field emission is a quantum mechanical tunneling phenomenon in which electrons escape from a solid surface into vacuum , as explained theoretically by r. h. fowler and l. nordheim in 1928 .
field emission is widely used in many kinds of vacuum electronic applications such as flat panel displays , microwave power tubes , electron sources , and electron - beam lithography . over the past decade
, research groups worldwide have shown that carbon nanotubes ( cnts ) are excellent candidates for electron emission .
cnts possess advantages in aspect ratios , tip radius of curvature , chemical stability , and mechanical strength .
however , issues related to the placement and throughput of cnt arrays has hampered the development of such arrays for commercial applications .
graphene is a two - dimensional honeycomb - structured single crystal showing ballistic transport , zero band gap , and electric spin transport characteristics [ 3 - 5 ] . in previous studies ,
graphene layers were randomly distributed on cathode electrodes for field emission display applications . however , further field emission studies are required using high - quality , planar graphene structure ( e.g. obtained from a highly oriented pyrolyzed graphite ( hopg ) block ) . in order to understand the fundamental behavior of graphene field emission and expand its application into vacuum nanoelectronics beyond the field emission display ,
the characterization and analysis of field emission from an individual graphene sheet is necessary . in this paper
such a graphene triode structure can be used as a fundamental unit for vacuum nanoelectronics .
the triode has an in - plane graphene tip ( emitter ) with the other in - plane electrodes used as source , drain , and gate on the substrate .
depending on the gate voltage applied , electrons are emitted from the graphene tip creating an electron current that can be modulated on and off .
to realize this conceptual device , the field emission characteristics of graphene layers with different thicknesses need to be characterized . conceptual schematic view of a graphene - based triode as a fundamental unit for vacuum nanoelectronics . depending on the gate voltage applied ,
electrons are emitted from the graphene tip creating an electron current that can be modulated on and off to create the graphene layer for this experimental study , graphene sheets were prepared by mechanical exfoliation and placed on insulating sio2 substrate .
a thermo - curable elastomer , polydimethylsiloxane ( pdms , sylgard 184 , dow corning co. ) film was prepared using a standard recipe on an oxidized si wafer ( see fig .
after peeling the film from the wafer , its polished side was scrubbed on a highly oriented pyrolyzed graphite ( hopg ) block ( see fig .
2b , c ) , and lifted off , transferring graphene layers to the pdms ( fig .
the exfoliated graphene layers were transferred onto sio2 thin film by scrubbing the pdms film and subsequently detaching , leaving behind thin graphene layers ( see fig .
2e , f ) . in order to find and evaluate the graphene layers , the thickness of sio2 layer on si was set to 300 nm considering optical interference .
the graphene sheets are transferred from hopg block to sio2layer a zyvex nanomanipulator operating inside a scanning electron microscope ( sem : xl-40 sem , fei co. ) was used to measure field emission from individual graphene sheets ( fig .
2 ) . figure 3 shows the schematic view of the experimental setup for measuring a field emission current from graphene sheets . in the sem vacuum chamber ,
two tungsten tips were located on the graphene sample ; one was contacted directly to the sample and grounded as a cathode , and the other was placed an arbitrary distance , d , apart from the edge of the sample as the anode .
the tungsten tips were connected to a keithley semiconductor measurement system via a feed - through in the vacuum chamber to apply and sense the electric signal for field emission .
the thickness of the layer was optically measured on 300 nm thick sio2 layer by using the change of color due to optical interference and transparency .
4a , cobalt blue , purple , and light purple stand for 8 , 4 and 2 nm thicknesses , respectively .
figure 4b shows an sem image of graphene sheets with a pair of tungsten tips controlled by the nanomanipulator .
schematic view of the experimental setup using a nanomanipulator graphene sampleaoptical image of graphene sheets on sio2 .
thecolorof graphene sheets determines thickness of the graphene layer.scale bar : 6 m.bsem image of a graphene sample with tungsten tips controlled by nanomanipulator after adjusting the position of the tips , a positive potential was applied to the second tip .
figure 5a shows ie curves of graphene for an arbitrary gap < 1 m . the graphene sheet started to emit electron current around 20 v and increased exponentially up to 170 na following the behavior of the fowler  nordheim relationship .
the field emission current fluctuated for applied voltages higher than 33 v. figure 5b shows fn curves obtained as a result of field emission from a graphene sheet .
5a , the emission current is increased exponentially , and the fn curve shows linear relationship following the field emission behavior .
the estimated turn - on voltages of the tested graphene sheet is 12.1 v , where the slope of fn curve is changed and the linear region ( red line ) begins as shown in fig .
fn parameters were evaluated by linear fit of the red line as shown in the equations .(1)(2)(3 ) where i : current , e : electric field ( v / d ) ,  : field - enhancement factor ,  : work function , a : area ,  : reduced planck constant , and m : electron mass . assuming the work function of graphene is 5 ev and the gap between the graphene sheet and the nanomanipulator tip is 1 m , the estimated field - enhancement factor ,  , is 3519 .
it is found that the measured field - enhancement factor is comparable with previous results of graphene film prepared by electrophoresis , and the field emission efficiency of graphene is twice as high as other carbon nanomaterials such as cnt and diamond film . from the experimental results
, it is found that one can further reduce the voltage for electron emission as the fabrication process is refined to create a fine emitter tip from graphene sheets .
the field emission properties of graphene need further investigation in terms of the number of graphene layers and crystallographic arrangement of the carbon lattice . in the near future
ai  eplot for emission current.bfnplot for emission current this field - emitting nanodevice based on the planar form of graphene potentially allows for top - down cmos compatible process flows , an advantage for potential industrial fabrication of electronic devices . for applications where high field emission currents or low turn - on voltages are required , nanodevices based on graphene would inherently provide the necessary alignment based on its crystallographic nature
this work has partially been supported by exchange student program by brain korea 21 , award no kuk - f1 - 038 - 02 made by king abdullah university of science and technology ( kaust ) and national science foundation ( major research instrumentation program , award no .
 this paper describes an experimental study on field emission characteristics of individual graphene layers for vacuum nanoelectronics . 
 graphene layers were prepared by mechanical exfoliation from a highly oriented pyrolyzed graphite block and placed on an insulating substrate , with the resulting field emission behavior investigated using a nanomanipulator operating inside a scanning electron microscope . a pair of tungsten tips controlled by the nanomanipulator enabled electric connection with the graphene layers without postfabrication . 
 the maximum emitted current from the graphene layers was 170 na and the turn - on voltage was 12.1 v. 
None
Acknowledgments