"Si-CH=CH-(C6H4)n-CH=CH-Si纳米线"对称异质结的导电性能研究

This proposal aims at fabricating Si-CH=CH-(C6H4)n-CH=CH-SiNW (silicon nanowire) symmetric junctions via two-step thermal hydrosilylation, and studying electron transport through the junctions..In molecular electronics, a basic unit consists of single or an assembly of molecules attaching to two electrodes at their both two ends simultaneously, for studying electron transport from one electrode to another through molecules. Au-S covalent bonds can be created spontaneously, hence are widely adopted to function as alligator clips for generating Au-S-molecules-S-Au symmetric junctions. However, it has been reported that Au-S contacts don't have good mechanical stability and electrical reliability, and have high contact resistance to shield the electronic contribution of molecules. Alternatively, having a higher bonding energy than Au-S, Si-C covalent bonds may improve mechanical and electrical stability, and reduce contact resistance ascribed to the natural continuity from the C-based molecules. Hence it is desired to fabricate Si-C-molecules-C-Si symmetric junctions, in which Si-C replaces Au-S as the alligator clip. However, the recent development of nano engineering limits the generation to Si-C-molecules-metal asymmetric junctions, in which high resistance at the molecules-metal contacts and the "diode" effect derived from the asymmetric structure may mask the electronic signatures of molecules. .This project first proposes to replace the metal electrode in the asymmetric junctions with a SiNW, and create a conjugated junction Si-CH=CH-(C6H4)n-CH=CH-SiNW symmetrically anchored by Si-C bonds. The symmetric junctions are generated in two steps: immobilize the molecule HC≡C-(C6H4)n-C≡CH onto a Si wafer to generate Si-CH=CH-(C6H4)n-C≡CH semi-junctions, and then graft SiNWs onto the free ends (C≡C) of the semi-junctions to create the symmetric junctions. Both two immobilizations are carried out by thermal hydrosilylation. Not only as an electrode, but also does a grafted SiNW act as a junction marker which can be readily located by a CAFM (conducting atomic force microscopy) tip for investigating charge transport through the symmetric junctions. It is elaborated how to reliably fabricate the junctions and evaluate molecular conductance, with consideration of the inevitable silicon oxidation and unpredictable contact/molecular configurations in the junctions. The mechanism of charge transport will be explored with applied bias, molecular length (n:1-3) and temperature..We believe that the success of this project will provide a novel, reliable strategy to study molecular electronics without electrical and mechanical perturbation from the electrode-molecules contacts, potentially open a new door to hybridize molecular electronics with the mature silicon-based manufacturing technology, and promote mass production.

分子电子学主要研究"电极-分子-电极"异质结对电荷传输的调控，实现电子器件的分子/纳米尺度微型化，从物理极限上提高器件集成度和运算能力。目前广泛使用Au-S键作为将分子与电极相链接的"鳄鱼夹"，但其缺点是机械稳定性差、接触电阻高而且变化大。为了避免由Au-S键引起的这一系列问题，本项目使用Si-C共价键作为"鳄鱼夹"，发展一套可靠的制备研究方法，试图完善两步热化学氢化硅烷化反应制备"Si-CH=CH-(C6H4)n-CH=CH-Si纳米线"对称异质结，建立一种使用导电原子力显微镜（AFM）研究电荷在异质结中传输的方法，通过研究异质结电流密度与偏压、分子长度以及工作温度的关系，揭示电子传输机理。该研究旨在为分子电子学发展新的Si-C键"鳄鱼夹"，从而提高分子异质结的机械稳定性和工作可靠性，降低接触电阻，推动分子电子学与成熟硅工艺相结合，促进其今后的产业化发展。

分子电子学主要研究“电极－分子－电极”异质结的电荷传输，将电子器件缩小至分子或纳米尺度，最大程度的提高电子器件集成度及信息运算、存储、处理能力。目前广泛使用Au－S键作为将分子与电极相链接的“鳄鱼夹”，但会导致分子异质结的机械稳定性差、接触电阻高且变化大、电学性能不稳定。Si－C键具有比Au－S键更高的键能，可以增强分子异质结的机械和电学稳定性；Si－C键作为硅电极和分子的自然延续，将降低“鳄鱼夹”的接触电阻。因此本项目将Au－S替换成Si－C键，制备Si－C－分子－C－SiNW（硅纳米线）对称异质结，使用导电原子力显微镜（CAFM）研究对称异质结的电荷传输。.通过两步法制备对称异质结。第一步，使用氢化硅烷化反应将分子通过形成Si－C键固定在高掺杂Si片表面，形成“Si－C－分子”不对称异质结；第二步，再次使用氢化硅烷化将高掺杂SiNW通过Si－C键固定在不对称异质结的分子自由端，形成Si－C－分子－C－SiNW。分子包括1,4-diethynylbenzene（即DEBZ）和Octa-1,7-diene（即C8）,分别使用水热和UV氢化硅烷化形成对称异质结。优化氢化硅烷化反应， 使得异质结中自组装分子排列紧密，从而充分抑制Si氧化，避免其影响分子异质结的电荷传输表征。.使用CAFM针尖接触异质结中的SiNW，施加偏压检测电流－偏压（I－V）曲线。 Si－C8－SiNW异质结具有正向整流效应，该整流效应随施加在SiNW上作用力的增加而增大。Si－DEBZ－SiNW没有正向整流效应，而随正负偏压变化呈现对称的I－V曲线。DEBZ较C8具有较高的单分子异质结电导。Si－DEBZ－SiNW异质结中具有“π－共轭”电子结构，较Si－C8－SiNW异质结的σ电子结构更容易导电，因此具有较高的电导。另外，“π－共轭”电子结构使得异质结中DEBZ形成致密的自组装分子结构，具有很高的异质结结构对称性，因此呈现对称的I－V曲线。相比之下，σ电子结构使得C8自组装膜结构比较松散，破坏了异质结的结构对称性，因此呈现不对称的正向整流效应。.本项目以高掺杂硅纳米线作为硅电极，使用氢化硅烷化法制备以Si－C键为“鳄鱼夹”、机械性能稳定、电学性能可靠的对称分子异质结；这将有助于将分子器件的制备与硅工艺相结合，有利于今后分子电子器件的大规模商品化生产。

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