**3. Redox molecules for nonvolatile memory applications**

In this section, the early logic forms of molecular memory—molecular logic switching devices will be reviewed. The integration of redox molecules in a liquid electrolyte‐involved nonvola‐ tile memory device will be discussed where we also consider the device‐related issues and limitations.

#### **3.1. Introduction**

#### *3.1.1. CMOS memory technology*

The continuous CMOS scaling and the impact on memory scaling has pushed for the investi‐ gation of alternative storage technologies, and many solutions have been proposed and stud‐ ied. Solid‐state mass‐storage memory is getting a large part of the market, as introduced in the first section, due to its consolidated technology, better reliability, nonvolatility, and tolerance to harsh environment. Currently, the main solid‐state reprogrammable nonvolatile memory is based on the floating‐gate structure, which is also known as flash memory. Flash memory has fast read access times, good retention and reliability, and CMOS compatible fabrication pro‐ cess [34, 35]. However, physical limitations related to the difficulty in shrinking the tunnel and interpoly dielectric layer have further reduced the margins for the memory cell size reduc‐ tion predicted by Moore's law. The floating‐gate memory will suffer short‐channel effects when the channel length is scaled below 100 nm. Leakage current will be significant during program/erase operations due to both drain‐induced barrier lowering (DIBL) and subsurface punch‐through effects.

In recent decades, charge‐trapping nonvolatile memory has attracted intensive attention to replace the conventional floating‐gate memory due to its advantages such as better scalabil‐ ity, lower power consumption, improved reliability, and simpler structure and fabrication process [36–38]. In a charge‐trapping memory, the electrons are stored in a trapping layer, instead of the conducting floating gate in the conventional floating‐gate memory. Different charge‐storage media have been well studied, including conventional nitride material, vari‐ ous nanocrystals, and high‐k dielectric materials. Incorporating redox‐active molecules as the charge‐storage medium in a Si‐based nonvolatile memory is very interesting, as it will lever‐ age the advantages afforded by a molecule‐based active medium with the vast infrastructure of traditional metal‐oxide‐semiconductor (MOS) technology.

#### *3.1.2. History of molecular memory*

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

In this section, the early logic forms of molecular memory—molecular logic switching devices will be reviewed. The integration of redox molecules in a liquid electrolyte‐involved nonvola‐ tile memory device will be discussed where we also consider the device‐related issues and

The continuous CMOS scaling and the impact on memory scaling has pushed for the investi‐ gation of alternative storage technologies, and many solutions have been proposed and stud‐ ied. Solid‐state mass‐storage memory is getting a large part of the market, as introduced in the first section, due to its consolidated technology, better reliability, nonvolatility, and tolerance to harsh environment. Currently, the main solid‐state reprogrammable nonvolatile memory is based on the floating‐gate structure, which is also known as flash memory. Flash memory has fast read access times, good retention and reliability, and CMOS compatible fabrication pro‐ cess [34, 35]. However, physical limitations related to the difficulty in shrinking the tunnel and interpoly dielectric layer have further reduced the margins for the memory cell size reduc‐ tion predicted by Moore's law. The floating‐gate memory will suffer short‐channel effects when the channel length is scaled below 100 nm. Leakage current will be significant during program/erase operations due to both drain‐induced barrier lowering (DIBL) and subsurface

In recent decades, charge‐trapping nonvolatile memory has attracted intensive attention to replace the conventional floating‐gate memory due to its advantages such as better scalabil‐ ity, lower power consumption, improved reliability, and simpler structure and fabrication process [36–38]. In a charge‐trapping memory, the electrons are stored in a trapping layer, instead of the conducting floating gate in the conventional floating‐gate memory. Different charge‐storage media have been well studied, including conventional nitride material, vari‐ ous nanocrystals, and high‐k dielectric materials. Incorporating redox‐active molecules as the charge‐storage medium in a Si‐based nonvolatile memory is very interesting, as it will lever‐ age the advantages afforded by a molecule‐based active medium with the vast infrastructure

of traditional metal‐oxide‐semiconductor (MOS) technology.

the CyV curves were obtained using a CHI600 electrochemical analyzer.

