Bertille Martinez

Clément Livache

Elisa Meriggio

Zhen Xiang

Hervé Xu

Emmanuelle Cruguel

Anna Lacaze

Sandrine Proust

Mathieu G Ithurria

Gregory Silly

Cabailh

Xu

Hervé Cruguel

Emmanuelle Lacaze

Anna Proust

Sandrine Ithurria

Mathieu G Silly

Gregory Cabailh

Florence Volatron

Emmanuel Lhuillier

Polyoxometalate as Control Agent for the Doping in HgSe Self-Doped Nanocrystals

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

Beyond their light emitting properties in the visible range, the broad tunability of optical absorption of colloidal quantum dots (CQDs) from UV to THz 1 make them promising for the design of low-cost optoelectronic devices. Among potential applications of CQDs, solar cells 2 and thermal imaging are two examples where infrared absorption is required. [START_REF] Lhuillier | Recent Progresses in Mid Infrared Nanocrystal Based Optoelectronics[END_REF] To push absorption of CQDs in the mid-infrared, two paths have been explored: either the use of interband transitions in narrow band gap semiconductors and semimetals or the use of doped semiconductors. [START_REF] Jagtap | Emergence of Intraband Transitions in Colloidal Nanocrystals[END_REF][START_REF] Deng | Colloidal Quantum Dots Intraband Photodetectors[END_REF] In the latter case, intraband transition appears in the mid-infrared when the material becomes degenerately doped. Mercury chalcogenides selfdoped nanocrystals especially HgS, [START_REF] Kim | High Electron Mobility of β-HgS Colloidal Quantum Dots with Doubly Occupied Quantum States[END_REF][START_REF] Shen | HgS and HgS/CdS Colloidal Quantum Dots with Infrared Intraband Transitions and Emergence of a Surface Plasmon[END_REF][START_REF] Jeong | Air-Stable N-Doped Colloidal HgS Quantum Dots[END_REF] HgSe [START_REF] Lhuillier | Infrared Photodetection Based on Colloidal Quantum-Dot Films with High Mobility and Optical Absorption up to THz[END_REF][START_REF] Wang | Transport in a Single Self-Doped Nanocrystal[END_REF] and HgTe [START_REF] Shen | Synthesis of Nonaggregating HgTe Colloidal Quantum Dots and the Emergence of Air-Stable N-Doping[END_REF] are certainly the most investigated materials because of their ability to combine intraband absorption in the mid-infrared with photoconductive properties. In the following we will focus on HgSe CQDs as model system for self-doped nanocrystals.

Self-doping [START_REF] Jagtap | Emergence of Intraband Transitions in Colloidal Nanocrystals[END_REF][START_REF] Kim | Self-Doped Colloidal Semiconductor Nanocrystals with Intraband Transitions in Steady State[END_REF] of nanocrystals is a mechanism leading to degenerate doping [START_REF] Araujo | Doped Colloidal PbSe Quantum Dots: Band Assignments and Electrostatic Effects[END_REF] which is specific to narrow band gap materials. Commonly doping is obtained through the introduction of extrinsic impurities into a semiconductor matrix (see the work on doped oxide [START_REF] Luther | Localized Surface Plasmon Resonances Arising from Free Carriers in Doped Quantum Dots[END_REF][START_REF] Agrawal | Control of Localized Surface Plasmon Resonances in Metal Oxide Nanocrystals[END_REF] ) or through the synthesis of non-stoichiometric material (this is the case of Cu2S(e) [START_REF] Kriegel | Tuning the Excitonic and Plasmonic Properties of Copper Chalcogenide Nanocrystals[END_REF][START_REF] Coughlan | Compound Copper Chalcogenide Nanocrystals[END_REF][START_REF] Dorfs | Reversible Tunability of the Near-Infrared Valence Band Plasmon Resonance in Cu2-xSe Nanocrystals[END_REF] ). In the case of mercury chalcogenides, the doping process is different. Indeed, most II-VI nanocrystals present a slight cation excess which leads to a n-type character. In the specific case of narrow band gap materials such as mercury chalcogenides, the conduction band state can be brought below the Fermi level of the environment: [START_REF] Robin | Surface Control of Doping in Self-Doped Nanocrystals[END_REF] as a result the stable form of these CQDs is negatively charged and the 1Se, 1Pe and even 1De states can be filled with electrons, which corresponds to strongly degenerate doping.

The control of the doping magnitude becomes of utmost importance in photodetection applications, [START_REF] Jeong | Singly and Doubly Occupied Higher Quantum States in Nanocrystals[END_REF] not only because it drives the optical spectrum but also because many other properties such as the dark current, the time response, their paramagnetic response…are connected to the exact number of carriers per particle. [START_REF] Deng | Colloidal Quantum Dots Intraband Photodetectors[END_REF][START_REF] Jeong | Singly and Doubly Occupied Higher Quantum States in Nanocrystals[END_REF] In particular infrared absorption based on intraband transition (moderate degenerate doping) and on plasmonic absorption (strongly degenerate doping) offer an interesting strategy to interband transition in narrow band gap material to address the mid infrared range of wavelengths. Because doping drives the optical spectrum in such materials, identifying a post synthesis method to tune it is of utmost interest.

Different strategies have been reported to tune the doping magnitude based on ligand exchange, [START_REF] Robin | Surface Control of Doping in Self-Doped Nanocrystals[END_REF] heterostructure growth [START_REF] Shen | HgS and HgS/CdS Colloidal Quantum Dots with Infrared Intraband Transitions and Emergence of a Surface Plasmon[END_REF][START_REF] Sagar | Core/Shell Nanocrystals by Colloidal Atomic Layer Deposition[END_REF] or nanoparticle size tuning. [START_REF] Chen | Reversible Electrochemistry of Mercury Chalcogenide Colloidal Quantum Dot Films[END_REF][START_REF] Martinez | HgSe Self-Doped Nanocrystals as a Platform to Investigate the Effects of Vanishing Confinement[END_REF] Among them, the surface gating consists in tuning the surface dipole at the nanoparticle surface to induce a band bending and thus control the relative doping. This approach has been widely used for PbS nanocrystals to change the nature of the majority carrier. [START_REF] Miller | Revisiting the Valence and Conduction Band Size Dependence of PbS Quantum Dot Thin Films[END_REF][START_REF] Brown | Energy Level Modification in Lead Sulfide Quantum Dot Thin Films through Ligand Exchange[END_REF] In this direction, Kroupa et al have proposed to design molecules with especially large dipole. [START_REF] Kroupa | Tuning Colloidal Quantum Dot Band Edge Positions through Solution-Phase Surface Chemistry Modification[END_REF] They reported a band displacement close to 2 eV. On the other hand in the case of mercury chalcogenides the variety of ligands which has been explored is much weaker and they typically consist in thiol-based molecules. [START_REF] Martinez | HgSe Self-Doped Nanocrystals as a Platform to Investigate the Effects of Vanishing Confinement[END_REF][START_REF] Martinez | Probing Charge Carrier Dynamics to Unveil the Role of Surface Ligands in HgTe Narrow Band Gap Nanocrystals[END_REF] In the case of HgSe nanoparticles, surface chemistry tuning has only led to a band shift of the order of 0.3 eV, which is nevertheless already close to the intraband transition energy. In this paper, we propose an alternative post synthesis method to boost the tunability of HgSe CQD doping, based on their hybridization with polyoxometalates (POMs).

