Jeanne Fichez

Cathia Soulié

Laurent Le Corre

Sophie Sayon

Stéphane Priet

Karine Alvarez

Olivier Delelis

Patrick Gizzi

Guillaume Prestat

Christine Gravier-Pelletier

Cathia Soulie

Anne- Geneviève Marcelin

Vincent Calvez

Patricia Busca

Discovery, SAR study and ADME properties of methyl 4-amino-3-cyano-1-(2-benzyloxyphenyl)-1 H -pyrazole-5-carboxylate as an HIV-1 replication inhibitor

Keywords: general procedures, compounds characterization, 1

come

Introduction

Human immunodeficiency virus (HIV) and the resulting acquired immunodeficiency syndrome (AIDS) still represent a major global public health threat. According to the joint united nations program on HIV and AIDS (UNAIDS), since the beginning of the epidemic, more than three decades ago, 77.3 million people have become infected with HIV and 35.4 million of them have died from AIDS-related illnesses. 1 Highly active anti-retroviral therapy (HAART), introduced in 1996, led to a significant decrease of AIDS-related deaths by 38% and helped to save 11.4 million lives. 2 HAART has also improved the life expectancy of HIV/AIDS patients which nowadays is approaching that of the general population. 3 However, HAART is still not curative, [START_REF] Deeks | International AIDS Society global scientific strategy: towards an HIV cure 2016[END_REF] and this life-long treatment suffers from several pitfalls such as inherent drug toxicity, 5 emergence or transmission of drug-resistant HIV-1 strains, 6,7,8 and the decline in patient adherence. 9 In response to these concerns, a global effort is required to pursue the development of novel and potent anti-HIV-1 drugs, which represents a continuing challenge. 10 In this context, prompted by the promising antiretroviral activities of several pyrazole-based inhibitors such as PNU-32945 I, 11 Lersivirine II 12 and benzylamino derivatives III 13 (Figure 1), we decided to screen a pyrazole-based chemical library, which was previously developed in our laboratory. 14,15 We report herein the discovery of new pyrazolic HIV-1 inhibitors, their synthesis, biological activities and ADMET properties. Interestingly, the methyl 4-amino-3-cyano-1-(2benzyloxyphenyl)-1H-pyrazole-5-carboxylate that was selected as a lead compound does not inhibit any of the viral enzymes that are classically targeted in antiviral therapy.

Results and discussion

Discovery of the hit compounds through a double screening

Our in-house chemical library, which was originally designed to target a kinase domain, is composed of 110 pyrazole-based molecules that are well balanced between four heteroaromatic collections: aminopyrazoles A, pyrazolopyridines B, pyrazolopyrimidines C and pyrazolooxadiazoles D (Figure 2). Interestingly, while compounds from families C and D behave like kinase inhibitors, those from series A and B are not active against this kind of target. 16 These compounds were tested for their potential inhibitory activity against HIV-1 replication in cell cultures. This primary screening was carried out using HeLa P4 cells containing a stably integrated LTR linked to a Lac Z reporter gene. These cells were infected with HIV-1 Laï viruses, and were incubated 48 h with 50 µM of each compound, using zidovudine as positive control (see ESI). This kind of phenotypic assay was chosen because it allows for several steps of the HIV replication cycle to be evaluated at once. In parallel, the cytotoxicity of each compound was evaluated in the same culture conditions according to the MTS standard protocol (see ESI). This test is also necessary to assess the relevancy of inhibition values. After this first screening at 50 µM, none of the 24 pyrazolopyrimidines C nor the 27 pyrazolo-oxadiazoles D showed enough activity to be selected (the full table of results can be found in ESI). However, three compounds displayed an inhibition activity higher than 50%, and a toxicity value lower than 15%: aminopyrazoles A. 20 and A.21, as well as pyrazolopyridine B.7 (Figure 3). Interestingly, the benzyloxy group selected twice with A. 20 and A.21 is likely to be part of the pharmacophoric requirements.

Dose-dependent assay

To further validate these hits, and to rule out the possibility of false positives, the inhibition assay was repeated in a dosedependent manner using a 1.0 to 50 µM concentration range of inhibitor (see ESI). The resulting dose-response curves allowed for the concentration at which 50% of the replication of HIV is inhibited to be determined (EC50 value). These experiments revealed that A.20 and A.21 are able to inhibit the HIV replication in a dose-dependent manner. On the contrary, it was found that B.7 is likely to be a false positive because its apparent inhibition potency is related to its toxicity and not to its activity; and therefore it was discarded. Encouragingly, the EC50 of A.20 and A.21 were found to be 19.4 µM and 15.2 µM respectively, which are values that are comparable to the 13.3 µM EC50 of tenofovir (TDF, clinically used inhibitor) which we determined under the same conditions. 17 Hence, A. 20 and A. 21 were validated as hits and selected for the next step of biological evaluation.

Multiple-round infection assay

With the aim to ascertain their antiviral potency, the validated hits were then evaluated in different cell-types, with another source of the virus, and also incubated for a longer amount of time. For this multiple-round infection assay, human lymphoblastoid MT2 cell line were incubated for 3 days with Laï virus at MOI 3 and the antiviral effect of A.20 and A.21 was monitored at a concentration range from 0 to 50 µM using zidovudine as a positive control (see ESI). According to these results, A. 20 and A.21 are undoubtedly efficient to counteract HIV-1 replication, even at this high concentration of virus. To further develop this series of aminopyrazoles, the synthesis of analogous compounds was therefore undertaken.

Chemical expansion of A series

To study and verify the impact of substituents on the phenyl ring of pyrazoles A, new analogues were synthesized according to the two-step strategy which we previously developed (Table 1). 14,15 Starting from commercially available anilines I.1-28, firstly a diazotization reaction with sodium nitrite in aqueous hydrochloric acid was performed. This was immediately followed by condensation with malononitrile in basic medium to afford the corresponding dicyanohydrazones II.1-28 in good to excellent yields. The second step is a Thorpe-Ziegler reaction which was performed with methyl bromoacetate and potassium carbonate in dioxane under microwave irradiation. Desired aminopyrazoles A.1-28 were obtained in moderate to good yields ranging from 16 to 83% over two steps. Thanks to this synthetic effort, we obtained an homogeneous series of aminopyrazoles with comparable substitution patterns which are suitable for structure-activity relationship study.

Structure-activity relationship (SAR)

To draw out the SAR pattern, inhibition and toxicity values of the previously and newly synthesized pyrazoles were gathered in the following table (Table 2). For the sake of clarity, we used a minimum threshold of 30% for inhibition, and a maximum one of 15% for toxicity. Indeed, beyond this value, the cells mortality distorts the value of inhibition significantly. The application of this double filter revealed 11 interesting compounds (highlighted in Table 2).

