# Continuous Anisotropic Growth of Plasmonic CsxWO3-δ Nanocrystals into Rods and Platelets

Jisoo Oh, Joshua Davis, Sandrine Tusseau-Nenez, Mathis Plapp, Alexandre Baron, Thierry Gacoin, Jongwook Kim

► To cite this version:

Jisoo Oh, Joshua Davis, Sandrine Tusseau-Nenez, Mathis Plapp, Alexandre Baron, et al. Continuous Anisotropic Growth of Plasmonic CsxWO3-δ Nanocrystals into Rods and Platelets. ACS Nano, 2025, 19 (14), pp.14445-14455. 10.1021/acsnano.5c02382. hal-05408643

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Distributed under a Creative Commons Attribution 4.0 International License
Continuous anisotropic growth of plasmonic

CsₓWO₃-δ nanocrystals into rods and platelets

Jisoo Oh¹, Joshua Davis², Sandrine Tusseau-Nenez¹, Mathis Plapp¹, Alexandre Baron²,³, Thierry

Gacoin*¹, Jongwook Kim*¹

¹ Laboratoire de Physique de la Matière Condensée, CNRS, École Polytechnique, Institut Polytechnique de Paris, 91128 Palaiseau, France

² Univ. Bordeaux, CNRS, CRPP, UMR 5031, F-33600 Pessac, France

³Institut Universitaire de France, 1 rue Descartes, 75231 Paris Cedex 05, France

**Corresponding Authors**

* (Jongwook Kim). jong-wook.kim@polytechnique.edu

* (Thierry Gacoin). thierry.gacoin@polytechnique.edu
ABSTRACT

Shape control during nanocrystal synthesis enables tunable physico-chemical properties that emerge at the nanoscale. While extensive efforts have been devoted to control shapes in various systems such as plasmonic metal nanoparticles or semiconductor quantum dots, the shape control of plasmonic doped semiconductor nanocrystals remains less explored and limited. Here, we report the synthesis of CsₓWO₃-δ nanocrystals with an exquisite shape control achieved through a continuous injection synthesis combined with precursor-mediated facet-selective growth. We demonstrate that the anisotropic growth of CsₓWO₃-δ nanocrystals is strongly dependent on the precursor injection rate, which we attribute to the material's intrinsic structural anisotropy and the contrasting reaction kinetics on different crystallographic facets. Furthermore, we reveal that the presence of halide ions in the reaction medium is critical for passivating and suppressing the growth of Cs-exposed basal planes. By systematically modulating the shape aspect ratio, we achieved an extended range of nanocrystal morphologies, leading to a broad tunability of LSPR spectra, spanning the entire near-infrared region and extending into the mid-infrared. Computational simulations effectively reproduce the observed shape-dependent optical properties and highlight the size-dependent damping behavior consistent with the free electron model. These findings provide a robust experimental methodology for shape control in structurally anisotropic nanocrystals and offer theoretical insights into the tunable LSPR properties of heavily doped plasmonic semiconductor systems.

KEYWORDS

Colloidal synthesis, shape control, anisotropy, growth kinetics, IR plasmonics
<figure><img src="image_2.png" /><figcaption>TOC GRAPHICS</figcaption></figure>
Anisotropy of nanocrystals give rise to novel physical and chemical properties that emerge at the nanoscale. Beyond size-dependent effects, variations in shape anisotropy can significantly enhance and extend the range of these properties. For instance, the wavelength of localized surface plasmon resonances (LSPR) in gold nanorods undergoes a pronounced redshift with increasing the rod aspect ratio, demonstrating a substantially wider optical range compared to increasing the diameter of gold nanospheres¹⁻⁵. Moreover, anisotropic shapes inherently exhibit directional properties, such as polarized luminescence of rod- or platelet-shaped quantum dots⁶, offering new functionalities such as for orientation probes⁷ or display devices with enhanced efficiency and colour purity⁸,⁹.

Various synthetic methodologies have been developed to tailor the shapes of nanocrystals across a wide range of chemical compositions. For example, seed-mediated growth allows for the selection of crystal growth direction by temporally separating the nucleation step from the subsequent growth. Since the crystal structure of seeds is pre-defined, it becomes straightforward to control the growth towards selected directions.⁴,¹⁰⁻¹⁵ A common strategy for the anisotropic growth is to employ molecular ligands that bind selectively on specific crystallographic facets, either promoting or inhibiting growth on those surfaces.¹³,¹⁴,¹⁶⁻²¹ For gold nanorods exhibiting polarized LSPR, cetyltrimethylammonium bromide (CTAB) is often used as a shape-directing agent, blocking the {1 0 0} faces directing the preferential growth on the {1 1 1} faces.²²,²³ In the case of PbS nanocrystals, tetradecahedron seeds naturally develop into cube shapes. However, rods or multipods are grown in presence of dodecanethiols due to their strong binding on the {1 0 0} faces.²⁴,²⁵ Inorganic ligands can also be employed.²⁶ Highly electronegative halide ions efficiently block {0 0 1} faces of TiO₂ nanocrystals leading to the 2D growth into platelets or sheets.²⁷⁻³⁰
Halides were also used to promote thickening of CdSe nanoplatelets by altering the surface energy of the basal plane.<sup>31-33</sup>

In this study, we focus on controlling the anisotropic growth of hexagonal cesium-doped tungsten oxide (Cs<sub>x</sub>WO<sub>3-δ</sub>) nanocrystals, which exhibit efficient LSPR in the near-infrared (NIR) range. Among the recently discovered plasmonic doped semiconductor nanocrystals<sup>34-37</sup>, Cs<sub>x</sub>WO<sub>3-δ</sub> stands out due to its exceptionally high free electron density attributed to the maximum values of x = 0.33 and δ = 0.3.<sup>38</sup> This leads to a strongly metallic behavior that manifests LSPR in the NIR close to the visible range, while its large band gap ensures full transmission of visible light. Such advantageous optical properties of Cs<sub>x</sub>WO<sub>3-δ</sub>, combined with its superior thermal stability, make this material a promising alternative to gold for various applications in NIR optics.

Controlling the shape of Cs<sub>x</sub>WO<sub>3-δ</sub> nanocrystals has long been a challenging task<sup>39-41</sup> despite its importance for exploiting their tunable LSPR properties. We recently demonstrated the synthesis of Cs<sub>x</sub>WO<sub>3-δ</sub> nanocrystals with various shapes by using empirically discovered one-pot methods.<sup>42</sup> However, this approach provided limited control over both the size and shape of nanocrystals, and did not offer fundamental understanding into the underlying growth mechanisms.

Here, we integrate the aforementioned two strategies of seed-mediated growth and facet-selective passivation to systematically control the anisotropic growth of hexagonal Cs<sub>x</sub>WO<sub>3-δ</sub> nanocrystals. This approach effectively separates the nucleation and growth phases by employing an initial hot-injection of molecular precursors to generate seeds, followed by a slow continuous injection of the same precursors to promote controlled growth of the seeds. The preferential growth faces are determined by the presence or absence of halides in the precursors, dictating the growth into either platelets or rods. Moreover, we show that the relative growth rates of the basal and prismatic facets of Cs<sub>x</sub>WO<sub>3-δ</sub> can be finely tuned by adjusting the precursor injection rate, enabling
precise and predictable control over the nanocrystals' shape anisotropy. We also demonstrate that the injection rate governs the growth pattern, involving a formation of dendritic platelets when it exceeds a certain limit.

The as-prepared $Cs_xWO_{3-δ}$ nanocrystals with varied shapes are shown to exhibit dramatic evolutions in their LSPR spectra. We observe contrasting yet consistent shifts of the two distinct longitudinal and transverse LSPR modes during the continuous growth of the nanocrystals, highlighting an unprecedented broad range of LSPR from the visible edge to the mid-infrared (750 ~ 6000 nm). These experimental findings are further corroborated by computational simulations that incorporate the anisotropic dielectric functions of $Cs_xWO_{3-δ}$⁴³ providing a comprehensive understanding of the observed spectral behavior.