POMs are molecular oxides formed by the early transition metals in their highest oxidation state (W VI , Mo VI …). They are well known for being electron attractors [START_REF] Sadakane | Electrochemical Properties of Polyoxometalates as Electrocatalysts[END_REF][START_REF] Huder | Evidence for Charge Transfer at the Interface between Hybrid Phosphomolybdate and Epitaxial Graphene[END_REF] and thus we can anticipate an electron transfer from the self-doped CQDs to the POMs. In this direction, a key challenge relies in the coupling between the POMs and the CQDs. There are a limited number of reports relative to this question [START_REF] Huang | Surface Functionalization of Semiconductor and Oxide Nanocrystals with Small Inorganic Oxoanions (PO43-, MoO42-) and Polyoxometalate Ligands[END_REF] and they are mostly motivated by the idea to use POMs as a charge transfer agent in dye sensitized solar cell. [START_REF] Yu | Polyoxometalate-CdS Quantum Dots Co-Sensitized TiO2 Nanorods Array: Enhanced Charge Separation and Light to Electricity Conversion Efficiency[END_REF][START_REF] Chen | The Improved Efficiency of Quantum-Dot-Sensitized Solar Cells with a Wide Spectrum and Pure Inorganic Donor-Acceptor Type Polyoxometalate as a Collaborative Cosensitizer[END_REF] Here we take benefit of the great affinity of mercury for thiols and design a thiol functionalized POM. By using infrared spectroscopy, photoemission and transport measurement, we bring evidence for the electron transfer from the HgSe CQDs to the POMs and show for the first time a change of doping from 5 electrons per nanocrystal to less than 1 while tuning the surface chemistry.

METHODS

Chemicals

Mercury acetate (Hg(OAc)2, Sigma-Amdrich), selenium powder (Se, Strem Chemicals), oleic acid (OA, Sigma-Aldrich, 90%), oleylamine (OLA, Sigma-Aldrich, 80-90%), trioctylphosphine (TOP, Sigma-Aldrich, 97%), n-hexane (VWR, 99%), ethanol absolute anhydrous (EtOH, Carlo Erba, 99.5%), methanol (MeOH, Carlo Erba, 99.8%), N,N-dimethylformamide (DMF, Sigma Aldrich), triethyloxonium tetrafluoroborate (Et3O + BF4 -, Sigma Aldrich), lithium perchlorate (LiClO4, Sigma-Aldrich, 98%), 1,2-ethanedithiol (EDT, Fluka, 98%), polyethylene glycol (PEG 6k, Mw = 6.10 3 g.mol -1 , Fluka).

Material synthesis

HgSe QD synthesis: In a 50 mL three neck flask, 0.5 g of mercury acetate (Hg(OAc)2), 10 mL of oleic acid (OA) and 25 mL of oleylamine (OLA) are degassed at 85 °C for 1 h. Under argon atmosphere at 90 °C, 1.6 mL of a 1 M solution of Selenium (Se) in trioctylphosphine (TOP) is quickly injected into the flask. After 4 min, the reaction is quenched by adding quickly 10 mL of hexane and cooled down to room temperature using ice bath. The obtained dark solution is then opened to air and precipitated by addition of ethanol and methanol and then centrifuged. The supernatant is discarded and the pellet is redispersed in hexane. The cleaning procedure is repeated two more times. At the last step, we perform a selective precipitation to split the batch over two fractions. To do so, the solution in pure hexane is first centrifuged without the addition of non-solvent to get rid of the lamellar phase resulting from the complexation of mercury with OLA. Then, only the stable solution of HgSe in hexane is kept while the gray precipitate is discarded. The nanoparticles in this fraction are 5 ± 0.7 nm, measured by transmission electron microscopy (TEM), see Figure 1b. The intraband absorption peak is at 2080 cm - 1 , see Figure 1c. Because of the absence of DDT ligands, the obtained solution is poorly stable, and the HgSe CQDs can no longer be redispersed in hexane after 30 minutes.

POM-SH (TBA)3[PW11O39{O(SiC3H6SH)2}

] synthesis: K7[PW11O39] (0.64 g, 0.2 mmol) is dissolved in a water/acetonitrile mixture (30 mL, 1:1). A 1 M HCl aqueous solution is added drop by drop until an apparent pH equals to 3. The solution is cooled in an ice bath and the (OCH3)3SiC3H6SH (0.157 mL, 0.8 mmol) is inserted. The 1 M HCl solution is added drop by drop again to reach pHapp=2. After an all night reaction, TBABr (0.26 g, 0.8 mmol) is added and the solution concentrated with a rotary evaporator to precipitate the product. The sticky solid is dissolved in the minimum of acetonitrile then precipitated again with an excess of diethylether. The solid is recovered by centrifugation and washed thoroughly with diethylether to obtain a white powder (0.55 g, 76%). OA/OLA towards POM-SH ligand exchange procedure: 500 µL of the solution of fresh HgSe QDs in hexane (10 mg.mL -1 ) is mixed with 1 mL of a solution of triethyloxonium tetrafluoroborate in DMF (50 mg.mL -1 ) to clean QD surface of any ligand. After vigorous agitation and sonication, the solution is centrifuged for 2 min at 5500 rpm. The supernatant is discarded while the QDs in the pellet are redispersed in 0.5 mL of a solution of POM-SH in DMF (50 mg.mL -1 ). The obtained solution is more colloidably stable than the initial HgSe QDs in hexane without the POM-SH. To remove the excess of POM-SH, the solution is centrifuged at 5500 rpm for 10 min. The POM-SH capped QDs fall in the pellet while the excess of POM-SH stay in the supernatant. The supernatant is then discarded and the pellet is redispersed in fresh DMF. The obtained solution is colloidably stable (several months).

OA/OLA towards EDT ligand exchange procedure: Solution of HgSe QDs in hexane is dropcasted on a clean substrate. Once the solvent is completely evaporated, ligand exchange is performed on the sample to remove OA/OLA and replace it by EDT. The EDT ligands are simply dissolved in ethanol to reach a 1 wt % solution. To perform ligand exchange, the film is dipped into the EDT solution for 90 s. Then the film is rinsed with ethanol for 30 s.

Transport measurement

Electrode fabrication: Electrodes are fabricated using standard optical lithography methods. Briefly, the surface of a Si/SiO2 (400 nm thick) wafer is cleaned by sonication in acetone. The wafer is rinsed with isopropanol and finally cleaned using an O2 plasma. AZ5214 resist is spin-coated and baked at 110 °C for 90 s. The substrate is exposed under UV through a pattern mask for 1.5 s. The film is further baked at 125 °C for 2 min to invert the resist. Then, a 40 s flood exposure is performed. The resist is developed using a bath of AZ726 for 32 s, before being rinsed with pure water. We then deposit a 5 nm Cr layer and a 80 nm gold layer using a thermal evaporator. The lift-off is performed by dipping the film for 1 h in acetone. The electrodes are finally rinsed using isopropanol and dried by air flow. The electrodes are 2.5 mm long and spaced by 20 µm.

Electrolyte preparation: LiClO4 (500 mg) is mixed with 2.3 g of polyethylene glycol on a hot plate in an argon-filled glove box at 170 °C for 2 h.

Low temperature DC transport: Temperature-dependent electrical measurements were carried out inside a He-flow cryostat, using a low current SourceMeter Keithley 2634B.

Transistor measurements: Transistor measurements are conducted in Ar filled glove box using two Keithley 2400 as source-meters, by biasing the drain source under low bias (between 5 and 100 mV) and applying a varying gate bias. The gate step bias is 1 mV at 2 mV.s -1 for electrolyte gating. A scheme of the setup is presented on Figure 3a.

