Silver nanorod arrays for photocathode applications
Subramanian Vilayurganapathy1,2, Manjula I. Nandasiri1,2, Alan G. Joly3, Patrick Z. ElKhoury3, Tamas Varga1, Greg Coffey4, Birgit Schwenzer3, Archana Pandey1, Asghar
Kayani2, Wayne P. Hess3*, Suntharampillai Thevuthasan1
1
Environmental and Molecular Sciences Laboratory, Pacific Northwest National
Laboratory, P.O Box 999, Richland WA 99352, USA.
2
Western Michigan University, 1903 West Michigan Ave, Kalamazoo MI 49008, USA.
3
Physical Sciences Division, Pacific Northwest National Laboratory, P.O Box 999,
Richland WA 99352, USA.
4
Energy Processes and Materials Division, Pacific Northwest National Laboratory, P.O
Box 999, Richland WA 99352, USA.
We explore the possibility of using plasmonic Ag nanorod arrays featuring enhanced
photoemission as high-brightness photocathode material. Silver nanorod arrays are synthesized
by the DC electrodeposition method and their dimensionality, uniformity, crystallinity and
oxide/impurity content are characterized. These Ag nanorod arrays exhibit greatly enhanced twophoton photoemission under 400 nm femtosecond pulsed laser excitation. Plasmonic field
enhancement in the array produces photoemission hot spots that are mapped using photoemission
electron microscopy (PEEM). The relative photoemission enhancement of nanorod array hot
spots relative to that of a flat Ag thin film is found to range between 102 and 3 x 103.
*Email: wayne.hess@pnnl.gov; 509-371-6140
1
High-brightness photocathode sources that emit intense electron pulses of low thermal
emittance and high coherence are needed for future light source development, such as xray free electron lasers and energy recovery linacs,1 and to enable dynamic transmission
electron microscopy (DTEM).2,
3
Important photocathode properties include high
quantum efficiency (QE) and consistent charge delivery under light source and electron
microscope operating conditions. High QE photocathode materials are typically sensitive
to background gas impurities which persist even under ultrahigh vacuum conditions1 such
that light source photocathodes are typically operated under rather expensive extreme
vacuum conditions (EUV). For example, negative electron affinity GaAs or multi-alkali
metal photocathodes are typically operated at pressures of 10-11 torr or below.4 Electron
microscopes, however, operate at much higher pressures, typically on the order of 10-8
torr. As such, photocathodes designed for DTEM applications must have significantly
greater tolerance to background gases. In this regard, there is a clear need for the
development of robust photocathode materials, capable of withstanding high background
pressures and prolonged use under light source and electron microscope operating
conditions.
Metal photocathodes have low QE, on the order of 10-4 to 10-5, but are robust with respect
to background gas degradation and sustained pulsed laser excitation.1 Several strategies
to increase the QE of metal photocathodes have been proposed and are presently under
investigation. For example, thin films of CsBr grown on flat metal photocathodes have
been shown to increase QE by one to three orders of magnitude under ultraviolet (UV)
laser irradiation.5-9 A second strategy uses plasmonic field enhancement to increase the
2
photoemission yield by incorporating metal nanostructures into the photocathode surface.
Enhanced photoemission yields from sub-wavelength rectangular groove arrays (gold
nanogratings)10 show a dramatic photoelectron yield increase of greater than 6 orders of
magnitude following 800 nm fs pulsed laser excitation over a flat gold surface. Similarly,
surface plasmon assisted photoemission from a nanohole array on a copper surface
displays a photoemission yield increase of greater than 100 when compared to a flat
copper surface operated under the same experimental conditions.11
Of particular relevance to our current investigation is another nanostructured
photocathode design, comprising arrays of vertically-aligned embedded silver nanorods.12
The nanorod structure supports two plasmonic resonances along the axial and
longitudinal directions.13 Aligned silver nanorods can be produced by various methods
including electrochemical growth. However, the template-assisted electrodeposition has
the advantages of nanorod size control and scalability to areas of several cm2.14, 15 Using
this technique, Ag nanorod arrays have been synthesized in anodized aluminum oxide
(AAO) templates.16-20 For photocathode applications, the nanorod array must have high
electrical conductivity and structural stability. Here, we investigate nanorod arrays for
plasmon enhanced photoemission under fs laser excitation. The plasmon enhanced
photoelectron yield of the nanorod array, and its potential for photocathode applications,
is studied using photoemission electron microscopy (PEEM). The surface morphology,
cross-section, crystal structure, and preferred growth orientation of the nanorod arrays
were characterized using scanning electron microscopy (SEM) and x-ray diffraction
(XRD).
