The Deposition and Characterization of Single Sheet Graphene Films

The properties of single-sheet graphene (SG) (Figure 1), such as superior tensile strength, high thermal and electrical conductivity, make it a potential material for applications, including building highly efficient electric batteries, creating more flexible yet stronger construction materials and developing faster and tinier electronic circuits.

Since, graphene can behave like both p and n type conductors as a result of its semiconducting nature, researchers consider it as a potential alternative solution for future electronics. This article describes the application of a KSV NIMA Langmuir-Blodgett Deposition Trough and a KSV NIMA PM-IRRAS in the deposition and characterization of graphene films.

Figure 1. Graphene is a form of carbon where the carbon atoms have formed a planar sheet.

Deposition Techniques

SG can be produced by means of a variety of methods, of which different liquid phase exfoliation techniques are used for large-scale production of the nanomaterial. The different liquid phase exfoliation technique involves producing SG or single-sheet graphene oxide (SGO) (Figure 2) in dispersed form.

Controlled transfer of the SG or SGO dispersion onto a support is the challenge involved in this method. Some of the promising methods for controlled preparation of graphene layers include layer-by-layer (LbL) assembly, dip coating, Langmuir-Schaefer (LS) deposition, and Langmuir-Blodgett (LB) deposition.

Figure 2. Graphene oxide is a similar sheet structured material to graphene.

Dip coating method requires a precise control over parameters such as rate of dipping, concentration, and temperature. The LS and LB film deposition methods involve preparing a floating monolayer of the precursor material on a subphase within in a Langmuir trough.

Here, the monolayer is compressed with barriers travelling along the surface in order to control the layer packing. The LS deposition method involves dipping the substrate through the surface parallel to the layer, whereas the LB deposition technique is at right angle to the surface (Figures 3, 4 and 5).

Figure 3. Langmuir-Schaefer deposition

Figure 4. Langmuir-Blodgett deposition

Figure 5. Multiple Langmuir-Blodgett depositions

The surface pressure and the rate of deposition need to be accurately controlled in both techniques in order to obtain a homogenous layer deposition. In all these methods, SGO is deposited for the preparation of SG layers.

Chemical treatment is used for reduction of SGO into SG. The treatments involve protective gases at elevated temperatures (600-1100°C) and the application of reductive agents at low or elevated temperatures. Cationic polymers such as poly(allylamine hydrochloride) (PAH) and polyaniline (PAN) are used for the Layer-by-Layer assembly of SGO, involving linear growth of the layer thickness following the deposition of the third pair of SGO-PAH.

The size of the SGO sheets and the layer quality are influenced by the concentration of the SGO. The resulting material exhibits an order of magnitude higher conductivity, thanks to the application of the hole-conducting PAN.

Both LB/LS instruments and dip coaters are offered by KSV NIMA for accurate material deposition for research purposes. The products range from instruments designed for tiny substrates and liquid volumes to instruments that can handle up to 500mm wide wafers and a large number of vessels.

The easy-to-use software allows the users to control deposition parameters accurately and independently of the technique selected, while maintaining the experiments easy to perform repeatedly.

Characterization Techniques

X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), atomic force microscopy (AFM) are some of the commonly applied techniques for graphene characterization. Ellipsometry for thickness, UV-VIS for optical transmittance measurement, and FT-IR in KBr disks or from film for identification of chemical composition are the other methods utilized to characterize SG and SGO.

The FT-IR of floating monolayers and thin films can be measured using polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) by collecting the surface-specific FT-IR spectra of materials due to the variations in the reflection of s and p polarized light at interfaces.

A sophisticated FTIR reflection spectrometer coupled to a polarization modulation system (PM-IRRAS) is offered by KSV NIMA for fast and highly specific thin film characterization. This method does not require protective gasses due to the absence of interference from environmental factors.

As a result, the KSV NIMA PM-IRRAS has a very open architecture, thereby facilitating measurements from solid samples and liquid surfaces of virtually any size. This open architecture enables the collective application of external UV light source and other complementary instruments such as heater, allowing the monitoring of SG or SGO prior to deposition, after possible reduction, and after deposition with a single device.

The combination of the KSV NIMA PM-IRRAS and KSV NIMA Lang-muir-Blodgett Trough can perform measurements on a floating graphene oxide layer grown on a pure water subphase prior to and following LS deposition to a gold-coated glass slide (Figures 6 and 7). In these illustrations, an SPR instrument was used to measure the deposited graphene layer thickness on a gold-coated glass slide.

Figure 6. PM-IRRAS spectrum of a GO layer at air-water interface

Figure 7. PM-IRRAS spectrum of a GO layer deposited on a gold substrate

Conclusion

Thin graphene and graphene oxide films can be prepared in a controlled manner using dip coating, LS and LB methods, which provide a high degree of control for deposition of a dispersion of SG and SGO made by the liquid exfoliation techniques. The liquid exfoliation techniques are considered for industrial scale production of graphene, whereas these deposition techniques are highly significant in graphene research. The use of KSV NIMA PM-IRRAS allowed acquisition of in-depth IR spectra of both floating and deposited layers to identify their chemical composition.

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