# Structural and optical properties of monocrystalline and polycrystalline
gold plasmonic nanorods

Lukáš Kejík∗,1, Michal Horák∗,1, Tomáš Šikola1,2, Vlastimil Křápek1,2

1 CEITEC, Brno University of Technology, Purkyňova 123, 612 00 Brno, Czech
Republic

2 Institute of Physical Engineering, Brno University of Technology, Technická
2, 616 69 Brno, Czech Republic

∗ These authors contributed equally.

∗ lukas.kejik(at)ceitec.vutbr.cz

∗ michal.horak2(at)ceitec.vutbr.cz

## Abstract

Plasmonic structures are often fabricated by lithographic patterning of a thin
metallic film. Properties of the thin film are intimately related to the
quality of the resulting structures. Here we compare two kinds of thin gold
films on silicon nitride membrane: a conventional polycrystalline thin film
deposited by magnetron sputtering, and monocrystalline gold microplates which
were chemically synthesised directly on the membrane’s surface for the first
time. Both pristine metals were used to fabricate plasmonic nanorods using
focused ion beam lithography. Structural and optical properties of the
nanorods were characterized by analytical transmission electron microscopy.

The dimensions of the nanorods in both substrates reproduced well the designed
size of $240\times 80\ \mathrm{nm^{2}}$ with the deviations up to 20 nm in
both length and width. The shape reproducibility was considerably improved
among monocrystalline nanorods fabricated from the same microplate.
Interestingly, monocrystal nanorods featured inclined boundaries while the
boundaries of the polycrystal nanorods were upright. Q factors and peak loss
probabilities of the modes in both structures are within the experimental
uncertainty identical. We conclude that the optical response of the plasmonic
antennas is not deteriorated when the polycrystalline metal is used instead of
the monocrystalline metal.

##  1 Introduction

Plasmonic antennas are metallic particles widely studied for their ability to
control, enhance, and concentrate electromagnetic field [1]. Strikingly, the
field in the vicinity of plasmonic antennas, the so-called near field, can be
focused into a deeply subwavelength region [2]. At the same time, the field is
strongly enhanced with respect to the driving field, which can be e.g. a plane
wave. Focusing of the field stems from the excitation of localized surface
plasmons (LSP) – quantized oscillations of the free electron gas in the metal
coupled to the evanescent electromagnetic wave propagating along the boundary
of the metal. Thanks to this light-concentrating ability, plasmonic antennas
have found applications in energy harvesting [3], construction of metasurfaces
[4, 5], luminescence enhancement [6], or biodetection [7].

Plasmonic antennas are conventionally prepared using electron beam
lithography, which utilizes a mask fabricated in electron-sensitive polymer
(resist), or more straightforwardly using direct milling by focused ion beam
(FIB) [8]. The energy of antenna’s plasmon resonance is highly dependent on
the choice of antenna material and its quality, surrounding material, as well
as antenna size and shape [9].

Typical plasmonic antennas fabricated from metallic layers deposited by
sputtering or evaporation techniques are polycrystalline by nature with
randomly oriented grains of varying sizes [10, 11, 12]. The fabrication
resolution and overall shape of the antenna are then significantly affected by
the size of grains present in the antenna [13] making each antenna of slightly
different shape. The deviations in shape or the orientation of the antennas
might then be detrimental e.g. for the performance of metasurfaces as they
introduce random noise into their phase response. Although by optimizing the
deposition parameters, the resulting grain size can be somewhat tuned [14],
the presence of grain boundaries within an antenna is related to the lower
resonance quality factor and higher relaxation rates [15, 16]. In combination
with other approaches, like template stripping, the layer quality might be
improved even further [17], and increased grain size can then allow antenna
fabrication from a single grain. This can be achieved readily by chemical
synthesis where each particle can be made as single crystal grain with great
shape variability [18], but their precise placement on the substrate is hard
to achieve. In an ideal case the whole set of antennas is fabricated from a
single grain, possibly large metallic 2D monocrystal [19, 20] placed on a
supporting substrate, alleviating the troubles with the size and shape
reproducibility in the follow-up lithographic process. In the case of gold,
there are several approaches to chemically reduce typically HAuCl4 into large
monocrystalline microplates in pathways based on aniline [21], two-component
ionic liquids [22], ethylene glycol [23, 24, 25] or tetraoctylammonium bromide
(TOABr) [26, 27, 28] where the latter two are the most prevalent. Although the
improvement in optical properties of monocrystalline versus polycrystalline
gold is not that radical in bulk [29], it has been shown as a useful platform
with improved propagation length of surface plasmons [30] even in complex
devices [31, 32, 33, 34]. Additionally, in terms of fabrication of antennas or
more complex structures, monocrystalline gold allows fabrication with higher
resolution, generally better defined shape and consequently better optical
performance [35, 36].