3
Silver nanorod arrays in AAO templates were synthesized by electrochemical deposition
using a two electrode cell, as schematically illustrated in Figure 1. AAO templates with a
diameter of 13 mm, nominal pore sizes of 200 nm, and a thickness of 60 µm were
purchased from Whatman Inc. Silver foil was used as the anode while the cathode was
constructed, on one side of the AAO, by magnetron sputtering a 1 µm layer of Ag
followed by applying an Ag metallic paste.14 This provides excellent electrical contact
that enables growth of uniform large surface area (~1cm2) nanorod arrays. An Ag plating
solution with a metal content of 28.1 g/l (Alfa Aesar) was used as the electrolyte. Electrodeposition was carried out under galvanostatic conditions using a Solartron
electrochemical interface (SI 1287) operated by Corrview software.
Figure 1. Schematic of the electrodeposition process for Ag metal nanorod arrays: (a)
bare AAO template with a magnetron sputtered metal layer and a coating of metallic
paste, (b) metal nanorods growing in the pores of the AAO template during
electrodeposition, (c) vertically aligned metal nanorod arrays upon template removal by
1.0 M NaOH AAO etch solution. The metallic paste and the sputtered layer provide a
strong electrical contact facilitating the nanorod growth.
Constant currents of 5 and 10 mA were maintained for various deposition times to control
nanorod array length. Following electro-deposition, the AAO template was rinsed in
4
deionized water to remove the plating solution and dried under ambient laboratory
conditions. The AAO template was then removed by an aqueous etch solution of 1.0 M
NaOH and the free standing Ag nanorods attached to the base Ag layer were rinsed in
deionized water and dried. Nanorod sections were adhered to a mica substrate covered
with a thin silver film to provide a rigid yet highly-conductive structure for
Photoemission measurements using PEEM.
Surface and cross-sectional images of Ag nanorod arrays were collected using an FEI
Helios Nanolab 600 SEM. Grazing incidence X-ray diffraction (GIXRD) data were
obtained using a Phillips X’Pert Multipurpose X-ray diffractometer equipped with a fixed
anode operating at 45 kV and 40 mA at a fixed 5-degree incident angle. The analysis of
the diffraction data was carried out using JADE 9.4.5 (Materials Data, Inc.) and phase
identification was performed using the PDF4+ database (ICSD). Lattice parameters were
determined from the angular positions of the peaks in the 20-90o range. Crystallite size
was estimated from broadening of the (111) reflections of the metals using JADE’s
pseudo-Voigt profile function calibrated with a LaB6 (SRM 660, NIST) internal standard
for the grazing-incidence geometry employed.
Ag nanorod arrays were imaged using Photoemission Electron Microscopy (PEEM III,
Elmitec, GmbH). Two photoemission sources were used to obtain PEEM images: an
ultraviolet mercury lamp and a frequency-doubled titanium–sapphire laser oscillator (~40
fs pulse duration at 400 nm; Griffin-10, KM Labs). Both sources were directed onto the
nanorod array surface at an incidence angle of 75° with respect to the surface normal. The
5
p-polarized femtosecond laser source has a pulse repetition frequency of 90 MHz and an
average power of ~ 3 mW. The photoelectrons generated from the sample surface were
accelerated in a 10 MV/m electric field towards an electrically grounded objective lens,
then steered using a series of magneto-optic lenses. The photoelectrons were then
projected onto a micro channel plate/phosphor screen detector and imaged using a CCD
camera. The smooth Ag film adjacent to the nanorod array was used to measure the
reference photoemission yield needed to calibrate plasmon-enhanced nanorod
photoemission.