DISCUSSION

We first synthetize 5 nm HgSe CQDs presenting intraband absorption feature at ≈ 2000 cm -1 using dodecanethiol ligands at the end of the reaction to ensure HgSe CQDs a strong colloidal stability. [START_REF] Lhuillier | Infrared Photodetection Based on Colloidal Quantum-Dot Films with High Mobility and Optical Absorption up to THz[END_REF] The infrared spectrum of this material presents two main features. In the near infrared (wavenumber above 6000 cm -1 ), there is an absorption edge corresponding to interband transition. In the mid infrared (from 3000 to 2000 cm -1 ), there is a narrow band absorption which corresponds to intraband features. The spectrum of this material is presented in reference [START_REF] Martinez | HgSe Self-Doped Nanocrystals as a Platform to Investigate the Effects of Vanishing Confinement[END_REF] and is very similar to the one presented on Figure 1c (with different capping ligands). According to previous combined photoemission and infrared spectroscopy study, these HgSe CQDs are electron rich with typically the 1Se fully filled and the 1Pe partly filled state once capped with their native dodecanthiol ligand [START_REF] Martinez | HgSe Self-Doped Nanocrystals as a Platform to Investigate the Effects of Vanishing Confinement[END_REF] . 1Se and the 1Pe are respectively twice and six times degenerated, thus a full filling of these two states corresponds to 8 electrons per nanocrystal. [START_REF] Martinez | HgSe Self-Doped Nanocrystals as a Platform to Investigate the Effects of Vanishing Confinement[END_REF] Assuming a randomly close packing for the CQDs, this correspond to carrier density of ≈8x10 19 carrier per cm -3 .

We then synthetize POMs with tungsten as metal, stabilized with tetrabutylammonium (TBA) (see Supporting Information for more information about synthesis and structure). The POMs have the following stoichiometry (TBA)3PW12O40 , where W atoms are highly oxidized (+VI oxidation state) and so can be used as electron attractors when grafted onto HgSe QDs.

In order to couple the POMs onto the HgSe CQD surface, we first try the method proposed by Huang et al. [START_REF] Huang | Surface Functionalization of Semiconductor and Oxide Nanocrystals with Small Inorganic Oxoanions (PO43-, MoO42-) and Polyoxometalate Ligands[END_REF] This procedure consists in two steps: first, the native ligands are removed from CQD surface using an alkylating agent; then the POMs are grafted onto QD nude surface. In our case, this procedure has been unsuccessful: we observe that the final solution of POM capped HgSe QDs is not stable in DMF despite the high solubility of POMs in this solvent, meaning that the grafting of ligands is not done. Moreover we notice that the alkylating agent that we use (triethyloxonium tetrafluoroborate, EtO3 + BF4 -) cannot remove the native dodecanethiol ligands at the surface of HgSe nanocrystals, probably due to the great affinity of mercury with sulfur.

Consequently, we update the HgSe CQD synthesis to get rid of any thiol ligand and replace them by oleylamine and oleic acid. The obtained nanocrystals keep a spherical shape despite the change of ligands (see Figure 1b), but their colloidal stability is very poor: typically after 30 minutes, the CQDs aggregate and can no longer be redispersed even with sonication. In addition, we choose to increase the affinity of the HgSe CQDs for the POM, by using a silyl-substituted POM terminated by two thiol groups (TBA)3[PW11O40(SiC3H6SH)2] (POM-SH). Using these POM-SH ligands, we successfully attach them at the surface of HgSe CQDs (a scheme of the obtained QDs is presented on Figure 1a) using a phase transfer [START_REF] Nag | Metal-Free Inorganic Ligands for Colloidal Nanocrystals: S2-, HS-, Se2-, HSe-, Te2-, HTe-, TeS32-, OH-, and NH2as Surface Ligands[END_REF] procedure and obtain at the end a stable colloidal solution in DMF. Note that we estimate the number of grated POM per QD to be in the 1 to 10 range.

c. FTIR spectra of HgSe QDs just after synthesis (in black) and after POM-SH grafting at the surface (in red). OA and OLA refer respectively to oleic acid and oleylamine, the only ligands used in the synthesis. Note that the peak at ≈2300 cm -1 is due to CO2. d. Energy levels of HgSe after synthesis (OA-OLA) and after POM-SH grafting. The dot line corresponds to Fermi level which is used as reference at 0 eV. 1Se and 1 Pe correspond respectively to first and second level of conduction band; 1Sh corresponds to last level of valence band.

As POMs are electron attractors, [START_REF] Sadakane | Electrochemical Properties of Polyoxometalates as Electrocatalysts[END_REF][START_REF] Huder | Evidence for Charge Transfer at the Interface between Hybrid Phosphomolybdate and Epitaxial Graphene[END_REF] we investigated their influence on the infrared spectrum of the HgSe CQDs and more specifically on their intraband feature. To do so, we recorded transmission spectra before and after POM-SH grafting, see Figure 1c. With their oleic acid (OA) and oleylamine (OLA) capping ligands, the HgSe CQDs present interband transition at wavenumbers above 6000 cm -1 which does not correspond to the band-edge feature [START_REF] Martinez | HgSe Self-Doped Nanocrystals as a Platform to Investigate the Effects of Vanishing Confinement[END_REF] as it is fully bleached (ie the 1Se level is full) but to the transition between valence band and deeper levels in the conduction band. Around 2000 cm -1 , the intraband peak is clearly noticeable. After POM grafting, the intraband feature at 2000 cm -1 mostly disappears whereas the interband band-edge feature emerges at 4900 cm -1 , meaning that the transition between 1 Sh and 1Se level is authorized. Additional peak at 3000 cm -1 and below 1500 cm -1 also clearly appear and can be attributed respectively to the organic tether, the counter cations and to the POM itself (see figure S1). This change of absorption weight between the band edge feature and the intraband feature is a clear signature of the dedoping of the CQDs 36 : the removing of electrons from conduction band levels leads to disappearance of intraband feature and unbleaching of the band edge absorption. It is also worth noting that evidence for the charge transfer from the QD to the POM is still preserved while the material is under film form. This is a key difference with previous reports on molecule to QD charge transfer which was possible in solution but unobserved while the QD where under a solid array form. [START_REF] Koh | Heavily Doped N-Type PbSe and PbS Nanocrystals Using Ground-State Charge Transfer from Cobaltocene[END_REF] The fact that charge transfer gets preserved under solid form is quite promising for the use of such doping control method at the device scale To further clarify the impact of the POMs on the carrier density of the HgSe CQDs, we conduct photoemission measurements on the TEMPO beamline of Synchrotron Soleil. The low binding energy part of the photoemission spectrum is used to reveal the position of the valence band edge with respect to the Fermi level. We obtain a value of 0.88 eV for the HgSe CQDs capped with their native dodecanethiol ligand (see Figure S2). This value has to be compared to the interband edge energy ≈0.60 eV confirming that initial HgSe CQDs are degenerately doped with the Fermi level deeply in the conduction band. This is consistent with the observation of the intraband feature on the infrared spectrum. After POM grafting, the valence band is only 0.5 eV below the Fermi level, meaning that the Fermi level is now within the band gap. Again this observation is consistent with the disappearance of the intraband feature and the recovery of the band edge transition. To further illustrate this shift of the band with respect to the Fermi level, the reconstructed energy levels in absolute energy for both materials are presented on Figure 1d. This graph demonstrates the possibility to tune the doping of HgSe. Initially the Fermi level is resonant with the 1Pe state, meaning that the latter is half filled. Since this state is 6 fold degenerated, there are 3 electrons in the 1Pe state and 2 more in the 1Se state for a total of 5 electrons per nanoparticle. After the grafting of the POM, the Fermi level is brought within the band gap and we can estimate the thermal population of the 1Se state to be given by 2. 𝑒𝑥𝑝 (-𝐸 1𝑆𝑒 -𝐸𝑓 𝑘𝑏𝑇 )=0.03 electron/nanoparticle with 𝐸 1𝑆𝑒 -𝐸𝑓 = 107 𝑚𝑒𝑉 the difference in energy between the 1Se state and the Fermi level and kbT the thermal energy (≈25 meV at room temperature). Almost 5 electrons per nanoparticle are transferred toward the POM, which is by far the greatest tuning of the carrier density reported for self-doped CQDs.