Please do not adjust margins

Please do not adjust margins First of all, the activity of N-phenyl-pyrazole A.1 (without any substituents) remains below the critical threshold of 30% inhibition. When compared to the activity of hits A. 20 and A.21 (63% and 65% respectively), this low value supports our hypothesis regarding the key role that might be played by the benzyloxy group. Then, nine compounds which show good inhibition are all sharing the common feature of bearing electron withdrawing groups such as NO2 (A.2-4), Br (A.5-6), Cl (A.8), F (A.12), or CO2tBu (A. 18-19). Beside pure electronic effect on the phenyl ring, these groups could also be involved in non-covalent interactions such as hydrogen or halogenbonding. Finally, none of the newly synthesized pyrazoles were found to be more efficient; with A.20 and A.21, bearing a benzyloxy substituent, being the best inhibitors within this series. When compared to their methyloxy counterparts (A. 23-24), these hits are at least twice as active, suggesting that the benzyl moiety is likely to interact with a hydrophobic pocket. These observations are summarized in the following figure (Figure 4). As no other compound appeared to be as potent as A. 20 and A.21, these two hits were further evaluated in order to assess their drugability.

ADME properties

A full pharmacokinetic profile has been established for A. 20 and A.21. The overall results are gathered in Table 3.

Although the structures of A. 20

Identification of the potential target of hit A.20

At present, 30 drugs have been officially approved for the treatment of HIV/AIDS. 18 These molecules target essential steps or viral enzymes that are involved in the HIV replication cycle: CCR5 co-receptor, 19 viral fusion, 20 reverse transcriptase (RT), 21 integrase (IN) 22,23 and protease (PR). 24 It is noteworthy to say that, the current WHO preferred first-line regimens predominantly consist of RT inhibitors (RTI), in some instances associated with PR or IN strand-transfer inhibitors (PRI or INSTI). [START_REF]Department of Health and Human Services -Guidelines for the Use of Antiretroviral Agents in Adults and Adolescents Living with HIV[END_REF][START_REF]World Health Organization -The Public Health Response to Pretreatment HIV drug resistance[END_REF] However, despite tremendous efforts to develop new therapeutic agents, no novel target exploitation is to be expected from the few inhibitors in clinical trials at the moment. 31 In this context, to explore the mechanism of action of A.20, we tried to discover its biological target. Since A.20 was selected through a screening assay that cannot detect PR inhibitors (one single cycle of HIV replication does not allow to detect post-integrative steps inhibitors), no further investigation was performed on this target. We therefore focused our study on RT and IN strand transfer (INST) activities.

To evaluate the inhibitory activity of compound A.20 on the RT of HIV-1, its relative efficiency of incorporation was measured using subtype B WT HIV-1 RT in an in vitro susceptibility assay, using nevirapin and AZT as positive controls (see ESI). Under these conditions, A.20 appeared to be completely ineffective toward RT (IC50 > 1000).

To check the propensity of A.20 to inhibit the reaction mediated by IN, strand transfer assays were carried out with increasing concentrations of inhibitors, from 100 nM to 333 µM, using raltegravir as positive control (see ESI). Whatever the concentration used, no effect on the strand transfer reactions efficiency was observed, thereby demonstrating that A.20 does not impact the catalytic properties of INST at all. From these results, we can conclude that our lead compound, A.20, does not belong to the three main classes of anti-HIV drugs (RTI, PI and INSTI), and thus is likely to inhibit an underexploited or unknown target.

Conclusion

It is still of key importance to develop new inhibitors of HIV replication in order to counteract the rapid and unavoidable virus mutations, with a preference for those which are orthogonal to the current drugs. In this study, we identified two promising hits A.20 and A.21 with interesting EC50 values, similar to that of tenofovir. Further multiple round infection assays confirmed their inhibitory activity on long term incubation. Based on the same aminopyrazole scaffold, a series of derivatives was synthesized to draw an SAR study that ascertained A.20 and A.21 as hits, and the OBn substituent as an essential group for activity. ADME evaluations allowed to select A.20 as the better lead compound. The final part of our study revealed that A.20 does not behave as usual anti-HIV drugs (RTI, PI and INSTI) and thus belongs to the sought-out category of inhibitors which could advantageously slow down the development of resistance. This study provided us with A.20 as a validated lead compound, and a solid SAR basis to design and develop more potent inhibitors. Moreover, fostered by the complete lack of activity on the classical targets, further studies will be dedicated to uncover its mechanism of action.

Figure 1 .Figure 2 .

 12 Figure 1. Structure of known pyrazole-based HIV inhibitors

Figure 3 .

 3 Figure 3. Structures of the three selected hits and biological results @50 µM

Figure 4 .

 4 Figure 4. General trends resulting from SAR analysis

Table 1 .

 1 Two-step synthesis of aminopyrazoles A.

1-28 a

	Compound	-R	Overall yield (%)
	A.1	H	69
	A.2	2-NO2	38
	A.3	3-NO2	48
	A.4	4-NO2	16
	A.5 b	2-Br	35
	A.6	3-Br	56
	A.7	4-Br	44
	A.8 b	2-Cl	35
	A.9	3-Cl	51
	A.10	4-Cl	38
	A.11 b	2-F	48
	A.12	3-F	32
	A.13	4-F	38
	A.14 b	2-I	44
	A.15 b	3-CF3	82
	A.16 b	4-CF3	83
	A.17 b	2-CO2tBu	35
	A.18	3-CO2tBu	68
	A.19	4-CO2tBu	42
	A.20	2-OBn	42
	A.21	3-OBn	61
	A.22 b	4-OBn	18
	A.23 b	2-OMe	50
	A.24	3-OMe	73
	A.25 b	4-OMe	22
	A.26 b	2-CCH	31
	A.27	3-CCH	77
	A.28	4-CCH	44

a Yields are calculated over two steps. b Hydrazones II.x and pyrazoles A.x are described in the ESI

Table 2 .

 2 In vitro evaluation of inhibitory activity and toxicity of aminopyrazoles A.1-28 a

	Compound	-R	Inhibition (%)	Toxicity (%)
	A.1	H	<30	7.0
	A.2	2-NO2	42	0
	A.3	3-NO2	47	0
	A.4	4-NO2	43	0
	A.5	2-Br	32	4.8
	A.6	3-Br	42	9.0
	A.7	4-Br	67	>15
	A.8	2-Cl	30	2,4
	A.9	3-Cl	<30	11,0
	A.10	4-Cl	56	>15
	A.11	2-F	<30	3,6
	A.12	3-F	48	0.0
	A.13	4-F	<30	6.0
	A.14	2-I	<30	>15
	A.15	3-CF3	<30	>15
	A.16	4-CF3	<30	14
	A.17	2-CO2tBu	<30	2.8
	A.18	3-CO2tBu	36	0
	A.19	4-CO2tBu	47	14,0
	A.20	2-OBn	63	9.0
	A.21	3-OBn	65	11.0
	A.22	4-OBn	<30	3.8
	A.23	2-OMe	<30	9.2
	A.24	3-OMe	<30	6.0
	A.25	4-OMe	<30	14.6
	A.26	2-CCH	<30	7.0
	A.27	3-CCH	<30	14.0
	A.28	4-CCH	nd	nd

a Data represent mean values of two experiments in duplicate.

