253106-2
Sathe et al.
Appl. Phys. Lett. 92, 253106 ͑2008͒
FIG. 4. ͑Color online͒ ͑a͒ I-t transients of Rh hexagons along with its ͑b͒
field emission micrograph. Bright spots on the screen represent emission at
an applied potential of 10 kV from the protrusions on the emitter surface.
and d is the separation ͑d=5 mm͒. However, the actual en-
hanced field at the apex of the hexagons can be estimated
from the equation as follows:13
 = − 6.8 ϫ 1033/2/m,
͑1͒
FIG. 2. ͑Color online͒ ͑a͒ XRD pattern of Rh hexagons reveling reflections
from the ͑111͒, ͑200͒, ͑220͒, ͑311͒, and ͑222͒ planes along with XP spectra
corresponding to ͑b͒ Rh and ͑c͒ oxygen, respectively.
where  is field enhancement factor and is the work func-
tion of the emitter material in eV. The field enhancement
factor  is calculated to be 9325 ͑by taking the work function
of Rh as 5.25 eV on HfO2 substrate8͒ and this high value of
 is attributed to the presence of nanoscale protrusions on
the edges/tips ͑surface heterogeneity͒, as seen in Fig. 1͑b͒,
which could be responsible for lowering the threshold lead-
ing to a final increase in the resultant current. Chen et al.
have reported a similar enhancement due to nanoprotrusions
on amorphous diamond films.14 In comparison, our hexagons
are much bigger in size and the areal density is about 100
times smaller ͑106 cm−2 compared to 108 cm−2͒ perhaps, due
to the use of single crystalline Si substrates.
nent fcc reflections corresponding to the ͑111͒, ͑200͒, ͑220͒,
͑311͒, and ͑222͒ planes suggest the single phase nature of
Rh͑0͒. Moreover, X-ray photoelectron spectra corresponding
to Rh3d and O1s also help to explain the predominance of
Rh͑0͒ peak with perhaps, a minimum amount of unavoidable
surface oxide as clearly seen ͓Figs. 2͑b͒ and 2͑c͔͒.
Field emission measurements were carried out in a con-
ventional microscopic tube evacuated at a base pressure
1ϫ10−9 mbar with interesting features. The cathode ͑nano-
structured Rh hexagons deposited on a silicon substrate͒ was
held at a distance ϳ10 mm from the transparent anode
screen in a vacuum chamber. Accordingly, Fig. 3͑a͒ shows
typical emission current density-applied field characteristics
for the diode configuration, where an onset field of
0.6 V/m, requiring an emission current of 1 nA ͑corre-
sponding to the current density of 4ϫ10−3 A/cm2͒ is re-
producibly observed. With the increase in the applied field,
the emission current density increases very rapidly, finally
reaching 40 A/cm2 at 1.76 V/m. This could be com-
pared with the recent results on metallic tungsten nanowires,
where an applied field of 5 V/m generates an emission
current density of 0.1 mA/cm2, which is important for many
Field emission current stability is one of the decisive
parameters in the context of practical applications of cold
cathodes. The field emission current stability of Rh/Si has
been investigated at a preset current of 1 A ͑corresponding
to the current density of 4ϫ10−3 A/cm2͒, over duration of
more than 3 h. Accordingly, Fig. 4͑a͒ shows the current-time
͑I-t͒ plot for this preset current value at a base pressure of
1ϫ10−9 mbar. Significantly, our Rh hexagons exhibit a re-
markable current stability for repeated performance without
any obvious signs of degradation, making an initial excur-
sion to ϳ1.5 A. Further, a good current stability with cur-
rent fluctuations within about Ϯ15% of the average value is
seen over a period of more than 3 h as also confirmed by
repetitive measurement of I-t transients. The observed cur-
rent fluctuations in the form of spikes could be attributed to
adsorption/desorption of the residual gas molecules at the
grain boundaries on the emitter surface. Moreover, self-
diffusion of atoms at the tip in the presence of high electric
field is also expected to contribute to these fluctuations. The
observed field emission pattern comprises of bright and sym-
metric oval shaped spots having higher image stability, as
revealed in Fig. 4͑b͒. Table I shows a comparison of the field
emission properties of common nanostructures along with
corresponding synthetic methods to highlight the importance
of the low threshold for our Rh hexagons. Interestingly, the
surface morphology of the Rh hexagons on Si substrate by
SEM after the field emission measurements shows no severe
deterioration even after a long-term operation of the emitter.
This signifies that the Rh hexagons are mechanically robust
and have strong resistance toward ion bombardment during
Figure 3͑b͒ shows Fowler–Nordheim plot, where a
straight line behavior indicates that the emission from the Rh
hexagons follows a quantum mechanical tunneling process,
similar to that reported for metallic emitters. The applied
electric field ͑F͒ is defined as F=V/d, where V is the voltage
FIG. 3. Field emission ͑a͒ J-F characteristics and ͑b͒ Fowler–Nordheim plot
for Rh hexagons/Si showing typical metallic behavior of the emitter.
the electron emission.
129.137.83.253 On: Sun, 07 Dec 2014 04:48:03
Sathe, Bhaskar R.