In order to confirm that HgSe CQD change of doping is the result of the POM grafting, we investigate the photoemission signal of the pristine HgSe CQDs, of the pristine POM-SH and of the grafted POM on HgSe CQDs. Experimental data are given in Figure 2.

The overviews obtained for both films are presented on Figure 2a. For POM-SH capped HgSe QDs, the presence of tungsten coming from POM-SH ligands on the overview confirms that the POM-SH are present into the sample. The observation of the W 4f core level from the pristine POM-SH ligands reveals the presence of a single contribution at a binding energy of 36.03 eV (see Figure 2b). This value is consistent with the + VI oxidation state of W. [START_REF] Volatron | Electron Transfer Properties of a Monolayer of Hybrid Polyoxometalates on Silicon[END_REF] The charge transfer from the HgSe electron rich CQDs to the oxidized POM is well evidenced by investigating the spectrum relative to the W 4f of the hybrid material, see Figure 2b. This state can no longer be fitted by a single contribution. More specifically, new contributions appear at lower binding energy, which means that W presents contribution from lower oxidation states [START_REF] Ng | XPS Studies of Oxides of Row Transition Metals of W[END_REF][START_REF] Mcguire | Core Electron Binding Energies in Some Group IIIA, VB, and VIB Compounds[END_REF] typically +V and possibly +IV. This multielectronic reduction of the POM-SH is a clear signature of a charge transfer ant that the POM-SH is responsible for the de-doping of the HgSe CQDs. In the following, we investigate how this modification of the HgSe CQD carrier density is impacting the transport properties of HgSe CQD arrays. HgSe is typically expected to be used as active material for low cost photodetection. [START_REF] Deng | Colloidal Quantum Dots Intraband Photodetectors[END_REF][START_REF] Lhuillier | Infrared Photodetection Based on Colloidal Quantum-Dot Films with High Mobility and Optical Absorption up to THz[END_REF] A key figure of merit is dark current. While doping is important to obtain intraband transition, it is also driving the magnitude of the dark current, reducing doping is then of utmost interest. In the case of OA/OLA capping ligands, we exchange OA/OLA with ethanedithiol ligands using the procedure described in the Supporting Information. This ligand exchange has very little effect on the doping level, as shown by Martinez et al. [START_REF] Martinez | HgSe Self-Doped Nanocrystals as a Platform to Investigate the Effects of Vanishing Confinement[END_REF] On the other hand, in the case of POM capped HgSe CQDs, we just deposited the QDs + DMF solution and dried at 100 °C for a minute to evaporate the solvent.

It was a priori unclear if the POM grafted HgSe CQD films were conductive or not. Indeed the large size of the POM may have prevented the coupling of the CQDs, leading to a poor hopping transport efficiency. Nevertheless, a conductive film is obtained.

Transport is probed in an electrolyte gating field effect transistor configuration, [START_REF] Lhuillier | Investigating the N-and P-Type Electrolytic Charging of Colloidal Nanoplatelets[END_REF][START_REF] Sahu | Electronic Impurity Doping in CdSe Nanocrystals[END_REF] see Figure 3a for a scheme of the device. The main motivation for such a gating approach comes from the ability of the electrolyte to inject a large number of carriers per nanoparticle. This is of utmost importance to tune the carrier density of strongly doped CQDs. Without POM, the HgSe CQDs show only a n-type behavior with a negative threshold voltage (ie the material is inherently doped), characterized by a rise of conductance while increasing gate bias, see Figure 3b. No hole transport is observed, which is consistent with the electron degenerate doping of this material.

While POM-SH are grafted on the CQD surface, the transfer curve (drain current as a function of gate bias) is strongly affected. The material turns to be ambipolar (see Figure 3c) with both hole and electron conductions. This confirms that the Fermi level is now closer to the valence band than it is without POMs.

The reduction of doping is also affecting the temperature dependence of the material. HgSe with conventional ligands typically suffer from very weak temperature dependence. Typically the Arrhenius fit of the current as a function of temperature in the vicinity of the room temperature leads to an activation energy (18 meV) even smaller than kb T with kb being the Boltzmann constant, see Figure 3d.

As a result, cooling the sample is not a viable strategy to reduce the dark current and improve photodetection performances. In the presence of POM-SH ligands, the activation energy (50 meV) is increased by a factor 2.5, Figure 3d. This is again consistent with the displacement of the Fermi level in a part of the electronic spectrum where the density of states is sparser.

ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: POM SH characterization, details for material characterization methods, and photoemission measurements.

CONCLUSION

Control of doping is of utmost importance for the use of doped CQDs in application where intraband and plasmonic transition appear. Previously the main strategy for tuning the doping was relying on surface gating based on the design of dipole [START_REF] Robin | Surface Control of Doping in Self-Doped Nanocrystals[END_REF][START_REF] Brown | Energy Level Modification in Lead Sulfide Quantum Dot Thin Films through Ligand Exchange[END_REF][START_REF] Kroupa | Tuning Colloidal Quantum Dot Band Edge Positions through Solution-Phase Surface Chemistry Modification[END_REF] to induce band bending at the nanocrystal surface.

Here we demonstrate that the grafting of electron attractor molecules such as POMs is a viable way for much broader tuning of the carrier density (here up to 5 carriers per nanoparticle). All infrared spectroscopy, photoemission and transport measurements bring a consistent picture for a charge transfer from the CQDs to the POMs leading to (i) doping decrease in the CQDs, (ii) reduction of the POM. Moreover we demonstrate that the presence of the POMs is compatible with electronic transport in POM-CQD hybrid array. This strategy is not limited to mercury chalcogenides and is also promising for the tuning of doping in degenerately doped nanoparticles such as oxide or silicon nanoparticles.

1 H

 1 NMR (400 MHz, CD3CN): δ(ppm) 3.12 (m, 24H), 2.64 (m, 4H), 1.87 (m, 4H), 1.63 (m, 25H), 1.40 (sex, 3J(H,H)=7.4 Hz, 24H), 0.99 (t, 3J(H,H)=7.4 Hz, 36H), 0.90 (m, 4H) ; 31 P NMR (121 MHz, CD3CN) δ (ppm) -12.37; IR (KBr pellet) : δ =2962 (m), 2934 (m), 2873 (w), 1483 (m), 1381 (w), 1111 (s), 1065 (s), 1052 (s), 1036 (s), 963 (vs), 870 (vs), 824 (vs), 711 (m), 585 (w), 522 (m), 383 (s) ; MS (ESI-), m/z (%): calculated for C6H14O40PS2Si2W11 : 966.56 [M] 4-; found : 966.75 (100) Elemental Analysis: calculated (%) for C70H158N4O40S2Si3W11: C 21.74, H 4.09, N 1.45; found: C 21.70, H 3.97, N 1.43 (TBA)3[PW12O40] 33 and K7[PW11O39] 34 was synthesized according to published procedures.

Figure 1 :

 1 Figure 1: a. Scheme of HgSe QDs capped with POM-SH ligands. The arrow corresponds to the electron transfer from doped HgSe QD to electron attractor POM-SH ligand. b. TEM picture of HgSe QDs capped with POM-SH. c. FTIR spectra of HgSe QDs just after synthesis (in black) and after POM-SH grafting at the surface (in red). OA and OLA refer respectively to oleic acid and oleylamine, the only ligands used in the synthesis. Note that the peak at ≈2300 cm -1 is due to CO2. d. Energy levels of HgSe after synthesis (OA-OLA) and after POM-SH grafting. The dot line corresponds to Fermi level which is used as reference at 0 eV. 1Se and 1 Pe correspond respectively to first and second level of conduction band; 1Sh corresponds to last level of valence band.