A detailed study, though, comparing the polycrystalline and monocrystalline
antennas in terms of material properties and correlating the antenna shape
with plasmon modes with high spatial and energy resolution, has been missing
in the literature.

Here we show direct comparison of gold plasmonic nanorods fabricated either
from monocrystalline or polycrystalline gold substrates using FIB lithography.
We selected the nanorod shape due to well-isolated modes and an existence of
analytical description [37]. We characterize the material properties of the
input substrates as well as the resulting nanorods, correlate the nanorod
shape with its plasmonic properties and visualize the supported plasmonic
modes by scanning transmission electron microscopy (STEM) in combination with
electron energy loss spectroscopy (EELS) with nanometer spatial resolution not
achievable by means of other methods.

##  2 Methods

###  2.1 Fabrication of nanorod antennas

Membranes with monocrystalline gold were prepared using the modified procedure
by Radha and Kulkarni [27]. Briefly, 4 ml of TOABr solution in toluene (50 mM)
was stirred vigorously while 1.6 ml of aqueous HAuCl4 (25 mM) was injected.
The resulting mixture of two immiscible phases, aqueous and toluene-based, was
stirred for approximately 2 minutes. Aqueous component then became colourless
while toluene component became red-coloured which is related to the phase
transfer of (AuCl4)- ions from aqueous solution into toluene using TOABr as a
phase transfer agent. Toluene solution containing (AuCl4)-– TOABr precursor
was then extracted and 1–2 $\mu$l were typically applied onto SiNx TEM
membrane, which was kapton-taped to a piece of silicon wafer for easier
manipulation, on a preheated hot plate (75∘C). When toluene evaporated, the
temperature was set to 140 or 145∘C held for 35–45 hours. Such procedure
resulted in monocrystalline gold microplates of various lateral sizes (tens of
micrometers) and thickness (tens to hundreds of nanometers) and the remaining
precursor residues were washed away in toluene, ethanol, and water baths,
respectively. Note that the thinnest microplates (below 100 nm) were pre-
selected using bright field optical microscopy due to their partial
transparency resulting in pink shade as opposed to thicker ones which
reflected the light completely (see Fig. S1). The final information about the
thickness was obtained using EELS.

Polycrystalline gold layer (thickness of 30 nm) was deposited by magnetron
sputtering using Leica coater EM ACE600 directly (i.e. with no adhesion layer)
on a 30-nm-thick SiNx membrane (Agar Scientific).

Gold nanorods on both samples were fabricated by FIB lithography in dual beam
FIB/SEM microscope FEI Helios using gallium ions with the energy of 30 keV and
ion beam current of 1.3 pA. Note that the energy (the highest available) and
the current (the lowest available) are optimized for the best spatial
resolution of the milling [8]. The nanorods were designed as rectangles with
the length of 240 nm and the width of 80 nm. They were situated in the middle
of a $1.5\times 1$ $\mu$m2 metal-free rectangle, which is perfectly sufficient
to prevent their interaction with the surrounding metallic layer. Note that
the amount of residual gallium ions is negligible and does not influence the
localized surface plasmon resonances [8]. Results of the energy-dispersive
X-ray spectroscopy (EDS) analysis are shown in Fig. S2.

