Jongtaek Lee

Improved field emission properties of CdS deposited single walled carbon nanotube emitter

The field emission (FE) properties of sigle-walled carbon nanotubes (SWNTs) are of great importance, especially in applications involving flat panel display devices such as field emission displays (FED) [1]. The electron emitters of the FED must be long-lived and stable, and possess a low turn-on threshold voltage and a high current density at a given external field. [2] In general, the emission source should have highly oriented and well-distributed tubes in order to utilize the characteristics of the nanotubes for FE. A notable method using plasma for the synthesis of the aligned CNTs was developed by Ren et al.[3] We report the modificated field emitter of CdS quantum-dot deposited on SWNTs (DcS/SWNTs). As a II–VI semiconductor compound, CdS has attracted considerable interest in optoelectronic application because of its relatively low electron affinity, high chemical inertness and sputter resistance.

Characteristic features of stone-wales defects in single-walled carbon nanotube; adsorption, dispersion, and field emission

Adsorption behaviors of dodecanethiol (C12H25SH) molecules are investigated on the surface of single-walled carbon nanotubes (SWCNTs) with vibrational and X-ray photoelectron spectrometers. The active adsorption sites are proved as Stone-Wales (SW) defects (5-7 ring defects). The SW defect-removed SWCNTs formed by reacting nanotubes with allyl acrylate molecules are compared with pristine SWCNTs in dispersion and field emission. The former shows higher dispersion and field emission than the latter.

Sensitive and Stable Gas Ionization Sensor Based on 3-D Single-Walled Carbon Nanotube Networks Suspended on ZnO Nanorods

We fabricated a gas ionization sensor based on 3-D single-walled carbon nanotube (SWNT) networks suspended between ZnO nanorods using a chemical vapor deposition method. ZnO nanorods were grown on the pyramid-like protrusions of a textured Si substrate. The breakdown voltages and stability of the 3-D SWNT networks were measured and compared with those of a SWNT film synthesized on Si substrate. Our results show that 3-D SWNT networks on ZnO nanorods enable ionization gas sensors to be sensitive, selective, and highly stable devices.

Field Emission and Electrical Properties of Perovskite

We investigate characteristic field emission properties of methyl ammonium mixed-halide perovskite (CH3NH3PbI3-xClx) and their current change under one laser pulse. To analyze these properties, we fabricated inverted-type mixed-halide perovskite solar cells which exhibit a device efficiency of 9.31% under A.M 1.5 condition. Under one laser pulse varying from 420 nm to 580 nm, perovskite layer considerably reacted from 420 nm to 440 nm and then gradually decreased in current. A turnon field of 5.56 V and a field enhancement factor of 3183 were obtained from one spin-coating perovskite layer and in eight times of perovskite spin-coating cycles, a turn-on field of 6.70 V and a field enhancement factor of 5110 were observed.

Photocurrent Enhancement of CdSe Quantum-Dot Sensitized Solar Cells Incorporating Single-Walled Carbon Nanotubes

We demonstrate quantum-dot sensitized solar cells (QDSSCs) which have colloidal CdSe quantum dots (TOPO-CdSe) as a sensitizer onto mesoporous TiO2 photoanodes. CdS quantum-dot (QD) layer plays a role of buffer layer for direct adsorption of TOPO-CdSe. We incorporate single-walled carbon nanotubes with TiO2 photoanode of our QDSSCs to facilitate efficient charge transfer. Shortcircuit current densities (Jsc) of our QDSSCs are enhanced while other parameters are maintained. Furthermore, we apply inert N2 pressure onto our sensitized photoanodes and observe 44% of Jsc enhancement with respect to pristine sample. Consequently, light-harvesting efficiency of our QDSSCs are increased. Significant series resistance reduction is observed from electrochemical impedance spectroscopy, indicating better interface contact between TiO2 photoanode and TOPOCdSe QD sensitizer are achieved.