## Results/Discussion

Cs-oleate and W-oleate precursors were prepared through a heating process of tungsten hexacarbonyl ($W(CO)_6$) and cesium carbonate ($Cs_2CO_3$), or tungsten and cesium chlorides ($WCl_4$, $CsCl$) in oleic acid. The seed particles were synthesized through a hot-injection of these oleate precursors, following methods adapted from our previous report.⁴² In this process, the rod-shaped seeds with large aspect ratios (AR > 1, length along the crystallographic c-axis divided by width across the basal plane) (Figure 1a,b) were made in the absence of chlorides, and the platelet-shaped seeds (AR < 1) (Figure 1c,d) were made with the chloride-containing precursors.⁴² Then, to further grow the seeds, the same precursors were continuously injected into the reactor at a constant rate by using a syringe pump. The aliquots collected during the continuous growth reactions (see detailed methods in the S.I.) show that the anisotropic growth directions were conserved: The rod-shaped seeds underwent further longitudinal growth increasing the AR
(Figure 1a,b), and the platelet-shaped seeds underwent further lateral growth decreasing the AR (Figure 1c,d). This result reveals that the precursor nature is a dominant factor governing the preferential crystal growth direction in both regimes of the rapid hot-injection and of the slow continuous injection.

<figure><img src="image_3.png" /><figcaption>Figure 1. Continuous growth of Cs<sub>x</sub>WO<sub>3-δ</sub> nanocrystals in different shapes. (a-d)</figcaption></figure>

TEM images of Cs<sub>x</sub>WO<sub>3-δ</sub> nanocrystals synthesized under varying injection rates with and without Cl<sup>-</sup> ions. The series of TEM images display the aliquots of seeds, two intermediate moments during the continuous injection, and the final product. The inset values are the amount of W-oleate added to the seeds at the moment of aliquoting. The
injection rate of W-oleate is noted on the left side of the first aliquot images. (e) HR-TEM images of rods prepared under slow precursor injection (left), platelets under slow injection (middle), and platelets under fast injection (right). The inset images are fast Fourier transformations (FFT). (f) XRD patterns of the final products obtained from each synthesis. The bottom pattern is the reference patterns of hexagonal $Cs_{0.32}WO_3$ (ICSD 100193) (g) Illustration of $Cs_{0.3}WO_3$ lattice structure projected on the basal plane (left) and the prismatic plane (right). Cs-Red, W-light blue, O-grey.

The X-ray diffraction (XRD) patterns of the final nanocrystals of **Figure 1a-d** confirm the hexagonal phase of the produced $Cs_xWO_{3-\delta}$ nanocrystals (**Figure 1f**). In the XRD patterns of nanorods, (**Figure 1f** red, blue) the (2 0 0) intense peak, which is more evident when compared to the (0 0 2) one, indicates that the nanorods were predominantly aligned with their long axis parallel to the substrate, inducing a preferred orientation. In contrast, the relative intensities of these two peaks in nanoplatelets (**Figure 1f** purple, yellow), followed the reference pattern more closely, suggesting less directional alignment. For both nanoplatelets and rapidly grown nanorods (**Figure 1b-d**), Le Bail analysis produced crystallite sizes that well matched those measured from TEM images (**Figure S1**, **Table S1**). However, there was a significant discrepancy between the two methods for the slowly grown long nanorods. This is probably due to slight crystal misalignments that occurred during the longitudinal growth, which may result in the grayscale contrast observed along the long axis of individual nanorods in the TEM images (**Figure 1a**).

Metal carbonyl and carbonate sources are expected to generate no additional species other than tungsten and cesium oleate precursors as they release gaseous CO or $CO_2$ upon decomposition. Consequently, the intrinsic crystal structure would govern the growth rates on different crystal
planes, each with distinct surface energy.⁴⁴ In this condition, longitudinal growth on the basal plane was apparently faster than the lateral growth on the prismatic plane, resulting in rod-shaped particles (Figure 1a,b). This anisotropic growth aligns with the highly anisotropic hexagonal crystal structure of CsₓWO₃₋₆ featuring a continuous repetition of Cs⁺ ions and corner-shared WO₆ octahedra along the c-axis (Figure 1g). In this particular tungsten bronze structure⁴⁵⁻⁴⁷, Cs⁺ readily occupy the void sites of the basal plane surrounded by six WO₆ octahedra. The cationic Cs⁺ exposed on top can then be rapidly covered by the next incoming WO₆ layer. This unidirectional longitudinal growth is presumed to be faster than the lateral growth on the prismatic planes that requires simultaneous incorporation of Cs⁺ and WO₆ in each layer without highly favorable Cs⁺ sites.

By contrast, in the presence of chloride ions, the longitudinal growth was nearly suppressed while lateral growth continued, resulting in a continuous decrease of aspect ratio (Figure 1c,d). Facet-selective adsorption of halide ions has been observed and simulated in various nanocrystal systems, where halides adsorption reduces surface energy and slows growth on specific planes.³⁰,⁴⁸,⁴⁹ We previously showed that ammonium chlorides selective bind to basal facets of CsₓWO₃₋₆ nanocrystals, facilitating their face-selective assembly.⁵⁰ It is thus plausible that Cl⁻ binds with surface-exposed Cs⁺ inhibiting successive growth or detaching Cs⁺ from the surface. Cs⁺ sites seen on the basal plane are large voids uncovered by WO₆ octahedra (Figure 1g), which should be easily accessible to Cl⁻ ions, likely suppressing longitudinal growth and promoting lateral growth to dominate formation of platelet-shaped nanocrystals. This hypothesis is supported by the observed precipitation of CsCl byproducts while using chloride precursors.⁴²

We found that a key factor to further control the nanocrystal morphology is the precursor injection rate. In the case of rod growth, slow continuous injection of precursors (1.5µmol/min,
**Figure 1a)** induced highly anisotropic growth resulting in thin elongated rods, whereas an order of magnitude faster injection produced shorter and thicker rods (11.6 µmol/min, **Figure 1b**). An intermediate injection rate (2.8 µmol/min) produced moderately long rods (**Figure S2**). The measured average length, width, and aspect ratio of aliquot samples (**Figure 2a-c**, **Figure S4-S6**) show a clear trend that the longitudinal growth is favored compared to the lateral growth when the injection rate decreases: The initial aspect ratio of 4.6 of the seeds increases to 17.0 during the slow injection, while the aspect ratio barely changes during the fast injection (**Figure 2c**).

These observations can be interpreted in the light of general theories of crystal growth.⁵¹ A common assumption for nanocrystal growth is that the growth rates on crystalline facets are linearly proportional to the supersaturation.⁵²,⁵³ However, if we assume that the growth rate of all crystal planes of $Cs_xWO_{3-\delta}$ nanocrystals are linearly proportional to the supersaturation, the aspect ratio would remain unaffected by the injection rate. (See **S.I.** for more details) The local supersaturation of monomers near the crystal increases with the incoming flux, which increases with injection rate. The growth kinetics at the crystal surface depend on its microscopic structure. For surfaces with readily available growth sites, such as the basal faces of $Cs_xWO_{3-\delta}$, the growth rate is a linear function of the supersaturation. In contrast, the kinetics become strongly nonlinear if growth is limited by a high nucleation energy barrier. Within this framework, the growth of the prismatic faces of $Cs_xWO_{3-\delta}$ may be quicker to slow down compared to that of the basal faces as the supersaturation drops below a certain threshold. Consequently, the aspect ratio of rods may increase significantly when the injection rate is lowered. Such differences in growth kinetics between two types of faces cause inhomogeneities in the local supersaturation around the crystal. Depletion regions generated near the rapid-growing faces can then drive a redistribution of
monomers by bulk or surface diffusion,⁵⁴⁻⁵⁸ thereby prompting a continuous anisotropic growth, as illustrated in **Figure 2g**.

<figure><img src="image_4.png" /><figcaption>**Figure 2. Evolution of the CsₓWO₃-δ nanocrystal dimensions during the growth.**</figcaption></figure>

Variation of the measured average length, width, and aspect ratio (AR) during the continuous injection synthesis with varied injection rates. (a-c) Plots for the rods synthesized from W(CO)₆ and Cs₂CO₃ (without Cl⁻). (d-f) Plots for the platelets synthesized from WCl₄ and CsCl. The plotted data points correspond to the TEM images
shown in Figure 1a-d and Figure S2-S3. The error bars show the standard deviation of the distribution. Details about the size distribution and measurement methods can be found in Figure S3-S9. Illustration depicting (g) the longitudinal growth of rods and (h) the lateral growth of platelets. Purple arrows indicate the growth directions. Yellow arrows indicate the preferential growth directions, and black arrows depict the tendency of diffusive movements of monomers.

