Mono- and few-layer nanocrystalline graphene grown on sapphire by molecular beam epitaxy

The implementation of graphene in different applications will only be possible if the large-area and high-quality production of this material becomes a reality. The ideal synthesis method would allow for precisely controlled formation of graphene layers directly on the insulating substrate of choice. In that context, molecular beam epitaxy (MBE) appears as a promising method. The growth by this technique does not involve catalytic processes and could in principle be extended to different substrates. One of the main advantages of MBE is that it offers the benefit of exact deposition rates and sub-monolayer thickness precision. This is especially attractive regarding the preparation of graphene with a specific number of atomic layers.

Figure 1 (a) Raman spectra measured for graphene films prepared at 1000 ºC with different growth times on Al2O3(0001). The spectra are normalized to the G peak intensity. (b) Raman mapping of the I2D/IG ratio acquired over 1.6X1.6 mm2 surface area for a graphene film grown for 300 min at 1000 °C.

Figure 2 Cross-sectional high-resolution TEM image of a few-layer graphene film on Al2O3(0001). The inset depicts a magnified image of the layers, which are separated by about 3.3 Å, as expected for multilayer graphene. This image was taken from a film grown at 1000 ºC for 300 min.

We have studied the growth of graphene by MBE. We have synthesized nanocrystalline graphene films directly on c-plane Al2O3 substrates by carbon evaporation from a highly-oriented-pyrolytic-graphite filament. The prepared graphene films are continuous over the entire substrate surface up to two-inch wafer sizes.

 

Raman spectroscopy was employed to access the structural characteristics of the MBE-grown graphene. Figure 1 displays typical Raman spectra obtained for films grown at a substrate temperature of 1000 ºC and growth times ranging from 240 to 480 min. The appearance of well-defined and intense G and 2D peaks is a clear evidence for the formation of a graphene structure. Its nanocrystalline nature is revealed by the existence of the D and D` peaks, which are mainly related to the existence of domain boundaries. A systematic analysis of the Raman peaks parameters shows that the films have a high in-plane order with crystalline domain sizes exceeding 30 nm. In addition, it reveals the homogeneity of the layers over large surface areas, which is exemplified here by a Raman mapping of the intensity ratio between the 2D and G peaks acquired over a surface area of 1.6X1.6 mm2 [see Fig. 1(b)]. X-ray photoelectron spectroscopy analysis (not shown) of the same samples reveals that the average thickness of the graphene films varies from about 1 to 3 monolayers for the growth time range from 240 to 480 minutes. This proves the principle that MBE can be used to synthesize not only mono- but also few layers of graphene of similar quality. It also suggests that the growth proceeds in a layer-by-layer manner.

 

The planarity as well as the layered nature of the graphene films is confirmed by transmission electron microscopy (TEM). Fig. 2 depicts an image where the few-layer graphene structure can be seen. Also, there are some local corrugations within the atomic layers. In the corrugation-free areas the graphene layers are separated by about 3.3 ± 0.2 Å, as expected for multilayer graphene structures.

 

Finally, transport measurements revealed that the nanocrystalline graphene layers exhibit mobility values up to 140 cm2/Vs at room temperature. Even though a mobility of 140 cm2/Vs is still low for what is expected for graphene as an electronic material, it shows the potential of MBE as a method to synthesize conducting graphene layers directly on an insulator.

 

The results demonstrate the feasibility of using MBE for achieving a controlled and large-area synthesis of graphene (mono- and few-layer) directly on an insulating substrate. Based on them, it is reasonable to state that the optimization of the growth conditions will certainly allow the growth of high-quality graphene layers (i.e. with crystalline domains exceeding hundreds of nanometers) not only on Al2O3 but on a variety of technologically relevant insulating templates. 

Publication
1 Autor M. H. Oliveira Jr. , T. Schumann , R. Gargallo-Caballero , F. Fromm , T. Seyller , M. Ramsteiner , A. Trampert , L. Geelhaar , J. M. J. Lopes , H. Riechert
Titel

Mono- and few-layer nanocrystalline graphene grown on Al2O3(0001) by molecular beam epitaxy

Source Carbon , 56 , 339 ( 2013 )
2374 Cite : Bibtex RIS
M. H. Oliveira Jr., T. Schumann, R. Gargallo-Caballero, F. Fromm, T. Seyller, M. Ramsteiner, A. Trampert, L. Geelhaar, J. M. J. Lopes, and H. Riechert