**3. Redox molecules for nonvolatile memory applications**

limitations.

**3.1. Introduction**

punch‐through effects.

*3.1.1. CMOS memory technology*

62 Redox - Principles and Advanced Applications

Memory application of molecules has been widely investigated, and one of the most common approaches in the molecular memory devices has been the bistable conductance switching devices. There are two major device architectures based on these devices: molecular crossbar circuit and nanocell molecular circuit.

In a crossbar circuit, the switching element is a metal/molecule/metal sandwich junction, wherein the molecules are located at the cross‐section of two nanoscale metal wires [39–41]. The early demonstration of such a junction‐switching device utilized the bistable rotaxane molecule that consists of two mechanically interlocked rings [39]. The molecular monolayers were deposited as a Langmuir‐Blodgett film. The mechanical motion of such a molecule is an activated redox process, and the two stable mechanical states can exhibit different tunneling currents, representing logic on and off states. Note that ∼100 on/off current ratio has been reported, but with limited endurance cycles. By taking advantage of the fabrication simplic‐ ity and high nanowire density, large‐scale crossbar circuit has been realized [42]. However, the controlling of metal‐molecule interface is the major challenge, upon which a new theory was put forward with experimental evidence indicating that the conductance switching in the metal/molecule/metal junction is independent of molecules, but attributed to the electrode reactions with molecules [43, 44].

Nanocell molecular circuit is a two‐dimensional network of self‐assembled metallic particles connected by molecules [45, 46]. The active component in a nanocell is also metal/molecule/ metal switch. The molecules in a nanocell can show reprogrammable negative differential resistance which was initially believed to be originated from the redox center in the mol‐ ecules. However, experimental results suggested that the electron transport phenomena is more likely due to the nanofilamentary metal switching, which is correlated with the similar metal‐molecule interface problems as the crossbar circuit [46].

In addition to the two‐terminal metal/molecule/metal junction devices, molecules have also been tested as the channel material in a standard three‐terminal metal‐oxide‐semiconduc‐ tor (MOSFET) architecture [47–49]. Even though organic field‐effect transistors (OFETs) with low‐power and low leakage current have been demonstrated, such OFETs suffer from the slower switching speed and limited device lifetime as compared with conventional silicon transistors. Moreover, the poor stability in a harsh environment is another disadvantage of these OFETs. Thus, the direct integration of molecules in advanced silicon CMOS technology is still under question and needs further research efforts.

#### **3.2. Electrolyte/molecule/Si capacitors for memory application**

#### *3.2.1. Introduction*

Based on the molecular logic switching devices, appropriate modifications of the molecular structures and switching elements have been designed to change the switching kinetics with enhanced memory performance. Different device platforms have been engineered to interface with molecules such as oxides, dielectrics, nanowires, and so forth. The memory effect achieved by integrating redox‐active molecules is very promising, due to the intrinsic redox behavior of the molecules giving birth to fast speed, low operation voltage, and high‐reliability memory devices.

Hybrid CMOS/molecular memory devices were proposed and studied by incorporating redox‐active charge‐storage molecules into Si structures through the self‐assembly process [32, 33, 50–52]. Different redox‐active components can be designed or synthesized for mul‐ tiple redox states, thus for complex and high memory density. The memory retention proper‐ ties can be effectively tuned through the engineering of the linkage component. The redox component such as the Fe in 4‐ferrocenylbenzyl alcohol (referred as Fc‐BzOH) and Zn in 5‐(4‐hydroxymethylphenyl)‐10,15,20‐trimesitylporphinatozinc(II) (referred as Por‐BzOH) as shown in **Figure 3a** and **b**, respectively, can be in neutral and positively charged states through losing electrons [32, 51]. The molecular components surrounding the redox center function as the barrier against charge loss. Such an alternative scenario replacing the con‐ ventional tunneling, charge‐trapping, and blocking layers in a charge‐trapping memory with molecular components provides a smooth transition from CMOS technology to molecular electronics technology.