In the case of platelet growth, the rapid longitudinal growth is almost completely suppressed by the Cl⁻ ions, letting the slower two-dimensional (2D) lateral growth dominate the formation of particles (Figure 2d-f, Figure S7-S8). We observe that the precursor injection rate has a major influence on the edge pattern of these platelets. Slowly grown platelets (2.3 µmol/min) maintain their hexagonal shapes with moderately jagged edges, while the rapidly grown ones (7.7 µmol/min) show dendritically branched shapes similar to snowflakes (Figure 1c-d, Figure S3). The small protrusions at early stages of growth gradually evolve into dendritic branches (Figure 1d). The high-resolution TEM images (Figure 1e) reveal that these platelets grow in a clear 2D dendritic structure maintaining the monocrystalline feature of the seeds.

Dendritic growth is generally understood to occur under high supersaturation conditions where the provision of monomers is faster than their surface migration to construct the lowest energy structure.⁵⁹⁻⁶³ This is in accordance with our observation where dendritic growth occurred while increasing the flux of precursors. The dendritic branching was predominant along the a- and b-axis directions, while the growth along the c-axis was minimal. It can thus be interpreted that the relatively slow lateral growth of CsₓWO₃₋δ quickly meets the supersaturation condition, while the Cl⁻ ions inhibit the longitudinal growth by binding with the Cs⁺ exposed on the basal faces (Figure
**2h)** This formation of 2D dendritic structures is distinguished from the other branched structures such as multipods$^{24,64-67}$, for which different growth mechanisms can contribute including oriented attachment$^{68-70}$, polytypism$^{71-73}$, and selective etching$^{74-76}$.

The evolutions of LSPR spectra during the growth of $Cs_xWO_{3-\delta}$ nanorods and nanoplatelets are presented in **Figure 3**. We note that the $Cs^+$ doping ratio remains close to the stoichiometric value of $x = 0.33$ as $Cs^+$ is essential to form the hexagonal bronze structure. Even though hexagonal $WO_3$ has been reported, the structure collapsed when the stabilizing ions were completely removed.$^{77}$ The $x$-value may slightly change for different syntheses, but it did not majorly influence the optical spectra. For the rods, one can clearly see a continuous redshift of the high wavelength peak – longitudinal LSPR mode ($\lambda_{sp-L}$) – while the AR increases during the continuous growth (**Figure 1a-b**). To precisely identify the wavelengths of LSPR peaks, peak deconvolution analyses were performed as described in the S.I. (**Figure S10**). For the rapidly grown rods, the initial $\lambda_{sp-L}$ of seeds (2264 nm) shifts moderately to 2547 nm as the AR changes from 3.9 to 5.0 (**Figure 3b**). For the slowly grown rods, almost the same $\lambda_{sp-L}$ of seeds (2401 nm) largely shifts to 3847 nm while the AR changes from 4.6 to 17 (**Figure 3a**). The maximum $\lambda_{sp-L}$ we obtained is 5150 nm for a sample with AR = 21 (**Figure S11**). Besides, the low wavelength peak – transverse LSPR mode ($\lambda_{sp-T}$) – slightly blueshifts initially but almost saturates around 1300 nm as the AR continuously increases beyond 10. This can be seen more clearly on the spectra plotted linearly in wavelength scale (**Figure S12**). For the platelets, $\lambda_{sp-T}$ of seeds at 1574 nm moderately redshifts and saturates near 1800 nm as AR decreases, while their $\lambda_{sp-L}$ continuously blueshifts from 960 nm to 802 nm (**Figure 3c**). The trends of $\lambda_{sp-L}$ and $\lambda_{sp-T}$, shifting in opposite directions during the growth of rods and platelets, is consistent with the LSPR behavior observed in gold nanorods and nanodisks$^{78-81}$,
as well as with the electrostatic approximation of the absorption behavior of anisotropic particles
such as ellipsoids.⁸²

<figure><img src="image_5.png" /><figcaption>Figure 3. The evolution of LSPR spectra during the continuous growth of Cs<sub>x</sub>WO<sub>3</sub>-δ nanorods and nanoplatelets. (a-d) correspond to the samples presented in Figure 1a-d, respectively. (e-f) are the LSPR peak positions found by peak deconvolution analyses as a function of aspect ratio for nanorods-(e) and nanoplatelets-(f).</figcaption></figure>
Meanwhile, the LSPR of dendritically grown platelets show a relatively small redshift of $\lambda_{sp-T}$ (1465 nm → 1518 nm, **Figure 3d**) compared to that of non-dendritic platelets (1574 nm → 1812 nm, **Figure 3c**). It also presents a broader transverse mode peak width. We attribute this discrepancy of the LSPR spectra of dendritic platelets to the perturbation of the transverse LSPR mode oscillation of electrons due to the spatial gaps between branches. Different oscillation modes with varied characteristic lengths as well as coupling modes between the adjacent branches are to be generated and averaged in the observed LSPR spectra. A similar tendency of LSPR broadening and reduced shifting was observed when sharp branches developed from gold nanospheres.⁸³

We perform computational electromagnetic simulations and quantitatively compare the results with the experimental observations. For the material parameters, the previously reported bi-anisotropic permittivities of $Cs_xWO_{3-\delta}$⁴³ were initially adopted, however, a size dependence is introduced through the damping coefficient ($\gamma$).⁴² This size dependence originates from enhanced surface scattering by electrons due to a decreased mean free path. Further details of the simulations are described in the Methods and S.I. We emphasize here that the optical response of the particles is fully encompassed by the size dependent permittivity. As displayed in **Figure 4a-c**, the simulated LSPR spectra for rods, platelets, and dendritic platelets (colored straight lines) agree well with the experimental spectra (dashed black lines), reproducing accurately the LSPR peak shift. Our simulations accounting for the size polydispersity of the nanocrystals (red lines) compared to a monodisperse system (blue lines) reveal that its impact on the LSPR peak shift and broadening is minor compared to those of the aspect ratio and $\gamma$ (**Figure 4a-c** and **S14-S17**). The plot of $\gamma$ against $A_{T,L}v_F/l_{eff}$ (**Figure 4d**), where $A_{T,L}$ is the dimensionless size parameter, $v_F$ is the Fermi velocity, and $l_{eff}$ is the effective mean free path of electrons, shows a good linear fit with the slope of 0.97 and 0.96 and the y-intercept, $\gamma_0$ (bulk damping coefficient), of 0.35 PHz and
0.36 PHz. This result is in a good agreement with the standard classical approach$^{84}$, which predicts a slope close to unity (see Methods). Given that $l_{eff}$ varies significantly between the transverse and longitudinal directions, the remarkably similar values of $\gamma_0$ and that both gradients are very close to unity, this graph reveals the nonlocal response of the electron plasma in the nanocrystals.

<figure><img src="image_6.png" /><figcaption>**Figure 4** Computational simulation of LSPR in different nanocrystal shapes. Plots of simulated and experimental LSPR spectra for seeds and final products of (a) rods, (b) platelets, and (c) dendritic platelets. Red lines account for a statistical ensemble of polydisperse particles based on the particle sizes measured in the real sample. Blue lines are for monodisperse systems based on the size averages. Black dashed lines are</figcaption></figure>
experimental spectra taken from Figure 4. (d) Plot of the damping coefficient $\gamma$ as a function of $\gamma_0 + Av_F/l_{eff}$ for all aliquot samples simulated. The slopes of the fitted linear lines are close to unity, reflecting the reliability of the simulation. (e) Middle cross section of the normalized scattered electric field in a typical dendritic $Cs_xWO_{3-\delta}$ nanoplatelet for a wavelength of the incident light of 1500nm.

The dendritic nanoplatelets are particularly interesting due to the thin gaps between branches where the electromagnetic field can be tightly confined and enhanced.⁸⁵⁻⁸⁷ The simulated scattered field map of the dendritic nanoplatelet at its resonance wavelength (Figure 4e) shows many hotspots between branches exhibiting a strongly enhanced local field. As in Figure S18, this enhancement remains high in a wide spectral range across 1200-3000nm. This may lead to new applications where these dendritic particles are used to enhance nonlinear optical effects, plasmon assisted catalysis, or single molecular sensing.

