Sohyeon Seo

Vertical alignments of graphene sheets spatially and densely piled for fast ion diffusion in compact supercapacitors.

Supercapacitors with porous carbon structures have high energy storage capacity. However, the porous nature of the carbon electrode, composed mainly of carbon nanotubes (CNTs) and graphene oxide (GO) derivatives, negatively impacts the volumetric electrochemical characteristics of the supercapacitors because of poor packing density (<0.5 g cm(-3)). Herein, we report a simple method to fabricate highly dense and vertically aligned reduced graphene oxide (VArGO) electrodes involving simple hand-rolling and cutting processes. Because of their vertically aligned and opened-edge graphene structure, VArGO electrodes displayed high packing density and highly efficient volumetric and areal electrochemical characteristics, very fast electrolyte ion diffusion with rectangular CV curves even at a high scan rate (20 V/s), and the highest volumetric capacitance among known rGO electrodes. Surprisingly, even when the film thickness of the VArGO electrode was increased, its volumetric and areal capacitances were maintained.

Nonvolatile memory device using gold nanoparticles covalently bound to reduced graphene oxide.

Nonvolatile memory devices using gold nanoparticles (AuNPs) and reduced graphene oxide (rGO) sheets were fabricated in both horizontal and vertical structures. The horizontal memory device, in which a singly and doubly overlayered semiconducting rGO channel was formed by simply using a spin-casting technique to connect two gold electrodes, was designed for understanding the origin of charging effects. AuNPs were chemically bound to the rGO channel through a π-conjugated molecular linker. The π-conjugated bifunctional molecular linker, 4-mercapto-benzenediazonium tetrafluoroborate (MBDT) salt, was newly synthesized and used as a molecular bridge to connect the AuNPs and rGOs. By using a self-assembly technique, the diazonium functional group of the MBDT molecular linker was spontaneously immobilized on the rGOs. Then, the monolayered AuNPs working as capacitors were covalently connected to the thiol groups of the MBDT molecules, which were attached to rGOs (AuNP-frGO). These covalent bonds were confirmed by XPS analyses. The current-voltage characteristics of both the horizontal and vertical AuNP-frGO memory devices showed noticeable nonlinear hysteresis, stable write-multiple read-erase-multiple read cycles over 1000 s, and a long retention time over 700 s. In addition, the vertical AuNP-frGO memory device showed a large current ON/OFF ratio and high stability.

Photo-switchable molecular monolayer anchored between highly transparent and flexible graphene electrodes

A molecular ultra-thin film (for example, a molecular monolayer) with graphene electrodes would allow for the realization of superior stable, transparent and flexible electronics. A realistic prospect regarding the use of graphene in two-terminal molecular electronic devices is to fabricate a chemically stable, optically transparent, mechanically flexible and molecularly compatible junction. Here we report on a novel photo-switchable molecular monolayer, one side chemically and the other side physically anchored between the two graphene electrodes. The photo-switchable organic molecules specified with an electrophilic group are chemically self-assembled into a monolayer on the graphene bottom electrode, while the other end is physically contacted to the graphene top electrode; this arrangement provides excellent stability for a highly transparent and flexible molecular monolayer device with a high device yield due to soft contacts at the top electrode interface. Thus, the transparent graphene electrodes allow stable molecular photo-switching due to photo-induced changes in the molecular conformational length.

An electrolyte-free flexible electrochromic device using electrostatically strong graphene quantum dot-viologen nanocomposites.

A strong electrostatic MV(2+) -GQD nanocomposite provides an electrolyte-free flexible electrochromic device wih high durability. The positively charged MV(2+) and negatively charged GQD are strongly stabilized by non-covalent intermolecular forces (e.g., electrostatic interactions, π-π stacking interactions, and cation-π electron interactions), eliminating the need for an electrolyte. An electrolyte-free flexible electrochromic device fabricated from the GQD-supported MV(2+) exhibits stable performance under mechanical and thermal stresses.

Nitrogen-doped partially reduced graphene oxide rewritable nonvolatile memory.

