Aqueous phase near-infrared emitters: water transfer of colloidal 2D PbS, PbSe and PbTe nanoplatelets

Abstract

Colloidal two-dimensional (2D) lead chalcogenide PbX (X = S, Se, Te) nanoplatelets (NPLs) are strongly confined narrow band gap semiconductors with tuneable efficent photoluminescence (PL) in the near-infrared (NIR). They hold high potential for the use as classical and quantum emitters in fiber-based photonics that operate at telecommunication wavelengths. Up to now, the insolubility of 2D PbX NCs in water and other polar solvents has been a challenge that complicates their post-synthesic processing, e.g. into future functional nanocomposites. Here, we describe a phase transfer protocol from hexane to water using 11-mercaptoundecanoic acid (MUA), which yields aqueous phase 2D PbS, PbSe, and PbTe NPLs with preserved shape, crystallinity and NIR PL (e.g. PbS: 724 nm, PbSe: 1023 nm and PbTe: 1184 nm). Water-soluble 2D PbSe shows efficient emission (up to 13% PL quantum yield at 1023 nm), thereby retaining 65% of the initial quantum yield and making it highly interesting as an aqueous NIR light source. By using X-ray photoelectron spectroscopy (XPS) and nuclear magnetic resonance (NMR), we follow the phase transfer on a molecular level and find two binding motifs of MUA to the 2D PbX surfaces: X-type bound thiolate and L-type bound thiol. Our results shine new light on mercaptocarboxylic acid based nanomaterial phase transfers and represent a crucial step for incorporating NIR-emissive 2D PbX into (fiber) optics.

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Introduction

Colloidal 2D semiconductor nanocrystals (NCs), so-called nano-platelets (NPLs) or flat quantum dots (fQDs), with the most prominent UV-Vis emitting example of CdX (X = S, Se, and/or Te) NPLs, are highly topical materials and exhibit fast radiative, spectrally narrow, and efficient directed photo-luminescence (PL) caused by their strong anisotropic quantum confinement.^1–4 However, for near-infrared (NIR) wavelength use such as (quantum) optical communication^5,6 or IR sensing,^7,8 materials with a smaller band gap than cadmium chalcogenides are required. NIR emitting 2D CdX-associated examples include Hg_xCd_1−xSe NPLs, HgX NC decorated HgX NPLs,⁹ and CdSe–PbSe NPLs as well as PbSe NPLs,¹⁰ all prepared via cation exchange from CdX NPLs.

On the other hand, direct colloidal synthesis routes for narrow band gap 2D lead chalcogenide (PbX; X = S, Se, Te) NPLs and fQDs have been developed in recent years. 2D PbX NPLs and fQDs exhibit efficient telecommunication band emission (1^st, 2^nd and 3^rd low-loss optical fiber windows at ∼850 nm, ∼1300 nm and ∼1550 nm, respectively)¹¹ at NIR to short-wave-infrared (SWIR) wavelengths (PbS: up to 19% PLQY at ∼720 nm,^12,13 PbSe: up to 61% between 860–1510 nm,^14–16 PbTe: up to 15% for 910–1460 nm).¹⁷ They are intriguing materials for example for fiber optics-based photonics. In addition, colloidal 2D PbS exhibits high carrier multiplication efficiencies (0.9–0.55 for 4–7 nm thick PbS nanosheets (NSs)),¹⁸ narrow linearly polarized emission (down to 615 μeV with a linear degree of polarization of up to 90%),¹⁹ efficient charge carrier mobilities (550–1000 cm² V⁻¹ s⁻¹ for 4–16 nm thick PbS NSs),²⁰ and Rashba-type band splitting.²¹ These unique properties may be leveraged for future optoelectronic or spintronic applications.

A shared aspect of the direct 2D PbX NPL syntheses is the use of low-polarity solvents and long-chain aliphatic amines, which are assumed to help template the anisotropic growth of the (isotropic) cubic rock salt crystal-structured materials.²²

As a consequence, the obtained 2D PbX flat fQDs, NPLs, or NSs, are covered by lipophilic ligands, typically L-type bound amines and X-type bound oleates,^14,23 which ensure their colloidal stability in organic environments and render them insoluble in aqueous (and other polar) solutions. While post-synthetic surface passivation with metal halides (e.g. CdCl₂ or PbI₂) has proven to be a powerful tool to enhance the colloidal stability and PL/PLQY of 2D lead chalcogenides,¹⁴ ligand engineering with respect to the solubility of the materials has not been investigated so far, limiting their general applicability. Up to now, the restrictions imposed by post-synthetic processing of hydrophobic 2D PbX NPLs and fQDs complicate numerous encapsulation techniques toward easy to handle solid-state composite materials feasible from aqueous solution.

