The field emission properties of LaS nanoprotrusions called nanodomes, formed by pulsed laser deposition on porous anodic alumina films, have been analyzed with scanning anode field emission microscopy. The voltage necessary to produce a given field emission current is ∼3.5 times less for nanodomes than for thin films. Assuming the same work function for LaS thin films and nanoprotrusions, that is, ∼1 eV, a field enhancement factor of ∼5.8 is extracted for the nanodome emitters from Fowler-Nordheim plots of the field emission data. This correlates well with the aspect ratio of the tallest nanodomes observed in atomic force micrograph measurements

B. Kanchibotla

D. J. Lockwood

J. W. Fraser

K. Garre

L. Grazulis

M. Cahay

S. Bandyopadhyay

S. Fairchild

S. Pramanik

V. Semet

Vu Thien Binh

Journal of Nanomaterials

Crossref

Hindawi Publishing Corporation
Journal of Nanomaterials
Volume 2008, Article ID 682920, 4 pages
doi:10.1155/2008/682920
Research Article
Field Emission from Self-Assembled Arrays of
Lanthanum Monosulfide Nanoprotrusions
V. Semet,1 Vu Thien Binh,1 M. Cahay,2 K. Garre,2 S. Fairchild,3 L. Grazulis,3 J. W. Fraser,4
D. J. Lockwood,4 S. Pramanik,5 B. Kanchibotla,5 and S. Bandyopadhyay5
1 Equipe Emission Electronique, LPMCN-CNRS, Universite Claude Bernard Lyon 1, 69622 Villeurbanne, France
2Department of Electrical and Computer Engineering and Computer Science, University of Cincinnati, Cincinnati, OH 45221, USA
3Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, OH 45433, USA
4 Institute for Microstructural Sciences, National Research Council, Ottawa, ON, Canada K1A OR6
5Department of Electrical and Computer Engineering, Virginia Commonwealth University, Richmond, VA 23284, USA
Correspondence should be addressed to M. Cahay, marc.cahay@uc.edu
Received 30 October 2007; Accepted 10 January 2008
Recommended by Donglu Shi
The field emission properties of LaS nanoprotrusions called nanodomes, formed by pulsed laser deposition on porous anodic
alumina films, have been analyzed with scanning anode field emission microscopy. The voltage necessary to produce a given
field emission current is ∼3.5 times less for nanodomes than for thin films. Assuming the same work function for LaS thin films
and nanoprotrusions, that is, ∼1 eV, a field enhancement factor of ∼5.8 is extracted for the nanodome emitters from Fowler-
Nordheim plots of the field emission data. This correlates well with the aspect ratio of the tallest nanodomes observed in atomic
force micrograph measurements.
Copyright © 2008 V. Semet et al. This is an open access article distributed under the Creative Commons Attribution License, which
permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
The promise of producing extremely high current densities
in near-vacuum with no external source (such as heater,
primary electron-beam, or intense light source) has gener-
ated significant interest in robust, reproducible, and field-
emission-based cold cathodes [1, 2]. Several promising ap-
proaches to achieving cold cathode emission from a dense ar-
ray of nanoscale field emitters have been investigated includ-
ing metallic field emitter arrays (FEAs) or Spindt emitters
[3], carbon nanotube (CNT) arrays [4], and arrays of semi-
conductor nanowires [5–9]. To date, metallic FEAs with uni-
formly high emission have been diﬃcult to fabricate. They
are prone to failure and present long-term reliability prob-
lems [3]. Carbon nanotubes (CNTs), on the other hand, can
be fabricated easily, but the emission current density that
they can support, while adequate for some applications, is
currently not enough for mainstream high power devices
such as traveling wave tubes (TWTs). Semiconductor and
semimetallic nanowires therefore appear as perhaps the most
promising route to realizing field emitters that can support
a large total emission current, in particular if regular high
density arrays of them can be fabricated in a self-assembled
growth mode. Moreover, to achieve high current densities
under low applied voltage, the field emitter material must
have low work function. Rare earth monosulfides are there-
fore ideal for this purpose.
Recently, we reported the first successful deposition of
lanthanum monosulfide (LaS) thin films on Si substrates
using pulsed laser deposition (PLD) [10, 11]. They were
flat layers composed of nanocrystallites having dimensions
of the order of 5 nm. The scanning anode field emission
microscopy (SAFEM) technique was used to measure the
field emission (FE) properties of these LaS thin films (about
0.1 μm thick) [12]. It was shown experimentally that elec-
trons are easily extracted by FE from patchwork areas of
few nanometer dimensions having a measured eﬀective
work function of ∼1 eV, corresponding to some outcrop-
ping LaS nanocrystallites with (100) orientation, embedded
in a ∼2.8 eV work function surface related to other crystal-
lographic orientations. A LaS thin film obtained by PLD can
therefore be an excellent field emitter material, especially if