  Being slightly less lipophilic, A.20 is ten times more soluble; it displays better membrane permeability by passive diffusion. As the compound doesn't contain any hydrolysable group, its plasma stability is excellent. Its low metabolic stability is offset by a high level of protein binding, which should ensure good availability over time. According to these results, A.20 has the best drugability potential, therefore we decided to pursue our study with this compound.

and A.21 are very similar, A.20 shows a better overall profile than A.21.

Table 3 .

 3 ADME properties of hits A.20 and A.21

	ADME properties

Acknowledgements

The author thanks the "Agence Nationale de Recherches sur le Sida" for support (ANRS, ref ECTZ20422), the "Ministère de l'Enseignement supérieur, de la Recherche et de l'Innovation" for J.F. PhD grant and the UMR 8601 CNRS, Université Paris Descartes for HRMS and NMR facilities.

Conflicts of interest

There are no conflicts to declare.


Federico Panciera

Zhaslan Baraissov

Gilles Patriarche

Vladimir G Dubrovskii

Frank Glas

Laurent Travers

Utkur Mirsaidov

Jean-Christophe Christophe Harmand

Phase Selection in Self-catalyzed GaAs Nanowires

Keywords: nanowires, in situ TEM, crystal growth, polytypism, crystal phase

come

by changing the group III and V fluxes. The zincblende phase forms at small (< 100 o ) and large (> 125 o ) contact angles, while pure wurtzite phase is observed for intermediate contact angles. Wurtzite nanowires are restricted by vertical sidewalls, while zincblende nanowires taper or develop the truncated edge at their top. These findings are explained within a dedicated model for surface energetics. These results give a clear route for the crystal phase control in Au-free III-V nanowires.

On a more general note, in situ growth monitoring with atomic resolution and at the technologicalrelevant growth rates is shown to be a powerful tool for the fine-tuning of material properties at the nanoscale.

Growth of III-V semiconductor nanowires (NWs) using the vapor-liquid-solid (VLS) method can result in the crystal structures different from their bulk phase. 1,2,3 In GaAs NWs, for example, stable zincblende (ZB) phase coexists with metastable wurtzite (WZ) structure. 4 Due to a relatively small difference in cohesive energy of the two phases, a modest change in the growth conditions is sufficient to switch from one phase to the other. The formation probabilities of the two phases are often close to each other, resulting in NWs having a mixed-phase structure and stacking faults.

Controlling the crystal phase purity is critical for technological applications, because different crystal structures of the same chemical compound present different electronic, photonic or phononic properties, with discontinuities at their common interfaces. At the same time, controlled switching between the two phases enables synthesis of novel heterostructures with different electronic and optoelectronic properties in each phase. 5,6 Remarkably, the valence and conduction bands of the two phases are misaligned, so that small sections of one phase within the other effectively confine charge carriers. In contrast to compositional heterojunctions, crystal phase heterostructures have intrinsically abrupt interfaces and hence do not suffer from the alloy intermixing at the interface. This feature has been proven crucial for the fabrication of crystalphase quantum dots with exceptional properties. 7,8,9,[START_REF] Assali | Crystal phase quantum well emission with digital control[END_REF] Despite the compelling need for phase control, the lack of fundamental understanding of the governing mechanisms prevents growing these nanostructures reliably. Several models have been proposed to date to explain the crystal phase switching. Even though the suggested mechanisms differ, there is a general consensus that the phase selection occurs at the moment of nucleation of each new monolayer (ML). [START_REF] Glas | Why does form in nanowires of III-V zinc blende semiconductors?[END_REF] It has also been highlighted that the key parameter that determines the choice of the phase is the contact angle between the droplet and the crystal (hereafter referred to as "the contact angle"). [START_REF] Glas | Why does form in nanowires of III-V zinc blende semiconductors?[END_REF][START_REF] Jacobsson | Interface dynamics and crystal phase switching in GaAs nanowires[END_REF][START_REF] Dastjerdi | Methods of Ga droplet consumption for improved GaAs nanowire solar cell efficiency[END_REF] A change in the contact angle alters the balance of capillary forces at the triple-phase line (TPL). It may thus render the nucleation site more favorable for one phase with respect to the other. Unfortunately, despite the evident importance of controlling the contact angle, this parameter cannot be monitored or directly adjusted during growth in a standard reactor.

Only the experiments conducted by in situ transmission electron microscopy (TEM) [START_REF] Jacobsson | Interface dynamics and crystal phase switching in GaAs nanowires[END_REF][START_REF] Panciera | Controlling nanowire growth through electric field-induced deformation of the catalyst droplet[END_REF][START_REF] Harmand | Atomic step flow on a nanofacet[END_REF] helped to shed light on the relationship between the growth parameters and the contact angle and provided valuable information for developing the droplet engineering strategies. However, these experiments treated Au-catalysed nanowires, not desirable for integration with silicon platform, and grown at rates much lower than in the real epitaxy systems.

Here, we present direct observations of the growth of self-catalyzed GaAs nanowires by molecular beam epitaxy (MBE) using in situ TEM. We chose to study self-catalyzed III-V NWs, as opposed to Au-catalysed NWs 12 due to their higher technological relevance and because this growth mode gives access to the full range of contact angles. Indeed, the droplet size can be continuously decreased until total consumption by supplying an excess of group V element. This property allows us to monitor the change of crystal phase over a wide range of contact angles and discover a mechanism for the phase transition that occurs at small angles. We show that multiple mechanisms are involved in the phase selection and develop a theoretical model to explain the observed behavior.

GaAs NWs were grown using specially designed MBE sources directly fitted on the microscope. [START_REF] Harmand | Atomic step flow on a nanofacet[END_REF] The material fluxes were adjusted to obtain an average growth rate on the order of 1 ML/s, which is the typical MBE growth speed in standard systems (see Methods). By changing the ratio of Ga and As fluxes, we deliberately modified the size of the catalyst droplet and hence the contact angle. Starting from a large droplet and gradually decreasing the droplet size, the first transition from ZB to WZ is observed at a contact angle of ~125°. Further decrease of the droplet size leads to the second transition, from WZ to ZB, at a contact angle of ~100°.