Figure 2 :

 2 Figure 2: XPS overviews of HgSe capped with OA/OLA (in black) and of HgSe capped with POM-SH (in red). The samples are deposited on gold substrate, which leads to the Au4f signal around 84 eV. b. W 4f core level of the POM-SH. Three doublets are necessary to fit this core level in the case of POM-SH capped HgSe, which are at lower binding energies so corresponding to lower oxidation states.

Figure 3 :

 3 Figure 3: Transport measurements on HgSe CQD thin films. a. Scheme of the field-effect transistor set-up. 41 In the following, the source is named S, the drain D and the gate G. b. FET measurement made on HgSe capped with EDT, VDS = 5 mV. c. FET measurement made on HgSe capped with POM-SH, VDS = 100 mV. d. Current as a function of decreasing temperature, VDS = 1 V. The solid curves correspond to raw data and the dot curves correspond to Arrhenius fit.

ACKNOWLEDEGMENTS

EL thanks the support ERC starting grant blackQD (grant n° 756225). We acknowledge the use of cleanroom facilities from the "Centrale de Proximité Paris-Centre". This work has been supported by the Region Ile-de-France in the framework of DIM Nano-K (grant dopQD). This work was supported by French state funds managed by the ANR within the Investissements d'Avenir programme under reference ANR-11-IDEX-0004-02, and more specifically within the framework of the Cluster of Excellence MATISSE and also by the grant Nanodose and H2DH.


Z Cansu Canbek

Robinson Cortes-Huerto

Fabienne Testard

Olivier Spalla

Simona Moldovan

Ovidiu Ersen

Andreas Wisnet

Guillaume Wang

Jacek Goniakowski

Claudine Noguera

Z Cansu Canbek

Nicolas Menguy

Twinned gold nanoparticles under growth: bipyramids shape controlled by environment

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

The unique physical and chemical properties of anisotropic metallic nanoparticles (NPs) have inspired researchers to improve the control over their shape and size distribution. 1,2 Today, beyond the several synthetic routes for anisotropic metallic NPs described in the literature, [3][4][5] it is still a challenge to optimize the yield for a given anisotropic shape and to fully explain the mechanisms promoting the anisotropy (in particular when considering the materials crystallizing in a FCC phase).

In noble metal NPs, the particular case of gold (Au) has drawn strong interest due to the plasmonic properties of Au NPs associated with their important potential applications in various fields (imaging, cancer therapy, electronic, detection, etc.). 6 The most classical way to control the anisotropic shape of Au NPs is the seed mediated approach, which separates the nucleation and growth stages in time. 2,7 This method involves the preparation of small precursors (seeds) in a first step, that grow into desired morphologies in a second step after their addition into a growth solution composed of complex chemical reagents. 1,7,8 The seeded growth method has been widely utilized to synthesize multiple geometries including nanorods, cubes, plates, spherical particles, bipyramids, etc. 4,9,10 with a main interest for nanorods.

It is only recently that bipyramids (first considered as a byproduct) appeared to be very promising due to the high plasmon enhancement properties of their sharp tips. 11 However, water synthetic processes are always polluted by other shapes with required postsynthetic purification. A better understanding of the reaction mechanism should help to improve the control over the final shapes.

Liu and Guyot-Sionnest 12 were the first to identify the crucial importance of the seed structure in seeded growth methods. From transmission electron microscopy (TEM) images, they compare the seed structure to their associated final shapes after growth in silver(I)-CTAB growth solution. Two populations were identified; single crystal seeds (produced in CTAB) with a preferential growth into single crystal nanorods, and multitwinned seeds (produced in citrate solution), which grew preferentially into bipyramids. Even if the resolution of the TEM observations was not sufficient to clearly identify the precise seed crystal structure, these experiments pointed out the importance of defects in the seeds prior to their growth.

The nature of the growth solution also plays an important role. For instance, previously mentioned citrate seeds (multitwinned), utilized to obtained bipyramids with a growth solution containing silver(I), could also form pentatwinned nanorods (with an extremely low yield) in the absence of added silver in the growth solution. 13 The anisotropic shape is clearly dictated by the complex mutual influence of the initial seed structure and the nature of the capping agent in the growth solution.

Concerning the precise role of the seeds, their polydispersity associated with their instability over time makes it difficult to define a clear roadmap to synthesize 100% of a desired anisotropic Au NP. The alternative seedless synthesis employed for the production of single crystalline gold nanorods also leads to a final state polluted by isotropic morphologies. 14,15 The only way to identify the precise role played by the seeds in the formation of anisotropy is to determine the crystal structure of the nanoparticles during their formation, even though this presents a real challenge. In 2010, Carbo-Argibay et al. 16 and Katz-Boon et al. 17 took advantage of the recent development of electron tomography to elucidate the precise crystal structure of the final gold nanorods obtained from CTAB capped seeds (single crystal). They evidenced the presence of high index facets on monocrystalline nanorods (obtained by the seed mediated method) in opposition to the structure of the pentatwinned nanorods (obtained by electrochemical method) described by Wang et al. in 1999. 8 Recently, Zhou et al. 18 utilized the high chemical and colloidal stability of large decahedral seeds (>25 nm) to follow their growth into bipyramids (D ≈ 73 nm, L ≈ 223 nm,) with good control over the shape and size. They have demonstrated that the bipyramids are derived from pentatwinned decahedra through a single step growth of the seeds. The use of such a strategy is, however, difficult to realize under classical conditions where the small size of seeds (<3-5 nm) limits their stability and characterization (particularly in CTAB water solution).

Furthermore, the structure of the bipyramids is still a matter of debate. Liu and Guyot-Sionnest 12 were the first to analyze the final bipyramids by high resolution transmission electronic microscopy (HRTEM) and selected area electronic diffraction (SAED). From analogies with copper pentatwinned nanorods, 5 they have stated that bipyramids (D ≈ 30 nm, L ≈ 110 nm, AR ≈ 3.6) bear penta-fold twinned structures around their growth axis with probably high order external side facets. Such a penta-fold twinned model for bipyramids has been supported by various groups. 12,19,20 Recently, Burgin and co-workers 21 have investigated the structure of Au bipyramids (D ≈ 36 nm, L ≈ 120 nm, AR ≈ 3.3) by electron tomography and revealed an irregular 6fold twinning structure with highly stepped, dominant {151} side facets without any detail concerning the type of twinning. This surprising hexagonal section is also mentioned by Yoo et al. 22 in their study on bipyramids obtained with silver(I)-assisted growth using cetyltrimethylammonium chloride (CTAC) and salicylic acid as surfactants. The versatility of the seeded growth synthesis and the difficulty in stabilizing the seed solution after their production (size < 3 nm) can explain the different structures found for the bipyramids and their precursors.

The goal of this present work is to report a full characterization of Au bipyramids and their associated formation mechanism, starting from initial isotropic decahedra (i-Dh). Here, combining multiple techniques, we have revisited the crystal growth of bipyramidal Au nanostructures. We have investigated the growth mechanism from seeds to bipyramids, employing state of the art of TEM (HRTEM and electron tomography) combined with a theoretical approach (based on an atomistic approach for metal environment interaction) in order to model the effect of the environment on the stabilization of anisotropic multitwinned nanoparticles.

EXPERIMENTAL SECTION

2.1. Materials. Cetyltrimethylammonium bromide (CTAB, 99%) was purchased from G-Biosciences; chloroauric acid (extra pure), silver nitrate (extra pure), sodium borohydride (98%), trisodium citrate, L- ascorbic acid (AA), and 5-bromosalycilic acid were obtained from Sigma. All these compounds were used without further purification. All glassware was cleaned with aqua regia and abundantly washed with distilled water before all the experiments.