For example, we have assembled aqueous core-crown CdSe/CdS NPLs via layer-by-layer deposition with polyelectrolytes²⁴ and have incorporated CdSe/CdS NPLs into zeolitic imidazolate frameworks (ZIF) to produce photoluminescent composite films.²⁵ Such hybrid materials (made from phase transferred NCs) combine the functionality of two different materials classes, e.g. the inertness or chemical function of a polymer or the high specific surface area of a ZIF with the unique photophysics of NPLs. On the other hand, inorganic–organic nanocomposites possess new application-oriented functionalities such as switching of the optical properties based on external stimuli, e.g. humidity changes or electric and magnetic fields.^25,26 Water phase transfer of NIR-emissive NCs is also frequently discussed in the context of biomedical imaging and labelling,^27,28 albeit we note that heavy metal-containing NCs require thorough matrix incorporation (which may be facilitated from aqueous solution). In addition, aqueous ligand exchange often coincides with the replacement of long insulating ligands (e.g. oleic acid) by shorter chained molecules, which are beneficial for charge transport in NC-based solid-state devices such as field-effect transistors, photodetectors, and solar cells.^29–33

A common strategy to phase transfer lipophilic emissive NCs, while retaining their PL, is the exchange of the pristine ligands with bifunctional mercaptocarboxylic acids (see Scheme 1). The latter coordinate to the NCs’ surface via the sulfur, while the free carboxylate headgroup provides solubility in water.^34–36

Scheme 1 Schematic representation of the organic to aqueous phase transfer of 2D PbX fQDs and NPLs (dark brown) using 11-mercaptoundecanoic acid at alkaline pH conditions. Upon vigorous shaking of the biphasic system, the fQDs and NPLs change to the MeOH phase (light blue); consecutive purification with chloroform and redispersion in 0.1 M KOH in water yields colloidally stable aqueous 2D PbX solutions.

For example, Tamang et al. reported the aqueous phase transfer of InP/ZnS core/shell NCs, CdSe/CdS/ZnS NCs, CuInS₂/ZnS NCs, and CdSe as well as CdSe/CdS nanorods using different hydrophilic thiols (e.g. cysteine, thioglycolic acid, and 11-mercaptoundecanoic acid).²⁸ The authors, and other reports, further emphasize the importance of alkaline pH conditions during ligand exchange and subsequent storage of the aqueous colloidal solution.^28,35,36 This is needed because the thiol and the carboxyl group of the mercaptocarboxylic acids must both be deprotonated, so that the thiolate can bind covalently to the surface of the NCs, while the carboxylate enables solubility in aqueous solution (MUA with a protonated carboxyl group is hardly soluble in water). Phase transferred NCs using MUA specifically have been used for imaging of human colon cancer cells (PbS QDs-MUA)²⁷ or as an active component in the aforementioned zeolitic imidazolate framework composites for PL-based gas sensing (aqueous core-crown CdSe/CdS NPLs).²⁵

Here, we demonstrate a ligand exchange protocol using 11-mercaptoundecanoic acid to water transfer highly confined 2D lead chalcogenide NPLs and fQDs (PbS, PbSe, and PbTe), which exhibit telecommunication band emission in the NIR. The obtained water-soluble 2D PbX NPLs and fQDs bridge the gap between classical and quantum emitters and solubility requirements of common downstream processing techniques. By applying X-ray photoelectron spectroscopy (XPS) and NMR, we disentangle X- and L-type binding motifs of thiolate and protonated thiols to the PbX NPLs’ surface after the universal phase transfer. These findings add new insights to the molecular understanding of mercaptocarboxylic acid-based water transfers of nanomaterials and lead the way for the post-synthetic processing of 2D PbX NPLs into nanocomposites with technologically relevant telecommunication band emission.