We systematically observe three distinct regimes that are characterized by different morphologies and the formation of a specific phase. Large contact angles result in the ZB phase, where the formation of each new ML is quasi-instantaneous and the edge truncation is present at the TPL (Figure 1a). At moderate contact angles below 125°, the WZ phase forms and the edge truncation is absent. The ML nucleates at the TPL and extends across the NW/droplet interface through a slow step flow (Figure 1b). For small contact angles below ~100°, the ZB phase forms again. This second phase transition was predicted earlier to occur at contact angles well below 90°, [START_REF] Jacobsson | Interface dynamics and crystal phase switching in GaAs nanowires[END_REF] but this was never confirmed experimentally. Remarkably, our observation reveals an important similarity between the growths of GaAs NWs at moderate (between 100° and 125°) and small (<100°) contact angles. In both cases, no edge truncation is observed in NWs, the ML nucleation occurs at the TPL, and lateral spreading of the ML is slow (Figure 1c). The two latter features are observed independently of the crystal phase, and hence must be entirely related to the morphology of the TPL. In our conditions, the amount of As from the liquid phase is not sufficient to form the entire ML, [START_REF] Harmand | Atomic step flow on a nanofacet[END_REF] thus the missing As must be supplied either from vapor or solid phase. If

As is supplied from vapour, the growth duration of the full ML is limited by the As refill from vapor, which is relatively slow. This must be the case for moderate and small contact angles, where no edge truncation is present. On the other hand, if As is transferred directly from the truncated facet (whose size oscillates within the ML growth cycle), the ML can be completed quasiinstantaneously. This situation corresponds to large contact angles above 125°, where the edge truncation serves as an extra source of material for the rapid completion of the whole ML. during growth of ZB NW section (with an As flux of 0.06 nm/s and a Ga flux of 0.15 nm/s). Each new ML grows quasi-instantaneously (in one frame) and its formation corresponds to the development of the edge truncation. The truncated facet is inclined at the angle 𝛼 with respect to the vertical and is wetted by the liquid catalyst. The amount of truncation oscillates with the period of ML growth, it decreases slowly but then jumps back to its maximum size with the next ML nucleation. Note that the truncation appears simultaneously on both sides of the NW; asymmetric truncations are observed in the presence of stacking faults or near the transition from ZB to WZ. The two nucleation events correspond to MLs #17 and #18. b) Images extracted from Movie 2 (MLs #226 and #227) recorded during growth of WZ NW section (with an As flux of 0.08 nm/s and a Ga flux of 0.15 nm/s). The step flows slowly across the planar top facet of the NW; the position of the step is indicated by the arrows. c) Images extracted from Movie 3 (MLs #377 and #378) recorded during growth of ZB NW section (with an As flux of 0.8 nm/s and Ga shutter closed). Similar to b), the growth occurs by a slow step flow and the edge corner is sharp. The two nucleation events correspond to MLs #377 and #378.

To understand the mechanisms for switching between these three regimes, we performed experiments in which the droplet volume was changed several times by modulating either Ga or As flux [see the Supporting Information (SI) 1 and 2 for more details]. The droplet volume can change by one of the two possible mechanisms. The first one is the change in NW diameter, where the system tends to maintain a constant contact angle. In this case, unbalanced capillary forces at the TPL lead to a change of the NW top diameter through the introduction of the inclined side facets. [START_REF] Schwarz | Elementary processes in nanowire growth[END_REF] Hence, the NW adjusts its diameter to keep the stable contact angle. [START_REF] Schwarz | Geometrical frustration in nanowire growth[END_REF][START_REF] Krogstrup | Structural phase control in self-catalyzed growth of GaAs nanowires on silicon (111)[END_REF][START_REF] Dubrovskii | Development of growth theory for vapor-liquid-solid nanowires: contact angle, truncated facets, and crystal phase[END_REF] The second one is changing the contact angle of the droplet at a constant NW diameter, leading to the phase transition when the contact angle reaches a specific critical value. Figure 2a shows that the phase transition occurs at the two critical angles, 𝜑 𝑚𝑖𝑛 = 100° and 𝜑 𝑚𝑎𝑥 = 125°, regardless of whether the droplet volume is increased or decreased. Between the two critical angles, the crystal phase is WZ, switching to ZB outside this range. This central result cannot be explained within the existing models of polytypism. [START_REF] Glas | Why does form in nanowires of III-V zinc blende semiconductors?[END_REF][START_REF] Jacobsson | Interface dynamics and crystal phase switching in GaAs nanowires[END_REF] Indeed, one central conclusion of both models is the maximum probability for the WZ phase formation at 𝜑=90°, which contradicts our in situ data. It should be noted that both models ignore possible tapering of the NWs, which modifies the surface energy balance at the TPL. We now consider in more detail the NW morphology to fully understand the phase switching mechanisms. Figure 2 shows that the two phases respond to the change in the droplet volume in very different ways. WZ NW maintains vertical sidewalls during a significant change in the contact angle (from 𝜑 𝑚𝑖𝑛 = 100° to 𝜑 𝑚𝑎𝑥 =125°). A slight increase in the top diameter visible in some cases is due to radial growth through the vertical step flow (see Movie 4), which occurs later than the axial growth and does not affect the crystal phase selection. Conversely, the diameter of ZB NW rapidly adjusts to the changing droplet volume by outward tapering (taper angle θ < 0) for large or inward tapering (θ > 0) for small contact angles. It is noteworthy that the phase transition from WZ to ZB occurs quasi-simultaneously with tapering of the NW. A natural question to ask is whether the phase change triggers the NW tapering or vice versa. In all our experiments, we observed that the phase transition occurs at the same time or slightly after developing of an inclined side facet in the WZ segment, as shown Figure 3 ( see also Figure S9 and Movie 4). After the phase switches to ZB, tapering proceeds by developing well defined {110} facets (see Figure 3 and Figure S9). At small contact angles, the phase change is thus promoted by tapering of the NW sidewalls. Each image was captured when the ML was half-grown to make its crystal phase clearly identifiable. The graphs show the NW diameter and the contact angle versus the NW length expressed in MLs. The initial conditions are: substrate temperature = 480 °C, As pressure = 1.2×10 -5 mbar, Ga shutter closed. After ML #14, the Ga source was opened and kept at a temperature of 940 °C. When the contact angle decreases, tapering starts within the WZ phase at ML #10. When the contact angle increases, the crystal phase switches to WZ at ML #20 and simultaneously the NW sidewalls become vertical. The diameter increase after ML #14 is not caused by an inverse tapering at the interface but is rather due to radial growth, which reorganizes the {110} facets into the vertical {111} facets. This mechanism has some features in common with the phase change at large contact angles, which, according to Reference 12, is determined by developing a truncated facet at the TPL. Even though the phase switching at the two critical contact angles is governed by different mechanisms (either tapering or truncation), a unifying pattern emerges. Indeed, in both cases the phase switching from WZ to ZB is accompanied by a modification of the growth front. At the large critical angle, the side facet at the NW top changes from vertical to truncated, with the truncation inside the liquid phase, while at the small critical angle the side facet changes from vertical to tapered, not wetted by the liquid phase. The development of these novel inclined facets changes the surface energy balance at the TPL, which in turn determines the preferred crystal phase. The ZB phase always emerges in presence of an inclined facet, suggesting that the edge line between the inclined facet and the top <111> facet represents the preferential nucleation site for ZB NWs.

Based on these observations, we develop a unifying model that predicts the morphology of the growth interface and the preferred crystal phase as a function the contact angle φ.

