Dong Yeol Yun

Electrical bistabilities and carrier transport mechanisms of write-once-read-many-times memory devices fabricated utilizing ZnO nanoparticles embedded in a polystyrene layer

High-resolution transmission electron microscopy images showed that ZnO nanoparticles were randomly distributed in a polystyrene (PS) layer. Current-voltage (I-V) curves at 300 K for Al/ZnO nanoparticles embedded in PS layer/indium tin oxide devices showed a current bistability with a large ON/OFF ratio of 103 for write-once-read-many-times (WORM) memory devices. The estimated retention time of the ON state for the WORM device was more than 10 years. The carrier transport mechanisms for the WORM memory device are described on the basis of the I-V results.

Multilevel charging and discharging mechanisms of nonvolatile memory devices based on nanocomposites consisting of monolayered Au nanoparticles embedded in a polystyrene layer

Capacitance-voltage (C-V) curves for Al/Au nanoparticles embedded in a polystyrene (PS) layer/p-Si devices at 300 K showed a metal-insulator-semiconductor behavior with flat-band voltage shifts of the C-V curves due to the existence of charge trapping. Memory windows between 2.6 and 9.9 V were observed at different sweep voltages, indicative of multilevel behavior. Capacitance-time measurements demonstrated that the charge-trapping capability of Au nanoparticles embedded in a PS layer was maintained for retention times larger than 1 × 104 s without significant degradation. The multilevel charging and discharging mechanisms of the memory devices are described on the basis of the experimental results.

Electrical stabilities and carrier transport mechanisms of flexible organic bistable devices based on CdSe-InP core-shell nanoparticle/polystyrene nanocomposites

Flexible organic bistable devices (OBDs) for the memory characteristics utilizing CdSe-InP core-shell nanoparticle/polystyrene nanocomposites were fabricated on indium-tin-oxide-coated polyethylene terephthalate substrates. Current-voltage measurements on Al/CdSe-InP nanoparticles embedded in polystyrene layer/indium-tin-oxide/polyethylene terephthalate devices without and with bending exhibit wide-range current hysteresis behaviours with ON/OFF ratios of 1 × 107 and 1 × 105, respectively. The endurance number of the ON/OFF switchings without bending was 1 × 105 cycles. The switching characteristics of the OBDs after bending were stable enough to distinguish the ON and OFF. The carrier transport mechanisms of the OBDs are described on the basis of the current-voltage results.

Electrical characteristics and operating mechanisms of nonvolatile memory devices fabricated utilizing core-shell CuInS2-ZnS quantum dots embedded in a poly(methyl methacrylate) layer

Nonvolatile memory devices were fabricated with core-shell CuInS2-ZnS quantum dots (QDs) embedded in poly(methyl methacrylate) (PMMA). Capacitance-voltage (C-V) measurements at 300 K on the Al/CuInS2-ZnS QDs embedded in PMMA layer/p-Si device showed capacitance hysteresis behaviors with a flatband voltage shift. The memory window of the device increased with increasing applied sweep voltage and saturated at high electric fields due to the current leakage. Capacitance-time measurements showed that the retention time was larger than 1 × 105 s that was more than 10 years. The operating mechanisms for the devices are described on the basis of the C-V curves.

Charge trapping process of nonvolatile memory devices based on CdTe and CdTe–CdSe core-shell nanoparticles/poly(methylmethacrylate) nanocomposites

Nonvolatile memory devices based on CdTe and CdTe–CdSe core-shell nanoparticles embedded in a poly(methylmethacrylate) (PMMA) layer were fabricated to investigate the variation in the carrier transport mechanisms due to a CdSe shell. Capacitance-voltage (C-V) curves for Al/CdTe nanoparticles embedded in PMMA/p-Si and Al/CdTe–CdSe nanoparticles embedded in PMMA/p-Si devices at 300 K showed that the flatband voltage shift of the C-V curve for the device with the CdTe–CdSe nanoparticles was relatively smaller than that for the device with the CdTe nanoparticle. Carrier transport mechanisms of the memory devices are described by using the C-V results, energy band diagrams, and capacitance-time retentions.

