devices in recent years. From the physical point of view, the transport properties of the isomeric quinoline molecular electronic devices connected with graphene nanoribbon electrodes can provide a theoretical basis for designing and manufacturing molecular electronic devices with excellent performance. Based on the first-principles calculation method combining the density functional theory and non-equilibrium Green's function, this paper systematically investigates the transport properties of the carbon-linked isomeric quinoline molecule electronic devices sandwiched between the graphene nanoribbon electrodes. The obtained results show that the device current presents a linear change in a bias voltage range [–0.3 V, +0.3 V], the current decreases with the increase of the absolute bias voltage, separately, in a range of [+0.5 V, +0.8 V] and [–0.4 V, –0.9 V], demonstrating a strong negative differential resistance effect. On the other hand, the interesting negative differential resistance effect is remained when there is an angle between the quinoline molecular plane and the graphene nanoribbon electrode; the current of the device is found to be independent of the rotation direction of quinoline molecule in the central region; the current of the device should be forbidden when the quinoline molecule plane is rotated to a direction vertical to the graphene nanoribbon electrodes. The obtained results can provide a theoretical basis for designing and manufacturing the molecular switches and negative differential resistance devices based on isomeric quinoline molecular electronic devices. Keywords:isomeric quinoline molecule/ graphene nanoribbon/ first-principles calculation/ electron transport
2.模型与方法半无限长锯齿型石墨烯纳米带-喹啉C9H5N分子-半无限长锯齿型石墨烯纳米带构成的分子电子器件如图1所示, 器件分为左电极、中心散射区(图1中的红色虚线框区域所示)和右电极三个部分, 喹啉C9H5N分子中氮原子N的位置编号如图1(a)所示. 喹啉C9H5N分子中氮原子N分别处于编号2, 3和5处时的模型称为M1, M2和M3, 如图1(a)—图1(c)所示. 将喹啉C9H5N分子平面垂直纸面向里旋转方向定义为正, 如图1(d)和图1(e)给出喹啉C9H5N分子平面与石墨烯纳米带电极平面成0°和90°时的模型. 图 1 由半无限长锯齿型石墨烯纳米带左电极/中心散射区/半无限长锯齿型石墨烯纳米带右电极组成的ZGNR/C9H5N/ZGNR分子电子器件结构示意图, 红色方框区域表示中心散射区 (a)—(c)分别对应喹啉C9H5N分子中氮原子N处于编号2, 3和5处; (d)和(e)给出喹啉C9H5N分子平面与石墨烯纳米带电极平面成0°和90°时的模型 Figure1. ZGNR/C9H5N/ZGNR molecular electronic device schematic diagram consisted of a semi-infinite ZGNR left electrode/a central scattering region/a semi-infinite right ZGNR electrode, the red dashed line area represents the central scattering region. (a)?(c) denotes the marked 2nd, 3rd and 5th N atom of the C9H5N molecular; (d) and (e) illustrates the model of the 0° and 90° angle between the C9H5N molecule and graphene nanoribbon electrodes, respectively.
为进一步分析图2中电流-电压曲线和电导变化规律, 图3给出0, ± 0.4 V, ± 0.9 V以及 ± 1.5 V 偏压下器件M1, M2和M3的透射谱, 费米能级取为能量参考点, 偏压窗为[–VB/2, +VB/2]即图中的黑色虚线之间的区域. Landauer-Büttiker公式表明, 分子器件的电流可以通过对偏压窗内的透射系数积分得到[56]. 从图3(a)—图3(c)可以看出, 零偏压下费米能级处存在透射峰, 器件是金属性的, 电流在0—±0.3 V低偏压范围内呈线性变化(如图2(a)所示); 零偏压下费米能级处的透射峰随着偏压的增加逐渐降低. 随着偏压的增加, 低于费米能级的第一隧穿峰逐渐增大并向费米能级方向移动, 导致随着偏压窗的增大, 积分面积呈现先增大后减小再增大的变化, 有力印证了图2中电流和电导随偏压的变化. 图 3 器件(a) M1、(b) M2和(c) M3在0, ±0.4 V, ±0.9 V以及±1.5 V偏压下的透射谱, 图中的黑色虚线和阴影部分面积分别表示偏压窗和偏压窗内的透射系数积分面积 Figure3. The transmission spectrum of the device (a) M1, (b) M2 and (c) M3 under the bias voltage of 0, ±0.4 V, ±0.9 V and ±1.5 V, where the (black) dashed lines and shaded area denote the bias window and the integrated area of the transmission coefficient in the bias window, respectively.
将喹啉C9H5N分子平面往垂直纸面向里旋转方向定义为正, 图1(d)和1(e)给出喹啉C9H5N分子平面与石墨烯纳米带电极成0°和90°时的模型. 图4中给出M1器件喹啉C9H5N分子平面与石墨烯纳米带电极成0°, 30°, 45°, 60°, 90°和–90°时的I-V曲线和电导. 在0—± 0.3 V偏压范围里, 如图4(a)中的方块标记的黑色实线、圆点标记的红色长虚线和上三角标记的蓝色短虚线所示, 喹啉C9H5N分子平面与石墨烯纳米带电极成0°, 30°和45°时的I-V曲线呈线性变化, 但电流在偏压绝对值超过0.3 V后随着角度的增大迅速减小; 当喹啉C9H5N分子平面与石墨烯纳米带电极成60°时, 如图4(a)中下三角标记的绿色点线所示, 电流在 ± 0.3 V偏压时大幅减小, 在 ± 0.8 V—± 1.2 V范围内趋近于0; 当喹啉C9H5N分子平面与石墨烯纳米带电极成 ± 90°时, 如图4(a)中的菱形标记的紫色点虚线和左三角标记的黄色点线所示, 电流几乎为0, 即电流截止, 且喹啉C9H5N分子平面与石墨烯纳米带电极成–90°时呈现与90°时同样的变化, 说明电流变化与喹啉分子平面旋转方向无关, 只与旋转的角度有关. 此外, 图4(b)中电导在偏压绝对值超过0.3 V后, 随着喹啉分子平面与石墨烯纳米带电极间夹角的增大迅速减小, 有力地印证了电流的变化规律. 图 4 M1器件喹啉C9H5N分子平面与石墨烯纳米带电极成0°, 30°, 45°, 60°, 90°和–90°的(a)I-V曲线和(b)电导 Figure4. The (a) I-V curve and (b) conductance of the M1 device when the angle between the C9H5N molecule and graphene nanoribbon electrodes is 0°, 30°, 45°, 60°, 90° and –90°, respectively.
