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Fabrication and Characteristics of Submicron Stacked-Junctions on Thin Graphite Flakes
Venugopal, Gunasekaran,Kim, Sang-Jae American Scientific Publishers 2011 Journal of Nanoscience and Nanotechnology Vol.11 No.2
<P>We report on the fabrication and transport characteristics of submicron-size stacks along c-axis of thin graphite flakes. The stacks were fabricated using a three-dimensional focused-ion-beam (FIB) etching technique. The stack with in-plane area A of 0.5 microm2 showed nonlinear concave-like I-V characteristics even at 300 K; however the stack with A of > 0.5 microm2 were shown an ohmic-like I-V characteristic at 300 K for both low and high-current biasing. It turned into nonlinear characteristics when the temperature goes down. The in-plane area dependence of stack capacitance were discussed and the observed capacitance of stack with A of 0.5 microm2 is smaller than the capacitance of stack with A of 1 microm2 which causes the nonlinear I-V characteristics in stack with A of 0.5 microm2 even at 300 K.</P>
Temperature Dependence of Planar-type Graphite Structures
Gunasekaran Venugopal,Sang-Jae Kim 한국물리학회 2009 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.55 No.3
We have characterized the temperature dependence of the transport behavior for planar-type structures along ab-plane fabricated in micron-scale graphite layers. The planar-type structures of graphite layers were fabricated by using a focused ion beam (FIB) etching method. In-plane areas of 10 μm × 10 μm, 6 μm × 5 μm, 6 μm × 2 μm, and 1 μm × 1 μm exhibit semi-conducting behaviors which is contradictory to conventional metallic behavior of graphite flakes and show a small drop in resistance around 49 K. The origin of this effect is suspected from Ga+ ion damage during FIB fabrication. The fabricated planar-type structures show a transition in the current (I) - voltage (V) curves from diode-like characteristics around 30 K to an Ohmic behavior around 300 K. We have characterized the temperature dependence of the transport behavior for planar-type structures along ab-plane fabricated in micron-scale graphite layers. The planar-type structures of graphite layers were fabricated by using a focused ion beam (FIB) etching method. In-plane areas of 10 μm × 10 μm, 6 μm × 5 μm, 6 μm × 2 μm, and 1 μm × 1 μm exhibit semi-conducting behaviors which is contradictory to conventional metallic behavior of graphite flakes and show a small drop in resistance around 49 K. The origin of this effect is suspected from Ga+ ion damage during FIB fabrication. The fabricated planar-type structures show a transition in the current (I) - voltage (V) curves from diode-like characteristics around 30 K to an Ohmic behavior around 300 K.
Venugopal, Gunasekaran,Jung, Myung-Ho,Suemitsu, Maki,Kim, Sang-Jae Elsevier 2011 Carbon Vol.49 No.8
<P><B>Abstract</B></P><P>We report on the fabrication and transport characteristics of nanoscale stacked-junctions of thin graphite flake. The stacked-junctions were fabricated using a three-dimensional focused-ion-beam milling. By varying the effective in-plane area down to submicron scale, the stacked-junctions with in-plane area <I>A</I> (from 2 down to 0.25μm<SUP>2</SUP>) and stack height–length (from 300 to 100nm) along <I>c</I>-axis were fabricated. The nano-stack shows perfect <I>c</I>-axis transport characteristics in which we observed a semiconducting behavior for <I>T</I>>65K and metallic behavior for <I>T</I><65K. The obtained results were well fitted with the <I>c</I>-axis electrical conduction mechanism. The stack with in-plane area <I>A</I> of 0.25μm<SUP>2</SUP> showed nonlinear concave-like <I>I</I>–<I>V</I> characteristics even at 300K; however the stack with <I>A⩾</I>1μm<SUP>2</SUP> were shown an ohmic-like <I>I</I>–<I>V</I> characteristic at 300K for both low and high-current biasing. It turned into nonlinear characteristics when the temperature goes down. The observation of this anomalous transport characteristics were discussed in detail with stack capacitance calculations. The nonlinear characteristics observed at 300K for the stack with <I>A</I> of 0.25μm<SUP>2</SUP> were shown best fit with Fowler–Nordheim tunneling model.</P>
Fabrication of planar junctions in Graphene sheets using Focused Ion Beam
Gunasekaran Venugopal,Sang-Jae Kim 대한기계학회 2008 대한기계학회 춘추학술대회 Vol.2008 No.5
In this paper we present about fabrication of planar-type structures in thinner graphite sheets (thickness ~ 300 ㎚) using focused ion beam (FIB). We have studied temperature dependent transport characterization for these planar-type structures. Highly oriented pyrolytic graphite (HOPG) is a periodical stack of two dimensional (2-D) graphite sheets along the c-axis. The interlayers of graphite are loosely bonded each other with weak Van der waals forces. We describe a technique for preparing micronscale graphite sheets out of bulk graphite, method of electrical ohmic contacts and fabrication of planar-type structures in graphite sheet using focused ion beam (FIB). We also report the temperature dependence transport characteristics for several sizes of planar-type structures patterned in graphite flakes. We have observed semiconducting behavior and a small drop in resistance during the temperature down to 30 K. This paper will further describe the current (Ⅰ) - voltage (Ⅴ) characteristics for those planar structures.
Investigation of transfer characteristics of high performance graphene flakes.
Venugopal, Gunasekaran,Krishnamoorthy, Karthikeyan,Kim, Sang-Jae American Scientific Publishers 2013 Journal of Nanoscience and Nanotechnology Vol.13 No.5
<P>In this article, we attempted a study on field effect transport characteristics of graphene flakes. These graphene flakes were exfoliated by mechanical peeling-off technique and the electrical contacts were patterned by photo-lithographic method. Graphene devices have shown better transfer characteristics which was obtained even in low-voltage (< 5 V). Back-gated graphene transistors were patterned on oxidized silicon wafers. A clear n-type to p-type transition at Dirac point and higher electron drain-current modulation in positive back-gate field with current minimum (the Dirac point) were observed at V(GS) = -1.7 V. The carrier mobility was determined from the measured transconductance. The transconductance of the graphene transistors was observed as high as 18.6 microS with a channel length of 68 microm. A maximum electron mobility of 1870 +/- 143 cm2/V x s and hole mobility of 1050 +/- 35 cm2/V x s were achieved at a drain bias 2.1 V which are comparatively higher values among reported for mechanically exfoliated graphene using lithographic method. The fabricated devices also sustained with high-current density for 40 hr in continuous operation without any change in device resistance, which could be applied for robust wiring applications.</P>