Finally, we remark that we have experienced issues with the mechanical
stability of the membranes with monocrystalline nanorods which proved to be
more brittle than untreated ones. In total we have fabricated five
monocrystalline samples containing microplates of suitable thickness (below
100 nm). Three samples have been destroyed during the fabrication process or
sample manipulation and the last two were characterized in the transmission
electron microscope (TEM). For the latter ones it was possible to obtain
majority of the properties, although no membrane survived complete analysis.
Therefore, a complete characterization of a single sample turned out
impossible and we used different samples instead. We have not experienced any
stability issues for the polycrystalline substrates. We suppose that the low
stability of the monocrystalline substrates is related to the membrane
heating, its loading with the growth solution or by the following washing
procedure in various liquid solvents. Conversely, a continuous thin film of
polycrystalline gold does not undergo any wet treatment and can even
contribute to the mechanical strength of the membrane unlike the finite-size
monocrystalline microplates with a rather low coverage.

###  2.2 Analytical methods

Surface roughness of the resulting polycrystalline and monocrystalline gold
substrates was analysed by atomic force microscopy (AFM) using Bruker
Dimension Icon in tapping ScanAsyst mode.

Analytical transmission electron microscopy characterization including STEM-
EELS was performed using TEM FEI Titan equipped with Super-X spectrometer for
EDS and GIF Quantum for EELS operated at the primary electron energy of 300
keV. Prior to STEM-EELS experiments all samples were cleaned in oxygen/argon
plasma for 10 seconds to prevent the carbon contamination [8]. Beam current
was set around 0.2 nA and the full-width at half-maximum (FWHM) of the zero-
loss peak (ZLP) was in the range from 0.1 eV to 0.15 eV. We set the
convergence semi-angle to 10 mrad and the collection semi-angle to 20.5 mrad.
We note that these parameters are not critical for the signal-to-background
ratio of the setup [38]. The dispersion of the spectrometer was 0.01 eV/pixel.
We recorded spectrum images with the pixel size of 2 nm. Every pixel consists
of 1 EEL spectrum whose acquisition time was adjusted to use the maximal
intensity range of CCD camera in the spectrometer and avoid its overexposure.
EEL spectra were integrated over rectangular areas consisting of tens of
pixels in the areas corresponding to maximum loss probability of studied LSP
resonances. Obtained spectra were divided by the integral intensity of the ZLP
(the energy window for integration was set as $-1$ eV to $+1$ eV) to transform
measured counts to a quantity proportional to the loss probability. We fitted
the peaks in the EEL spectra by Gaussian function to determine the peak energy
and its spectral width (i.e., FWHM). EEL maps were calculated by dividing the
map of integrated intensity at the plasmon peak energy with the energy window
of 0.1 eV by the map of the integral intensity of the ZLP.

Simulations of EELS spectra have been carried out using boundary element
method (BEM) [39] with a software package MNPBEM [40]. The dielectric function
of gold was taken from Ref. [41] and the dielectric constant of the silicon
nitride membrane was set equal to 4, which is a reasonable approximation in
the considered spectral region [42] taking into account non-stoichiometric
composition of the membrane [8]. The polycrystalline nanorods have been
represented by a rectangular prism and the monocrystalline nanorods by a
truncated pyramid with the top face reduced by 30 nm from each side in
comparison to the bottom face (i.e., with the length and width smaller by 60
nm). The edges of the nanorods have been rounded with a radius of 10 nm.

##  3 Structural properties of the nanorods

Figure 1:  (a,b) STEM ADF image of (a) a monocrystalline microplate and (b) a
polycrystalline thin film after the fabrication of nanorods. Insets show
electron diffraction patterns recorded at the area of the film far from the
nanorods. (c,d) AFM images of the surface of a typical (c) monocrystalline
microplate and (d) a polycrystalline thin film.

Thin monocrystalline microplates and polycrystalline films of gold have been
characterized by selected-area electron diffraction (SAED) in TEM and by AFM.
Fig. 1(a) shows a monocrystalline microplate with fabricated nanorods and
corresponding electron diffraction pattern recorded at an area away from the
nanorods. The diffraction pattern exhibits a 6-fold rotational symmetry and
corresponds to the face-centered cubic (FCC) Au crystal lattice viewed along
the [111] direction. A point-like symmetric character of the pattern indicates
the single-crystalline nature of the synthesized Au microplates laying flat on
the membrane with (111)-oriented basal plane.

