Hyeong-Rag Lee

Enhancement of the field emission of carbon nanotubes straightened by application of argon ion irradiation

The field emission properties of carbon nanotubes (CNTs) were enhanced by argon ion irradiation. Argon irradiation treatment led to an enhancement in the emission properties of CNTs which showed a decrease in turn-on field and an increase in total emission current after the treatment. The irradiation treatment permanently straightened as-grown curly CNTs, and, as a result, the local electric field was increased, due to the increased aspect ratio and reduced mutual shield effect. In addition, increased defects, produced by the argon irradiation are likely to make their effective surfaces more active, thus emitting more electrons.

Novel emission degradation behavior of patterned carbon nanotubes by field emission

Abstract Field emission behavior of photolithography-based patterned carbon nanotubes (PP-CNTs) was irreversibly degraded, showing a gradual turn-on voltage shift to the high voltage region as a result of field emission measurements. The PP-CNTs show a flat region in Fowler–Nordheim (F–N) plots, which can be attributed to residue-induced emission suppression. The observed degradation in field emission can be attributed to a subsequent degradation of carbon nanotubes emitters by a combination of the high electric-field-induced straightening out effect and the high current-induced burning of PP-CNTs. The local emission current and the stability of electron emission were observed in an attempt to investigate the residue effect of the PP-CNTs on the field emission behavior.

Effects of gas exposure on the field-emission properties of ZnO nanorods

The effect of diverse gases on the field emission (FE) properties of ZnO nanorods was investigated. The FE properties of nanorods were fully recovered after evacuating to the initial pressure, even after an abrupt current drop under severe vacuum conditions of Torr induced by the presence of O2, N2, Ar and air. The reversible and sensitive response of the FE of the nanorods with variation in pressure was found for all gases tested except for H2. Exposure of H2 causes a permanent increase in the FE current and a decrease in the turn-on field. The pressure-dependent field emission behaviour of nanorods must be considered when field emission data are compared and characterized.

Effect of the in situ Cs treatment on field emission of a multi-walled carbon nanotube

Abstract The effect of an in situ Cs treatment on the field emission of a multi-walled carbon nanotube (MWNT) was investigated. The field emission of as-grown MWNT shows two Fowler–Nordheim (F–N) slopes in current–voltage characteristics. Both slopes in the two regions (high voltage and low voltage) were lowered as the result of the Cs treatment. The turn-on voltage was significantly decreased by a factor of about 1.3 and the total emission current was increased by over one order of magnitude after Cs deposition. In addition, the emission current was decreased beyond the optimum Cs deposition time. A stable emission current was observed after Cs deposition and during the no-Cs-deposition state. A decrease in work function is the major cause of the electron field enhancement as the result of the formation of a surface dipole layer induced by Cs deposition.

The growth of freestanding single carbon nanotube arrays.

Regular arrays of freestanding single carbon nanotubes (CNTs) were prepared on Ni dot arrays by dc plasma-enhanced chemical vapour deposition. The size of the Ni dot was reduced for single CNT growth by means of conventional photolithography and a lateral wet-etch process. The vertical alignment of a single CNT was directly dependent on the location of the catalyst metals. Using this method, well-separated and well-defined regular arrays of freestanding CNTs can be fabricated and the process can be scaled up at a lower cost than electron beam lithography, which is encouraging for applications in field emitters and nanoelectrodes.