As memory materials, two-dimensional (2D) carbon materials such as graphene oxide (GO)-based materials have attracted attention due to a variety of advantageous attributes, including their solution-processability and their potential for highly scalable device fabrication for transistor-based memory and cross-bar memory arrays. In spite of this, the use of GO-based materials has been limited, primarily due to uncontrollable oxygen functional groups. To induce the stable memory effect by ionic charges of a negatively charged carboxylic acid group of partially reduced graphene oxide (PrGO), a positively charged pyridinium N that served as a counterion to the negatively charged carboxylic acid was carefully introduced on the PrGO framework. Partially reduced N-doped graphene oxide (PrGODMF) in dimethylformamide (DMF) behaved as a semiconducting nonvolatile memory material. Its optical energy band gap was 1.7-2.1 eV and contained a sp2 C═C framework with 45-50% oxygen-functionalized carbon density and 3% doped nitrogen atoms. In particular, rewritable nonvolatile memory characteristics were dependent on the proportion of pyridinum N, and as the proportion of pyridinium N atom decreased, the PrGODMF film lost memory behavior. Polarization of charged PrGODMF containing pyridinium N and carboxylic acid under an electric field produced N-doped PrGODMF memory effects that followed voltage-driven rewrite-read-erase-read processes.

Solution-processed reduced graphene oxide films as electronic contacts for molecular monolayer junctions.

In monolayer-based molecular electronics, functionalized molecules are utilized as active components; in electronic devices, these functionalities are accessed through stable electrical contacts. In such molecular junctions, a metallic contact can be used to complete a molecular electronic circuit. For practical applications, however, the formation of a soft contact is highly desired to avoid the possibility of a metal filamentary short circuit or molecular damage caused by the penetration of metal atoms into molecular monolayers during direct metal deposition. Therefore, contact fabrication techniques such as indirect metal evaporation, surfacediffusion-mediated deposition, and soft organic interlayer coating have been developed for molecular electronic devices. One successful technique used for the formation of stable molecular junctions involves the use of a conducting layer of an organic material such as single-walled nanotubes (SWNTs), a conducting polymer (e.g., poly(3,4-ethylenedioxythiophene):poly(4-styrenesulphonic acid), PEDOT:PSS), or multilayered graphene as a top electrode or a bridging interfacial layer between the active molecules and a top electrode. These layers can provide proper contact resistance to facilitate true molecular effects in monolayer-based devices composed of alkanethiols/alkanedithiols, photoisomers, p-conjugated organic molecules, or metal complexes. Nonetheless, although molecule-dependent electronic transport has been thoroughly investigated, previous interlayer junctions have not allowed for a clear elucidation of the intrinsic properties of functionalized molecules such as memory components in molecular monolayer devices. A reliable device system with a highly sensitive interlayer with molecular functionality is required for the further development of these molecular devices. To develop a highly sensitive interlayer with a high device yield, it is crucial to create an interlayer that is stable, both chemically and electrically, with a soft contact that is able to communicate between the functional molecular monolayer and a metal electrode. To optimize the sensitivity of the interlayer, its thickness should be easily controllable by varying the number of layers. For a stable junction, the interlayer should protect the molecular monolayers from the penetration of hot metal nanoparticles during the vapor deposition of a metallic top electrode because most functional molecular monolayers, which are only a few nanometres thick (< 2–3 nm), are vulnerable to this contamination. Chemically exfoliated graphene oxide (GO) consists of atomically thin sheets of oxidized graphite that are dispersible in various solvents and can be used to produce dispersible reduced graphene oxide (rGO) by chemical reduction. The conductivity of rGO is comparable to that of SWNT and is increased by thermal treatment, similar to graphite. Graphene and rGO are composed of sp carbon networks and can act as electrodes for electronic devices. One notable advantage of the use of rGO for organic electronics is its solution processability compared with that of graphene. For example, an rGO film electrode can be easily prepared by spin-coating an rGO solution onto a substrate or by spincoating with GO followed by vapor reduction. Additionally, the strong p–p interactions between rGO nanosheets result in a graphite interlayer distance of approximately 0.34 nm, leading to a high conductivity that is comparable to that of highly oriented pyrolytic graphite (HOPG). Electrical conduction in rGO thin films is treated as that through a semimetal such as graphite, and the contact resistance in rGO devices is negligible. Because rGO has high chemical stability, mechanical strength, and a work function comparable to that of gold, rGO has been considered to be a promising candidate for interfacial electronic contacts in monolayer molecular electronics. Herein, we report the development of a solution-processed electronic contact between rGO thin films and molecular components in monolayer-based devices. The rGO contact allows for stable monolayer junctions of alkanethiol monolayers (e.g., molecular resistors) and redox-active metal complex monolayers (e.g., molecular memories) that prevent the formation of metallic short circuits and exhibit excellent junction preservation. The effects of monolayer thickness and molecular functionality were examined in novel molecular junctions using rGO interlayers. Through the semimetallic rGO interlayer contact, the current hysteresis loops and threshold conductance [*] Dr. S. Seo, M. Min, Dr. J. Lee, Dr. H. Lee National Creative Research Initiative, Center for Smart Molecular Memory, Department of Chemistry, Samsung-SKKU Graphene Center, Sungkyunkwan University 300 Cheoncheon-dong, Jangan-gu, Suwon, Gyeonggi-do 440-746 (Korea) E-mail: hyoyoung@skku.edu

Can Commonly Used Hydrazine Produce n‐Type Graphene?