Similar characterization of a polycrystalline thin film is shown in Figure
1(b). For the selected scale, the individual grains of the polycrystal are not
well visible [see Fig. 2(b) for a zoomed-in images]. Nevertheless, a circular
diffraction pattern clearly corresponds to a polycrystalline sample with
random orientation of individual grains.

AFM has been performed for a different set of pristine samples fabricated
under the same growth conditions as the samples with nanorods. It reveals
rather flat surface of both monocrystalline [Fig. 1(c)] and polycrystalline
[Fig. 1(d)] samples, with root mean square surface roughness on average around
0.6 nm for the monocrystalline and 1.2 nm for the polycrystalline substrates.
Larger roughness of the polycrystalline film is related to a stochastic nature
of the gold sputtering.

The overview of resulting nanorod shapes which underwent EELS analysis is
shown in Fig. 2. Monocrystalline nanorods are rather regular, with smooth
boundaries. However, the tips of the rods are faceted and deviate from the
rectangular shape. The area surrounding the nanorods is clean and smooth
(i.e., it shows no features in the STEM). Polycrystalline nanorods are of more
irregular shape, with coarse boundaries and overall larger deviations from the
desired rectangular shape. The irregularity and larger variance are related to
the random orientation of grains in the polycrystalline metal layer, for which
different sputtering rates apply during the FIB lithography. Individual grains
of gold are visible in the STEM images, with typical sizes between 20 and 50
nm. Nevertheless, polycrystalline nanorods lack the faceted tips and some may
reproduce the desired rectangular shape better than monocrystalline nanorods
[e.g. the first and the second rod in Fig. 2(b)]. The area around the
polycrystalline nanorods is covered by gold crystallites due to incomplete
removal of the gold layer again related to different removal rate of randomly
orientated gold grains. We note that it is not possible to remove these
crystallites simply by increasing the removal time as it would result into
thinning and finally a destruction of the membrane underneath. Upon close
inspection, monocrystalline nanorods fabricated from the same gold monocrystal
exhibit very similar overall shape with the tips faceted in the same manner
which is also reflected in much smaller deviations in terms of e.g. antenna
length (Tab. 1). Across the different microplates though the facets look very
different. When comparing the respective orientation of gold monocrystal and
the resulting nanorods, some of the monocrystal’s side facets seem to be
imprinted into the antenna shape (see Fig. S3). Therefore, the fabrication
results might be further improved by a suitable alignment of the monocrystal
side facets with the FIB milling direction.

Figure 2: STEM ADF images of gold nanorods fabricated from (a) monocrystalline
and (b) polycrystalline gold substrates. Note that monocrystalline nanorods
are divided into columns according to the microplate from which they were
fabricated.

Representative nanorods with the targeted length and width $240\times 80$ nm2
are shown in Fig. 3(a,c) with the corresponding thickness profiles along their
length and width in Fig. 3(b,d). We have used EELS to retrieve the thickness
of the nanorods using the same approach described in Ref. [8] based on Refs.
[43, 44, 45]. Polycrystalline nanorods exhibit upright boundaries with a steep
change in the thickness from zero to maximum over a distance of about 10 nm
related to the diameter of the ion beam whose nominal FWHM is around 3 nm. On
the other hand, monocrystalline nanorods have strongly inclined boundaries
mostly due to microplate’s anisotropy with possible gold redeposition.