2.2. Methods. 2.2.1. Chemical Synthesis of Au Bipyramids. 2.2.1.1. Synthesis of Seeds. Preparation of the so-called multitwinned seeds stabilized by citrate followed the pioneering work of Murphy's group. 1 Typical synthesis involves the addition of 0.3 mL of ice cold 0.01 M NaBH 4 to a mixture of 10 mL of 0.25 mM HAuCl 4 and 0.25 mM sodium citrate under vigorous stirring at 28 °C. The mixture color immediately turns to orange red. Citrate-stabilized seeds are aged for 3 h prior to use to ensure the full decomposition of excess borohydride in the solution.

2.2.1.2. Synthesis of Bipyramids. The chemical conditions for the reference synthesis are defined as follows: a growth solution is prepared by addition of 45 μL of 10 mM AgNO 3 and 100 μL of 0.05 M HAuCl 4 to a 10 mL solution of 0.1 M CTAB, followed by the addition of 75 μL of 0.1 M ascorbic acid. Finally, 400 μL of seed is added to the growth solution. The resulting mixture is gently mixed for 15 s and kept at 28 °C without disturbance overnight for the growth stage.

The final nanostructure solution is cleaned from excess reactants by two cycles of centrifugation (8694g, 40 min), and sediment is redispersed in pure water. The resulting solution is concentrated by a factor 10 through a final centrifugation to 1 mL.

Different synthetic conditions have been used to promote the anisotropy. First, two different AgNO 3 concentrations were used while keeping all the other conditions constant. Second, the pH of the growth solution was controlled by two different methods: (1) via addition of a small amount (100 μL) of 0.1 M HCl and (2) via the use of 5bromosalicylic acid (0.01M) with a lower CTAB, AA, and seed concentration (0.05 M, 0.25 mM, and 60 μL, respectively). All the chemical conditions used during preparation of different aspect ratio (AR) bipyramids are presented in the Supporting Information (Table S1 and section 1-ii).

2.2.2. Characterization. 2.2.2.1. UV-vis Spectroscopy. Absorption spectra of final particles were measured in a 1 mm Helma cell using a Shimadzu UV-2550 UV-vis spectrophotometer. Data were analyzed with UV Probe Software.

Transmission Electron Microscopy (TEM)

. Characterization of the nanoparticles was performed by TEM. Low resolution electron observations were carried out using a Philips CM 30 operating at 300 kV. An 8 μL drop of concentrated gold nanoparticle solution was dried on a carbon coated copper grid overnight before TEM analysis. HRTEM observations were achieved using two different microscopes: a JEOL 2100F microscope equipped with Gatan US4000 CCD camera operating at 200 kV (installed at IMPMC, Paris UPMC) and a JEOL ARM 200 F Cold FEG equipped with a CEOS Cs-image corrector operating at 200 kV and 80 kV (installed at MPQ-UniversitéParis Diderot, Paris France).

2.2.2.3. Electron Tomography. Tomography data were acquired in a JEOL 2100F transmission electron microscope with a field emission gun, equipped with a spherical aberration (Cs) probe corrector and operating at 200 kV (installed at ICPMS-Strasbourg, France). The acquisition of bright field (BF) and dark field (DF) tilt series was carried out simultaneously in the scanning mode (STEM), by using a circular detector for the BF and a high angle annular detector for the HAADF. A camera length of 10 cm was chosen for this experiment. It corresponds to inner and outer semiangles of 60 and 160 mrad, respectively, for the HAADF detector. A 100 μm condenser aperture was employed, allowing one to reach a probe diameter of about 0.12 nm with a current density of 0.5 pA•Å -2 . Under these conditions, the tomography series were acquired using the Digital Micrograph software (tomography plugin), giving access to an automatic increment of the tilt angles and a sharp control of the specimen drift and defocusing. A high tilt specimen holder from Gatan was employed for a tilting range of -65°to 65°, with an equal angular step of 2.5°. From analytical relations described by Midgley et al., 23 the tomogram resolution was estimated at about 0.16 nm in the direction perpendicular to the electron beam. For the direction of the electron beam, the resolution diminishes with a factor of 1.22, due to the missing wedge from data acquisition.

Once the series acquisition was completed and given the high contrasts achieved in the HAADF mode, a cross-correlation algorithm was sufficient to properly align the images from the series. The volume reconstruction was carried out by discrete algebraic reconstruction techniques (DART) [START_REF] Herman | Discrete Tomography: Foundations, Algorithms, and Applications Series: Applied and Numerical Harmonic Analysis[END_REF] using the Tomo J plugin under the Fiji (ImageJ) software. The same software was used for the volume visualization and analysis.

RESULTS

The as-prepared bipyramids were produced by the classical silver(I)-seeded growth method. [START_REF] Jana | [END_REF] Using the reference conditions given in section 2.2.1, a dispersion of bipyramids in coexistence with spheres (Supporting Information, Supp.Info-F8) was obtained. We retrieve the polymorphism generally obtained in bipyramid or nanograin syntheses based on the seeded growth method, a consequence of their high sensitivity to reshaping. 26,27 In the chosen synthetic method, the aspect ratios of the bipyramids were improved by (a) increasing the AgNO 3 concentration (Supporting Information, Supp.Info-F1), (b) adding 5-bromosalicylic acid which is classically used for nanorods, 28 employed here for the first time as an additive to CTAB to produce bipyramids (as shown in Figure 1 with TEM and UV-vis characterizations), (c) decreasing the pH 4,[START_REF] Jana | [END_REF] (Supporting Information, Supp.Info-F2), or (d) decreasing the volume of added seeds 4,29 (Supporting Information, Supp.Info-F2). In all these conditions, it appears that a rounding of the apexes is generally observed for small aspect ratio bipyramids.

3.1. Morphology of the Bipyramids. The morphology of the bipyramids has been obtained from the analysis of HRTEM images and SAED patterns. Bipyramids with large aspect ratios (AR ≥ 2) and sizes greater than 20 nm are most often observed with the elongation direction parallel to the carbon grid, that is, perpendicular to the electron beam (Figure 2, Supporting Information, Supp.Info-F3 and -F4). They are always characterized by a nonuniform contrast and the presence of planar defects. From the analysis of several nanoparticles, it appears that the large bipyramids are always twinned and only two main observation directions separated by 18°are accessible (Figure 2; Supporting Information, Supp.Info-F3 and -F4, for other orientations and bipyramids). For the particle shown in Figure 2 (D = 17.1 nm, L = 45.7 nm, AR = 2.7), two crystal orientations of the multitwinned bipyramid can be deduced from the fast Fourier transform (FFT) analysis, ⟨110⟩ and ⟨111⟩ (Figure 2b). The SAED pattern corresponds to a superimposition of three, ⟨111⟩, ⟨110⟩, and ⟨114⟩, zone axis diffraction patterns, which is consistent with the multiple twinning of a face-centered cubic structure (Figure 2c). This type of twinning, common in isotropic gold nanoparticles with decahedral (i-Dh) morphologies, [30][31][32][33] has also been observed for gold 34 and copper 5 nanorods. We can conclude that the large bipyramids are based on pentatwinned structures showing agreement with the initial TEM observations by Liu and Guyot-Sionnest. 12 In contrast, smaller bipyramids (size ≤ 20 nm) with smaller aspect ratios offer a wider range of orientations. Thus, this type of particle may be observed, as in Figure 3a, with an apparent 5-fold axis almost parallel to the electron beam (i.e., with an angular shift within the tilt capabilities of the double tilt specimen holder) or as in Figure 3b, exhibiting a similar orientation observed for large sized bipyramids (as in Figure 2). From the FFT analysis, a pentatwinned structure can be unambiguously attributed to low aspect ratio bipyramids (also called "nanograins") (D = 11.9 nm, L = 16.9 nm, AR = 1.4 for nanograin given in Figure 3b). Hence, such typical twinning is observed for different elongated nanoparticles with different aspect ratios going from nanograins (small aspect ratio) to bipyramids (large aspect ratio).

