Atomic Layer Deposition of Titanium Oxide on Single-Layer Graphene: An Atomic-Scale Study toward Understanding Nucleation and Growth

Abstract

Controlled synthesis of a hybrid nanomaterial based on titanium oxide and single-layer graphene (SLG) using atomic layer deposition (ALD) is reported here. The morphology and crystallinity of the oxide layer on SLG can be tuned mainly with the deposition temperature, achieving either a uniform amorphous layer at 60 °C or ∼2 nm individual nanocrystals on the SLG at 200 °C after only 20 ALD cycles. A continuous and uniform amorphous layer formed on the SLG after 180 cycles at 60 °C can be converted to a polycrystalline layer containing domains of anatase TiO2 after a postdeposition annealing at 400 °C under vacuum. Using aberration-corrected transmission electron microscopy (AC-TEM), characterization of the structure and chemistry was performed on an atomic scale and provided insight into understanding the nucleation and growth. AC-TEM imaging and electron energy loss spectroscopy revealed that rocksalt TiO nanocrystals were occasionally formed at the early stage of nucleation after only 20 ALD cycles. Understanding and controlling nucleation and growth of the hybrid nanomaterial are crucial to achieving novel properties and enhanced performance for a wide range of applications that exploit the synergetic functionalities of the ensemble.

Evidence for Chemicals Intermingling at Silicon/Titanium Oxide (TiOx) Interface and Existence of Multiple Bonding States in Monolithic TiOx

“…However, from 2 to 56 nm, the O‐K edge represented only the TiO x . Furthermore, by close inspection and following standard literature, we observed very interesting behavior occurring at the 2–3 nm region. From 1 to 3 nm, the Ti‐L peak continuously shifted toward higher energy loss values in both samples.…”

“…Between 3 and 56 nm, the structure of TiO x changed further and the Ti L 3 and L 2 peaks were found to further split into two peaks, namely t 2g (peaks A′ and B′ in Figure b) and e g (peaks A and B in Figure b), via the crystal field splitting phenomenon. However, unlike the usual rutile or anatase form of TiO 2 which shows strong crystal field splitting as shown in Figure c (taken from the EELS database) and following many other references, the crystal field splitting effect was observed to be weaker in our TiO x samples. To understand this and investigate the structure of TiO x between 3 and 56 nm, we examined our films by Raman spectroscopy and X‐ray diffraction (XRD), discussed the molecular orbital theory for oxides, and correlated our EELS results with standard literature.…”

“…This shift of the Ti‐L peak along with the O‐K edge spectra in this region (Figure b) indicates that the structure of TiO x evolves when moving away from the mixed oxide interface layer. We found that at the 2–3 nm location, the spectral shape of Ti‐L is similar to TiO with the possibility of minor amounts of higher oxidation states such as Ti 3+ and Ti 4+ being present, indicating a region of oxygen‐deficient oxide when considering TiO 2 as a reference . XPS analysis (discussed later) also supports this observation.…”

“…Based on the spectral shape of the Ti‐L peak, EELS spectra provided evidence for the existence of multiple bonding states of TiO x , extending from near the interface to the top (56 nm) region of the TiO x layer. In general, the Ti‐L edge EELS spectra reveal two characteristic peaks, namely L 3 and L 2 . In our case, the Ti‐L edge spectra demonstrated mainly two well‐defined L 3 and L 2 peaks at and near the mixed oxide/TiO x interface (up to 2–3 nm into the TiO x layer), which were generated as a consequence of a changeover from spin–orbit splitting levels 2p 3/2 and 2p 1/2 to unoccupied 3d states; 2p 3/2 → 3d (L 3 ) and 2p 1/2 → 3d (L 2 ).…”

Preparation of graphene oxide/semiconductor oxide composites by using atomic layer deposition

“…These latter studies involved depositing oxides (e.g. A 2 O 3 [22]; TiO 2 [23][24][25][26][27][28][29][30]) and metals (e.g. Pt [31]).…”