反荷阳离子对Preyssler型多金属氧酸盐纳米器件电导影响的实验观察
Experimental observation of the role of countercations on the electrical conductance of Preyssler-type polyoxometalate nanodevices
AI总结：
本研究合成了四种反荷离子稳定的Preyssler型多金属氧酸盐，发现反荷离子显著影响其分子结的电子传输特性，低偏置电流随反荷离子改变增加约100倍，并揭示了LUMO能级和电极耦合能的变化规律。
摘要：
多金属氧酸盐是具有良好应用前景的纳米级分子氧化物，目前正被探索用于基于分子的存储器件。在本研究中，我们合成了一系列Preyssler多金属氧酸盐，(Na-P5W30O110)14-，并用四种不同的反荷离子（H+、K+、NH4+和四丁基铵（TBA+））进行稳定。我们研究了分子结的纳米级电子传输特性（导电原子力显微镜，C-AFM），这些分子结由静电沉积在预先用带正电的胺端接烷基硫醇链SAM功能化的超平整金表面上的POMs自组装单分子层形成。我们报告P5W30基分子结的电子传输特性取决于反荷离子的性质，低偏置电流（在-0.6 V至0.6 V电压范围内）通过按K+、NH4+、H+和TBA+的顺序改变反荷离子而逐渐增加约100倍。通过使用纳米级器件电荷传输的简单分析模型对数百条电流-电压曲线进行统计研究，我们表明P5W30的最低未占分子轨道（LUMO）相对于电极费米能级的能量位置从约0.4 eV增加到0.7 eV，并且电极耦合能在从K+、NH4+、H+到TBA+的相同顺序下也从约0.05增加到1 meV。我们讨论了关于这些特征可能起源的几种假设，例如POM/电极界面处依赖反荷离子的偶极子和反荷离子调制的分子/电极杂化，在两种情况下，TBA+反荷离子的效应最大。
英文摘要：
Polyoxometalates are nanoscale molecular oxides with promising properties that are currently explored for molecule-based memory devices. In this work, we synthesize a series of Preyssler polyoxometalates (POMs), (Na-P5W30O110)14-,stabilized with four different counterions, H+, K+, NH4+ and tetrabutylammonium (TBA+), and we study the electron transport properties at the nanoscale (conductive atomic force microscopy, C-AFM) of molecular junctions formed by self-assembled monolayers (SAMs) of POMs electrostatically deposited on ultraflat gold surface prefunctionalized with a positively charged SAM of amine-terminated alkylthiol chains. We report that the electron transport properties of P5W30-based molecular junctions depend on the nature of the counterions, the low-bias current (in the voltage range -0.6 V to 0.6 V) gradually increasing by a factor ca. 100 by changing the counterion in the order K+, NH4+, H+ and TBA+. From a statistical study (hundreds of current-voltage traces) using a simple analytical model for charge transport in nanoscale devices, we show that the energy position of the lowest unoccupied molecular orbital (LUMO) of the P5W30 with respect of the Fermi energy of the electrodes increases from ca. 0.4 eV to 0.7 eV and that that electrode coupling energy also increases from ca. 0.05 to 1 meV in the same order from K+, NH4+, H+ to TBA+. We discuss several hypotheses on the possible origin of these features, such as a counterion-dependent dipole at the POM/electrode interface and counterion-modulated molecule/electrode hybridization, with, in both cases, the largest effect in the case of TBA+ counterions.
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