## Conclusions

We have developed a novel synthesis of $Cs_xWO_{3-\delta}$ nanocrystals based on seed-mediated, face-selective, continuous growth. This approach allows to continuously grow monocrystalline seeds with a well-defined hexagonal structure towards selected crystallographic directions depending on the precursor nature. The presence of halides ($Cl^-$) strongly influences the dominant growth direction by favorably interacting with alkaline ions ($Cs^+$) that are incorporated in the crystal structure. Furthermore, controlling the precursor injection rate offers a precise control over the relative growth rates on the basal and prismatic faces of $Cs_xWO_{3-\delta}$ for longitudinal and lateral growths towards rods and platelets. The strategy of leveraging the ionic interaction of halides and
alkalines to create reaction barriers presents potential applications to other material compositions such as halide-containing perovskites.

The resulting LSPR obtained from the series of CsₓWO₃-δ nanocrystals with widely varied aspect ratios extends its spectral range across the entire near-infrared region and beyond, up to the mid-infrared (750 ~ 6000 nm) unlocking new possibilities for applications such as chemical sensing or thermal imaging. Computational simulations confirm the consistent LSPR peak shift with changing aspect ratio. We show that there is a significant surface scattering effect leading to large variations in the overall damping coefficient. We also highlight that the branched structure of dendritic platelets, obtained under high precursor supersaturation, creates plasmonic hotspots where the nearfield is strongly amplified. Such rich control and fine tunability of LSPR achieved over an unprecedented wide spectral range significantly broaden the potential fields for application, that include solar-energy harvesting, biomedical imaging, telecommunications, and IR sensing, where specific target wavelengths are paramount.

# Methods/Experimental

## Synthesis

All synthesis was performed by using a standard Schlenk line setup under a nitrogen atmosphere. For the synthesis of nanorods (AR > 1), Cs₂CO₃ (Sigma Aldrich, 99.95%), W(CO)₆ (Sigma Aldrich 99.9%) and oleic acid (OLAC, 90% from Sigma Aldrich) were used. 0.1 M W-oleate precursor solution was prepared by heating the mixture of 1 mmol (352 mg) of W(CO)₆ with 10 mL of OLAC at 180 °C in an oven for 16 hours. The obtained 0.1 M W-oleate was diluted with pure OLAC to the desired concentrations. Cs-oleate (0.2 M) precursor solution was prepared by heating Cs₂CO₃ in oleic acid at 120 °C for 30 minutes under nitrogen atmosphere. As prepared precursor solutions were collected without exposure with air and stored inside a glove box. For
the synthesis of seed nanocrystals, W-oleate solution was rapidly injected to Cs-oleate at 300 °C under nitrogen atmosphere and vigorous stirring. After 30 minutes, a mixture of W-oleate and Cs-oleate was injected drop-by-drop at a constant rate by using a syringe pump for the continuous growth of the seeds.

For the synthesis of nanoplatelets (AR < 1), WCl₄ (Sigma Aldrich, 95%), CsCl (Sigma Aldrich 99.9%), Cs₂CO₃ (Sigma Aldrich, 99.95%) and Oleic acid (OLAC, 90% from Alfa Aesar) were used. To prepare W-oleate, a mixture of 1 mmol (326 mg) of WCl₄ and 5 mL of OLAC, or 1 mmol (326 mg) WCl₄ and 20mL OLAC were heated at 120 °C for 30 minutes under nitrogen atmosphere to yield 0.2 M or 0.05 M W-oleate solutions, respectively. Cs-oleate (0.2 M) was prepared from Cs₂CO₃ using the same method as above for nanorods. For the synthesis of seed nanocrystals, the required amount of CsCl was dissolved in methanol and added to OLAC as CsCl is insoluble to pure OLAC. The methanol was then removed from the reactor by applying vacuum at 80 °C for 10 minutes. The solution was then heated up to 300 °C under nitrogen atmosphere, and W-oleate was rapidly injected. After 30 minutes, a mixture of W-oleate and Cs-oleate was injected at a constant rate for the continuous growth of the seeds.

As-synthesized OLAC-capped CsₓWO₃-δ nanocrystals are not stable in common organic solvents. The nanocrystals were thus functionalized with mPEG5k-Si (methoxy-polyethylene-glycol silane) and stably dispersed in chloroform. Further details of the synthesis of each sample presented in Figure 1 are provided in the S.I.

## Characterization

The transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) were performed using Titan Themis 300 G3 (Thermo Fischer) with an accelerating voltage of 300kV. Nanocrystal size analysis was performed using the ImageJ software on more than 150 particles for
each sample. The powder XRD patterns were obtained with drop-casted nanocrystals on zero background silicon wafer using a high-resolution D8 Advance diffractometer (Bruker). The diffractometer was equipped with a Cu Kα X-ray source operating at 40 kV and 40 mA and measuring $2\theta = 10 - 90^{\circ}$ with a step size of 0.02° and a scan speed of 0.9 s/step. The Le Bail refinement (Figure S1) was performed using the Maud software.

Cary 5000 UV-vis-NIR spectrometer (Agilent) was used to obtain the optical extinction spectra of colloidal nanocrystals in the range of 200 to 3300 nm. An air-tight quartz cuvette (Hellma, optical pathlength 1cm) was used for measurement. For this, the mPEG5k-Si-capped nanocrystals were dispersed in a 2 vol% mixture of chloroform in tetrachloroethylene (TCE). For obtaining optical extinction spectra above 3000 nm, Equinox 55 FTIR (Bruker) was used in the transmission mode, and 100 scans were taken in the range of 5000 to 500 cm⁻¹. For this measurement, solution of nanocrystals was injected into a 1mm gap between two CaF₂ windows using SPECAC Omni-cell.

## Computational simulation

The simulations were conducted using a combination of the finite element method, solved with COMSOL Multiphysics to generate the near-field maps of the dendritic structure, and analytic expressions based on the quasi-static approximation⁸⁸. The size-dependent damping parameter is described by the equation $\gamma(l_{eff}) = \gamma_0 + A \nu_F / l_{eff}$, adapted from the standard classical approach⁸⁴. Here, $\nu_F$ is the Fermi velocity, $\gamma_0$ is the bulk damping coefficient, and A is a dimensionless parameter, which is typically close to unity. The Fermi velocities in the longitudinal and transverse directions are calculated from the Fermi energies, reported to range between 2.4 and 3.0 eV.⁸⁹⁻⁹¹ $l_{eff}$ is adapted from Coronado et al,⁹² and expressed as $l_{eff,T} = \frac{\pi w}{4}$, $l_{eff,L} = \frac{\pi lw/2}{w+l}$ where l and w are the average length and width of the nanocrystals. As previously
demonstrated for spherical gold nanoparticles, A varies quadratically with nanoparticle radius.⁹³

Similarly, we assume $A_{T,L}$ of CsₓWO₃-δ nanocrystals to be quadratically proportional to the width or length, respectively. For each synthesis, we fit all parameters by minimizing a least-squares-objective function between experiment and simulation results, using the downhill simplex method.

⁹⁴ The material parameters, $ω_p$ and $γ_0$, are constrained to be within 20% of those found in Bouchard et al.⁴⁶ Importantly, all fitting coefficients represent physical parameters and as such are constrained to ensure self-consistency across all aliquots, (e.g. same sign and similar magnitude). This ensures that the reported non-locality is due to the intrinsic bi-anisotropy of the material and the effective mean free path $l_{eff}$.

## ASSOCIATED CONTENT

### Supporting Information.

Experimental methods including the synthesis details, Le Bail analysis of XRD results, TEM images and optical spectra of nanorods (2.8µmol/min), deconvolution study of experimentally obtained optical spectra, and computational simulations. The Supporting Information is available free of charge on the ACS Publications website.

### Preprint

A preprint version of this manuscript is available on ChemRxiv⁹⁵

## AUTHOR INFORMATION

### Corresponding Author

*(Jongwook Kim). jong-wook.kim@polytechnique.edu
## Author Contributions

J.O. performed nanocrystal synthesis, surface chemistry, optical characterizations, and TEM analysis. J.O. and S.T-N. performed XRD and Le Bail analyses. J.D. and A.B. performed computational LSPR modelling and simulations. J.K. and T.G. designed the study and supervised the research. J.O., J.K., J.D., and M.P. wrote the manuscript, which was revised and approved by all authors.

