Protopopova V S, Wester N, Caro M A, Gabdullin P G, Palomäki T, Laurila T, Koskinen J
Dept. of Materials Science and Engineering, School of Chemical Technology, Aalto University, P.O. Box 16200, FI-00076 Aalto, Espoo, Finland.
Phys Chem Chem Phys. 2015 Apr 14;17(14):9020-31. doi: 10.1039/c4cp05855k. Epub 2015 Mar 9.
In this paper we show that the electronic properties of ultrathin tetrahedral amorphous carbon (ta-C) films are heavily dependent on their thickness. By using scanning tunnelling spectroscopy, Raman spectroscopy, and conductive atomic force microscopy, it was found that a decrease of ta-C thickness from 30 to 7 nm leads to (i) the narrowing of the band gap; (ii) appearance of shallower monoenergetic traps as well as the increase of their concentration; (iii) the increase of the equilibrium concentration of free charge carriers and their mobility; which were caused by (iv) the increase in the sp(2) fraction. However, beyond a certain ta-C thickness (7 nm) the electronic properties of the studied samples start to deteriorate, which is highly likely related to titanium oxide formation at the Ti/ta-C interface. The same tendency is observed for the sample with beforehand air-formed native titanium oxide at the interface. With respect to the last point, it is suggested that the ta-C layer has no uniform coverage if its thickness is small enough (less than 7 nm). The experimental results were rationalized by detailed atomistic simulations. By using the so-called "Tauc plot" we introduce the possibility of the coexistence of bulk and surface band gaps originating from the large increase in sp(2) bonded carbon atoms in the surface region compared to that in the bulk ta-C. The results from the simulations were found to be consistent with the experimental measurements. The previously stated variation in the electronic properties of the layers as a function of their thickness was also exhibited in the electrochemical properties of the samples. It appears that the thinner ta-C layers had more facile electron transfer kinetics as determined with a ferrocenemethanol (FcMeOH) outer sphere redox system. However, if the ta-C layer thickness was reduced too much, the films were not stable anymore.
在本文中，我们表明超薄四面体非晶碳（ta-C）薄膜的电子特性强烈依赖于其厚度。通过使用扫描隧道光谱、拉曼光谱和导电原子力显微镜，发现ta-C厚度从30 nm减小到7 nm会导致：（i）带隙变窄；（ii）出现较浅的单能陷阱以及其浓度增加；（iii）自由电荷载流子的平衡浓度及其迁移率增加；这些是由（iv）sp(2) 组分分数增加引起的。然而，超过一定的ta-C厚度（7 nm）后，所研究样品的电子特性开始变差，这很可能与Ti/ta-C界面处氧化钛的形成有关。对于在界面处预先形成空气氧化的天然氧化钛的样品也观察到相同的趋势。关于最后一点，有人认为如果ta-C层厚度足够小（小于7 nm），它没有均匀覆盖。通过详细的原子模拟使实验结果合理化。通过使用所谓的“Tauc图”，我们引入了体带隙和表面带隙共存的可能性，这源于表面区域中sp(2) 键合碳原子与体相ta-C相比大幅增加。模拟结果与实验测量结果一致。先前所述的层电子特性随厚度的变化在样品的电化学特性中也有体现。似乎用二茂铁甲醇（FcMeOH）外球氧化还原体系测定时，较薄的ta-C层具有更易的电子转移动力学。然而，如果ta-C层厚度减小过多，薄膜就不再稳定。