Figure 2 displays SEM images of a dense Ag nanorod array with a rather uniform 20 to
22 m length distribution. Depending on the choice of current and deposition time, the
nanorod length can be varied from a few hundred nanometers to few tens of µm, and the
yielded nanorod tip diameters are in the 150-300 nm range. The GIXRD pattern for the
Ag nanorod arrays shows (111), (200), (220), and (311) reflection peaks, suggesting that
nanorods are polycrystalline (See supplementary material).21 The average crystallite size
was estimated to be ~29 nm. The lattice parameter for the crystallites obtained from the
location of the reflection peaks is 4.09Å, which is identical to the literature value of bulk
Ag.22 No significant residual contamination of the templates due to immersion in the
plating solution was observed. The Ag nanorod arrays do not exhibit reflection peaks for
oxides, indicating the absence of oxidized Ag in the bulk of the sample.
6
Figure 2. Secondary electron SEM images of the (a) surface (scale bar: 5 µm) and (b)
cross-section (scale bar: 10 µm) of uniform Ag nanorod arrays grown on the AAO
templates via the DC electro-deposition technique. Note that in the cross-sectional images
(b) nanorods on the surface are fractured during sample preparation for imaging.
Figure 3 displays the PEEM images of Ag nanorods following mercury lamp and 400 nm
fs laser excitation of an identical region of the sample. The maximum photon energy
emitted from the mercury lamp is near 5.0 eV, sufficiently above the work function of
silver, such that the image represents the spatial distribution of electrons emitted through
single-photon excitation. The lamp image shows regions of high nanorod density
consistent with the SEM images. The electron counts per second range from roughly 1 to
10 across the image as shown in the Figure 3(a) with brightest photoemission likely
correlated with large nanorod tips or pairs of proximal tips. The variation in
photoemission intensity under mercury lamp excitation can be due to several factors
including array topography and variations in crystalline orientation, phase, or work
function of individual rod tips.23
7
The corresponding fs laser excited image (Figure 3(b)) however displays distinct hot
spots of very high photoelectron yield and image contrast but lower spot density. Laser
excitation at 400 nm (3.1 eV) produces photoemission through a two-photon excitation
process as the single photon energy is well below the silver work function (~4.4 eV).24
For a two-photon photoemission process, the electron yield follows a |E|4 dependence,
that is, a quadratic dependence of photoelectron yield on laser power (See supplementary
material).21 Therefore, doubling the field intensity as a result of the localized surface
plasmon resonance enhancement effect would lead to a 16 fold increase in photoemission
yield. A clear indication of plasmon-induced field enhancement is also evidenced by
surface enhanced Raman spectroscopy (SERS) results obtained using Ag nanorod arrays
(See supplementary material).21 Overall, the SERS data indicate strong plasmonic field
enhancement due to the nanorod array structure and strongly support our interpretation of
the PEEM results.
8
Figure 3. Photoemission electron microscopy images of the Ag nanorod sample surface
illuminated with the mercury lamp and integrated for 10 seconds for a 25 micron field-ofview (a); and a 400 nm femtosecond laser integrated for 0.2 seconds (b). The scale bars
represent photoelectron counts per second per pixel. We note the much higher contrast in
3b than 3a even though the integration time is 50 times longer in 3a than 3b.
Representative hot spots within the box in Figure 3 are displayed as a photoemission
enhancement image in Figure 4a.
A photoemission enhancement image can be calculated by dividing each pixel of the
photoelectron yield by the average per pixel yield from a smooth Ag thin film under
identical laser excitation conditions,25 see Figure 3b. This treatment reveals that the
enhancement of photoemission at hot spots ranges from 102 to ~ 3x103 times that of a
smooth silver film (See supplementary material) REF 21. Figure 4(a) displays the
enhancement factor image calculated as described above for hot spots residing within the
box shown in Figure 3b and line profiles of a hot spot in this region. Horizontal and
vertical line profiles of the hot spot display a full width half maximum (FWHM) of 250
to 300 nm and a round profile consistent with emission from individual rod tips (Figure
4a). Figure 4(b) shows the photoemission enhancement factor image and line profiles for
an oblong shaped hot spot selected from a different region in the PEEM image. This hot
spot displays line profiles of roughly 300 and 600 nm FWHM for short and long axis
respectively. The oblong emission structures likely result from pairs of adjacent tips in
close proximity, either by forming a larger tip or by coupling proximal nanorod
plasmonic eigenmodes. Since the tips are not perfect cylinders, the oblong hot spots
could also be generated by specific tip asperities. We will test these hypothesis in future
work using correlated PEEM and SEM imaging.25 Various (non-round) hot spot shapes
have been observed by PEEM and correlated with specific nanostructures from a
9
collection of nominally spherical nanoparticles25 supporting the nanostructural
explanation for hot spot shapes on a <10 nm scale.