## Funding Sources

This research was supported by French national agency (ANR) in the framework of Plasmoglaz (ANR-18-CE05-0038), Shearprobe (ANR-23-CE42-0007-01), and Melodie (ANR-22-CE09-0027) projects.

## ACKNOWLEDGMENT

The authors thank Dr. Kassiogé Dembélé for assisting in TEM experiments, Mr. Rabei Mohammedi for assisting in chemistry work, and Dr. Clément Marchat for glove box operation.

## ABBREVIATIONS

LSPR, localized surface plasmon resonance; NIR, near infrared; AR, aspect ratio; PHz, petahertz; OLAC, oleic acid.

<footnote>*(Thierry Gacoin). thierry-gacoin@polytechnique.edu</footnote>
REFERENCES

(1) Link, S.; El-Sayed, M. A. Size and Temperature Dependence of the Plasmon Absorption of Colloidal Gold Nanoparticles. *J. Phys. Chem. B* **1999**, *103* (21), 4212–4217.
https://doi.org/10.1021/jp984796o.

(2) Link, S.; El-Sayed, M. A. Spectral Properties and Relaxation Dynamics of Surface Plasmon Electronic Oscillations in Gold and Silver Nanodots and Nanorods. *J. Phys. Chem. B* **1999**, *103* (40), 8410–8426. https://doi.org/10.1021/jp9917648.

(3) Jana, N. R.; Gearheart, L.; Murphy, C. J. Seed-Mediated Growth Approach for Shape-Controlled Synthesis of Spheroidal and Rod-like Gold Nanoparticles Using a Surfactant Template. *Adv. Mater.* **2001**, *13* (18), 1389–1393. https://doi.org/10.1002/1521-4095(200109)13:18<1389::AID-ADMA1389>3.0.CO;2-F.

(4) Murphy, C. J.; Jana, N. R. Controlling the Aspect Ratio of Inorganic Nanorods and Nanowires. *Advanced Materials* **2002**, *14* (1), 80–82. https://doi.org/10.1002/1521-4095(20020104)14:1<80::AID-ADMA80>3.0.CO;2-%23.

(5) Murphy, C. J.; Gole, A. M.; Hunyadi, S. E.; Orendorff, C. J. One-Dimensional Colloidal Gold and Silver Nanostructures. *Inorg. Chem.* **2006**, *45* (19), 7544–7554.
https://doi.org/10.1021/ic0519382.

(6) Hu, J.; Li, L.; Yang, W.; Manna, L.; Wang, L.; Alivisatos, A. P. Linearly Polarized Emission from Colloidal Semiconductor Quantum Rods. *Science* **2001**, *292* (5524), 2060–2063.
https://doi.org/10.1126/science.1060810.
(7) Ohmachi, M.; Komori, Y.; Iwane, A. H.; Fujii, F.; Jin, T.; Yanagida, T. Fluorescence Microscopy for Simultaneous Observation of 3D Orientation and Movement and Its Application to Quantum Rod-Tagged Myosin V. *Proc. Natl. Acad. Sci. U.S.A.* **2012**, *109* (14), 5294–5298.
https://doi.org/10.1073/pnas.1118472109.

(8) Cunningham, P. D.; Souza, J. B.; Fedin, I.; She, C.; Lee, B.; Talapin, D. V. Assessment of Anisotropic Semiconductor Nanorod and Nanoplatelet Heterostructures with Polarized Emission for Liquid Crystal Display Technology. *ACS Nano* **2016**, *10* (6), 5769–5781.
https://doi.org/10.1021/acsnano.5b07949.

(9) Jeltsch, K. F.; Schädel, M.; Bonekamp, J.; Niyamakom, P.; Rauscher, F.; Lademann, H. W. A.; Dumsch, I.; Allard, S.; Scherf, U.; Meerholz, K. Efficiency Enhanced Hybrid Solar Cells Using a Blend of Quantum Dots and Nanorods. *Adv Funct Materials* **2012**, *22* (2), 397–404.
https://doi.org/10.1002/adfm.201101809.

(10) Wu, H.-L.; Chen, C.-H.; Huang, M. H. Seed-Mediated Synthesis of Branched Gold Nanocrystals Derived from the Side Growth of Pentagonal Bipyramids and the Formation of Gold Nanostars. *Chem. Mater.* **2009**, *21* (1), 110–114. https://doi.org/10.1021/cm802257e.

(11) Luo, M.; Ruditskiy, A.; Peng, H.; Tao, J.; Figueroa-Cosme, L.; He, Z.; Xia, Y. Penta-Twinned Copper Nanorods: Facile Synthesis via Seed-Mediated Growth and Their Tunable Plasmonic Properties. *Adv Funct Materials* **2016**, *26* (8), 1209–1216.
https://doi.org/10.1002/adfm.201504217.

(12) Rice, K. P.; Saunders, A. E.; Stoykovich, M. P. Seed-Mediated Growth of Shape-Controlled Wurtzite CdSe Nanocrystals: Platelets, Cubes, and Rods. *J. Am. Chem. Soc.* **2013**, *135* (17), 6669–6676. https://doi.org/10.1021/ja402240m.
(13) Gordon, T. R.; Cargnello, M.; Paik, T.; Mangolini, F.; Weber, R. T.; Fornasiero, P.; Murray, C. B. Nonaqueous Synthesis of TiO₂ Nanocrystals Using TiF₄ to Engineer Morphology, Oxygen Vacancy Concentration, and Photocatalytic Activity. *J. Am. Chem. Soc.* **2012**, *134* (15), 6751–6761. https://doi.org/10.1021/ja300823a.

(14) Fiore, A.; Mastria, R.; Lupo, M. G.; Lanzani, G.; Giannini, C.; Carlino, E.; Morello, G.; De Giorgi, M.; Li, Y.; Cingolani, R.; Manna, L. Tetrapod-Shaped Colloidal Nanocrystals of II–VI Semiconductors Prepared by Seeded Growth. *J. Am. Chem. Soc.* **2009**, *131* (6), 2274–2282. https://doi.org/10.1021/ja807874e.

(15) Prado, Y.; Qu, J.; Gréboval, C.; Dabard, C.; Rastogi, P.; Chu, A.; Khalili, A.; Xu, X. Z.; Delerue, C.; Ithurria, S.; Lhuillier, E. Seeded Growth of HgTe Nanocrystals for Shape Control and Their Use in Narrow Infrared Electroluminescence. *Chem. Mater.* **2021**, *33* (6), 2054–2061. https://doi.org/10.1021/acs.chemmater.0c04526.

(16) Kim, M.; Choi, M.; Choi, S.; Jeong, S. Semiconductor Nanocrystals: Unveiling the Chemistry behind Different Facets. *Acc. Chem. Res.* **2023**, *56* (13), 1756–1765. https://doi.org/10.1021/acs.accounts.3c00123.

(17) Puntes, V. F.; Zanchet, D.; Erdonmez, C. K.; Alivisatos, A. P. Synthesis of Hcp-Co Nanodisks. *J. Am. Chem. Soc.* **2002**, *124* (43), 12874–12880. https://doi.org/10.1021/ja027262g.

(18) Puzder, A.; Williamson, A. J.; Zaitseva, N.; Galli, G.; Manna, L.; Alivisatos, A. P. The Effect of Organic Ligand Binding on the Growth of CdSe Nanoparticles Probed by Ab Initio Calculations. *Nano Lett.* **2004**, *4* (12), 2361–2365. https://doi.org/10.1021/nl0485861.
(19) Panda, A. B.; Glaspell, G.; El-Shall, M. S. Microwave Synthesis and Optical Properties of Uniform Nanorods and Nanoplates of Rare Earth Oxides. *J. Phys. Chem. C* **2007**, *111* (5), 1861–1864. https://doi.org/10.1021/jp0670283.

(20) Dinh, C.-T.; Nguyen, T.-D.; Kleitz, F.; Do, T.-O. Shape-Controlled Synthesis of Highly Crystalline Titania Nanocrystals. *ACS Nano* **2009**, *3* (11), 3737–3743. https://doi.org/10.1021/nn900940p.