The dimensions of all the antennas have been determined as the maximum extent
of the white contrast in the STEM annular dark field (ADF) images along the
longitudinal (length $L$) and transverse (width $W$) direction, and the height
$H$ has been determined as the average height of the central part of the
nanorod. The dimensions of the monocrystalline nanorods were $L=(232\pm 9)$
nm, $W=(82\pm 7)$ nm, and $H=(52\pm 5)$ nm, while the dimensions of the
polycrystalline nanorods were $L=(238\pm 14)$ nm, $W=(84\pm 3)$ nm, and
$H=(30\pm 3)$ nm (see Tab. 1 for a complete summary). The targeted lateral
dimensions have been reproduced well. The height of the monocrystalline
microplates cannot be quantitatively controlled and due to low mechanical
stability of the monocrystalline samples we measured the height only after the
fabrication of the antennas together with the EELS characterization of plasmon
modes. We could fabricate the polycrystalline nanorods (for which the height
is controlled rather accurately) after the characterization of the
monocrystalline antennas, assuring the same thickness. However, this would
necessarily result into some deviation in the alignment and adjustment of the
electron microscopes used for the fabrication of the nanorods and their
characterization, restricting the comparability of both types of the nanorods.
Therefore, we preferred fabrication and characterization of both samples
without any delay and need for realignment of the microscopes at the price of
different nanorod heights. In addition, the inclined boundaries of the
monocrystalline nanorods resulted in the LSP energies nearly the same as for
the polycrystalline nanorods, which is optimum for the comparison of both
systems despite their different height.

Figure 3:  (a,c) STEM ADF images of representative plasmonic nanorods
fabricated from (a) a monocrystalline microplate and (c) a polycrystalline
thin film with cross-sections corresponding to (b,d) thickness profiles along
the long and short axes of (b) the monocrystalline and (d) polycrystalline
nanorod determined by EELS. The color of thickness profile curve corresponds
to the color of the axis indicated in (a,c).

##  4 Optical properties of the nanorods

Figure 4:  Loss probability spectra for (a,b) monocrystalline and (d,e)
polycrystalline nanorods recorded for the electron beam located at (a,d) the
center of the short edge and (b,e) the center of the long edge for two
different (c) monocrystalline and (f) polycrystalline nanorods. Experimental
spectra are displayed by red or blue lines. Calculated spectra are displayed
by gray lines, calculated spectra broadened by a Gaussian with 0.3 eV FWHM to
reproduce the instrumental broadening are displayed by gray shaded area. Most
significant LSP modes are labelled as LD (longitudinal dipole), LQ
(longitudinal quadrupole), and TD (transverse dipole) mode. (c,f) STEM ADF
images of the selected nanorods with color spots indicating the position of
the electron beam with colors corresponding to the respective EEL spectra.

Two red and blue curves in Fig. 4 represent typical experimental loss
probability spectra of two monocrystalline and two polycrystalline nanorods.
The lowest-order LSP mode is observed around 1.1 eV and by inspecting energy-
filtered maps of the loss intensity in Fig. 5(a,c), it is identified as the
longitudinal dipole (LD) mode. The peak at 1.8 eV is attributed to two
spectrally overlapping modes, the longitudinal quadrupole (LQ) mode and the
transverse dipole (TD) mode. It is difficult to resolve the LQ and TD modes in
the experiment, as well as the higher-order modes. Nevertheless, the energy-
filtered maps of the loss intensity for the peak energy [Fig. 5(b,d)] suggest
LQ mode to be dominant at the peak energy.

Both main peaks of experimental spectra corresponding to monocrystalline
antennas are slightly blue-shifted with respect to polycrystalline ones. Since
the differences in dielectric function of monocrystalline and polycrystalline
gold have been previously found as insignificant [29], we attribute the blue
shift to the differences in the shape and thickness of the nanorods. The
experimental loss probability spectra are accompanied by those calculated
using MNPBEM shown as grey lines. The simulations take into account actual
height profiles of the antennas as determined by EELS, i.e., the height of 50
nm with inclined boundaries for the monocrystalline nanorods and the height of
30 nm with upright boundaries for the polycrystalline nanorods. Note that the
larger thickness causes a shift towards higher mode energies while the
presence of inclined boundaries causes shift to lower energies, as shown in
Fig. S4. To account for the experimental spectral broadening of our EELS
system, the simulated spectra were convoluted with a Gaussian of 0.3 eV FWHM
shown as shaded gray spectra. These broadened spectra are then in good
agreement with the experimental ones.

Figure 5:  Spatial distribution of the loss probability at the energy of (a-d)
the LD mode and (e-h) the LQ mode. Left panels (a,c,e,g) show experimental
maps for nanorods displayed in upper part of Fig. 4(c,f). Right panels
(b,d,f,h) show calculated values with the contour of the nanorod displayed by
the black lines. The energy of the mode is indicated at the top part of each
panel. The color scale at the bottom is common to all panels.