To completely assess the internal structure of the bipyramids described above, a simulated model of a pentatwinned bipyramid of ca. 3000 atoms exhibiting high index {11n} stepped side faces was produced. The different views (Supporting Information, Supp.Info-F6) from varied orientations reproduce the observed TEM images of bipyramids. The variation in the area of the twinning zone along the center of the particles results from the contribution of the different crystals oriented along the horizontal axis (experimental differences are visible from comparison between Figure 2a and Supp.Info-F3a and -F4a, Supporting Information).

Further observations were achieved by STEM-HAADF electron tomography on a large size bipyramid. Figure 4 shows the 3-D morphology reconstructed from DART algorithms. [START_REF] Herman | Discrete Tomography: Foundations, Algorithms, and Applications Series: Applied and Numerical Harmonic Analysis[END_REF] The bipyramid (D = 18 nm, L = 64 nm, AR = 3.5) presents rounded tips with an angle of 27°between the side edges. Our results differ significantly from those obtained by Burgin et al. 21 who observed a hexagonal section. In our case, despite the fact that some angles may be rounded due to the missing wedge, a clear pentagonal section can be extracted.

Characterization of Seeds and Small

Size Nanoparticles. The low concentration of seeds associated with their small size make their shape and structure analysis quite difficult. Contrary to the Turkevich method known to produce stable isolated nanoparticles, 35 the small nanoparticles produced by borohydride addition are quite unstable. Combinations of DLS, SAXS, and UV-vis spectroscopy (Supporting Information, Supp.Info-F7), used on a freshly prepared solution, evidence a large size distribution (σ/R = 0.37) of NPs with an average particle size of 5 nm. Further attempts for direct TEM analysis on these NPs lead to the formation of large sized aggregates on the TEM support. To overcome such a problem, we have imaged the residual seeds unaffected by the reduction of additional metal salt in the growth solution (Supporting Information, Supp.Info-F8) 36 by assuming that these small particles represent the seeds after their addition to CTAB solution.

A HRTEM image of sub-5 nm pentatwinned particles found in the final solution after growth is shown in Figure 5a. The particle is constituted in such a way that five individual crystals oriented along a common ⟨110⟩ direction (parallel to the electron beam) exhibit 5-fold symmetry. Similarly, a small, multitwinned nanoparticle shown in Figure 5b exhibits the same structural feature as the one described for large and small aspect ratio bipyramids, that is, adjacent subunits with the same relative crystallographic orientation (Figures 2 and3).

It is remarkable that for all the different sizes of nanoparticles representative of the particles in formation, HRTEM images evidence pentatwinned structure either for initial isotropic seeds or for elongated shapes (nanograins and bipyramids).

DISCUSSION

From HRTEM and SAED results, the structure of gold bipyramids obtained by silver(I)-assisted seeded growth processes have been determined. Analyses have been conducted on a final sample containing different sizes of representative growing nanoparticles. A pentatwinned structure was determined for both highly elongated bipyramids with sizes largely greater than 5 nm and for the smallest apparently isotropic nanoparticles with sizes below 5 nm.

Previously, Zhou et al. 18 have shown strong evidence of growth of big decahedral seeds (25-49 nm) into bipyramids with the conservation of 5-fold twinning. However, for smaller size seeds, such direct evidence of conservation of the pentatwinned structure during growth of bipyramids was not obtained due to size limitations.

For considerably small sizes (<5 nm), the isotropic decahedral (i-Dh) particles are known to be most stable in vacuum and have been largely described in literature. 32,[37][38][39] However, in solution containing surface ligands, experimental evidence of this structure is more tricky. 12,40,41 The use of low voltage (80 kV) during HRTEM observations allowed us to explore the decahedral structure of the small size seeds added in silver(I)assisted synthesis of bipyramids, even in the presence of CTAB surfactant. From these experiments, we show the conservation of the pentatwinned structure during the growth of seeds into bipyramids.

It is known that for a face-centered cubic structure, the only way to obtain elongated shapes is to break the symmetry by incorporating twinning defects or stacking faults into the nanocrystal. 36,42 For nanoparticles in solution, the ligands can promote this process by inverting the energy of different facets through selective adsorption. In the CTAB silver(I) solution, small, elongated nanoparticles bearing pentatwinned structures with extended {100} facets are observed in comparison to the initial i-Dh structure of seeds (Figure 5, see Supporting Information Supp.Info-F9 and -F10 for details). There have been theoretical attempts to account for the modification of surface energies of certain facets on gold NPs in the presence of surfactants and solution ions. Almora-Barrios et al. 43 have analyzed the role of silver on gold surface stability, in relation to experimental features. The promotion of the monocrystalline nanorod anisotropy experimentally results from the combination of face-capping by the CTA + /[AgBr 2 ] - ions 15,44 (hereafter called CTASB) and the stabilization effect of the CTAB/CTASB surfactant double layer. 45 Using density functional theory simulations, they demonstrate the preferential adsorption of the CTASB complex on open {100} facets on a gold nanorod surface in comparison to {111} facets, and its strong stabilizing effect for higher index {520} facets presented on the lateral sides of the monocrystalline nanorods. Beyond arguments on surface energies by the use of a simplified atomistic model of CTAB, Grochola et al. 46 have simulated the adsorption of the surfactant on gold nanoparticles to explain the growth of nanorods by preferential adsorption on chosen facets.

At this stage, in order to account for multitwinned nanorod or bipyramid stability, arguments based solely on surface energies (and deduced Wulff shapes) turn out to be insufficient. On the one hand, for small size NPs that are in the first stages of growth, besides surface energies, the energy contributions of edges and corners are important. On the other hand, the presence of twinning planes and of a 5-fold symmetry axis destabilizes the core of the particles as their size increases. This energy term is an important contribution, absent in monocrystalline particles.

Conversely, stability of the multitwinned gold nanorods or gold bipyramids in a complex environment can be qualitatively accounted for by using the atomistic approach for metalenvironment interaction that we have recently developed. 47 The total energy (E) of a NP is expressed as the sum of many-body gold-gold and gold-environment interactions. The former interaction is given by the second-moment approximation (SMA) to the tight binding Hamiltonian, [START_REF] Pettifor | Bonding and Structure of Molecules and Solids[END_REF] and the goldenvironment interaction is the sum of all under-coordinated atoms i of the form: eq 1.

= -ϵ -

- E Z Z ( ) i i p M E B (1)
The difference Z B -Z i between the coordination number Z B in the bulk (Z B = 12 for gold fcc) and the Z i of an atom i is equal to the number of free bonds available to bind to species in the environment. The pair of positive parameters (ϵ, p) defines the strength and the nature of the gold-environment interaction. 47,[START_REF] Cortes-Huerto | [END_REF] In order to focus on gold pentatwinned nanostructures, a motif optimization technique 50,51 was used to generate families of closed-shell NPs in the size range 2500 < N < 3500 atoms, terminated by different proportions of {111}, {100}, {110}, {311}, and {210} facets. For each initial configuration, quenched molecular dynamics runs were performed under conditions that preserve their shape and symmetry. In line with the conclusions of our recent study, [START_REF] Cortes-Huerto | [END_REF] we have found two sets of parameters (ϵ, p) for which the reduction of NP surface and edge energies due to the interaction with the environment diminishes drastically the stability gap between elongated particles and the isotropic decahedral seeds (i-Dh).