(21) Chang, J.; Waclawik, E. R. Facet-Controlled Self-Assembly of ZnO Nanocrystals by Non-Hydrolytic Aminolysis and Their Photodegradation Activities. *CrystEngComm* **2012**, *14* (11), 4041. https://doi.org/10.1039/c2ce25154j.

(22) Johnson, C. J.; Dujardin, E.; Davis, S. A.; Murphy, C. J.; Mann, S. Growth and Form of Gold Nanorods Prepared by Seed-Mediated, Surfactant-Directed Synthesis. *J. Mater. Chem.* **2002**, *12* (6), 1765–1770. https://doi.org/10.1039/b200953f.

(23) Murphy, C. J.; Thompson, L. B.; Alkilany, A. M.; Sisco, P. N.; Boulos, S. P.; Sivapalan, S. T.; Yang, J. A.; Chernak, D. J.; Huang, J. The Many Faces of Gold Nanorods. *J. Phys. Chem. Lett.* **2010**, *1* (19), 2867–2875. https://doi.org/10.1021/jz100992x.

(24) Lee, S.-M.; Cho, S.-N.; Cheon, J. Anisotropic Shape Control of Colloidal Inorganic Nanocrystals. *Advanced Materials* **2003**, *15* (5), 441–444. https://doi.org/10.1002/adma.200390102.

(25) Lee, S.-M.; Jun, Y.; Cho, S.-N.; Cheon, J. Single-Crystalline Star-Shaped Nanocrystals and Their Evolution: Programming the Geometry of Nano-Building Blocks. *J. Am. Chem. Soc.* **2002**, *124* (38), 11244–11245. https://doi.org/10.1021/ja026805j.
(26) Ha, T. H.; Koo, H.-J.; Chung, B. H. Shape-Controlled Syntheses of Gold Nanoprisms and Nanorods Influenced by Specific Adsorption of Halide Ions. *J. Phys. Chem. C* **2007**, *111* (3), 1123–1130. https://doi.org/10.1021/jp066454l.

(27) Yang, H. G.; Sun, C. H.; Qiao, S. Z.; Zou, J.; Liu, G.; Smith, S. C.; Cheng, H. M.; Lu, G. Q. Anatase TiO2 Single Crystals with a Large Percentage of Reactive Facets. *Nature* **2008**, *453* (7195), 638–641. https://doi.org/10.1038/nature06964.

(28) Han, X.; Kuang, Q.; Jin, M.; Xie, Z.; Zheng, L. Synthesis of Titania Nanosheets with a High Percentage of Exposed (001) Facets and Related Photocatalytic Properties. *J. Am. Chem. Soc.* **2009**, *131* (9), 3152–3153. https://doi.org/10.1021/ja8092373.

(29) Liu, S.; Yu, J.; Jaroniec, M. Anatase TiO₂ with Dominant High-Energy {001} Facets: Synthesis, Properties, and Applications. *Chem. Mater.* **2011**, *23* (18), 4085–4093. https://doi.org/10.1021/cm200597m.

(30) Ma, X.; Dai, Y.; Wei, W.; Huang, B.; Whangbo, M.-H. Insights into How Fluorine-Adsorption and n-Type Doping Affect the Relative Stability of the (001) and (101) Surfaces of TiO₂: Enhancing the Exposure of More Active but Thermodynamically Less Stable (001). *J. Phys. Chem. Lett.* **2015**, *6* (10), 1876–1882. https://doi.org/10.1021/acs.jpclett.5b00595.

(31) Christodoulou, S.; Climente, J. I.; Planelles, J.; Brescia, R.; Prato, M.; Martín-García, B.; Khan, A. H.; Moreels, I. Chloride-Induced Thickness Control in CdSe Nanoplatelets. *Nano Lett.* **2018**, *18* (10), 6248–6254. https://doi.org/10.1021/acs.nanolett.8b02361.

(32) Cho, W.; Kim, S.; Coropceanu, I.; Srivastava, V.; Diroll, B. T.; Hazarika, A.; Fedin, I.; Galli, G.; Schaller, R. D.; Talapin, D. V. Direct Synthesis of Six-Monolayer (1.9 Nm) Thick Zinc-
Blende CdSe Nanoplatelets Emitting at 585 Nm. *Chem. Mater.* **2018**, *30* (20), 6957–6960.
https://doi.org/10.1021/acs.chemmater.8b02489.

(33) Meerbach, C.; Wu, C.; Erwin, S. C.; Dang, Z.; Prudnikau, A.; Lesnyak, V. Halide-Assisted Synthesis of Cadmium Chalcogenide Nanoplatelets. *Chem. Mater.* **2020**, *32* (1), 566–574.
https://doi.org/10.1021/acs.chemmater.9b04438.

(34) Choi, S.-I.; Nam, K. M.; Park, B. K.; Seo, W. S.; Park, J. T. Preparation and Optical Properties of Colloidal, Monodisperse, and Highly Crystalline ITO Nanoparticles. *Chem. Mater.* **2008**, *20* (8), 2609–2611. https://doi.org/10.1021/cm703706m.

(35) Buonsanti, R.; Llordes, A.; Aloni, S.; Helms, B. A.; Milliron, D. J. Tunable Infrared Absorption and Visible Transparency of Colloidal Aluminum-Doped Zinc Oxide Nanocrystals. *Nano Lett.* **2011**, *11* (11), 4706–4710. https://doi.org/10.1021/nl203030f.

(36) De Trizio, L.; Buonsanti, R.; Schimpf, A. M.; Llordes, A.; Gamelin, D. R.; Simonutti, R.; Milliron, D. J. Nb-Doped Colloidal TiO₂ Nanocrystals with Tunable Infrared Absorption. *Chem. Mater.* **2013**, *25* (16), 3383–3390. https://doi.org/10.1021/cm402396c.

(37) Gordon, T. R.; Paik, T.; Klein, D. R.; Naik, G. V.; Caglayan, H.; Boltasseva, A.; Murray, C. B. Shape-Dependent Plasmonic Response and Directed Self-Assembly in a New Semiconductor Building Block, Indium-Doped Cadmium Oxide (ICO). *Nano Lett.* **2013**, *13* (6), 2857–2863.
https://doi.org/10.1021/nl4012003.

(38) Mattox, T. M.; Bergerud, A.; Agrawal, A.; Milliron, D. J. Influence of Shape on the Surface Plasmon Resonance of Tungsten Bronze Nanocrystals. *Chem. Mater.* **2014**, *26* (5), 1779–1784.
https://doi.org/10.1021/cm4030638.
(39) Guo, C.; Yin, S.; Yan, M.; Sato, T. Facile Synthesis of Homogeneous CsxWO3 Nanorods with Excellent Low-Emissivity and NIR Shielding Property by a Water Controlled-Release Process. *J. Mater. Chem.* **2011**, *21* (13), 5099. https://doi.org/10.1039/c0jm04379f.

(40) Liu, J.; Luo, J.; Shi, F.; Liu, S.; Fan, C.; Xu, Q.; Shao, G. Synthesis and Characterization of F-Doped Cs0.33WO3–F Particles with Improved near Infrared Shielding Ability. *Journal of Solid State Chemistry* **2015**, *221*, 255–262. https://doi.org/10.1016/j.jssc.2014.10.016.

(41) Zhang, B.; Liu, J. Q.; Wang, D. Y.; An, F. H.; Xu, L. Boosted Near-Infrared Shielding Properties of Cesium-Tungsten Bronze Based on Solid State Reaction with Optimized Precursor. *J. Phys. D: Appl. Phys.* **2024**, *57* (14), 145501. https://doi.org/10.1088/1361-6463/ad17a2.

(42) Cheref, Y.; Lochon, F.; Daugas, L.; Cleret De Langavant, C.; Larquet, É.; Baron, A.; Gacoin, T.; Kim, J. Dual-Band LSPR of Tungsten Bronze Nanocrystals Tunable over NIR and SWIR Ranges. *Chem. Mater.* **2022**, *34* (21), 9795–9802. https://doi.org/10.1021/acs.chemmater.2c02879.