we can take advantage of a field enhancement factor at the
tip of nanoscale protrusions formed from such material.
2 Journal of Nanomaterials
Nanodomes
Nanodots
500nm
Figure 1: FE-SEM images of a 1 × 1 μm2 area of an anodic porous
alumina film after PLD of LaS. The nanoprotrusions are the nan-
odomes and nanodots indicated in the figure.
Z
:8
6.
5
n
m
Y
:4
μm
X : 4 μ
m
Nanodots
Nanodomes
Figure 2: Three-dimensional AFM image of an area of 4×4 μm2 of
the alumina template after PLD of LaS. The tallest irregular features
are the LaS nanodomes. The smaller and more regular pyramidal
structures are the LaS nanodots forming in the regions between ad-
jacent pores.
In this letter, we report FE properties of arrays of LaS
nanoprotrusions self-assembled with a density as high as
109/cm2. These nanoprotrusions, called nanodomes and
nanodots hereafter, are produced when LaS is pulse-laser-
deposited on nanoporous anodic alumina films containing
hexagonal arrays of cylindrical pores that are ∼50 nm wide
and ∼500 nm deep. The details of the self-assembly growth
of arrays of nanoprotrusions using a porous anodic alumina
film as a template are given in [13].
The nanodomes are cone-shaped LaS structures with
base diameter 2r ranging from 50 to 100 nm and height
h ranging from 100 to 150 nm. They were found to grow
on the boundaries separating regions of the anodic alu-
mina film that have near perfect pore ordering. Their density
is ∼109/cm2, as extracted from the field emission-scanning
electron micrographs (FE-SEM) and atomic force micro-
graphs (AFM) such as those shown in Figures 1 and 2, re-
spectively. In addition to nanodomes, nanodots (or quantum
dots) of LaS nucleate on top of the alumina islands between
adjacent pores. These are sites of minimal strain. The dots
have a diameter 2r∼ 50 nm, a density equal to the pore den-
sity (1010/cm2), and an aspect ratio h/2r less than 1.
The SAFEM technique was used to measure the FE
current-voltage (I-V) characteristics at diﬀerent surface lo-
cations. As the nanowires are buried inside the pores, they
contribute very little, if any at all, to the net FE current. More-
over, FE measurements from the arrays of nanodomes and
nanodots were possible because the regions joining adjacent
nanodomes and nanodots are covered with a thin percolating
network of LaS over the entire array. Its presence was con-
firmed by measuring a low resistance value of about 2 Ohms
between two electrical contacts about 1 cm apart. For each
location, a full set of I-V characteristics (total measured cur-
rent versus applied voltage) for diﬀerent values of d, the dis-
tance between the cathode surface and the probe ball, was
measured. This set of measurements was then analyzed in
order to extract the apparent current density αJ versus ac-
tual applied local field γF, where γ is the local geometrical
field enhancement at the surface of the cathode [14], F is the
notional electric field at the corresponding flat LaS cathode
surface (when γ = 1), α a factor taking into account the ac-
tual emitting area, and J the notional FE current density. In
Figure 2, we see that the nanodome structures dominate the
landscape and therefore they have the highest field enhance-
ment factor γ. The relative contribution of the nanodots to
the total FE current is minor due to a smaller value of the
γ associated with them. In all probability, the majority of the
measured FE current is then extracted from the highest nan-
odomes.
In order to assess the FE properties from LaS nanopro-
trusions, we have compared the FE from nanodome cathodes
with that from planar thin film LaS cathodes. This compar-
ative methodology was chosen because a precise quantitative
estimation of γ from topographic measurements is still sub-
ject to controversy, except for a flat surface. These experimen-
tal measurements were performed for both LaS thin films of
100 nm thickness on Si wafers and LaS nanodomes on anodic
alumina films. The analysis consisted of the following steps.
(1) In order to restrict the analysis of the diﬀerences in the
I-V data to the surface morphology of the cathodes,
both SAFEM measurements were performed with the
same probe ball-to-cathode distance d = 3.65 μm.
Typical I-V characteristics are shown in Figure 3 for
both thin film and nanodome cathodes. We found that
in order to extract the same total FE current, the ap-
plied voltage for nanodome cathodes was ∼3.5 times
less than that for thin film cathodes. This indicates
clearly the contribution of the protruding geometry of
the nanodomes to the FE current.
(2) The total FE current versus applied voltage (I-V) char-
acteristics were measured for diﬀerent values of the
probe-ball-to-cathode-surface distance d. From these
data, the apparent current densities (αt f J and αndJ)
as a a function of the applied local field were ex-
tracted [15, 16], assuming that the applied local elec-
tric field included the local enhancement factor γ,
that is, γt f F and γndF, for LaS thin film and nan-