By fitting the spectral profile of the modes by a Gaussian, we obtained the
LSP resonance energy $E$ and Q factor defined as the LSP resonance energy
divided by FWHM for both main modes. Mean energies of the LD mode in
experiment averaged to $(1.14\pm 0.03)~{}\mathrm{eV}$ for the monocrystalline
nanorods and $(1.02\pm 0.05)~{}\mathrm{eV}$ for the polycrystalline nanorods.
Taking into account the experimental resolution of EELS, estimated as 0.1 eV
for our setup, no significant difference in the energy of LD mode has been
observed between monocrystalline and polycrystalline nanorods. The simulations
accounting for the dispersion in length and width of nanorods [10 % deviations
from the mean value are assumed, i.e., $L=(240\pm 24)~{}\mathrm{nm}$,
$W=(80\pm 8)~{}\mathrm{nm}$] yielded the energies of the LD mode in the range
$(1.06\pm 0.08)~{}\mathrm{eV}$ for monocrystalline and $(1.08\pm
0.07)~{}\mathrm{eV}$ for polycrystalline nanorods, in good agreement with the
experimental values taking into account the experimental error and uncertainty
of the shape and dimensions of model nanorods.

To confirm the nature of the LSP modes induced in gold nanorods, we extracted
EEL maps to visualize the modes at the peak energies. Fig. 5(a,c) shows EEL
maps at the 1.1 eV energy for monocrystalline and polycrystalline gold
nanorods with a loss probability distribution typical for the LD mode.
Similarly, the EEL maps at the higher energy peak at 1.8 eV for
monocrystalline and 1.7 eV for polycrystalline nanorods [Fig. 5(e,g)] imply
that the longitudinal quadrupole mode is more prevalent than the transverse
dipole mode at the peak energy. EEL maps for both modes show only minute
differences between monocrystalline and polycrystalline nanorod, although the
modes in polycrystalline one can be resolved a bit easier. For detailed
interpretation of the EEL maps we refer to Ref. [46]. The experimental EEL
maps were complemented by the simulated EEL maps with a very good agreement in
both LD [Fig. 5(b,d)] and LQ [Fig. 5(f,h)] mode after taking into account the
differences in the nanorod morphology.

Table 1 summarizes morphological as well as optical properties of all studied
antennas. In total, we have analysed 7 polycrystalline and 9 monocrystalline
nanorods which were fabricated using in total three monocrystalline
microplates. In terms of the nanorods’ size the average length of the
polycrystalline nanorods was closer to the desired length (240 nm) though with
larger deviations, the width of the monocrystalline nanorods was closer to the
desired width (80 nm) also with larger deviations. The average energy of LD
mode for the monocrystalline nanorods is slightly larger and with smaller
overall dispersion than for polycrystalline ones, the difference is even more
pronounced in the case of LQ mode. Similarly, extracted Q factors are slightly
larger in case of monocrystalline nanorods for both modes. Overall the average
parameters of the nanorods’ plasmon resonance for monocrystalline and
polycrystalline nanorods are quite similar. Although when we focus on the
nanorods fabricated from within the same gold microplate, the dispersion of
measured parameters is significantly reduced, especially in terms of LD and LQ
mode energy.

