Younjoo Lee

DNA-Base Guanine as Hydrogen Getter and Charge-Trapping Layer Embedded in Oxide Dielectrics for Inorganic and Organic Field-Effect Transistors

DNA-base small molecules of guanine, cytosine, adenine, and thymine construct the DNA double helix structure with hydrogen bonding, and they possess such a variety of intrinsic benefits as natural plentitude, biodegradability, biofunctionality, low cost, and low toxicity. On the basis of these advantages, here, we report on unprecedented useful applications of guanine layer as hydrogen getter and charge trapping layer when it is embedded into a dielectric oxide of n-channel inorganic InGaZnO and p-channel organic heptazole field effect transistors (FETs). The embedded guanine layer much improved the gate stability of inorganic FETs gettering many hydrogen atoms in the gate dielectric layer of FET, and it also played as charge trapping layer to which the voltage pulse-driven charges might be injected from channel, resulting in a threshold voltage (Vth) shift of FETs. Such shift state is very ambient-stable and almost irrevocable even under a high electric-field at room temperature. So, Boolean logics are nicely demonstrated by using our FETs with the guanine-embedded dielectric. The original Vth is recovered only under high energy blue photons by opposite voltage pulse (charge-ejection), which indicates that our device is also applicable to nonvolatile photo memory.

Energy level alignment at the interfaces between typical electrodes and nucleobases: Al/adenine/indium-tin-oxide and Al/thymine/indium-tin-oxide

We investigated the interfacial electronic structures of Al/adenine/indium-tin-oxide (ITO) and Al/thymine/ITO using in situ ultraviolet and x-ray photoemission spectroscopy and density functional theory calculations. Adenine shows both an interface dipole and level bending, whereas thymine shows only an interface dipole in contact with ITO. In addition, thymine possesses a larger ionization energy than adenine. These are understood with delocalized π states confirmed with theoretical calculations. For the interface between nucleobases and Al, both nucleobases show a prominent reduction of the electron injection barrier from Al to each base in accordance with a downward level shift.

An efficient collective communication method for grid scale networks

This research is to design a collection of effective collective operations for the Grid scale network environment. In the Grid, several network features should be specified and adapted into the algorithmic design of collective operations. For this, we characterize the Grid and design hierarchical latency optimal tree algorithm for the MPI (message passing interface) library. The experimental results show that performance gain in performing collective operations can be achieved by around 160% compared with MPICH-G2 which is based on the flat algorithm.

Vacuum-integrated electrospray deposition for highly reliable polymer thin film

Vacuum electrospray deposition (ESD) equipment was designed to prepare polymer thin films. The polymer solution can be injected directly into vacuum system through multi-stage pumping line, so that the solvent residues and ambient contaminants are highly reduced. To test the performance of ESD system, we fabricated organic photovoltaic cells (OPVCs) by injecting polymer solution directly onto the substrate inside a high vacuum chamber. The OPVC fabricated has the structure of Al∕P3HT:PCBM∕PEDOT:PSS∕ITO and was optimized by varying the speed of solution injection and concentration of the solution. The power conversion efficiency (PCE) of the optimized OPVC is 3.14% under AM 1.5G irradiation without any buffer layer at the cathode side. To test the advantages of the vacuum ESD, we exposed the device to atmosphere between the deposition steps of the active layer and cathode. This showed that the PCE of the vacuum processed device is 24% higher than that of the air exposed device and confirms the advantages of the vacuum prepared polymer film for high performance devices.

Increasing the effective work function of multilayer graphene films using silver nanoparticles

The authors report a practical method to increase the effective work functions of multilayer graphene (MLG) films for high work function transparent electrodes. MLG films were prepared using a chemical vapor deposition and transfer process. The effective work functions of MLG films covered with different amounts of silver (Ag) nanoparticles were measured using ultraviolet photoelectron spectroscopy (UPS). The formation of Ag nanoparticles on the MLG surface was investigated using scanning electron microcopy. The sheet resistance and transmittance of the MLG films were also measured with different amounts of Ag nanoparticles on the MLG surfaces. The UPS measurements revealed that the effective work function of an MLG film could be increased from 4.39 to 4.55 eV by adding Ag nanoparticles onto the MLG surface with minimal changes in the sheet resistance and transmittance.

Strong interfacial dipole formation with thermal evaporation of lithium cobalt oxide for efficient electron injections

We investigated the electronic structures at the interface of Al/lithium cobalt oxide (LiCoO2)/tris(8-hydoxyquinoline) aluminum (Alq3) to elucidate the origin of the electron injection enhancement with the insertion of the LiCoO2 layer in organic light-emitting devices using in situ photoelectron spectroscopy experiments. We discovered that LiCoO2 was decomposed into lithium oxide (Li2O) by thermal evaporation, and only Li2O was deposited on the desired substrate. Li2O forms a strong interfacial dipole, which reduces the surface potential on Alq3 due to its extremely low work function. As a result, the electron injection barrier was dramatically decreased by the Li2O layer. Furthermore, there is no strong chemical interaction at the interface of Al/Li2O/Alq3; hence, this would contribute to extend the device lifetime.