Magnetic Properties of Cr-doped LiNbO₃ by Using the Projection Operator Technique

The electron spin resonance lineshape (ESRLS) function for the electron spin resonance linewidth (ESRLW) of Cr³? (S = 3/2) in ferroelectric lithium niobate single crystals doped with 0.05 wt% of Cr, is obtained by using the projection operator technique (POT), developed by Argyres and Sigel. The ESRLS function is calculated to be axially symmetric about the c ? axis and analyzed by using the spin Hamiltonian H SP = μB (BㆍgㆍS) + SㆍDㆍS with the parameters g = 1.972 and D = 0.395 ㎝?¹. In the ca plane, the linewidths show a strong angular dependence, whereas in the ab plane, they are independent of the angle. This result implies that the resonance center has an axial symmetry along the c ? axis. Further, from the temperature dependence of the linewidths that is shown, it can be seen that the linewidths increase as the temperature increases, at a frequency of v = 9.27㎓. This result implies that the scattering effect increases with increasing temperature. Thus, the POT is considered to be more convenient to explain the scattering mechanism as in the case of other optical resonant systems.

In situ characterization of the field-emission behaviour of individual carbon nanotubes

The current–voltage (I–V) characteristics of carbon nanotubes (CNTs) during field emission were investigated by in situ imaging and field-emission (FE) measurement inside a field-emission scanning electron microscope (FE-SEM). A primary electron of the FE-SEM could induce and enhance a large stimulated electron emission from a CNT which might be due to the strong local field on the tip of the CNT in the presence of an applied voltage. FE of bent nanotubes (BNTs) can initially occur after they are fully straightened or it can start in the bent state (during geometrical straightening) as the applied field increases. The FE from a single CNT follows FN (Fowler–Nordheim) behaviour with a single linear slope in the FN plot. The FE from two nanotubes with a geometrical change during FE showed transition of the FN slope from the low voltage to the high voltage region, which could be due to interactions between two dynamic neighbouring BNTs.

Field emission properties of vertically aligned iron nanocluster wires grown on a glass substrate

Vertically aligned nanocluster wires (NCWs) were synthesized on indium-tin-oxide-coated glass substrates via the thermal decomposition of Fe(CO)5 with a resistive heater under a magnetic field. The density of the aligned NCWs was controlled by varying the flow rate of the carrier gas. The low-density NCWs showed better field emission characteristics, with a low turn-on field of about 4V∕μm and a current density as high as 3mA∕cm2 at 7.6V∕μm. The field enhancement factor (γ) was determined to be ∼1200 for high-density NCWs and ∼1600 for low-density NCWs, which is comparable to those of carbon nanotubes.

In situ optical characterization of the alignment and density of carbon nanotubes

Various characteristics of the growth of carbon nanotubes (CNTs) were examined by in situ optical interference patterns. A dc plasma chemical vapour deposition technique was used to grow the CNTs. The optical interference curves provided considerable in situ information such as the length, the alignment and related growth properties of CNTs. Curled CNTs have relatively small numbers of oscillations compared to well-aligned nanotubes during the same growth time. Sparse and short CNTs have shown a high reflectance with no interference oscillations. The alignment of the CNTs was strongly dependent on variations in the NH3 pre-treatment current. The electric field has a large effect on the direction of growth of well-aligned CNTs with high packing density. However, curled CNTs do not appear to be affected by the presence of the electric field. This optical interference method provides an in situ and effective method for monitoring the growth characteristics of CNTs.

Application of a Continued-Fraction-Based Theory to Line-Profile in Mn-Doped GaN Film

Starting with the Kubo formalism and using the projection operator technique (POT) introduced by Kawabata, the optical quantum transition line-profiles (QTLPs) formula for a Mn-doped wurtzite GaN film was derived as a function of temperature at a frequency of 9.49 GHz (X-band), on the basis of continued fraction representation (CFR) which is a counterpart of the conventional series expansion (CSE). Utilizing this formula we obtained the fine-structure parameter a - F = 9.4 ×10-4 cm-1 and fitting parameter ζ= 4.1. The optical quantum transition half-widths (QTHWs) obtained with the use of these parameters agrees quite well with the existing experimental result in the temperature region T > 20 K. The QTHWs increase with increasing temperature due to the interaction of electrons with optical phonons. Thus, the present technique is considered to be more convenient to explain the resonant system as in the case of other optical transition problems.