Fund Project:Project supported by the Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi, China, STIP (Grant No. 2020L0371), the Scientific and Technological Innovation Team Programs of Changzhi Medical College, China (Grant No. CX201904), the Fund for Shanxi “1331Project” Key Innovative Research Team, China, “1331KIRT”, and the Shanxi Province Service Industry Innovation Discipline Group Construction Plan, China (201809)
Received Date:21 June 2020
Accepted Date:28 September 2020
Available Online:01 February 2021
Published Online:20 February 2021
Abstract:Although the one-dimensional non-conjugated alkane chain, which has an important influence on the electron transport process, does not possess the characteristics of electron-rich and electron-deficient, it often exists in single-molecule devices and biological molecules such as peptides and proteins. In order to understand the electron transport characteristics of alkane chain, a one-dimensional linear non-conjugate (CH2)n molecular junction model is designed in this study. Subsequently, we conduct the systematic study of the electronic transport behavior of (CH2)n (n = 1–12) molecular linear chain coupling to two graphene electrodes, based on the density functional theory and nonequilibrium Green’s function formalism. The results reveal that the structure and conductance of CH2 chain are highly sensitive to the odevity of CH2 unit. When the value of n is odd, the groups of CH2 extend in a zigzag way from the left electrode to the right electrode in the plane of graphene, while the value of n is even, what is different is that the groups of CH2 are arranged above and below the electrode plane. For this reason, the odd-even behavior of conductance occurs in the (CH2)n (n = 1–12) molecular chain. Furthermore, n is also an important factor to affect their transport properties (odd or even behavior of conductance). The longer the (CH2)n chain, the deeper the suppression in transmission spectrum and the lower the equilibrium conductance. What is more, the conductance decreases exponentially with the increase of molecular length, with a decay constant β of 0.67 and 0.60 for odd and even, respectively, which is in good agreement with the experimental research. Additionally, by analyzing their eigenchannels of odd and even (CH2)n molecular chain, we find that the coplanar σ electron with graphene electrode makes a major contribution to the electronic transport channel. The current-voltage curve of (CH2)n molecular chain exhibits nonlinearity, implying their semiconductor characteristics. The interesting mechanical and electronic transport properties are expected to conduce to further experimental synthesis, design and operation of the single molecular nanodevices. Keywords:methylene/ first-principles calculation/ electron transport/ non-equilibrium Green’s function
为了更好地模拟亚甲基分子链的电子传输特性, 在“石墨烯-(CH2)n-石墨烯”分子结模型中, 我们参考游离烷烃分子中C—C键和C—N键的键长, 首先对两头连接有氨基的(CH2)n分子链进行了结构优化, 然后根据优化的N-N之间的距离设置了石墨烯两侧耦合点的间距, 最后将含有氨基的亚甲基分子链通过酰胺键连接于两侧羧基化的石墨烯电极间, 并再次进行结构优化, 得到如图2所示的n从1到12的一维线性非共轭亚甲基分子结稳定构型. 进一步地, 我们依据公式ΔE = E [GMG] – E [石墨烯电极]– E [分子链], 计算了这些分子结的结合能并列于表1中, 结果发现, 其结合能在数值上非常接近, 均介于–10.90 eV到–11.72 eV之间, 说明优化得到的这些一维非共轭(CH2)n分子结为稳定的平衡结构. 图 2 (CH2)n分子结(n = 1?12)稳定结构图(每个图中下方为沿y轴方向的俯视图, 上方和右方插图分别为沿x和z轴方向的侧视图, 坐标方向和小球颜色说明见图1) Figure2. The stable structure of (CH2)n (n = 1?12) molecule junction (on each diagram, the bottom figure is a top view along y axis, the upper and right side inset is a side view along x and z axis, respectively. Coordinate direction and color description of the ball are shown in Fig. 1).
n
d1N—N /?
d2C—C /?
d3C—C /?
α/(°)
ΔE/eV
1
2.44
—
—
—
–10.90
2
3.67
1.55
—
—
–11.04
3
5.10
1.55
2.54
2.25
–11.12
4
6.22
1.53
2.55
18.81
–11.45
5
7.71
1.55
2.63
0.91
–11.52
6
8.91
1.54
2.59
15.12
–11.55
7
10.30
1.55
2.62
0.41
–11.66
8
11.51
1.54
2.60
14.61
–11.57
9
12.90
1.55
2.62
0.80
–11.69
10
14.11
1.54
2.61
14.93
–11.60
11
15.51
1.55
2.62
1.85
–11.72
12
16.71
1.54
2.61
14.19
–11.60
表1(CH2)n分子结(n = 1—12)的平均键长、键角和结合能(d1, d2, d3, α如图2所示) Table1.The average bond length, bond angle and binding energy in (CH2)n (n = 1–12) molecule junction (the bond length of d1, d2, d3 and the bond angle α are shown in Fig. 2).