Flexible organic bistable devices based on [6,6]-phenyl-C85 butyric acid methyl ester clusters embedded in a polymethyl methacrylate layer

Flexible organic bistable devices (OBDs) consisting of [6,6]-phenyl-C85 butyric acid methyl ester ([84]PCBM) blended with a polymethyl methacrylate (PMMA) layer were fabricated on indium-tin-oxide (ITO) coated polyethylene terephthalate (PET) substrates. Current-voltage curves of the Al/[84]PCBM:PMMA/ITO/PET device at 300 K showed a current bistability. The maximum ON/OFF ratios of the OBDs at flat and bent conditions were about 7.5 × 102 and 2.7 × 103, respectively. The cycle endurance was larger than 1 × 105 cycles. The memory mechanisms of the OBDs were attributed to trapping and detrapping processes of electrons into and from the PCBM clusters.

Electrical stabilities and memory mechanisms of organic bistable devices fabricated utilizing a poly(3,4-ethylene-dioxythiophene): Poly(styrenesulfonate) layer with a poly(methyl methacrylate) buffer layer

Organic bistable devices (OBDs) based on a poly(3,4-ethylene-dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layer with a poly(methyl methacrylate) (PMMA) buffer layer were fabricated on indium-tin-oxide (ITO)-coated polyethylene terephthalate (PET) flexible substrates. Current-voltage curves for the Al/PEDOT:PSS/PMMA/ITO/PET device showed current bistabilities with an ON/OFF current ratio of 1 × 103, indicative of a significant enhancement of memory storage. The endurance number of the ON/OFF switchings for the OBDs was above 1 × 105 cycles showing high potential applications in read only memory devices. The memory mechanisms for the OBDs on the basis of oxidation and reduction operations were attributed to the filament processes.

Multilevel characteristics and memory mechanisms for nonvolatile memory devices based on CuInS2 quantum dot-polymethylmethacrylate nanocomposites

Nonvolatile memory devices based on CuInS2 (CIS) quantum dots (QDs) embedded in a polymethylmethacrylate (PMMA) layer were fabricated using spin-coating method. The memory window widths of the capacitance-voltage (C-V) curves for the Al/CIS QDs embedded in PMMA layer/p-Si devices were 0.3, 0.6, and 1.0 V for sweep voltages of ±3, ±5, and ±7 V, respectively. Capacitance-cycle data demonstrated that the charge-trapping capability of the devices with an ON/OFF ratio value of 2.81 × 10−10 was maintained for 8 × 103 cycles without significant degradation and that the extrapolation of the ON/OFF ratio value to 1 × 106 cycles converged to 2.40 × 10−10, indicative of the good stability of the devices. The memory mechanisms for the devices are described on the basis of the C-V curves and the energy-band diagrams.

Effects of CdSe shell layer on the electrical properties of nonvolatile memory devices fabricated utilizing core-shell CdTe-CdSe nanoparticles embedded in a poly(9-vinylcarbazole) layer

Nonvolatile memory devices were fabricated with core-shell CdTe-CdSe nanoparticles embedded in a poly(9-vinylcarbazole) (PVK) layer to investigate the variations in the electrical properties due to a CdSe shell layer. Capacitance-voltage measurements on Al/CdTe nanoparticles embedded in PVK layer/p-Si devices and on Al/core-shell CdTe-CdSe nanoparticles embedded in PVK layer/p-Si devices at 300 K showed hysteresis behaviors with a flatband voltage shift due to the existence of the CdTe and the CdTe-CdSe nanoparticles. Capacitance-time measurements showed that the retention time for devices fabricated utilizing core-shell CdTe-CdSe nanoparticles was larger than that for devices fabricated utilizing CdTe nanoparticles.

Optical and electronic properties of type-II CdSe/CdS core–shell quantum dots

CdSe/CdS core–shell quantum dots (QDs) were synthesized using a facile method in aqueous phase. X-ray diffraction pattern, high-resolution transmission electron microscopy images, and energy dispersive spectroscopy profiles showed that stoichiometric CdSe/CdS QDs were formed. Temperature-dependent photoluminescence spectra showed that the activation energy of CdSe/CdS core–shell QDs was 15 meV. The potential profiles and interband transition energies of the strained type-II CdSe/CdS core–shell QDs were calculated. The calculated interband transition energies slightly decreased from 2.061 to 2.007 eV when the shell thickness increased from 10 to 17 A. The theoretical interband transition energy of 2.007 eV was in reasonable agreement with the photoluminescence excitonic transition energy of 1.98 eV.