Generalizing the methods developed by Tersoff [START_REF] Schwarz | Geometrical frustration in nanowire growth[END_REF][START_REF] Jacobsson | Interface dynamics and crystal phase switching in GaAs nanowires[END_REF] and Dubrovskii, [START_REF] Dubrovskii | Development of growth theory for vapor-liquid-solid nanowires: contact angle, truncated facets, and crystal phase[END_REF] we derive the two equations that represent the difference of surface energy between a given morphology, which has either tapered (t) sidewalls or truncated (tr) edges, and the reference state having vertical sidewalls and 90° edges at the TPL. In addition to the previous models, we take into account that the NW can adopt either ZB or WZ structure at the growth front and hence the solid-vapor surface energies are phase-dependent (k = ZB or WZ). These equations are given by

𝛥𝐹 𝑡 𝑘 (𝜃, 𝜑) = 𝛾 𝜃𝑉 𝜅 𝑐𝑜𝑠 𝜃 -𝛾 0𝑉 𝜅 -(𝛾 𝑆𝐿 + 𝛾 𝐿𝑉 𝑐𝑜𝑠 𝜑) 𝑡𝑎𝑛 𝜃 (1)
𝛥𝐹 𝑡𝑟 𝑘 (𝛼, 𝜑) = 𝛾 𝛼𝐿 𝑐𝑜𝑠 𝛼 -𝛾 0𝑉 𝜅 -𝛾 𝑆𝐿 𝑡𝑎𝑛 𝛼 + 𝛾 𝐿𝑉 𝑠𝑖𝑛 𝜑 (2)
Here, 𝛾 𝑆𝐿 and 𝛾 𝐿𝑉 are the phase-independent surface energies of the horizontal solid-liquid and liquid-vapour interfaces; 𝛾 𝛼𝐿 is the solid-liquid surface energy of a ZB truncated facet, inclined at the angle 𝛼 to the vertical; 𝛾 0𝑉 𝑍𝐵 is the surface energy of vertical {110} sidewall facet of ZB NW, which represents the reference; 𝛾 0𝑉 𝑊𝑍 is the surface energy of vertical (11 ̅ 00) sidewall facet of WZ NW, which is lower than 𝛾 0𝑉 𝑍𝐵 ; and 𝛾 𝜃𝑉 𝜅 is the surface energy of a tapered facet, inclined at the angle θ to the vertical. Positive or negative angles θ correspond to narrowing or widening facets, leading to the NW tapering or inverse tapering, respectively. To model the NW morphology and phase versus the contact angle φ, we use a single set of surface energies for Ga-GaAs system gathered from the literature, [START_REF] Dubrovskii | Development of growth theory for vapor-liquid-solid nanowires: contact angle, truncated facets, and crystal phase[END_REF][START_REF] Pankoke | Calculation of the diameter-dependent polytypism in GaAs nanowires from an atomic motif expansion of the formation energy[END_REF][START_REF] Moll | GaAs equilibrium crystal shape from first principles[END_REF][START_REF] Dubrovskii | Gibbs-Thomson and diffusion-induced contributions to the growth rate of Si, InP and GaAs nanowires[END_REF]23 the measured angles of the inclined facets (θ and α), and the knowledge on the phase trends deduced from our experimental observations (see Methods). By doing so, the equations contain two unknowns, the solid-liquid surface energies 𝛾 𝑆𝐿 and 𝛾 𝛼𝐿 . 𝜑 𝑚𝑎𝑥 =125°, while for smaller and larger angles, the tapered and truncated ZB configurations are more favorable. Above 125°, the growth front is truncated and the formation of WZ phase is prevented. There is a narrow range of contact angles (from 125° to 127°) where ZB NWs have vertical {110} sidewalls, while for larger  ZB NWs exhibit inverse tapering, as often seen in self- catalyzed GaAs NWs 23,[START_REF] Dubrovskii | New mode of vapor-liquid-solid nanowire growth[END_REF] . Our model also contemplates a possible formation of a truncation at very small contact angles (smaller than 55 o , not shown in Figure 4), as predicted earlier in Reference 12. However, at contact angles smaller than 𝜑 𝑚𝑖𝑛 , development of the tapered nonwetted facet is more favorable and hence the truncation is suppressed. Furthermore, our experiments show that when the contact angle approaches 55°, the droplet unpins from the TPL and moves freely on the top (111) surface (Figure S9). We found, however, that the minimum angles 𝜑 𝑚𝑖𝑛 for the WZ-to-ZB phase switching vary in a range between 85° and 100°. We attribute this variability to different sidewall structures of the NWs (see SI, paragraph S3).

Precise control of the mo rphology of the ZB and WZ segments in crystal phase heterostructures is crucial for various applications. In this context, the phase change at large angles has two main advantages. First, it is more predictable since, for all the investigated growth conditions, it occurs in a very narrow range of angles around 125° (Figure S6). Second, it does not induce any significant change in the NW diameter, particularly if the maximum contact angle is smaller than 127°. Nevertheless, the phase change at small angles cannot be ignored as it occurs almost inevitably at the two crucial steps of NW growth. This transition should be observed at the beginning of growth [START_REF] Glas | Why does form in nanowires of III-V zinc blende semiconductors?[END_REF] , because the droplet has a much smaller contact angle when sitting on the planar substrate surface than on top of developed NWs. It also occurs at the end of growth, when the contact angle decreases when the droplet is completely [START_REF] Plissard | Goldfree growth of GaAs nanowires on silicon: arrays and polytypism[END_REF] or partially [START_REF] Lehmann | Crystal phase control in GaAs nanowires: opposing trends in the Ga-and As-limited growth regimes[END_REF] consumed. Furthermore, the formation of quantum-thin GaAs NWs requires a stage of droplet shrinking under high As fluxes, where the contact angle is small. [START_REF] Kim | Bi-stability of contact angle and its role in achieving quantum-thin self-assisted GaAs nanowires[END_REF] It is worth noticing that when the droplet volume decreases rapidly, the WZ insertion between the two ZB segments becomes very short and virtually disappears in the most extreme case (see Figure 2a, ML #200). This provides a clear route for the fabrication of pure ZB GaAs NWs, avoiding the phase mixing at the NW top. Previous studies [START_REF] Dubrovskii | New mode of vapor-liquid-solid nanowire growth[END_REF] suggested that self-catalyzed GaAs NWs should be almost exclusively ZB due to a low surface energy of liquid Ga compared to Au.

Our results show, however, that the WZ phase in such NWs is easily achievable under relatively high V/III ratios. Predominantly ZB phase observed previously is explained by effectively Ga-rich growth conditions employed in these works. Ga-rich environment guarantees that the Ga droplet is not consumed by an excessive As flux. On the other hand, it leads to droplet inflation, truncated growth interface and inverse tapering, following the described growth scenario at large contact angles.

In conclusion, our in situ growth monitoring and modeling of self-catalyzed GaAs and GaSb (see S9) NWs clearly shows the crystal phase switching depending on the sole parameter, the droplet contact angle, which can easily be regulated by the incoming material fluxes. The crystal phase can be changed in a regular way from pure ZB to pure WZ with the well-controlled lengths of crystal phase segments, which gives a clear route for obtaining crystal phase heterostructures in the Au-free approach. The method can be extended to other III-V NWs and demonstrates the capability of in situ method for delicate manipulations with the morphology, crystal phase and ultimately physical properties of nanomaterials.