#### *3.2.2. Characterization and performance discussion*

The SAM attachment of the two redox molecules to the silicon surface was carried out by using the similar chemical solution deposition as described in the previous section. Benzonitrile was used as the organic solvent in the attachment process [32, 51]. The sample was placed in the solution for 80 min at 100°C. **Figure 3c** shows the structure of a fabricated electrolyte/ molecule/Si capacitor memory cell. **Figure 4a** and **b** shows the CyV curves of Fc‐BzOH and Por‐BzOH molecular structures, both exhibiting oxidation and reduction peaks. Fc‐BzOH shows one pair of redox peaks because the redox center Fe has only one neutral and one

**Figure 3.** Molecular structure of redox molecules (a) Fc‐BzOH and (b) Por‐BzOH. (c) Schematic structure of the electrolyte/molecule/Si capacitor for memory application with a simplified equivalent circuit [32, 51].

**Figure 4.** CyV of the electrolyte/molecule/Si structures with (a) Fc‐BzOH and (b) Por‐BzOH molecules at different voltage scan rates. *C*‐*V* and *G*‐*V* curves of the capacitor structures involving (c) Fc‐BzOH and (d) Por‐BzOH molecules at 100 Hz [31, 33, 52].

 monopositive state. The Zn redox center in Por‐BzOH exhibits one neutral and two (mono‐ and di‐) positive states, thus, two pairs of redox peaks were observed.

Clear capacitance and conductance peaks related to the oxidation and reduction processes have been observed from the capacitance‐voltage (*C*‐*V*) and conductance‐voltage (*G*‐*V*) characteris‐ tics, as shown in **Figure 4c** and **d**, respectively [33, 52]. The *C*‐*V* and *G*‐*V* curves can be divided into three regions: depletion, accumulation, and redox regions. The depletion and accumula‐ tion are associated with the Si substrate, in a similar manner as MOS capacitors. The charac‐ teristics in the redox regions are due to the charging and discharging of molecules, resembling the CyV characteristics. Capacitance and conductance peaks were also observed at higher fre‐ quencies up to 5 kHz, but with lower capacitance peaks and higher conductance peaks. The lower capacitance peak at higher frequency is attributed to a lower effective capacitance of the electrolyte in series, while the higher conductance peak is due to the increasing frequency.

#### *3.2.3. Multibit molecular memory in one cell*

with molecules such as oxides, dielectrics, nanowires, and so forth. The memory effect achieved by integrating redox‐active molecules is very promising, due to the intrinsic redox behavior of the molecules giving birth to fast speed, low operation voltage, and high‐reliability

Hybrid CMOS/molecular memory devices were proposed and studied by incorporating redox‐active charge‐storage molecules into Si structures through the self‐assembly process [32, 33, 50–52]. Different redox‐active components can be designed or synthesized for mul‐ tiple redox states, thus for complex and high memory density. The memory retention proper‐ ties can be effectively tuned through the engineering of the linkage component. The redox component such as the Fe in 4‐ferrocenylbenzyl alcohol (referred as Fc‐BzOH) and Zn in 5‐(4‐hydroxymethylphenyl)‐10,15,20‐trimesitylporphinatozinc(II) (referred as Por‐BzOH) as shown in **Figure 3a** and **b**, respectively, can be in neutral and positively charged states through losing electrons [32, 51]. The molecular components surrounding the redox center function as the barrier against charge loss. Such an alternative scenario replacing the con‐ ventional tunneling, charge‐trapping, and blocking layers in a charge‐trapping memory with molecular components provides a smooth transition from CMOS technology to molecular