(43) Kim, J.; Agrawal, A.; Krieg, F.; Bergerud, A.; Milliron, D. J. The Interplay of Shape and Crystalline Anisotropies in Plasmonic Semiconductor Nanocrystals. *Nano Lett.* **2016**, *16* (6), 3879–3884. https://doi.org/10.1021/acs.nanolett.6b01390.

(44) Peng, X.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Shape Control of CdSe Nanocrystals. *Nature* **2000**, *404* (6773), 59–61. https://doi.org/10.1038/35003535.

(45) Magnéli, Arne. Studies on the Hexagonal Tungsten Bronzes of Potassium, Rubidium and Cesium. *Acta Chemica Scandinavica* **1953**, No. 7, 315–324.
(46) Bouchard, R. J.; Gillson, J. L. Hexagonal Indium Tungsten Bronze. *Inorg. Chem.* **1968**, *7* (5), 969–972. https://doi.org/10.1021/ic50063a025.

(47) Stanley, R. K.; Morris, R. C.; Moulton, W. G. Conduction Properties of the Hexagonal Tungsten Bronze, Rb x WO 3. *Phys. Rev. B* **1979**, *20* (5), 1903–1914. https://doi.org/10.1103/PhysRevB.20.1903.

(48) Manna, L.; Wang; Cingolani, R.; Alivisatos, A. P. First-Principles Modeling of Unpassivated and Surfactant-Passivated Bulk Facets of Wurtzite CdSe: A Model System for Studying the Anisotropic Growth of CdSe Nanocrystals. *J. Phys. Chem. B* **2005**, *109* (13), 6183–6192. https://doi.org/10.1021/jp0445573.

(49) Meena, S. K.; Sulpizi, M. Understanding the Microscopic Origin of Gold Nanoparticle Anisotropic Growth from Molecular Dynamics Simulations. *Langmuir* **2013**, *29* (48), 14954–14961. https://doi.org/10.1021/la403843n.

(50) Cleret De Langavant, C.; Oh, J.; Lochon, F.; Tusseau-Nenez, S.; Ponsinet, V.; Baron, A.; Gacoin, T.; Kim, J. Near-Infrared Dual-Band LSPR Coupling in Oriented Assembly of Doped Metal Oxide Nanocrystal Platelets. *Nano Lett.* **2024**, *24* (10), 3074–3081. https://doi.org/10.1021/acs.nanolett.3c04849.

(51) Markov, I. V. *Crystal Growth for Beginners: Fundamentals of Nucleation, Crystal Growth and Epitaxy*, 3rd ed.; WORLD SCIENTIFIC, 2017. https://doi.org/10.1142/10127.

(52) Jansons, A. W.; Hutchison, J. E. Continuous Growth of Metal Oxide Nanocrystals: Enhanced Control of Nanocrystal Size and Radial Dopant Distribution. *ACS Nano* **2016**, *10* (7), 6942–6951. https://doi.org/10.1021/acsnano.6b02796.
(53) Thanh, N. T. K.; Maclean, N.; Mahiddine, S. Mechanisms of Nucleation and Growth of Nanoparticles in Solution. *Chem. Rev.* **2014**, *114* (15), 7610–7630.
https://doi.org/10.1021/cr400544s.

(54) Chen, P. Z.; Skirzynska, A.; Yuan, T.; Voznyy, O.; Gu, F. X. Asymmetric Interfacet Adatom Migration as a Mode of Anisotropic Nanocrystal Growth. *J. Am. Chem. Soc.* **2022**, *144* (42), 19417–19429. https://doi.org/10.1021/jacs.2c07423.

(55) Zhang, H.; Li, W.; Jin, M.; Zeng, J.; Yu, T.; Yang, D.; Xia, Y. Controlling the Morphology of Rhodium Nanocrystals by Manipulating the Growth Kinetics with a Syringe Pump. *Nano Lett.* **2011**, *11* (2), 898–903. https://doi.org/10.1021/nl104347j.

(56) Wang, X.; Liu, M.; Chen, Y.; Fu, W.; Wang, B.; Guo, L. Symmetry Breaking in Semiconductor Nanocrystals via Kinetic-Controlled Surface Diffusion: A Strategy for Manipulating the Junction Structure. *Nanoscale* **2016**, *8* (35), 15970–15977.
https://doi.org/10.1039/C6NR04063B.

(57) Peng, Z. A.; Peng, X. Mechanisms of the Shape Evolution of CdSe Nanocrystals. *J. Am. Chem. Soc.* **2001**, *123* (7), 1389–1395. https://doi.org/10.1021/ja0027766.

(58) Plummer, L. K.; Crockett, B. M.; Pennel, M. L.; Jansons, A. W.; Koskela, K. M.; Hutchison, J. E. Influence of Monomer Flux and Temperature on Morphology of Indium Oxide Nanocrystals during a Continuous Growth Synthesis. *Chem. Mater.* **2019**, *31* (18), 7638–7649.
https://doi.org/10.1021/acs.chemmater.9b02467.
(59) Bales, G. S.; Chrzan, D. C. Transition from Compact to Fractal Islands during Submonolayer Epitaxial Growth. *Phys. Rev. Lett.* **1995**, *74* (24), 4879–4882.
https://doi.org/10.1103/PhysRevLett.74.4879.

(60) Libbrecht, K. G. Explaining the Formation of Thin Ice Crystal Plates with Structure-Dependent Attachment Kinetics. *Journal of Crystal Growth* **2003**, *258* (1–2), 168–175.
https://doi.org/10.1016/S0022-0248(03)01496-9.

(61) Tan, T.; Zhang, S.; Wang, C. Branched Ag Nanoplates: Synthesis Dictated by Suppressing Surface Diffusion and Catalytic Activity for Nitrophenol Reduction. *CrystEngComm* **2017**, *19* (42), 6339–6346. https://doi.org/10.1039/C7CE01421J.

(62) Chowdhury, S.; Roy, A.; Bodemann, I.; Banerjee, S. K. Two-Dimensional to Three-Dimensional Growth of Transition Metal Diselenides by Chemical Vapor Deposition: Interplay between Fractal, Dendritic, and Compact Morphologies. *ACS Appl. Mater. Interfaces* **2020**, *12* (13), 15885–15892. https://doi.org/10.1021/acsami.9b23286.

(63) González, C.; Silva, J. P. B.; Viana, A. S.; Gwozdz, K.; Conde, O. Shape-Controlled Monolayer MoSe2 Flakes by Chemical Vapor Deposition towards Tuning the Photoluminescence Emission. *Applied Surface Science* **2022**, *605*, 154742.
https://doi.org/10.1016/j.apsusc.2022.154742.

(64) Li, H.; Kanaras, A. G.; Manna, L. Colloidal Branched Semiconductor Nanocrystals: State of the Art and Perspectives. *Acc. Chem. Res.* **2013**, *46* (7), 1387–1396.
https://doi.org/10.1021/ar3002409.
(65) Teng, X.; Yang, H. Synthesis of Platinum Multipods: An Induced Anisotropic Growth. *Nano Lett.* **2005**, *5* (5), 885–891. https://doi.org/10.1021/nl0503072.

(66) Zhou, G.; Lü, M.; Xiu, Z.; Wang, S.; Zhang, H.; Zhou, Y.; Wang, S. Controlled Synthesis of High-Quality PbS Star-Shaped Dendrites, Multipods, Truncated Nanocubes, and Nanocubes and Their Shape Evolution Process. *J. Phys. Chem. B* **2006**, *110* (13), 6543–6548. https://doi.org/10.1021/jp0549881.

(67) Chandra, K.; Culver, K. S. B.; Werner, S. E.; Lee, R. C.; Odom, T. W. Manipulating the Anisotropic Structure of Gold Nanostars Using Good's Buffers. *Chem. Mater.* **2016**, *28* (18), 6763–6769. https://doi.org/10.1021/acs.chemmater.6b03242.

(68) Zitoun, D.; Pinna, N.; Frolet, N.; Belin, C. Single Crystal Manganese Oxide Multipods by Oriented Attachment. *J. Am. Chem. Soc.* **2005**, *127* (43), 15034–15035. https://doi.org/10.1021/ja0555926.

(69) Narayanaswamy, A.; Xu, H.; Pradhan, N.; Peng, X. Crystalline Nanoflowers with Different Chemical Compositions and Physical Properties Grown by Limited Ligand Protection. *Angew Chem Int Ed* **2006**, *45* (32), 5361–5364. https://doi.org/10.1002/anie.200601553.

