Ultrathin undoped tetrahedral amorphous carbon films: thickness dependence of the electronic structure and implications for their electrochemical behaviour

Abstract

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.

Multi-walled carbon nanotubes (MWCNTs) grown directly on tetrahedral amorphous carbon (ta-C): An interfacial study

“…With respect to this, we have observed a very strong correlation between the thickness and the average current going through the ta-C layers [12]. In addition, this same thickness dependency is strongly reflected in the electrochemical behavior (especially separation between the peak potentials in FcMeOH redox system) as discussed in more detail in [24].…”

Characterization and electrochemical properties of iron-doped tetrahedral amorphous carbon (ta-C) thin films

“…20 The I D /I G of 0.37 AE 0.08 measured for the 10 at% Fe sample, is over twice than the 0.15 AE 0.02 measured for the reference ta-C, but still comparable to I D /I G values reported for ta-C lms in literature. 20,31,49 The higher I D /I G with increased Fe content points to an increased amount of sp 2 -clustering or total sp 2 -fraction in the Fe/ta-C matrix. 37 Therefore, added iron appears to have an effect on the carbon-carbon bonding in ta-C.…”

“…Complementary to the resistive sp 3 -rich bulk of ta-C, the surface of the lm is characteristically sp 2 -rich due to "ion peening"-growth model, as proposed by Caro et al 40 in a recent computational study utilizing machine-learned interatomic potential trained from ab initio data. Several other computational 31,41,42 as well as XAS, XPS, and EELS studies 18,43 have investigated the nature of ta-C and its sp 2 -rich surface structures. For example, Caro et al 42 showed that the surface of ta-C lms consisted of clustered olenic chains, with a lower ratio of ringed carbon structures.…”

“…4(B) and Table 3, is typical for a pure high sp 3 -fraction ta-C matrix. 13,31 With an addition of 2 at% Fe, the anodic branch exhibits a negative shi from +2.7 V to +2.4 V, while the cathodic branch experiences a more noticeable shi of +1.3 V from À2.5 V to À1.2 V. In total, the cathodic and anodic branches shi by +1.4 V and À0.5 V, respectively to a potential window of 3.3 V for the 10 at% Fe sample. The peak current of a redox couple shown in the inset of Fig.…”

“…DLC and ta-C lms have been used in a broad range of analyte detection applications, such as enzymatic hydrogen peroxide sensing, In this work, the addition of iron into a pure ta-C matrix (Fe/ ta-C) is investigated to enable the adjustment of both electrical and electrochemical properties, while retaining other properties, such as the wide water window. Although facile outer sphere electron transfer has been achieved for 7 nm thick ta-C electrodes, 20,31 such ultrathin lms commonly lack uniformity and are not feasible for application on non-planar surfaces. Thicker lms on the other hand, such as the 30 nm thick reference ta-C in this study, become too resistive for most applications.…”