Conductivity Properties of Single-Walled Carbon Nanotubes Upon the Encapsulation of TTF and TCNQ

N-type and p-type single-walled carbon nanotubes (SWNTs) were formed via the encapsulation of tetrathiafulvalene (TTF) and 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) inside SWNTs, respectively. Raman, near-infrared, and X-ray photoelectron spectrometer were used to confirm the encapsulation. From measurements of the current-voltage curves in a vacuum, it was revealed that current of TTF-encapsulated SWNTs decreased and TCNQ-encapsulated SWNTs increased comparing with that of pristine SWNTs. This was resulted from electron-donating (TTF) and withdrawing (TCNQ) character into SWNTs.

Suppressed Interfacial Charge Recombination of PbS Quantum Dot Photovoltaics by Graphene Incorporated into ZnO Nanoparticles

Single-layer graphene (SLG) was incorporated into ZnO nanoparticles (NPs), and use of this material in photovoltaic devices generated significant changes. The Fermi level of ZnO NPs underwent a downshift, whereas the conduction and valence bands were maintained with increasing SLG concentrations. Furthermore, the effective defect densities were reduced and carrier mobility was enhanced. Colloidal quantum dot photovoltaics (CQDPVs) with the SLG-incorporated ZnO NP layer as an electron transporting layer achieved significant performance enhancement. Poor performing CQDPVs were also observed with incorporation of an excess amount of SLG. This trend paralleled the interfacial charge recombination trends of CQDPVs. Effective suppression of interfacial recombination was achieved for CQDPVs with an appropriate SLG concentration, whereas dramatically increased interfacial recombination was observed for CQDPVs with an excess of SLG. For CQDPVs with appropriate SLG incorporation, efficient defect passivation and enhanced electron mobility of ZnO NPs facilitated loss-less electron transfer and efficient electron extraction without compromising the favorable energy level alignment. Excess SLG incorporation led to an increase in recombination within the PbS QD layer due to the presence of an energy barrier. This simple and powerful strategy provides an effective method for modulating the interfacial properties of CQDPVs.

Improved field emission properties of GaAs/MgO-coated single-walled carbon nanotube

We measured the field emission (FE) properties and life time stability of single walled carbon nanotubes (SWNTs) after coating with wide-band-gap material (WBGM) and successive negative electron affinity (NEA) material, i.e., magnesium oxide and successive gallium arsenide. Their turn-on field and emission current densities were measured and compared with pristine SWNT film. The FE was increased successively after MgO coating and subsequent GaAs coating for SWNTs. A lifetime test revealed that GaAs/MgO/SWNTs protects SWNT film during FE under exposure to O2 gas. The field-enhancement factor, β, was calculated from the Fowler-Nordheim equation of FE results.

Field emission properties of a three-dimensional network of single-walled carbon nanotubes inside pores of porous silicon

This paper reports the characteristic field emission (FE) properties of single-walled carbon nanotubes (SWNTs) synthesized on the inside pores of a porous silicon (PS) substrate, as well as on the top surface of a PS substrate. Their turn-on fields and emission current densities are measured and compared with those of other types of SWNTs in similar environments. Investigation of field emission properties of single-walled carbon nanotubes (SWNTs) synthesized on the inside pores of a porous silicon (PS) substrate reveals a low turn-on field of about 2.25 V μm−1 at 10 μA/cm2 and a high field-enhancement factor (6182) compare with other samples. A life time stability test is performed by monitoring the current density change before and after repeated exposure to O 2 , suggesting that the pore channel can effectively prevent O 2 + ion etching from destroying the SWNTs within the pores of the PS layer.

P2–13: Field emission characteristics of carbon nanotube forest on etched Si substrate

In this work, we investigatethe field emission characteristics of carbon nanotube forest on three types substrate: (1) mirror polished, (2) chemically etched (large pattern) and (3) chemically etched (small pattern) Si substrate. The surface morphology of CNTs forest was characterized by scanning electron microscopy (SEM), surface chemical state and electronic structure phase analyzed by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy.