Thermal and electrical conductance and thermopower at the nanoscale

在之前的资助阶段，我们已经为详细研究由有机分子功能化的半导体材料组成的杂化异质结构的电学和热学输运性质奠定了基础。在实验上，新的方法被开发出来。建立了一种新的自组装单分子膜(SAM)软接触方法，首次获得了两个半导体电极接触的SAM的可靠输运测量。我们确定了不同长度的烷烃链的不同低温输运模式。我们还成功地“卷起并压回”了不同材料组成的超晶格堆栈，主要是半导体和金属层。利用热反射光谱和局域3ω测量得到了横截面电导率和热导率的初步结果。在第二阶段，我们将重点放在(I)制备与台面的微图案接触以获得完整的品质因数，以及(Ii)加入具有量身定制的电和热性能的合适的分子层，这将与理论合作伙伴密切合作来确定。从模型的角度来看，我们的工作朝着两个方向发展：(A)发展有效的计算方法，在第一原理的基础上处理大(实验相关的)系统中的热输运；(B)应用第一原理方法结合格林函数技术来研究硅-分子-硅结的热电性质。因此，一个特别的焦点是证明通过对分子的化学组成进行量身定制的修改来调节结的热电势的可能性。在第二个资助期，我们将与实验伙伴密切合作，应用这些方法研究(I)由半导体电极和预先定义的分子自组装膜组成的混合异质结构中的电和热输运，以及(Ii)开发一种方法来计算在原子水平上确定热电优值系数的相关量。

In the previous funding phase, we have set the basis for a detailed study of the electrical and thermal transport properties of hybrid heterostructures consisting of semiconducting materials functionalized by organic molecules. Experimentally, novel methodologies were developed. A new self-assembled monolayers (SAM) soft-contacting method was established which allowed us to obtain the first reliable transport measurements of SAMs contacted by two semiconducting electrodes. We identified different low temperature transport regimes of alkane chains with varying lengths. We also succeeded to "roll-up and press-back" superlattice stacks of different material compositions, mainly semiconducting and metal layers. Preliminary results of the cross-sectional electrical as well as thermal conductivity were obtained using thermoreflectance and local 3ω measurements. In the second period, we will focus on (i) the preparation of micro-patterned contacts to the mesas in order to obtain a complete figure of merit, and (ii) the incorporation of suitable molecular layers with tailored electrical and thermal properties, which will be identified in close cooperation with the theoretical partners. From the modeling point of view, our work progressed in two directions: (a) development of efficient computational methodologies to treat, on a first principle basis, thermal transport in large (experimentally relevant) systems and (b) application of first principle methods combined with Green function techniques to investigate the thermoelectric properties of silicon-molecule-silicon junctions. Hereby a special focus was to demonstrate the possibility of tuning the thermopower of the junction via tailored modifications of the chemical composition of the molecules. In the second funding period, we will apply these methodologies, in close cooperation with the experimental partners, to study (i) electrical and thermal transport in hybrid heterostructures consisting of semiconductor electrodes and pre-defined molecular SAMs, and (ii) to develop a methodology to compute the relevant quantities determining the thermoelectric figure of merit on an atomistic level.