Table 1:  Summary of average dimensions and plasmon resonance parameters of polycrystalline and monocrystalline gold nanorods characterized by EELS followed by parameters averaged only over nanorods originating from the same gold monocrystal. Note that the numbers in parentheses in the left column correspond to the number or analysed nanorods. | Length (nm) | Width (nm) | LD energy (eV) | LD Q factor | LQ energy (eV) | LQ Q factor
---|---|---|---|---|---|---
Polycrystalline - all (7) | $238\pm 14$ | $84\pm 3$ | $1.01\pm 0.05$ | $3.2\pm 0.4$ | $1.67\pm 0.08$ | $4.5\pm 0.6$
Monocrystalline - all (9) | $232\pm 9$ | $82\pm 7$ | $1.14\pm 0.03$ | $3.4\pm 0.3$ | $1.78\pm 0.03$ | $5.1\pm 0.4$
microplate 1 (3) | $243\pm 3$ | $90\pm 5$ | $1.12\pm 0.03$ | $3.1\pm 0.2$ | $1.81\pm 0.01$ | $4.9\pm 0.4$
microplate 2 (3) | $231\pm 2$ | $80\pm 2$ | $1.13\pm 0.01$ | $3.4\pm 0.1$ | $1.75\pm 0.02$ | $5.0\pm 0.4$
microplate 3 (3) | $223\pm 2$ | $76\pm 5$ | $1.17\pm 0.02$ | $3.7\pm 0.2$ | $1.77\pm 0.01$ | $5.3\pm 0.4$
Theory polycrystalline | $240\pm 24$ | $80\pm 8$ | $1.08\pm 0.07$ | $2.8\pm 0.2$ | $1.88\pm 0.08$ | $5.5\pm 0.1$
Theory monocrystalline | $240\pm 24$ | $80\pm 8$ | $1.06\pm 0.08$ | $2.9\pm 0.2$ | $1.62\pm 0.07$ | $4.4\pm 0.5$

##  5 Conclusions

We have prepared monocrystalline and polycrystalline plasmonic nanorods and
compared their structural and optical properties. Most pronounced differences
have been identified regarding the vertical interfaces of nanorods, inclined
for the monocrystalline nanorods but upright for polycrystalline ones.
Monocrystalline nanorods outperform the polycrystalline ones in terms of
slightly reduced size and shape fluctuations. They also benefit from clean
metal-free parts, while for polycrystalline nanorods these parts contain metal
crystallites due to incomplete metal removal. On the other hand,
monocrystalline nanorods systematically deviate from the desired shape due to
preferable termination with specific crystal planes. The membranes with
monocrystalline nanorods also suffered from increased brittleness.

As for the optical properties, we have observed no significant differences for
both crystallinities. The plasmon resonance energies, Q factors, and loss
probability magnitudes detected by EELS show no difference up to experimental
accuracy. The optical response of nanorods is not deteriorated when the
polycrystalline metal replaces the monocrystalline metal. To conclude,
polycrystalline plasmonic antennas represent a fully-fledged alternative of
monocrystalline antennas.

## Acknowledgements

We acknowledge the Czech Science Foundation (grant No. 19-06621S), and
Ministry of Education, Youth and Sports CR (projects CzechNanoLab Research
Infrastructure, No. LM2018110 and CEITEC 2020, No. LQ1601). M. H. acknowledges
the support of Thermo Fisher Scientific and CSMS scholarship 2019.

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## Supporting information

Figure S1:  (a,b,c) Bright field optical images of gold microplates on 30 nm
SiNx membrane (square $250\times 250$ $\mu$m). Membrane containing (a) thick
microplates with more than 100 nm thickness which are completely opaque for
the incident light, (b) combination of completely opaque thicker and partially
transparent thinner microplates which have slight pink shade, (c) thin
microplates with the thickness well below 100 nm - the thinner the microplate
the darker the shade of pink color. Figure S2:  (a,b) Energy-dispersive X-ray
spectroscopy (EDS) analysis of fabricated (a) monocrystalline and (b)
polycrystalline gold nanorods with the maps of all relevant elements. Figure
S3:  (a,b) ADF STEM images of selected gold microplates after FIB fabrication
of nanorod antennas. (c,d) Details of fabricated monocrystalline nanorods with
the respective ”mother” microplate’s facets highlighted: nanorods in (c)
correspond to the microplate in (a) and nanorods in (d) correspond to the
microplate in (b). Figure S4:  (a,b) Calculated EEL spectra for gold nanorods
of 30 nm thickness (red curves) and 50 nm thickness (blue curves) with upright
boundaries (solid lines) and inclined boundaries (dashed lines) for electron
beam positioned at the (a) shorter and (b) longer nanorod’s edge. Note that
all the spectra were broadened by a Gaussian with 0.3 eV FWHM to reproduce the
experimental broadening.