Mahapatro Ajit K, Janes David B
School of Electrical and Computer Engineering, Purdue University, West Lafayette, Indiana-47907, USA.
J Nanosci Nanotechnol. 2007 Jun;7(6):2134-8. doi: 10.1166/jnn.2007.783.
Electrical conduction through molecular junctions are measured in different local environments through two test beds that are ideal for single/few molecule and molecular monolayer systems. A technique has been developed to realize Au films with approximately 1.5 A surface roughness comparable to the best available techniques and suitable for formation of patterned device structures. The technique utilizes room temperature e-beam evaporated Au films over oxidized Si substrates silanized with (3-mercaptopropyl)trimethoxysilane (MPTMS). The lateral (single/few molecule) and vertical (many molecules) device structures are both enabled by the process for realizing ultraflat Au layer. Lateral metal-molecule-metal (M-M-M) device structures are fabricated by forming pairs of Au electrodes with nanometer separation (nano-gap) through an electromigration-induced break-junction (EIBJ) technique at room temperature and conductivity measurements are carried out for dithiol functionalized single molecules. We have used the flat Au layer (using the current technique) as the bottom contact in vertical M-M-M device structures. Here, molecular self-assembly are formed on the Au surface, and patterned (20 x 20 microm2) top Au contacts were successfully transferred on to the device using a stamping technique (where the Au is deposited on a polydimethylsiloxane (PDMS) pad and following a physical contact on the thiolated Au layer). The single molecular property of XYL, a highly conductive molecule and many molecular property of HS-C9-SH, an insulating molecule in its molecular monolayer form are measured. Observation of enhanced conduction following molecular deposition, and comparison of conductance-voltage characteristics to those predicted theoretically, confirms the success of trapping single/few molecules in the nano-gap. The observed approximately 10(2) less conductance through the molecular monolayer of HS-C9-SH compared to the estimation of a linear sum of single molecule conductances over large area indicate that either all the molecules are not in physical contact with the top stamping electrode or electrode-molecule coupling has a less broadening in presence of it own environment or both.
通过两个适用于单分子/少数分子和分子单层系统的测试平台，在不同的局部环境中测量分子结的导电情况。已开发出一种技术来制备表面粗糙度约为1.5埃的金膜，该粗糙度与现有最佳技术相当，且适合形成图案化的器件结构。该技术利用室温电子束蒸发在经（3-巯基丙基）三甲氧基硅烷（MPTMS）硅烷化的氧化硅衬底上制备金膜。实现超平金层的工艺使得横向（单分子/少数分子）和纵向（多分子）器件结构均得以实现。横向金属-分子-金属（M-M-M）器件结构是通过在室温下利用电迁移诱导断裂结（EIBJ）技术形成纳米间距（纳米间隙）的金电极对来制造的，并对二硫醇功能化的单分子进行电导率测量。我们已将（采用当前技术制备的）平整金层用作纵向M-M-M器件结构的底部接触。在此，分子自组装在金表面形成，并且使用压印技术成功地将图案化（20×20微米²）的顶部金接触转移到器件上（其中金沉积在聚二甲基硅氧烷（PDMS）垫上，随后与硫醇化金层进行物理接触）。测量了高导电分子XYL的单分子特性以及绝缘分子HS-C9-SH在其分子单层形式下的多分子特性。分子沉积后观察到导电增强，并将电导-电压特性与理论预测值进行比较，证实了在纳米间隙中捕获单分子/少数分子的成功。与大面积上单分子电导的线性总和估计值相比，观察到通过HS-C9-SH分子单层的电导大约低10²倍，这表明要么所有分子都未与顶部压印电极物理接触，要么电极-分子耦合在其自身环境存在时展宽较小，或者两者皆是。