A simple chemical method to obtain bulk quantities of N-doped, reduced graphene oxide (rGO) sheets (see figure) as an n-type semiconductor through the treatment of as-prepared GO sheets with the commonly used reducing reagent hydrazine, followed by rapid thermal annealing (RTA) is described.

Synthesis of Highly n‐Type Graphene by Using an Ionic Liquid

The development of an n-type, graphene-based semiconductor is currently a significant research interest. Graphene is easily p-doped by adsorbates such as oxygen and moisture and thus a p-type semiconductor can be easily prepared. However, the development of an n-type semiconductor is required to fabricate a complementary circuit, and nitrogendoped (N-doped) materials would be useful in real device applications. Chemical doping is an important method used to modulate the electrical properties of graphene. Both theoretical calculations and detailed experiments have proved that chemical doping with foreign atoms, such as nitrogen, is an effective approach to achieve n-type semiconductors. [1] nType semiconductors can be obtained by replacing carbon atoms with nitrogen atoms in the graphene framework. The lone electron pairs of nitrogen atoms play an important role in producing a delocalized conjugated system with sp 2 hybridized carbon frameworks [2–4] that can enhance the reactivity and electrocatalytic properties of graphene. Substitutional N-doped multilayer graphene sheets were synthesized by adding NH3 gas during the chemical vapor deposition (CVD) growth of graphene, [1] and the monolayer growth of N-doped graphene sheets by using poly(methyl methacrylate) (PMMA) and pyridine was recently reported. [5, 6] The nitrogen-doping behavior of graphene and reduced graphene oxide (rGO) through electrical joule heating and thermal annealing in NH3, respectively, has been reported to produce an n-type semiconductor. [7, 8] However, the systematic investigation of graphene doping is required to achieve a large N-doping effect in real device applications. A high atomic percentage of dopant nitrogen is important in the fabrication of n-type rGO with a large shift of the Dirac point (DP). The catalyst-free synthesis of N-doped graphene by thermal annealing of graphene oxide (GO) with melamine through a bulk reaction was reported to provide a high atomic percentage of nitrogen on the rGO surface. [9] How

Voltage‐Controlled Nonvolatile Molecular Memory of an Azobenzene Monolayer through Solution‐Processed Reduced Graphene Oxide Contacts

The solution-processed fabrication of an azobenzene (ABC10) monolayer-based nonvolatile memory device on a reduced graphene oxide (rGO) electrode is successfully accomplished. Trans--cis isomerizations of ABC10 between two rGO electrodes in a crossbar device are controlled by applied voltage. An rGO soft-contact top electrode plays an important role in the conformational-change-dependent conductance switching process of an ABC10 monolayer.

Atomic Dopants Involved in the Structural Evolution of Thermally Graphitized Graphene

Thermally doped nitrogen atoms on the sp(2)-carbon network of reduced graphene oxide (rGO) enhance its electrical conductivity. Atomic structural information of thermally annealed graphene oxide (GO) provides an understanding on how the heteroatomic doping could affect electronic property of rGO. Herein, the spectroscopic and microscopic variations during thermal graphitization from 573 to 1,373 K are reported in two different rGO sheets, prepared by thermal annealing of GO (rGO(therm)) and post-thermal annealing of chemically nitrogen-doped rGO (post-therm-rGO(N(2)H(4))). The spectroscopic transitions of rGO(N(2)H(4)) in thermal annealing ultimately showed new oxygen-functional groups, such as cyclic edge ethers and new graphitized nitrogen atoms at 1,373 K. During the graphitization process, the microscopic evolution resolved by scanning tunneling microscopy (STM) produced more wrinkled surface morphology with graphitized nanocrystalline domains due to atomic doping of nitrogen on a post-therm-rGO(N(2)H(4)) sheet. As a result, the post-therm-rGO(N(2)H(4))-containing nitrogen showed a less defected sp(2)-carbon network, resulting in enhanced conductivity, whereas the rGO(therm) sheet containing no nitrogen had large topological defects on the basal plane of the sp(2)-carbon network. Thus, our investigation of the structural evolution of original wrinkles on a GO sheet incorporated into the graphitized N-doped rGO helps to explain how the atomic doping can enhance the electrical conductivity.