Sanjoy Mukherjee

Fabrication of silver nanostructures using femtosecond laser-induced photoreduction

Silver nanostructures were fabricated by femtosecond laser-induced reduction of silver ions and the impact of solution chemistry on the fabricated structures was evaluated. By investigating the exact photochemistry of the nanofabrication solutions, which contained varying amounts of diamine silver ions, trisodium citrate, and n-lauroylsarcosine sodium, and optimizing the laser processing parameters, we fabricated two-dimensional silver pads with surface roughness values of 7 nm and stable 2.5-dimensional shell structures with heights up to 10 μm and aspect ratios of 20 in a ready manner. Moreover, thermal annealing of these structures afforded materials where the average resistivity value was only a factor of 4 greater than that of bulk silver. In this way, the work presented here provides for a methodology that can be used for laser direct fabrication of metal nanostructures for applications in plasmonics and micro- and nano-electronics.

Design of a three-state switchable chromogenic radical-based moiety and its translation to molecular logic systems

Three distinctly different and stable colored states of phenylgalvinoxyl (i.e., neutral phenolic, radical, and anionic species) small molecule and macromolecular systems are evaluated as a function of the solution pH. A clear halochromic effect is readily observed in the design of the polymer, in a manner that is distinct from the more oft-studied small molecule analogs. This key design paradigm allows the chemical nature of the phenylgalvinoxyl moieties to be comparable to a standard AND logic function, which evolves based upon the structural constitution of the material. Moreover, this crucial change in behavior allows for the revelation that the formation of the radical polymer that bears a galvinoxyl pendant group occurs through base-promoted step-wise oxidation, and this, in turn, provides a critical handle by which to design future radical polymer archetypes and to build molecular logic systems based upon this emerging class of functional materials.

Radical polymers as interfacial layers in inverted hybrid perovskite solar cells

We report high performance hybrid perovskite solar cells (PSCs) through the introduction of a radical polymer-based copolymer that contains a second moiety capable of undergoing crosslinking through simple exposure to ultraviolet (UV) light, poly(2,2,6,6-tetramethylpiperidinyloxy-4-yl methacrylate)-co-(4-benzoylphenyl methacrylate) (PTMA-BP). The PTMA-BP thin film engineered the surface of a poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) hole extraction layer (HEL). Systematic investigations indicate that PTMA-BP can induce better band alignment between PEDOT:PSS and perovskite hybrids, reduce interfacial charge carrier recombination, and improve the crystallization of perovskite hybrids that are cast on the top of the HEL. As a result, the stable PSCs incorporated with PTMA-BP exhibit a 15% power conversion efficiency, which is more than a 15% enhancement compared to cells that lacked the PTMA-BP interfacial modifying layer.

Stable Radical Materials for Energy Applications

Although less studied than their closed-shell counterparts, materials containing stable open-shell chemistries have played a key role in many energy storage and energy conversion devices. In particular, the oxidation-reduction (redox) properties of these stable radicals have made them a substantial contributor to the progress of organic batteries. Moreover, the use of radical-based materials in photovoltaic devices and thermoelectric systems has allowed for these emerging molecules to have impacts in the energy conversion realm. Additionally, the unique doublet states of radical-based materials provide access to otherwise inaccessible spin states in optoelectronic devices, offering many new opportunities for efficient usage of energy in light-emitting devices. Here, we review the current state of the art regarding the molecular design, synthesis, and application of stable radicals in these energy-related applications. Finally, we point to fundamental and applied arenas of future promise for these designer open-shell molecules, which have only just begun to be evaluated in full.

Highly Transparent Crosslinkable Radical Copolymer Thin Film as the Ion Storage Layer in Organic Electrochromic Devices

A highly transparent crosslinkable thin film made of the radical polymer poly(2,2,6,6-tetramethyl-4-piperidinyloxy methacrylate)- co-(4-benzoylphenyl methacrylate) (PTMA- co-BP) has been developed as the ion storage layer in electrochromic devices (ECDs). After photo-crosslinking, the dissolution of PTMA- co-BP in electrolytes was mitigated, which results in an enhanced electrochemical stability compared with the homopolymer PTMA thin film. Moreover, the redox capacity of PTMA- co-BP increased because of the formation of a crosslinked network. By matching the redox capacity of the PTMA- co-BP thin film and bis(alkoxy)-substituted poly(propylenedioxythiophene), the ECD achieved an optical contrast of 72% in a small potential window of 2.55 V (i.e., switching between +1.2 and -1.35 V), and it was cycled up to 1800 cycles. The ECD showed an excellent optical memory as its transmittance decayed by less than 3% in both the colored and bleached states while operating for over 30 min under open-circuit conditions. Use of crosslinkable radical polymers as the transparent ion storage layer opens up a new venue for the fabrication of transmissive-mode organic ECDs.