Figure 4. (a) Photoemission enhancement image of a typical Ag nanorod hot spot. The
line profiles show that the hot spot is about 300 nm FWHM in both dimensions,
indicating it likely originates from a single nanorod. (b) Photoemission enhancement
image of an oblong shaped Ag nanorod hot spot. The FWHM horizontal size of the hot
spot is approximately 600 nm indicating it derives from two nanorod tips in close
proximity.
Although field enhancement leading to hot spot photoemission is well understood on the
basis of nanostructure in principle,26, 27 producing uniform plasmonic nanostructures that
display consistent high field enhancement remains a challenge. The “hot spot’
phenomenon, as visualized in Figure 3b is a general result of our inability to control
10
structure on a 1-10 nm length scale. The concept behind our array design is to take
advantage of the plasmon enhancement derived from longitudinal nanorod resonance and
possible resonant couplings between proximal nanorods. Efficient plasmon propagation
over a distance of several microns has been demonstrated following near infrared
excitation of isolated gold nanowire structures.26 Herein, surface plasmons excited along
the length of a nanorod can efficiently propagate to localize and enhance the local electric
fields at the tips, thereby greatly enhancing photoemission yield.
The localized nature of hot spot photoemission has several inescapable consequences for
photocathode design and applications, as the regions of highly enhanced photoelectron
yield are a minute fraction of the nanorod array area. When the relative yield
enhancement is calculated, on an area basis, the overall yield enhancement is much
smaller than that of individual hot spots, roughly a factor of 12 (for comparison we note
that the nanorod enhancement factor under Hg lamp illumination is only a factor of 2).
We note that if greater control of nanorod growth can be achieved it should be possible to
generate arrays with a far greater density of hot spots such that the yield per unit surface
area could be dramatically increased. Furthermore, there are advantages of the localized
emission. It is possible to synthesize arrays of small sizes, on the order of tens of
microns, such that photoelectron emission is confined to the array area. Since
photocathode emittance1 is proportional to the area of the emitter, emittance can be
simply reduced by employing a smaller area array, perhaps even a single hot spot. This
concept is nicely embodied in a recent study of copper nanohole array photocathode.11
Similarly, a gold nanograting photocathode displayed a highly nonlinear P4 (fourth-
11
order?) photoelectron yield under 800 nm fs laser excitation.10 Power dependences
reported in the early report10 indicate that higher laser powers could generate sufficient
electron beam currents required to operate modern FEL designs.
In summary, aligned Ag nanorods were synthesized using AAO templates and the DC
electrodeposition method. The SEM images show the growth of nanorods with relatively
uniform width and length distributions ranging from 150 to 300 nm and 20-22 μm,
respectively. These nanorods are polycrystalline and show no indication of silver oxides
or other contaminants. Intense photoelectron emission is evidenced from the PEEM
images collected following 400 nm fs laser excitation. Hot spots of varying sizes and
shapes featuring extremely high photoelectron yields are apparent in the PEEM images.
The calculated enhancement factors at these hot spots in different regions of the sample
vary from several hundred to several thousand consistent with SERS studies on identical
arrays. Line profiles cross-sections of the hotspots indicate that they originate from
individual nanorods or adjacent nanorod pairs. In future, better control of nanorod growth
and structure should lead to much higher intensity photoemission from nanorod array
photocathodes. We conclude that aligned Ag nanorod arrays are promising candidates for
photocathode applications.
Acknowledgements
This research used EMSL, a national scientific user facility sponsored by the Department
of Energy’s Office of Biological and Environmental and located at Pacific Northwest
National Laboratory (PNNL) and is supported by the Chemical Imaging Initiative
12
conducted under the Laboratory Directed Research and Development Program. PNNL is
a multiprogram national laboratory operated by Battelle Memorial Institute for the U.S.
Department of Energy.
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