(70) Ren, P.; Lu, Z.; Song, M.; Liu, L.; Wang, B.; Wei, N.; Bowden, M. E.; Wirth, M. G.; Perea, D. E.; Zhang, D.; Buck, E. C.; Han, Y.; Sushko, P. V.; Li, D. Formation of Pyrophosphates across Grain Boundaries Induces the Formation of Mismatched but Oriented Interfaces in Silver Phosphate Polypods. *Applied Surface Science* **2021**, *563*, 149980. https://doi.org/10.1016/j.apsusc.2021.149980.
(71) Manna, L.; Milliron, D. J.; Meisel, A.; Scher, E. C.; Alivisatos, A. P. Controlled Growth of Tetrapod-Branched Inorganic Nanocrystals. *Nature Mater* **2003**, *2* (6), 382–385.
https://doi.org/10.1038/nmat902.

(72) Huang, J.; Kovalenko, M. V.; Talapin, D. V. Alkyl Chains of Surface Ligands Affect Polytypism of CdSe Nanocrystals and Play an Important Role in the Synthesis of Anisotropic Nanoheterostructures. *J. Am. Chem. Soc.* **2010**, *132* (45), 15866–15868.
https://doi.org/10.1021/ja105132u.

(73) McDaniel, H.; Shim, M. Size and Growth Rate Dependent Structural Diversification of Fe₃O₄/CdS Anisotropic Nanocrystal Heterostructures. *ACS Nano* **2009**, *3* (2), 434–440.
https://doi.org/10.1021/nn800737a.

(74) Cheong, S.; Watt, J.; Ingham, B.; Toney, M. F.; Tilley, R. D. In Situ and Ex Situ Studies of Platinum Nanocrystals: Growth and Evolution in Solution. *J. Am. Chem. Soc.* **2009**, *131* (40), 14590–14595. https://doi.org/10.1021/ja9065688.

(75) Mulvihill, M. J.; Ling, X. Y.; Henzie, J.; Yang, P. Anisotropic Etching of Silver Nanoparticles for Plasmonic Structures Capable of Single-Particle SERS. *J. Am. Chem. Soc.* **2010**, *132* (1), 268–274. https://doi.org/10.1021/ja906954f.

(76) Lim, S. J.; Kim, W.; Jung, S.; Seo, J.; Shin, S. K. Anisotropic Etching of Semiconductor Nanocrystals. *Chem. Mater.* **2011**, *23* (22), 5029–5036. https://doi.org/10.1021/cm202514a.

(77) Szilágyi, I. M.; Madarász, J.; Pokol, G.; Király, P.; Tárkányi, G.; Saukko, S.; Mizsei, J.; Tóth, A. L.; Szabó, A.; Varga-Josepovits, K. Stability and Controlled Composition of Hexagonal WO₃. *Chem. Mater.* **2008**, *20* (12), 4116–4125. https://doi.org/10.1021/cm800668x.
(78) Nikoobakht, B.; El-Sayed, M. A. Preparation and Growth Mechanism of Gold Nanorods (NRs) Using Seed-Mediated Growth Method. *Chem. Mater.* **2003**, *15* (10), 1957–1962. https://doi.org/10.1021/cm020732l.

(79) Vigderman, L.; Zubarev, E. R. High-Yield Synthesis of Gold Nanorods with Longitudinal SPR Peak Greater than 1200 Nm Using Hydroquinone as a Reducing Agent. *Chem. Mater.* **2013**, *25* (8), 1450–1457. https://doi.org/10.1021/cm303661d.

(80) Wei, M.-Z.; Deng, T.-S.; Zhang, Q.; Cheng, Z.; Li, S. Seed-Mediated Synthesis of Gold Nanorods at Low Concentrations of CTAB. *ACS Omega* **2021**, *6* (13), 9188–9195. https://doi.org/10.1021/acsomega.1c00510.

(81) Cui, X.; Qin, F.; Ruan, Q.; Zhuo, X.; Wang, J. Circular Gold Nanodisks with Synthetically Tunable Diameters and Thicknesses. *Adv Funct Materials* **2018**, *28* (11), 1705516. https://doi.org/10.1002/adfm.201705516.

(82) Gans, R. Über die Form ultramikroskopischer Goldteilchen. *Annalen der Physik* **1912**, *342* (5), 881–900. https://doi.org/10.1002/andp.19123420503.

(83) Blanch, A. J.; Döblinger, M.; Rodríguez-Fernández, J. Simple and Rapid High-Yield Synthesis and Size Sorting of Multibranched Hollow Gold Nanoparticles with Highly Tunable NIR Plasmon Resonances. *Small* **2015**, *11* (35), 4550–4559. https://doi.org/10.1002/smll.201500095.

(84) Genzel, L.; Martin, T. P.; Kreibig, U. Dielectric Function and Plasma Resonances of Small Metal Particles. *Z Physik B* **1975**, *21* (4), 339–346. https://doi.org/10.1007/BF01325393.
(85) Hao, E.; Schatz, G. C. Electromagnetic Fields around Silver Nanoparticles and Dimers.
*The Journal of Chemical Physics* **2004**, *120* (1), 357–366. https://doi.org/10.1063/1.1629280.

(86) Schuck, P. J.; Fromm, D. P.; Sundaramurthy, A.; Kino, G. S.; Moerner, W. E. Improving the Mismatch between Light and Nanoscale Objects with Gold Bowtie Nanoantennas. *Phys. Rev. Lett.* **2005**, *94* (1), 017402. https://doi.org/10.1103/PhysRevLett.94.017402.

(87) Talley, C. E.; Jackson, J. B.; Oubre, C.; Grady, N. K.; Hollars, C. W.; Lane, S. M.; Huser, T. R.; Nordlander, P.; Halas, N. J. Surface-Enhanced Raman Scattering from Individual Au Nanoparticles and Nanoparticle Dimer Substrates. *Nano Lett.* **2005**, *5* (8), 1569–1574. https://doi.org/10.1021/nl050928v.

(88) Bohren, C. F.; Huffman, D. R. *Absorption and Scattering of Light by Small Particles*; Wiley-VCH, 1998.

(89) Bange, K. Colouration of Tungsten Oxide: A Model for Optically Active Coatings. *Solar Energy Materials* **1999**, *58* (1), 1–131. https://doi.org/10.1016/S0927-0248(98)00196-2.

(90) Gesheva, K.; Szekeres, A.; Ivanova, T. Optical Properties of Chemical Vapor Deposited Thin Films of Molybdenum and Tungsten Based Metal Oxides. *Solar Energy Materials and Solar Cells* **2003**, *76* (4), 563–576. https://doi.org/10.1016/S0927-0248(02)00267-2.

(91) Ingham, B.; Hendy, S. C.; Chong, S. V.; Tallon, J. L. Density-Functional Studies of Tungsten Trioxide, Tungsten Bronzes, and Related Systems. *Phys. Rev. B* **2005**, *72* (7), 075109. https://doi.org/10.1103/PhysRevB.72.075109.
(92) Coronado, E. A.; Schatz, G. C. Surface Plasmon Broadening for Arbitrary Shape Nanoparticles: A Geometrical Probability Approach. *The Journal of Chemical Physics* **2003**, *119* (7), 3926–3934. https://doi.org/10.1063/1.1587686.

(93) Amendola, V.; Meneghetti, M. Size Evaluation of Gold Nanoparticles by UV-vis Spectroscopy. *J. Phys. Chem. C* **2009**, *113* (11), 4277–4285. https://doi.org/10.1021/jp8082425.

(94) Nelder, J. A.; Mead, R. A Simplex Method for Function Minimization. *The Computer Journal* **1965**, *7* (4), 308–313. https://doi.org/10.1093/comjnl/7.4.308.

(95) Oh, J.; Davis, J.; Tusseau-Nenez, S.; Plapp, M.; Baron, A.; Gacoin, T.; Kim, J. Continuous anisotropic growth of NIR-plasmonic $Cs_xWO_{3-\delta}$ nanocrystals into rods and platelets. *ChemRxiv*, https://chemrxiv.org/engage/chemrxiv/article-details/6797852181d2151a02280fd7 (accessed January 29, 2025)