Methods

Growth

We observed the growth of GaAs NWs in situ using a Cs corrected Titan environmental TEM (ETEM), equipped with custom-made MBE sources. These sources are highly collimated so that the evaporated material only deposits onto the sample. The substrate was a Protochips heating SiC membrane with holes of 10 µm diameter. Pure Ga and As were loaded into boron nitride crucible and heated to about 930 and 320 °C, respectively, to obtain a volumetric flux on the order of 0.3 (nm 3 /s) per nm 2 , which corresponds to a growth rate of about 1 ML per second (see SI, paragraph S2). This growth rate was selected as a trade-off between realistic growth conditions for a standard reactor and a growth speed that allows recording high-quality data. By keeping the source temperature constant, it was possible to adjust the flux of As through a needle valve and consequently control the growth rate. Ga was initially deposited on the substrate at a temperature of 500 °C and when the Ga droplets reached the size of 20-30 nm, Ga was introduced and temperature decreased to 420 °C to promote the growth of NWs. Due to the polycrystalline nature of the SiC substrate, NWs started growing in arbitrary directions on the substrate or freely suspended in vacuum in correspondence to the holes. The sample was then tilted to orient the selected NW on a specific zone axis, generally <110>. Typical experiments were carried out at a growth rate between 0.05 and 1 ML/s and high-resolution movies were recorded using a Gatan US1000 camera at a rate of 4 frames per second. Images were analyzed using an automated script that determines the relevant parameters such as the NW diameter, the volume of catalyst droplet and its contact angle (see SI S1).

Model

Equations 1 and 2 are derived from theoretical work [START_REF] Dubrovskii | Development of growth theory for vapor-liquid-solid nanowires: contact angle, truncated facets, and crystal phase[END_REF] and plotted in Figure 4 using the surface energy values reported in Table 1. Here, thanks to the experimental determination of the critical contact angles corresponding to the crystal phase switching and identification of different facets, we can assign the accurate values for the surface energies. For the angles θ, we distinguish between narrowing (n), vertical (v) and widening (w) facets. The angle θ(n) and the truncation angle  are directly measured form the TEM images. For widening of ZB GaAs NWs, we deduced that the inverse tapering proceeds by an alternation of vertical (110) and outward tapered (111)B sidewall facets. Therefore, in the model, we assign θ(w) and the corresponding surface energy value to the (111)B facet. The "interface" kymowire is obtained by stacking together the cross-sections at the interface. Boundaries of the kymowire are highlighted in green. For the ZB phase, the diameter measurements will be affected by the corner oscillations. c. The "Shifted" kymowire is similar to the "interface" kymowire, but the cross-sections are shifted away from the interface. Edges of the kymowire are shown in red. Measuring the diameter at the shifted interface ensures that the readings are insensitive to the corner oscillations. However, these measurements are unable to track the fast diameter changes at small contact angles.

𝛾 𝜃𝑉 𝑍𝐵 (w) = 𝛾 (111 )𝐵 (J/m 2 ) 𝛾 𝜃𝑉 𝑍𝐵 (n) = 𝛾 (110) (J/m 2 ) 𝛾 𝜃𝑉 𝑊𝑍 (v) = 𝛾 (11 ̅00) (J/m 2 ) 𝛾 𝐿𝑉 = 𝛾 𝐺𝑎 (J/m 2 ) 𝜑 𝑚𝑖𝑛 (deg) 𝜑 𝑚𝑎𝑥 (deg) θ (w) (deg) θ (n) (deg) 𝛼 (deg) 𝛾 𝑆𝐿 (J/m 2 ) 𝛾 𝛼𝐿 𝑍𝐵 (J/m 2 )
Finally, after finding the diameters of both kymowires, we make the selection of the true nanowire diameter based on the value of the contact angle. For contact angles greater than 125°, the shifted diameter is selected, while for smaller angles, we measure the diameter at the interface. In order to calculate the Ga flux from the MBE source, we perform a special experiment, where Ga is deposited on a SiNx membrane, which is heated up to 420 °C. Once the deposition starts, spherical Ga nanodroplets begin to nucleate and grow on the substrate surface (Fig. S3a).

S2. Calculation of Ga and

By segmenting the droplets and assuming hemispherical shape, we are able to calculate the total volume of Ga as a function of time. Experiments conducted at different temperatures allow us to conclude that desorption from the droplet surface and from the substrate is negligible for temperature below 500 °C. Then the Ga volume per unit area exactly equals the Ga flux onto the surface (Figure S3b).

The increase of the droplet volume on the nanowire top can be due to the direct impingement of Ga atoms onto the droplet surface or their diffusion along the nanowire sidewalls, while the volume reduction can be due to evaporation, incorporation into the crystal phase or reverse diffusion. We can estimate the contribution of all these processes using the data shown in Figure 2. In the range from layer #200 to #240, the maximum rate of changing the droplet volume is ~ 150 nm 3 /s (in both directions). The axial nanowire growth rate is 0.1 ML/s, the nanowire diameter is 30 nm, and the contact angle is 100°. First, we can any contribution from evaporation as it is negligible at this temperature. 9 The volume of Ga impinging onto the droplet from the direct flux is < 10 nm 3 /s, while the volume of Ga consumed to grow the nanowire is ~ 10 nm 3 /s. Both

S3. Variability of the critical contact angles

Here, we systematically investigate the variability of the critical contact angles 𝜑 𝑚𝑖𝑛 and 𝜑 𝑚𝑎𝑥 . We observed the phase switching in nanowires having diameters in the range from 15 nm to 65 nm, growth rates from 0.05 ML/s to 1 ML/s, and grown at temperatures between 350 °C and 480 °C. The measured critical angles are plotted in Figure S6 and show no clear trend with these parameters. We speculate that the observed variability could be due to the different sidewall structure. Pure WZ or ZB nanowires have different stable sidewall facets that are rotated by 30°.

When the phase changes, the nanowire sidewalls require a certain length to reconfigure. Therefore, the short WZ segments shown in Figure 2 may still maintain the sidewall orientation of the ZB phase or even have a more complex structure. On the other hand, the long WZ segment shown in

S4 Details of the model

In modeling, we use Eq. ( 1) of the main text, assuming that inward tapering of NWs at small contact angles is preferred in the ZB phase, that is, the (110) ZB inclined facet is the lowest energy one. The curves in Fig. 4 (which should be very similar for gold-and gallium-catalyzed growths) strongly suggests that phase transitions occur quite abruptly whenever allowed by surface energetics. In other words, chemical potential near the transition points quickly exceeds the energy of stacking fault, which is required for switching from ZB to WZ phase, [START_REF] Glas | Why does form in nanowires of III-V zinc blende semiconductors?[END_REF] and do not significantly affect the critical contact angles for the transition.

Figure 1 .

