**2. Cellulose substrates for electronic devices: characterization techniques and properties**

Despite the technological applications of paper, there are few contributions regarding cellulose-based substrates characterization, and its applicability in electronic devices. The device and material requirements often limit the choice of the paper substrate, thus a complete understanding of its characteristics, namely, surface morphology, roughness, thermal stability, flexibility, mechanical strength, and hydrophobicity, is essential to assure the proper device operation.

Paper substrates are generally classified based on their weight or *grammage* and can be divided in three categories, following the suggestion of Tobjörk and Österbacka [43]: Light paper, whose *grammage* is in the range of 12–30 g m−2, or for thickness below 75 µm; standard paper, such as office paper, which is around 80 g m−2 and 100 µm thick; and cardboard (or paperboard), when the *grammage* exceeds 200–800 g m−2, or when the thickness is above 300 µm:

• **Light paper**. In this category, one can place two cellulose substrates that are attracting much attention in the recent years: bacterial cellulose (BC) and nanocrystalline cellulose (NCC).

Bacterial cellulose is a bacteria-produced biopolymer composed of ultrafine nanofibers (<100 nm wide); the major perks of this material, comparing to the nanocellulose obtained from wood, is its purity and crystallinity since it is free of lignin, hemicellulose, and other components present in the vegetable cellulose [64–66]. This material can be obtained from several cellulose-producing bacteria, such as *Gluconacetobacter genus, Agrobacterium tumefaciens*, bacteria of the genera *Pseudomonas*, among others; the cellulose is produced extracellularly since the bacteria excretes the cellulose into an aqueous culture medium, of low molecular weight sugars, directly as nanofibers, which form a porous three-dimensional nanocellulose mesh structure [67–69]. The grown layer is then harvested from the medium, cut and dried.

Nanocrystalline cellulose can be obtained primarily from cotton. NCC membranes are prepared through the acid hydrolysis of cellulose process [70]. Cotton is one of the purest cellulose sources, given its higher amount of cellulose (90 wt%), when compared with other vegetal sources that usually contain a mass fraction between 50 wt% (wood) and 80 wt% (flax or hemp) [40, 67, 71]. These membranes are highly transparent, lightweight, and have a smooth surface [40]. The flexibility of some NCC papers and the unique optical properties of nanocrystalline cellulose open a wide range of cost-efficient applications, for instance, smart labels, RFID, smart packaging, or even as support for bio-applications.


material widely used in the food packaging and beverage industry throughout the world. This material comprises three layers: the pressed cellulose fibers (cardboard, 240 g m−2), an adhesive layer of low density polyethylene (LDPE, 12 g m−2), and the aluminum sheet (6–7 mm). One major particularity is its robustness to withstand harsh environments, as evidenced by its use as substrate for solar cell deposited by plasma-enhanced chemical vapor deposition (PECVD) [48] and as an efficient surface-enhanced Raman spectroscopy (SERS) platform supporting metal nanoparticles arrays fabricated through thin film annealing [73].

Based on this classification, the following sections present a comprehensive characterization overview and comparison between paper samples of each category.

The assessment of the surface morphology of cellulose substrates can be achieved by scanning electron microscopy (SEM) [39], backed by three-dimensional (3D) profilometry [74] and AFM [38], to evaluate the paper surface, dimension of fibers, thickness, and porosity. Other techniques, such as X-ray diffraction (XRD) [40, 41], differential scanning calorimetry (DSC), thermogravimetric analysis measurements (TGA) [40, 41], and Fourier transform infrared (FTIR) spectroscopy [75, 76], provide information regarding the paper structure and contents. This information in turn can help to understand the optical properties (such as transmittance and haze factor), studied by spectrophotometry [40], or the electrical properties of the cellulose fibers and overall electrical behavior of the paper material. One method to study the electrical properties of cellulose is the fabrication of different transistors (with and on paper) [38–41, 74] and its analysis by impedance spectroscopy [38, 40, 41, 74].
