VASE of Graphene-Based Films | Encyclopedia MDPI

VASE of Graphene-Based Films: Comparison

Please note this is a comparison between Version 1 by Grazia Giuseppina Politano and Version 2 by Nora Tang.

The interaction of graphene oxide (GO) with magnetron-sputtered metals is a promising research area. VASE optical models of GO thin films deposited on magnetron-sputtered titanium (Ti), silver (Ag) and gold (Au) are discussed. Moreover, the optical properties of graphene nanoplatelet (GNPS) films and reduced graphene oxide (RGO) stabilized with Poly(Sodium 4-Styrenesulfonate) (PSS) films, which are less studied graphene-related materials, are shown. Finally, different optical behaviors of chemical vapor deposition (CVD)-grown monolayer, bilayer, and trilayer graphene films on silicon and polyethylene terephthalate (PET) substrates are recapitulated.

ellipsometry
graphene oxide
reduced graphene oxide
graphene
CVD
optical properties
magnetron sputtering
thin films

1. Introduction

A transparent conductor is a relevant constituent in several photoelectronic appliances. Indium tin oxide (ITO) is principally used for fabricating transparent conductors due to its properties ^[1]. Nevertheless, ITO has many disadvantages; for instance, it is expensive and it does not find application in flexible devices because of its brittle nature ^[2]. Consequently, consideration in the semiconductor field has been drawn to graphene ^[3], which shows broadband light absorption, linear dispersion band structure and an ultrahigh charge-carrier mobility. Graphene-based materials are thus advantageous materials that can be produced in ultrathin sheet form and may be used in several applications ^{[4][5][6][7][8][9][10][11]}.

A transparent conductor is a relevant constituent in several photoelectronic appliances. Indium tin oxide (ITO) is principally used for fabricating transparent conductors due to its properties [1]. Nevertheless, ITO has many disadvantages; for instance, it is expensive and it does not find application in flexible devices because of its brittle nature [2]. Consequently, consideration in the semiconductor field has been drawn to graphene [3], which shows broadband light absorption, linear dispersion band structure and an ultrahigh charge-carrier mobility. Graphene-based materials are thus advantageous materials that can be produced in ultrathin sheet form and may be used in several applications [4,5,6,7,8,9,10,11].

Chemical vapor deposition (CVD) is a technique used for high-quality graphene production ^[12]. The employment of CVD technology in ultradense photonic, optoelectronic, and electronic instruments has been reported ^[13][14][15].

Chemical vapor deposition (CVD) is a technique used for high-quality graphene production [12]. The employment of CVD technology in ultradense photonic, optoelectronic, and electronic instruments has been reported [13,14,15].

Graphene oxide (GO) is a graphene-based material that has more oxygen-containing groups and defects in comparison with mechanically exfoliated or CVD-grown graphene. These defects are advantageous to enhance the performance of photodetectors [16]. Additionally, GO thin films show high optical transmittance in the visible region that allows their use as protective coatings and optically transparent electrodes, crucial in solar cells and for optical applications [17].

GO reduction is a method for large scale graphene manufacturing [18]. Reduced graphene oxide (RGO) is achieved using chemical methods [19], which eliminate or diminish the oxygen-containing groups. Additional reduction methods are thermal annealing that should be carried out above 200 °C [20] and “green reducers” (for instance vitamin C) [21].

RGO can be functionalized with Poly(Sodium 4-Styrenesulfonate) (PSS), which is a polyelectrolyte that avoids RGO aggregation. PSS interacts with graphene by means of π-π interactions, is soluble in water and safe to use [22].

Presently graphene nanoplatelets (GNPs) have arisen as a new graphene-based material. GNPs show some of the beneficial properties of single layer graphene [23]. GNPs are composed of mono- to few-layer sheets of sp

bonded carbon atoms that overlap creating nanometers thick 2D particles [24]. They can be obtained by means of exfoliation of cheap graphite flakes and then through chemical oxidation and graphite oxide nanoplatelets reduction [24].

Graphene-based materials show notable optical properties such as highly transparency in visible spectrum, photo-response up to Terahertz frequency range and tunable infrared optical absorbance [25].

2. Optical Properties

Spectroscopic Ellipsometry (SE) [26] is an highly precise optical method designed for studying the optical properties of materials.

SE has been extensively used to study graphene-based films. The complex refractive index of monolayer graphene has been investigated using SE ^[25][27][28]. In Ref. ^[29] the optical constants of graphene were studied by means of a phenomenological Fano model. The optical properties of thick as well as few-layer GO and RGO were studied using SE ^[30][31].

SE has been extensively used to study graphene-based films. The complex refractive index of monolayer graphene has been investigated using SE [25,27,28]. In Ref. [29] the optical constants of graphene were studied by means of a phenomenological Fano model. The optical properties of thick as well as few-layer GO and RGO were studied using SE [30,31].

We present a review of the authors’ research works on Variable-Angle Spectroscopy (VASE) of graphene-based films ^{[32][33][34][35][36][37][38][39]}.

We present a review of the authors’ research works on Variable-Angle Spectroscopy (VASE) of graphene-based films [32,33,34,35,36,37,38,39].

Despite the availability of literature on SE of graphene-based materials [40], there are not reviews about the optical interaction of GO with magnetron-sputtered metals studied using VASE, which is a promising research area. Moreover, we report about VASE optical model of less studied graphene-based materials such as GNPs and RGO stabilized with PSS films; particular attention to CVD-grown graphene on flexible substrates is given.

3. Conclusions and Outlook

It is no doubt that VASE is among the most valuable tools for studying graphene-based films and it is well-suited for many industrial applications. As in other fields, research on graphene-based applications using VASE has seen dramatic development and it is expanding fast. The advances made in this area are stimulating and hopeful; nevertheless, the challenges are also enormous and should be overcome. Future investigation on VASE of GO films should mainly focus on a much deeper understanding of the reduction mechanism. In fact, further studies on the controllable oxidation and reduction of GO could improve its use as semiconductor for transistor and photoelectronic devices. Moreover, there are several ways that graphene-based films can be functionalized for use in different applications. For instance, VASE could be used to study the optical properties of compounds made by combining graphene-based material with other 2D materials. For instance, multilayers made alternating GO and Molybdenum disulfide (MoS

₂

) could be used for metamaterials with application in energy storage. Another possible application of VASE may be the research on mixing graphene-based materials with matrix polymers, such as polyvinyl alcohol (PVA), to provide an original synthesis route to make graphene-polymers nanocomposites. It would be also interesting studying functional hybrid material composed of CVD-grown graphene on PET substrates and magnetron-sputtered Au and/or Ag for flexible high-performance graphene photodetectors.