The SAM attachment of the two redox molecules to the silicon surface was carried out by using the similar chemical solution deposition as described in the previous section. Benzonitrile was used as the organic solvent in the attachment process [32, 51]. The sample was placed in the solution for 80 min at 100°C. **Figure 3c** shows the structure of a fabricated electrolyte/ molecule/Si capacitor memory cell. **Figure 4a** and **b** shows the CyV curves of Fc‐BzOH and Por‐BzOH molecular structures, both exhibiting oxidation and reduction peaks. Fc‐BzOH shows one pair of redox peaks because the redox center Fe has only one neutral and one

**Figure 3.** Molecular structure of redox molecules (a) Fc‐BzOH and (b) Por‐BzOH. (c) Schematic structure of the

electrolyte/molecule/Si capacitor for memory application with a simplified equivalent circuit [32, 51].

memory devices.

64 Redox - Principles and Advanced Applications

electronics technology.

*3.2.2. Characterization and performance discussion*

One of the simple yet effective approaches to increase the memory density is to employ charge‐storage element containing molecules with multiple redox states. There are different approaches to realize multiple redox states, including mixed or stacked SAMs of different redox‐active molecules with distinct voltage levels, and synthesis of molecule with multiple redox centers. Here, we review the first method by mixing the Fc‐BzOH and Por‐BzOH mole‐ cules in one memory cell [51]. The second approach will be reviewed in the following section.

Mixed SAMs of Fc‐BzOH and Por‐BzOH on silicon surfaces to achieve a four‐state memory element were achieved by using the chemical deposition method of a mixture solution of Fc‐ BzOH and Por‐BzOH in benzonitrile [51]. Molar ratios (Fc‐BzOH/Por‐BzOH) of 1:0.35, 1:1.4, and 1:3.5 were selected. The CyV curves of the mixed molecules are shown in **Figure 5a**. The results from the nonmixed pure molecules were also illustrated for comparison. One pair of Fc‐BzOH peaks and two pairs of Por‐BzOH peaks were clearly observed for the three mixed SAMs. With decreasing Fc‐BzOH molar percentage, the corresponding peak height decreased substantially due to the changing surface coverage density. The coverage of Fc‐BzOH was higher than that of Por‐BzOH in the pure SAMs and even mixed SAMs with 1:1 molar ratio, because of the smaller size and the faster attachment of smaller molecule of Fc‐BzOH. The redox peaks exhibited a scan rate‐dependent peak separation, as shown in **Figure 5b**. Such phenomena could arise from either an increasing resistive drop in the electrolyte or limitations imposed by the intrinsic electron‐transfer rate of redox centers on the Si substrate [51]. **Figure 5c**–**e** shows the *C*‐*V* and *G*‐*V* characteristics of the SAMs at 100 Hz. Each of the mixed SAMs shows three pairs of peaks corresponding to the charging and discharging behaviors of the Fc‐BzOH and Por‐BzOH mole‐ cules. As the molar ratio decreases, the capacitance and conductance peaks of Por‐BzOH increase while those of Fc‐BzOH decrease. This is also in agreement with the CyV measurement results.

Such a mixed SAM approach is very attractive owing to its simpler synthesis and more dis‐ tinct potential of the redox states, as compared with a single molecule with multiple redox

**Figure 5.** (a) CyV of SAMs of pure and mixed Fc‐BzOH and Por‐BzOH with different molar ratios. The scan rate was 5 V/s. (b) CyV at increasing scan rates of 10, 50, and 100 V/s. *C*‐*V* and *G*‐*V* characteristics with mixed SAMs with molar ratios of (c) 1:0.35, (d) 1:1.4, and (e) 1:3.5 [51].

states. However, the disadvantage of this method is that the density of a given peak goes down. Nevertheless, this approach still paves the way for constructing multibit information storage devices.
