**2. Redox molecules, attachment, and characterization techniques**

#### **2.1. Redox‐active molecules**

As described above, the generic structure of a redox‐active molecule consists of redox‐ active components, linkage components, and surface attachment groups. **Figure 1** illustrates two examples of redox molecules with different components [24, 25]. α‐Ferrocenylethanol (referred as ferrocene) molecule shown in **Figure 1a** has one Fe redox center and ‐OH linker component. Ru<sup>2</sup> (ap)4 (C2 C6 H4 P(O)(OH)<sup>2</sup> ) (referred as Ru<sup>2</sup> ) possesses two Ru redox centers, and the phosphonic acid component functions as the linker part. Such redox molecules have dis‐ crete energy states, which are accessible with relatively low, distinct, and quantized voltages. Ferrocene has a single cationic‐accessible state and, therefore, can exhibit two states: neutral and monopositive. Ru<sup>2</sup> has two Ru metal atoms as redox centers; thus, it has two cationic states leading to three charge states: one neutral and two positive states. This is very attractive for multibit memory storage applications.

Despite the redox‐active and linker components shown in **Figure 1**, a variety of different redox‐ active and linkage parts can be specifically designed and engineered for purposes including more redox states and attachment on desired surfaces via covalent bonds [26–29]. The mol‐ ecules reviewer here are more focused on the attachment on Si and oxide structures with specific tethers and linkers, due to the prevalence of silicon in current microelectronic devices and the subsequent implementation in CMOS compatible nonvolatile memory devices.

**Figure 1.** Structure of redox‐active molecules (a) α‐ferrocenylethanol and (b) Ru<sup>2</sup> (ap)4 (C2 C6 H4 P(O)(OH)<sup>2</sup> ) [24].

#### **2.2. Molecule attachment and characterization techniques**

#### *2.2.1. Attachment methods*

is, oxidation and reduction of the redox centers. Distinct charged or discharge states can be deemed as logic on and off states, at different voltage with very fast write and erase speeds. It has been demonstrated that the redox molecules attached on silicon structures are stable and can endure more than 1012 program/erase cycles [23]. Such advantageous properties of redox molecules make them very attractive for future applications of fast speed, low‐power,

There have been great efforts to effectively integrate molecules as the active component for future micro‐/nanoelectronic devices. The following sections will review the attachment and characterization of redox molecules on active surfaces, and the strategies to involve such

As described above, the generic structure of a redox‐active molecule consists of redox‐ active components, linkage components, and surface attachment groups. **Figure 1** illustrates two examples of redox molecules with different components [24, 25]. α‐Ferrocenylethanol (referred as ferrocene) molecule shown in **Figure 1a** has one Fe redox center and ‐OH linker

) (referred as Ru<sup>2</sup>

has two Ru metal atoms as redox centers; thus, it has two cationic

(ap)4 (C2 C6 H4

P(O)(OH)<sup>2</sup>

) [24].

the phosphonic acid component functions as the linker part. Such redox molecules have dis‐ crete energy states, which are accessible with relatively low, distinct, and quantized voltages. Ferrocene has a single cationic‐accessible state and, therefore, can exhibit two states: neutral

states leading to three charge states: one neutral and two positive states. This is very attractive

Despite the redox‐active and linker components shown in **Figure 1**, a variety of different redox‐ active and linkage parts can be specifically designed and engineered for purposes including more redox states and attachment on desired surfaces via covalent bonds [26–29]. The mol‐ ecules reviewer here are more focused on the attachment on Si and oxide structures with specific tethers and linkers, due to the prevalence of silicon in current microelectronic devices and the subsequent implementation in CMOS compatible nonvolatile memory devices.

) possesses two Ru redox centers, and

attractive medium in low‐power and high‐density nonvolatile memory applications.

**2. Redox molecules, attachment, and characterization techniques**

P(O)(OH)<sup>2</sup>

**Figure 1.** Structure of redox‐active molecules (a) α‐ferrocenylethanol and (b) Ru<sup>2</sup>

high‐endurance, and high‐density nonvolatile memory.

**2.1. Redox‐active molecules**

60 Redox - Principles and Advanced Applications

(ap)4 (C2 C6 H4

for multibit memory storage applications.

component. Ru<sup>2</sup>

and monopositive. Ru<sup>2</sup>

The attachment of well‐ordered and tightly packed layers of molecules on active surfaces is important for the application of redox molecules in electronic devices. The attachment tech‐ nique reviewed here is by using the self‐assembled monolayers (SAMs) [24, 30–33]. SAMs are formed on active surfaces via covalent bonds to the atoms on the surface, and specific tether groups can be designed such that they can only attach to specific surfaces. The attachment of SAMs of redox molecules is via the use of chemical solution deposition [30]. A layer of 110 nm SiO2 was first thermally grown on the Si substrate followed by the definition of square‐shaped active areas (100 μm wide) using photolithography and wet etching. Then, a thin layer of SiO<sup>2</sup> was grown in the active area for the molecule attachment on the oxide surface, and it will also function as part of the tunnel barrier in the memory device. The solutions for deposition are prepared by dissolving redox molecules in organic solvents. The SAMs were then formed by placing droplets of the solution on the active areas with each drop kept in place for 3–4 min. The samples were held at elevated temperature in an N<sup>2</sup> environment during the attachment process. Saturated SAMs will be formed after ∼30 min. An alternative approach to form SAMs is by immersing the substrate into the solution under same condition for a certain period of time. During the attachment process, redox molecules are covalently bonded to the desired surface through the linkage component. After the self‐assembly process, the same organic solvent and the cleaning organic solvent were used to rinse the substrates in order to remove any residual molecules that are not bonded to the surfaces.

#### *2.2.2. Characterization techniques*

After the molecule attachment, cyclic voltammetry (CyV) was used to characterize the attach‐ ment quality and measure the molecule surface density. **Figure 2** shows the schematic of the CyV characterization setup [24]. The measurements were performed in a standard probe

**Figure 2.** Schematic of the CyV characterization setup. "∼" represents the voltage source and "M" represents the electrometer [24].

station, with the backside contact made via the probe station chuck. A solution of 1.0‐M tet‐ rabutylammonium hexafluorophosphate (TBAH) in propylene carbonate (PC) was used as the conducting gate electrolyte. Polypropylene micropipette tip containing the silver counter electrode and the electrolyte was lowered until only a small amount of electrolyte was spread across the active area. During the measurement, the voltage was applied on the substrate, and the CyV curves were obtained using a CHI600 electrochemical analyzer.