为进一步解释图4中电流和电导变化规律, 图5给出0, ± 0.3 V, ± 0.9 V以及 ± 1.5 V偏压下喹啉C9H5N分子平面与石墨烯纳米带电极成0°, 30°, 45°, 60°和90°的透射谱, 其中的黑色虚线和阴影部分面积分别表示偏压窗和偏压窗内透射系数的积分面积. 从图5(a)—(d)可以看出, 在0偏压下费米能级处存在透射峰, 器件是金属性的, 电流在低偏压范围内呈线性变化(如图4(a)所示), 费米能级处的透射峰随着偏压的增加逐渐降低, 直至最后消失. 随着偏压的增加, 低于费米能级的第一和第二隧穿峰逐渐增强并向费米能级移动, 导致随着偏压窗的增大, 积分面积先增大后减小再增大, 从而解释了图4中电流和电导变化的原因以及负微分电阻现象. 当喹啉C9H5N分子平面与石墨烯纳米带电极成60°角时, 积分面积在偏压为± 0.3 V附近大幅度减小, 当偏压增大到 ± 1.0 V时积分面积接近于0; 当喹啉C9H5N分子与石墨烯纳米带电极成90°时, 积分面积在所有偏压下趋于零, 即没有电流通过. 我们进一步研究了电极宽度对结果的影响, 发现电极宽度变大时, 所观测到的负微分电阻效应及开关效应仍然存在. 图 5 偏压0, ± 0.3 V, ± 0.9 V以及 ± 1.5 V下喹啉C9H5N分子平面与石墨烯纳米带电极分别成 (a) 0°, (b) 30°, (c) 45°, (d) 60°和(e) 90°时的透射谱, 图中的黑色虚线和阴影部分面积分别表示偏压窗和偏压窗内透射系数积分面积 Figure5. The transmission spectra for the angle between the C9H5N molecules and graphene nanoribbon electrodes is (a) 0°, (b) 30°, (c) 45°, (d) 60° and (e) 90°, respectively, under the bias voltage of 0, ± 0.3 V, ± 0.9 V and ± 1.5 V, where the (black) dashed lines and shaded area denote the bias window and the integrated area of the transmission coefficient in the bias window, respectively.
图6中给出零偏压下喹啉C9H5N分子平面与石墨烯纳米带电极成0°, 30°, 45°, 60°, 90°和–90°时的透射谱. 从图中可以看出, 喹啉C9H5N分子平面与石墨烯纳米带电极成0°, 30°, 45°和60°角度时, 费米能级处均呈现尖锐的透射峰; 当喹啉C9H5N分子平面与石墨烯纳米带电极成90°和–90°时, 透射谱在非常宽的带隙范围内为0, 很好地印证了图4中的电流-电压曲线以及电导变化规律. 图 6 零偏压下, 喹啉C9H5N分子平面与石墨烯纳米带电极成0°, 30°, 45°, 60°, 90°和–90°角度下的透射谱, 其中红色虚线表示费米能级 Figure6. The transmission spectrum of the C9H5N molecule and the ZGNR electrodes at the angle of 0°, 30°, 45°, 60°, 90° and –90° under the 0 bias, where the (red) dashed line denotes the Fermi level.
图7给出零偏压下喹啉C9H5N分子平面与石墨烯纳米带电极成0°, 60°, 90°, –90°时的实空间电荷密度, 直观地展现电荷的分布情况. 从图7(a)和图7(b)可以看出, 当喹啉C9H5N分子平面与石墨烯纳米带电极成60°时, 中心区喹啉C9H5N分子电荷分布较多, 但是分子与电极连接处存在电荷缺口, 这就解释了喹啉C9H5N分子平面与石墨烯纳米带电极成60°时, 电导在0偏压下较大但随着偏压的增加急剧减小的原因. 比较图7(c)和图7(d)可以发现, 当喹啉C9H5N分子平面与石墨烯纳米带电极成90°或–90°时, 中心区喹啉C9H5N分子电荷分布较多, 电极上电荷分布较少, 不利于电荷输运, 从另一方面解释了喹啉C9H5N分子平面与石墨烯纳米带电极成90°或–90°, 电流几乎为0. 图 7 零偏压下, 喹啉C9H5N分子平面与石墨烯纳米带电极成 (a) 0°, (b) 60°, (c) 90°和(d) –90°时的实空间电荷密度 Figure7. The real space charge density for the angle between the C9H5N molecule and graphene nanoribbon electrodes is (a) 0°, (b) 60°, (c) 90° and (d) –90°, respectively under the 0 bias voltage.