Figure 6 displays the evolution of the excess energy of the most stable particles with respect to an equal number N of bulk atoms (energy E coh ) defined as

Δ = - N E N NE N ( ) ( ) NP coh 2/3 (2)
in vacuum and in environments A (ϵ = 0.05 eV/atom, p = 1.5) and B (ϵ = 0.022 eV/atom, p = 2.0), as well as their equilibrium shapes. Environments A and B favor {100} and high order {210} facets, respectively, which qualitatively mimics ab initio results of Almora-Barrios et al. 43 for CTAB and CTASB adsorption on gold, respectively. Even though the strength, ϵ, of the goldenvironment interaction is smaller in environment B, it is more strongly coordination dependent.

In that way, the weight of the edges over facets is higher in environment B than in A.

In environment A, the NP has an aspect ratio of 2.2, with fairly long lateral {100} facets and relatively small tips bordered by five {110} facets. Their shape is that of pentatwinned nanorods, similar to those observed by Johnson and co-workers, 34 with largely extended {100} side facets. The pentatwinned nanorods bearing large {100} facets can experimentally be obtained in a very low yield (<1%) by the growth of citrate seeds in the absence of silver nitrate. 8,[START_REF] Jana | [END_REF] Such low yield can be explained by the fact that only a few seeds developing small {100} facets in relatively small sizes can be transformed into the small nanorods. As soon as the anisotropy is developed, gold nanorods continue to grow by increasing their {100} surfaces of the lateral facets by an autocatalytic process. 34 In this case, higher index facets are not stabilized and the transition of small nanorods into bipyramids cannot be observed.

In environment B, particles with an aspect ratio of 1.9 are stabilized. At their tips, these NPs expose {210} facets, representative of higher index facets. These pentatwinned NPs resemble the so-called nanograins (low AR bipyramids, shown in Figure 3 and Supporting Information Supp.Info-F5) obtained in experiments based on silver-(I)-assisted growth of citrate seeds (i.e., when CTASB (CTA + /[AgBr 2 ] -) is present in the growth solution). In this case, higher facets are stabilized, and bipyramid shapes are favored. We have seen experimentally that nanograins and bipyramids bear the same internal structure (Figures 2 and3). We can thus assume that as soon as nanograins are formed, further monomer feeding can proceed along twinning axis along the tip of the nanoparticles (due to less compact surfactant layers 29 ) leading to an increase in the surface of high index facets (which can be stabilized by the CTASB). The nanograins then can continue to evolve into higher aspect ratio bipyramids.

The two growth mechanisms are summarized in Figure 7, highlighting the growth of decahedral isotropic seeds into either gold pentatwinned nanorods without silver in the CTAB growth solution or bipyramids when silver is present. This schematic representation illustrates the inherited structure of the final nanoparticles from initial seeds as a function of the added ligand to the growth solution (i.e. high index versus low index facet stabilization by external ligands).

CONCLUSION

From a synthetic aspect, we have demonstrated that 5bromosalicylic acid, which is known to promote anisotropy in gold nanorod synthesis, can also be used as an additive in a CTAB environment to promote anisotropy of bipyramids. From a structural aspect, by HRTEM and electron tomography, the internal structure of the bipyramids was determined and definitively attributed to a pentatwinned structure. Even though this structure was initially proposed in the literature, 12 recent tomographic studies have opened the way to discuss another possible hexagonal symmetry for the bipyramid structure. 21 Here, we have ruled out this option for the gold bipyramids synthesized in CTAB solution from a silver(I)-assisted growth process. Through a complete HRTEM, SAED, and tomography study, we have also investigated different size NPs from initial decahedral seeds to the final bipyramidal shape. The pentatwinned structure was demonstrated for all these intermediate nanoparticles. Our experimental approach, combined with an atomistic model and taking into account the environment of the gold nanoparticle, has indeed shown that different kinds of elongated nanoparticles (either nanorods or bipyramids) can be stabilized by changing the nature of the ligand used during the synthesis. This consolidates the importance of a fine-tuning of surface chemistry for the control of (exotic) shaped nanoparticles.

ASSOCIATED CONTENT

. Formation mechanism of bipyramids and nanorods. In the present case, the bipyramids are formed by a silver(I)-assisted process in CTAB solution and deduced from HRTEM; pentatwinned structured is maintained along the growth. For pentatwinned nanorod formation, they are obtained without silver from decahedra seeds in a very low yield.

Figure 1 .

 1 Figure 1. (a) TEM image of Au bipyramids synthesized by addition of 60 μL of seed solution into growth solution including 5-bromosalicylic acid (AR ≈ 2.7) (TableS1, no. 3 experimental conditions detailed in Supporting Information, section 1-ii) and (b) corresponding UV-vis spectra of the solution.

Figure 2 .

 2 Figure 2. (a) HRTEM observation of a large bipyramid with elongation direction perpendicular to the electron beam. (b) In this orientation, crystals located on the sides of the bipyramid have different orientations: ⟨110⟩ and ⟨111⟩ zone axis deduced from FFT analysis. (c) This result can be interpreted in agreement with a pentatwinned structure of the bipyramid. (d) The corresponding SAED pattern may be analyzed as the superimposition of ⟨111⟩, ⟨110⟩, and ⟨114⟩ zone axis diffraction pattern.

Figure 3 .

 3 Figure 3. (a) Structural analysis of intermediate sized bipyramids (nanograins) and HRTEM image of the pentatwinned particle, constituting five individual crystals oriented along a common ⟨110⟩ direction (parallel to the electron beam), exhibiting a 5-fold symmetry. The overall fast Fourier transform can be interpreted as the superimposition of five ⟨110⟩ zone axis diffraction patterns (the color of the calculated diffraction pattern is related to the corresponding labeled crystal). (b) HRTEM image of a multitwinned particle constituting at least two crystals oriented along a common ⟨110⟩ direction (perpendicular to the electron beam). The FFT corresponding to the left part of the particle is related to a distorted ⟨110⟩ zone axis diffraction pattern. The white arrows indicate supplementary 220-type reflections likely due to the adjacent grains. The FFT corresponding to the right part of the particle is related to a ⟨111⟩ zone axis diffraction pattern. Red and green arrows indicate supplementary reflections related to upper and lower first order Laue zone for the [111] zone axis.

Figure 4 .

 4 Figure 4. (a) Figure e-tomo 3D volume of a bipyramid obtained from a DART reconstruction procedure and (b) the bipyramid viewed along the elongation direction. A clear pentagonal section is evidenced.

Figure 5 .

 5 Figure 5. Structural analysis of twinned seeds. (a) Ultra-HRTEM image of a 3.3 nm particle with FFT analysis of selected areas. (b) Ultra-HRTEM image of a 2.7 nm seed particle with different orientation.

Figure 6 .

 6 Figure 6. (top) Excess surface energies of the most stable elongated pentatwinned gold particles with respect to the isotropic decahedral (i-Dh) ones Δ -Δ Dh (eV/N 2/3 ). (bottom) i-Dh, type I, and type II particles corresponding, respectively, to the most stable structures in vacuum and environments A and B, created with an average of 3000 gold atoms per particle.

ACKNOWLEDGMENTS

The authors thank Olivier Tachéfor the 3D printed nanoparticles used in the graphical abstract. The authors acknowledge the French CNRS (Centre National de la Recherche Scinetifique) (Grant FR3507) and CEA (Commissariat al 'Energie Atomique et aux Energies Alternatives) METSA (Microscopie Electronique et Sonde Atomique) network. The authors acknowledge the financial support from Agence Nationale de la Recherche (ANR) under Contract ANR-11_BS10_006. This work has been supported by the Region Ile-de-France in the framework of DIM C'Nano IdF.

Supporting Information

Further chemical conditions used to synthesize varied aspect ratio bipyramids with additional diffraction analysis and a summary of the evolution of structure is provided. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b00121.

AUTHOR INFORMATION

Corresponding Author *E-mail: fabienne.testard@cea.fr.

Notes

The authors declare no competing financial interest.