odome cathodes, respectively. αt f and αnd are parame-
ters taking into account the actual emitting areas from
low work function nanocrystallite zones [12] and the
density of emitting nanodomes. The corresponding
Fowler-Nordheim plots (ln(αJ/(γF)2) versus 1/γF) of
the data are shown in Figure 4. As a result of the field
V. Semet et al. 3
0
20
40
60
80
100
120
×10−9
To
ta
lfi
el
d
em
is
si
on
cu
rr
en
t
I
(A
)
750 800 850 900 950 1000 1050
V (volts)
LaS thin film
z distance = 3.65 μm
Probe-ball diameter = 200 μm
(a)
0
20
40
60
80
100
120
×10−9
To
ta
lfi
el
d
em
is
si
on
cu
rr
en
t
I
(A
)
180 200 220 240 260 280 300 320
V (volts)
LaS nanoprotrusions
z distance = 3.65 μm
Probe-ball diameter = 200 μm
(b)
Figure 3: (a) Typical I-V characteristic recorded using the SAFEM technique for FE from LaS thin film deposited via PLD on a Si substrate.
(b) The same for LaS nanostructures self-assembled on a porous anodic alumina template. To allow immediate comparison, the sample to
SAFEM probe distance is kept the same in both measurements.
−2
0
2
4
6
ln
(J
/(
γ t
fF
2
))
(J
(A
/c
m
2
))
30 35 40 45
1/(γtf F) (V/A˚)−1
LaS thin film cathode
J-F characteristics
Mean slope ∼ − 0.4155
(a)
−3
−2
−1
0
1
2
ln
(J
/(
γ n
p
F
2
))
(J
(A
/c
m
2
))
100 120 140 160
1/(γnp F) (V/A˚)−1
LaS nanoprotrusion cathode
J-F characteristics
Mean slope ∼ − 0.0716
(b)
Figure 4: Fowler-Nordheim plots of the FE data corresponding to the cathodes shown in Figure 3.
enhancement factor, the emission area for nanodome
cathodes was restricted to the nanodome apexes. The
actual current densities J extracted from each nan-
odome are then much more important than the val-
ues of the apparent current densities in the plot of
Figure 4. The exact estimation of the ratio αnd/αt f
would require a self-consistent approach, which is in
progress, but one can estimate it in first approximation
to be proportional to the inverse of the density of the
emitting nanodomes. This ratio could be in the range
of 104 to 105.
(3) The FE analysis for both cathodes, presented in the
previous paragraph, was limited only to the region
where the analytical FE relation developed by Murphy
and Good is applicable [15, 16]. Under such condi-
tions, the slopes mt f and mnd of the Fowler-Nordheim
plots are, respectively, proportional to Φ3/2/γt f and
Φ3/2/γnd, where Φ is the eﬀective work function of the
cathodes. In Figure 4, a straight line fit to the Fowler-
Nordheim plots of the data gave slope values mt f =
−0.4155 and mnd = −0.0716 for LaS/Si and nanodome
cathodes, respectively.
(4) Under the assumption that the nanodome cathodes
have the same eﬀective work function as the planar
cathodes on Si, that is, Φ ∼ 1eV, [12] the ratio of the
slopes extracted from the Fowler-Nordheim plots is
equal to the inverse ratio of the field enhancement fac-
tors. Since AFM measurements of LaS thin films on Si
substrates have shown that surface corrugation is neg-
ligible [10, 11], the error introduced by taking γt f = 1
is minor. Under this assumption, γnd = 5.8. This value
is in very good agreement with the geometrical aspect
ratio of the tallest nanodomes observed by AFM [13].
In conclusion, the SAFEM technique was used to demon-
strate successful FE from LaS nanodomes grown by PLD
on top of flexible alumina templates. The SAFEM measure-
ments were found to be reproducible after a period of several
months without taking any special precaution to store the
template. LaS nanodome arrays are therefore very stable field
emitters with a work function around 1 eV. Even though not
discussed here, cross sectional FE-SEM images have shown
that LaS nanowires grow inside the pores with a density of
1010/cm2. The nanowires have a diameter of 50 nm and a
maximum length equal to the length of the pores (∼500 nm)
[13]. This density could approach 1011/cm2 by using tem-
plates with pores size down to 10 nm. Actually, the expo-
sure of the tips of the LaS nanowires filling the pores can
be done by etching the back aluminum substrate and the
4 Journal of Nanomaterials
small alumina barrier at the bottom of the pores, which could
lead to field enhancement factors for the LaS nanowires more
comparable to those reported for CNTs. This would lead to
a much lower threshold voltage for field emission compared
to the one reported here and a large increase in the density
of nanoemitters culminating in the realization, therefore the
realization of flexible cold cathodes using a self-assembling
process with reliable and low cost production.
ACKNOWLEDGMENT
The work at the University of Cincinnati and Virginia Com-
monwealth University is financially supported by the US Na-
tional Science Foundation under the collaborative GOALI
Grant ECS-0523966. Part of this work was performed by
M. Cahay under the Air Force Summer Fellowship Program,
Contract No. FA9550-07-C-0052.
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Field Emission from Self-Assembled Arrays of Lanthanum Monosulfide Nanoprotrusions

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