 1 Figure 1. Growth mode and phase selection at different contact angles. All images are recorded from the same self-catalysed GaAs NW having a diameter of ~30 nm and growing at 420 °C under different fluxes of As and Ga. The As/Ga flux ratio was changed during growth to tune the volume of the catalyst droplet and consequently the contact angle. Scale bars are 5 nm and the ML numbers refer to Figure 2 a. a) Images extracted from Movie 1 recorded during growth of ZB NW section (with an As flux of 0.06 nm/s and a Ga flux of 0.15 nm/s). Each new ML grows quasi-instantaneously (in one frame) and its formation corresponds to the development of the edge truncation. The truncated facet is inclined at the angle 𝛼 with respect to the vertical and is wetted by the liquid catalyst. The amount of truncation oscillates with the period of ML growth, it decreases slowly but then jumps back to its maximum size with the next ML nucleation. Note that the truncation appears simultaneously on both sides of the NW; asymmetric truncations are observed in the presence of stacking faults or near the transition from ZB to WZ. The two nucleation

Figure 2 .

 2 Figure 2. Time evolution of the NW morphology under varying As and Ga fluxes. a) The droplet volume (top), contact angle (middle) and base diameter (bottom) are plotted as functions of the NW length. b) Image of a slice of the NW obtained by composing several TEM images. Each data point is measured on the image in which the ML nucleation is observed and color-coded for the two crystal phases (blue for ZB and red for WZ). The effective fluxes of As and Ga are calculated from the axial growth rate of the NW and the change in the droplet volume (see SI 2). Data points corresponding to ML # 129-133 were not recorded.

Figure 3 .

 3 Figure 3. Sequence of images showing the phase switching caused by the NW tapering at small contact angles.Each image was captured when the ML was half-grown to make its crystal phase clearly identifiable. The graphs show the NW diameter and the contact angle versus the NW length expressed in MLs. The initial conditions are: substrate temperature = 480 °C, As pressure = 1.2×10 -5 mbar, Ga shutter closed. After ML #14, the Ga source was opened and kept at a temperature of 940 °C. When the contact angle decreases, tapering starts within the WZ phase at ML #10. When the contact angle increases, the crystal phase switches to WZ at ML #20 and simultaneously the NW sidewalls

Figure 4 .

 4 Figure 4. Model for the NW morphology and phase selection, showing the transitions from tapered ZB NWs at small contact angles to vertical WZ NWs at intermediate contact angles and back to ZB NWs with wetted truncated edges at large contact angles. Blue zero-level line corresponds to vertical (110) ZB facets and the red horizontal line to vertical (11 ̅ 00) WZ facets. The increasing curve for the narrowing (110) ZB facet intercepts with the WZ one at 𝜑 𝑚𝑖𝑛 = 100°, corresponding to the preferred ZB structure below 100°. The curve for the wetted truncated facet crosses with the WZ one at 𝜑 𝑚𝑎𝑥 = 125°, showing that the crystal phase is ZB above 𝜑 𝑚𝑎𝑥 . The morphology of these ZB NWs is first vertical, transitioning to inverse tapered when the contact angle further increases. In both cases, the growth front is truncated. The region between the two critical angles on the WZ line corresponds to vertical WZ NWs.

Figure S2. a .

 a Figure S2. a. Contact angle (top) and nanowire diameter (bottom) versus time. Diameters are measured from the "shifted" (red circles) and "interface" (green circles) kymowires and the true diameter is selected (black solid curve) based on the contact angle value. Note how the two diameters are overlapping at the contact angles close 125°. b.The "interface" kymowire is obtained by stacking together the cross-sections at the interface. Boundaries of the kymowire are highlighted in green. For the ZB phase, the diameter measurements will be affected by the corner oscillations. c. The "Shifted" kymowire is similar to the "interface" kymowire, but the cross-sections are shifted away from the interface. Edges of the kymowire are shown in red. Measuring the diameter at the shifted interface ensures that the readings are insensitive to the corner oscillations. However, these measurements are unable to track the fast diameter changes at small contact angles.

As fluxes from the kinetic data S2. 1

 1 Calculation of Ga flux from the droplet growth rate We modified the droplet volume by changing both Ga and As fluxes. During growth of the first 175 MLs, we kept the Ga flux constant and modulated the As flux. After that, we kept the As flux constant and periodically turned on and off the Ga flux. Figure 2a of the main text shows that the V/III flux ratio is the key parameter for controlling the droplet volume. While the droplet volume changes at different rates under different growth conditions, it consistently increases under an excess of Ga and decreases under an excess of As. A simple calculation (see below) shows that, in our conditions, the Ga diffusion along the nanowire sidewalls to the top is the main mechanism governing the droplet volume change.

Figure S3. a .

 a Figure S3. a. Evolution of the Ga droplet size and density with time. The nanodroplet boundaries are highlighted in red. b. Equivalent thickness of the deposited Ga versus time.

Figure S4 .

 S4 FigureS4shows that the axial nanowire growth rate depends on the As pressure in the source as well as on the droplet size. Higher As flux at higher pressure yields a larger amount of As reaching the droplet surface, while larger droplet represents a larger collection area. In the Aslimited regime, as in our case, the instantaneous axial growth rate equals the As flux times the cross-sectional area of the droplet (provided that the As evaporation is negligible).Let us consider cylindrical nanowire with spherical cap droplet of the radius r and height h seated on the top. The total flux of As entering the droplet, 𝛷, equals the flux per unit area, j, times the cross-sectional area of the droplet, S, which gives 𝛷 = 𝑗𝑆. The nanowire growth direction is parallel to the As flux, so for contact angles smaller than 90° the cross-section equals exactly 𝑆 = 𝜋𝑟 2 . The volume of each new monolayer equals the (111) interlayer distance, b = 0.326 nm, multiplied by the interface area, 𝐴 = 𝜋𝑑 2 /4. The time interval between the two successive nucleation events, τ, equals the volume of the monolayer divided by the volume of material entering the droplet per unit time:

Figure S5 .

 S5 Figure S5. Nanowire axial growth rates plotted as functions of 𝑑 2 /𝑟 2 and extrapolated by linear fits. The slope of each line is inversely proportional to the As flux at a given As pressure in the source. The inset shows the obtained dependence of the As flux on the As pressure in the source, and its linear fit.

Figure 3

 3 Figure 3 should have the typical sidewall structure of the WZ nanowire. For this reason, the development of tapering and hence switching from the ZB to WZ phase can occur at different contact angles.

Figure S6 .

 S6 Figure S6. Critical angles for the phase transition at different temperatures. Each point corresponds to nucleation of the first monolayer of a different phase. Data at 350, 400 and 450 °C are collected from a sequence of experiments on the same nanowire; data at 420 °C are the average of the measurements represented in Fig.2; data at 480 °C are from Fig.3. Empty symbols represent the transition at large angles, while solid symbols represent the transition at small angles. Downward (upward) triangles indicate that the droplet is shrinking (swelling). Red (blue) colour represents the phase change from WZ to ZB (ZB to WZ). The critical angles seem to be independent of temperature in the investigated range.

  of the main text are obtained for the same (110) facet for tapered and truncated NWs, making an angle of 54.7° to the vertical. The two crossing points at the minimum and maximum contact angles are fitted by adjusting the two surface energies of the liquid-solid interfaces, as explained in the main text. Figure S7 shows how the curves change at a fixed max  = 125 o and min  decreasing from 100° to 90°. The fitting values of the solid-liquid surface energies 𝛾 𝑆𝐿 and 𝛾 𝛼𝐿 are 0.540 J/m 2 and 0.522 J/m 2 at 𝜑 𝑚𝑖𝑛 = 95°, and 0.477 J/m 2 and 0.471 J/m 2 at 𝜑 𝑚𝑖𝑛 = 90°. It is seen that decreasing the 𝜑 𝑚𝑖𝑛 suppresses the possibility of forming vertical GaAs NWs in ZB phase at a fixed growth temperature of 420 °C.