根据优化得到的(CH2)n分子结(n = 1—12)平衡结构, 本文计算了零偏压时, 各(CH2)n分子链的透射谱. 结果如图3所示, 所有分子结的透射谱表现出了类似的峰形特征, 并且由于亚甲基的非共轭结构, 使得透射谱在费米能级附近没有明显的隧穿共振峰, 从而导致了这一分子链较差的导电性质. 同时, 随着分子链n值的增加, 这一分子结的透射系数明显地降低. 尤其值得注意的是, 比较图3(a)奇数分子链和图3(b)偶数分子链的透射谱, 能够发现, 偶数个CH2基团的分子链导电性明显差于奇数分子链, 并且由于结构上的奇偶性, 这一分子结的电导值也表现出明显的振荡衰减(见图3(a)中插图), 这一现象在Si[14], C[28]等原子链的电子输运研究中也被发现, 但Si原子链的电导振荡现象会随着原子数的增加而逐渐减弱, 而在CH2分子链中却并没有发现这一趋势. 图 3 奇数 (a)和偶数(b) (CH2)n分子结在零偏压下的透射谱图 Figure3. Transmission coefficient as a function of energy for odd (a) and even (b) (CH2)n molecule junction under zero external bias.
其中: G为分子电导, G0为量子电导常数, L是分子的长度, β为衰减常数. 因此, 本文根据零偏压下(CH2)n分子结在费米能级处的透射系数得到了n从1到12的不同分子链导电值, 并将分子链中N-N键的键长d1作为分子的长度, 总结了(CH2)n分子链的电导和长度之间的关系, 结果如图4所示. 由于该分子链随着长度的增加, 其电导表现出了明显的振荡现象. 因此, 从图4可以看出, 奇数或偶数分子链的电导均随分子长度变化呈指数性衰减的趋势, 并且其线性方程分别为: ln(G) = –0.67L – 7.78 (图4中奇数a, 线性相关系数r = 0.9966)和ln(G) = –0.60L –11.00 (图4中偶数b, 线性相关系数r = 0.9978). 根据线性方程, 可知奇数和偶数(CH2)n分子链的衰减常数基本一致, 分别为0.67和0.60, 相比较于实验报道的衰减常数0.94而言[19], 尽管β值要小一些, 但实验中使用的电极为金电极. 相关研究也证实, β值也将会因耦合基团、溶剂环境等测试条件的改变而改变[1]. 图 4 奇数 (a)和偶数(b) (CH2)n分子结零偏压电导与分子链长度关系图 Figure4. The plot of conductance versus chain length for odd (a) and even (b) molecular junction of (CH2)n at the bias of 0 V.
为了更好地理解(CH2)n奇偶分子链导电性质的差异和振荡现象, 本文分别以n为11和12的分子结为例, 计算了这个一维非共轭体系的导电本征通道(图5). 从图5可以看出, 不论是奇数还是偶数分子链, 其导电作用主要都是通过C—C键之间的定域σ电子进行电荷传输, 相对于π键的共轭体系, 导电性质要更差一些, 因而, 计算得到的衰减常数β值也要更大(一般来说, β值越小, 共轭体系越明显, 导电性也更好). 此外, 通过比较图5(a)和图5(b), 可以发现两者最大的区别是奇数分子链中, 每个CH2基团及对应的σ键与石墨烯电极基本处于同一平面, 因而也更有利于电子传输, 而偶数分子链中, 两者正好为垂直关系. 这充分说明了一维非共轭(CH2)n分子结出现电导奇偶振荡的本质原因. 进一步分析其电子输运的本质, n为奇数时, 应该是以声子散射机制为主; n为偶数时, 应该以定程跳跃散射为主. 因此, 表现出的奇数电导比偶数电导大1个数量级以上[29]. 图 5 奇数 (a)和偶数(b) (CH2)n分子结导电通道图(上方插图为沿x轴的侧视图, 蓝色和黄色区域分别代表得失电荷密度) Figure5. The eigenchannels of odd (a) and even (b) molecular junction of (CH2)n (The upper inset is a side view along x axis, blue and yellow areas denote the gain and loss of electron density)
23.4.电流电压关系 -->
3.4.电流电压关系
外偏压也是影响单分子器件导电性质的一个重要因素. 本文进一步计算了不同链长(CH2)n分子结在外偏压为0—2.0 V范围内不同电压下的电流值, 结果如图6所示. 可以看出, (CH2)n分子结的I-V曲线与文献报道的石墨二炔纳米带的I-V曲线特征相似, 具有半导体特征[30], 尽管奇数分子链相较偶数分子链的电流值要大10倍以上, 但由于烷烃链的导电性较差, 导致该分子链整体电流值均较小, 仅为几到几十nA的数量级. 此外, 从图中还可以看出, 奇数分子链的导通电压约为1.0 V(图6(a)), 而偶数分子链约为1.5 V (图6(b)). 这一偶数分子链较差的导电特性也与之前对导电通道的分析相吻合. 值得注意的是, 当分子链长度为n = 4, 外电压大于1.6 V时, 其电流值超过n = 2的分子链电流值, 根据Landauer-Buttiker公式, 分析其原因为偏压窗口增大后, n = 4的分子链积分计算所得到的透射率增大, 导致了电流的增加[31]. 图 6 奇数 (a)和偶数(b) (CH2)n分子结在不同电压下的电流图 Figure6. The current of odd (a) and even (b) (CH2)n molecular junction under different external bias.