Syntheses of Radical Polymers

What forms, follows functions; however, even the best of intentions can only be realized if and only if a successful formation (i.e., synthesis) of the targeted material can be achieved. Thus, a discussion of the viable and facile synthetic routes for the formation of the desired radical polymers is itself an important field of research. This is because one needs to understand and contain the reactivity of the pendant radical units [1]. Ambient stable radicals can be, and often are, reactive towards various chemical species including other radicals [2]. For instance, radical polymerizations of radical-containing stable monomers may not be a viable approach for the synthesis of radical polymers due to their reactivity. Thus, the choice of polymerization and the associated choice of monomer units are crucial in design and formation of any given radical polymer or polyradical (Fig. 2.1).

Applications of Radical Polymers in Solid-State Devices

The properties of organic materials in bulk solids and thin films are key to their applications in modern optoelectronic systems [1]. That is, unlike most commercial batteries, other devices (e.g., field-effect transistors, light-emitting devices, and organic photovoltaic cells) often demand the application of functional materials in the solid state. As most such applications of radical polymers have only been examined in less than the last 5 years, this area of research is practically the youngest frontier of radical polymers. Nevertheless, even in this relatively short timeframe, there have been several significant discoveries of the properties of radical polymers, which are further boosting the research efforts of these classes of compounds. In fact, the journey of the community started with the idea to utilize the spin systems in radical-containing macromolecules. In the earliest of these examples, which occurred at the dawn of twenty-first century, radical-containing macromolecules (e.g., polyradicals) formed the early examples of organic polymer magnets [2].

Applications of Radical Polymers in Electrolyte-Supported Devices

The modern revolution of organic material sciences cannot be justly described if one does not account for the contributions of polymeric materials [1]. Even at this fast pace of development, no other class of materials can match the versatility of macromolecules with regard to their fine-tuneable physical or chemical properties and ease of processing. It is not an exaggeration to claim that any specific functional property of any given polymer would eventually find (if it has not already found) its own importance in the upcoming avenues of science and engineering. In most cases, demand drives discovery of such materials. However, in many cases, mere curiosity drives discovery, broadening the scopes of the applications of various materials. In the case of radical polymers, the story is quite unique, as are the materials [2]. As noted in an earlier chapter, even though the successful synthesis of PTMA was known since 1972 [3], it required three decades for the community to appreciate the vast opportunity of such materials in any viable application. This spark encouraged an entire generation of researchers towards the broader opportunities of radical polymers, and the most significant impact of radical polymers has been in the development of modern approaches towards fully organic energy storage devices [4]. This opportunity is feasible due to the inherent redox-active electronic properties of this class of compounds.

An Introduction to Radical Polymers

In the modern era, organic compounds (i.e., carbon-based small molecules and polymers) have earned a status of great import across various fields of modern material sciences. Beyond the realms of mere curiosity and fundamental research, the commercialization of organic electronics [e.g., organic light-emitting diodes (OLEDs)] has boosted the impetus to elucidate the fundamentals regarding the vast chemistry and related physical properties and device opportunities of organic materials (Fig. 1.1) [1]. This is because organic materials are often preferred over their inorganic counterparts in electronic devices where their relatively lower costs of production, earth-abundant materials compositions, ease of fine-tuning and processing, mechanical robustness and flexibility, and relatively benign environmental hazards are of primary concern [2].

Conclusions and Future Outlook

After being shelved for a rather long time after their initial discovery in the 1970s, radical polymers are currently finding their unique identity in the modern frontiers of organic electronics. The intriguing chemistry associated with the radical sites opens a number of opportunities in a variety of unique and exhilarating directions. Assuredly, open-shell-bearing stable radical polymer structures have their own emerging opportunities in the field of organic electronics. The redox-active radical sites in the polymers allow for reversible redox reactions, opening opportunities in charge storage, which in turn diverges towards the broader opportunities of organic batteries, supercapacitors, and memory devices. The role of organic radical polymers in revolutionizing the field of organic energy storage systems cannot be emphasized enough. The initial reports on the opportunities of organic polymers in such systems have had a literal domino effect, crushing many of conventional prejudices about the versatility of organic materials. That is, they are not only restricted to the proof-of-concept, and radical polymers are proving to be pioneering in the development of large-scale yet cost-effective redox flow batteries. Furthermore, the emergence of flexible device systems is redefining the borders of flexible organic electronics.