Figure S7 .

 S7 Figure S7. Surface energy curves for different 𝜑 𝑚𝑖𝑛 from 90 o to 100 o . The arrows show modifications of the lines as 𝜑 𝑚𝑖𝑛 decreases, which closes the region of vertical ZB NWs at large contact angles.

Figure S9 .

 S9 Figure S9. Phase change at the small contact angle. a. Sequence of images showing the switch from WZ to ZB at the small contact angle and the corresponding details of the TPL with the taper angle θ and finally depinning of the droplet. The tapering starts in the WZ phase, and the phase switching to ZB occurs successively. When the contact angle reaches a critical value of ~55°, the droplet unpins from the TPL and moves on the {111} surface until the full consumption. b. Graphs showing the nanowire diameter and the contact angle versus time until the droplet unpinning from the TPL.

  The curves obtained from equation 1 and 2 are plotted by using the known surface energy values reported in Table1and by adjusting the two unknown values (𝛾 𝑆𝐿 and and 𝛾 𝛼𝐿 ) in order to fit the two critical contact angles corresponding to the ZB-WZ phase transitions. With this method, we are able to deduce the plausible values of 𝛾 𝑆𝐿 = 0.593 J/m 2 and 𝛾 𝛼𝐿 = 0.566 J/m 2 , which appear very close to each other (more details are given in SI, paragraph S4). Overall, the model explains very well our experimental observations and agrees with the earlier results. It is seen that the vertical WZ configuration is preferred for intermediate contact angles from 𝜑 𝑚𝑖𝑛 = 100° to

Table 1 :

 1 Parameter values used to plot Figure 4 based on Eqs. (1) and (2). All authors have given approval to the final version of the manuscript.

	Model
	Para-
	meters
	AUTHOR INFORMATION
	Corresponding Author
	Funding Sources
	We acknowledge the ANR (French National Research Agency) for funding the NanoMAX ETEM
	through the TEMPOS grant, project number 10-EQPX-0050 and the Singapore National Research
	Foundation's Competitive Research Program funding (NRF-CRP16-2015-05). VGD thanks the
	Russian Science Foundation for financial support under the Grant 19-72-30004.

*E-mail: federico.panciera@c2n.upsaclay.fr (F.P.) *E-mail: mirsaidov@nus.edu.sg (U.M.) Author contribution: F.P., G.P., and J.-C.H. designed and conducted the experiments. Z.B., F.P., and U.M. developed the image analysis scripts and analyzed the data. V.G.D., F.P. and F.G. developed the model. J.-C.H. and L.T. developed the MBE sources.

ACKNOWLEDGMENT

We acknowledge Jean-Luc Maurice and Odile Stephane, directors of NanoMAX and TEMPOS, respectively, for their continuous support, and Ileana Florea for technical assistance.

Supporting Information

S1. Image processing and data extraction

The image processing algorithm for the analysis of in situ image sequence file were written in Python-2.7 using scipy, 1 numpy, 2 scikit-image, 3 opencv, 4 mpi4py 5 and matplotlib 6 libraries.

The image processing routine can be divided into two main parts: image segmentation and Hough transform applied to the nanodroplet and the nanowire. Both steps were done separately for identification of the droplet and nanowire boundaries, and the liquid-solid interface.

For image segmentation, the intensity-based thresholding algorithms are unsuitable for our datasets because the contrast around the droplet is non-uniform. Here, the background subtraction cannot effectively remove the contrast gradient because the object size is comparable to the background. To address this challenge in the segmentation, we employed the Canny edgedetection algorithm, 7 a widely used gradient-based segmentation method, from the scikit-image library. As a preliminary image processing step, this function also applies the Gaussian blurring to remove the high-frequency noise. However, the choice of σ, the standard deviation of Gaussian function is different for the droplet-nanowire (σ = 5 pixels) and droplet boundary (σ = 7-10 pixels), which are low and high-frequency image features, respectively. Subsequently, in order to detect the edges, the Canny algorithm requires an input of low and high thresholds, which were chosen manually for each movie. Later these two sets of binary images were extensively used, and, for the sake of clarity, we refer to them as the droplet edges (Figure S1b) and interface edges (Figure S1c) throughout the text. Once the edges of the droplet and the interface were detected, we executed the Hough transformations on these sequences in order to measure the dimensions of the droplet and nanowire during the growth. Note that the growth and orientation of the nanowire is perpendicular to the electron beam. Consequently, the projection of the spherical nanodroplet becomes a circular segment, whose center and radius can be measured using the Hough circles transform. 8 The projection of the interface is a straight line, which was identified using the Hough lines transform. 8 Since both the droplet and interface edges contain segmented objects from the entire field of view, it is necessary to perform masking in order to isolate the area of interest for each set. For this reason, the first execution of the Hough transforms delivers masks for the second iteration.

Likewise, the Hough lines transform of the interface edge returns all the lines that parallel to the interface and represent layers of the nanowire with the highest intensity gradient. Knowing this region, we can create a mask to separate the droplet from the nanowire and, hence, by applying this mask to the droplet edges, a more accurate result of the Hough circle transform is achieved (Fig S1d). Similarly, the result of the Hough circle transform on the droplet edges is used to create a circular mask, which eliminates the area outside of the droplet on the interface edges.

Subsequently, we detect the interface with the second iteration of the Hough lines transform (Fig. S1e).

After the radius 𝑟 and positions of the droplet centre and the interface line have been identified, we measure the minimum distance ℎ from the centre to the interface, and find the total height 𝐻 of the droplet and its volume 𝑉(𝑟, 𝐻) for each frame using

2𝜋𝐻 2 (3𝑟-𝐻)

3

.

Another important dimension that characterizes the morphology is the nanowire diameter,

given by the projected length of the droplet-nanowire interface. Note that in the case of large ZB NW sections, when the contact angle is above 125 o , the tip of the nanowire is submerged into the droplet, so the measurement at the interface would overestimate the diameter of the growing front.

Therefore, for every frame, we look at the two cross-sections: at the interface and at a fixed distance of 25 pixels (1.36 nm) away from the interface. For both cross-sections, we calculate the mean intensity within 5 pixels (0.27 nm), and by stacking these 1D arrays together for each movie, we create two sets of kymowires: the interface (Figure S2b) and shifted (Figure S2c). Segmentation of the two kymowires is similar. During the imaging, the lower corner of the nanowire is always kept at the focus, and the upper corner is kept under the focus. Therefore, in order to find the edges, we divided the kymowire into two halves. For the upper half, we search for the local maximum using the scipy package, while the lower half is segmented by setting a manual intensity threshold at 100.

S5 Supplementary Figures