Experimental

Chemicals

Acetonitrile (≥99.5%), cadmium(II) chloride (99.99%), chloroform (≥99%, contains 0.5–1% ethanol as stabilizer), ethanol (EtOH, max. 0.01% H₂O), iso-propanol (≥99.5%), lead(II) oxide (≥99.99%), 11-mercaptoundecanoic acid (MUA, 95%), methanol (MeOH, ≥99.8%), n-octylamine (99%), tellurium powder (30 mesh, 99.99%), tetra-chloroethylene (TCE, ≥99%), triethylamine (≥99%), tri-fluoroacetic acid (99%), trifluoracetic anhydride (≥99%), and tris(dimethylamino)-phosphine (97%) were purchased from Sigma-Aldrich/Merck. Lead(II) iodide (99.99%), selenourea (99.97%), and thiourea (99%) were purchased from Alfa Aesar. Oleic acid (90%) was purchased from ABCR. n-Hexane (97%) was purchased from Acros Organics. Potassium hydroxide pellets were purchased from Avantor/VWR. Deuterium oxide (D₂O, 99.9 atom% D) was purchased from Carl Roth. n-Octylamine and oleic acid were degassed using the freeze–pump–thaw technique prior to being stored and handled inside a nitrogen-filled glove box. All other reagents were used as received from the listed suppliers. Lead oleate was synthesized according to an established method by Hendricks et al.³⁷ All steps of the NPL syntheses and phase transfers not involving deionized water or D₂O were performed under inert gas conditions in a nitrogen-filled glove box, unless stated otherwise.

PbS NPL synthesis

PbS NPLs were synthesized using a method similar to that of Manteiga Vázquez et al.¹² Seven days before the actual synthesis, a solution of thiourea (240 mg, 3.15 mmol) in octylamine (6 ml) was prepared and stirred at 35 °C until use in the PbS NPL synthesis. For a typical synthesis, lead oleate (183 mg, 0.24 mmol) was dissolved in a mixture of octylamine (0.75 ml), oleic acid (0.4 ml), and hexane (1.15 ml) at 35 °C. After complete dissolution, 0.25 ml of thiourea solution was rapidly injected and the reaction was allowed to run for 16 min, during which the colorless mixture became bronze colored. The PbS NPLs were subsequently passivated by injecting 1.25 ml of a 0.1 M CdCl₂ solution (in octylamine and oleic acid with a molar ratio of 9 : 1), followed by stirring for 40 min at 35 °C. For thorough purification, the NPLs were precipitated by the dropwise addition of EtOH until the colloidal solution was visibly destabilized (typically requiring 2 ml), centrifuged at 2500 rcf for 10 min. The supernatant was discarded, and the precipitate was redispersed in dry hexane (2 ml). This process was repeated three times before the purified PbS NPLs were sealed under N₂ atmosphere and stored in a refrigerator.

PbSe fQD synthesis

PbSe fQDs were synthesized using a method previously described by our group.^14–16 Two days before the actual synthesis, a solution of selenourea (193 mg, 1.57 mmol) in a mixture of octylamine (2.03 ml), oleic acid (0.23 ml), and hexane (0.75 ml) was prepared and stirred at 35 °C until use in the PbSe fQD synthesis. For a typical synthesis, lead oleate (1.83 mg, 2.7 mmol) was dissolved in a mixture of octylamine (2 ml), oleic acid (4 ml), and hexane (18 ml) at 35 °C. After complete dissolution, the sealed mixture was cooled to 0 °C using an ice bath. Shortly after, the selenourea solution (2.5 ml) was rapidly injected into the vigorously stirred lead oleate mixture. After a reaction time of 10 min, the dark brown reaction mixture was quenched by adding dry EtOH (18.5 ml). For purification, the destabilized colloidal PbSe fQD solution was processed as described above for PbS NPLs.

PbTe NPL synthesis

PbTe NPLs were synthesized using a method previously described by us.¹⁷ Before the actual synthesis, an aminophosphine telluride precursor solution was prepared by heating a stirred mixture of tellurium powder (255 mg, 2 mmol), tris(dimethylamino)phosphine (2 ml, 11 mmol), and octylamine (2 ml, 12.1 mmol) to 100 °C for 3 h under a N₂ atmosphere. After cooling to room temperature, the solution was passed through a 0.2 μm polytetrafluoroethylene syringe filter and stored in a refrigerator until use. For a typical PbTe NPL synthesis, 0.4 ml of a lead oleate stock solution (lead oleate (2.31 g, 3 mmol) in octylamine (6 ml)) was diluted in hexane (3.6 ml) and cooled down to 0 °C under vigorous stirring. Shortly after, 0.2 ml of the aminophosphine telluride precursor solution was rapidly injected, causing a color change to light brown. After a reaction time of 30 min, the dark brown mixture was quenched by adding 2 ml of a 0.1 M PbI₂ solution (in octylamine and oleic acid with a molar ratio of 9 : 1) and diluted with hexane (6 ml). For purification, the PbTe NPL solution was processed as described above for PbS NPLs.

Phase transfer

2D PbX fQDs and NPLs were phase transferred by adapting a protocol for Cd-based nanorods and NPLs described by Kodanek et al.³⁵ First, a phase transfer stock solution containing MUA (1.21 g, 5.56 mmol) and KOH (0.4 mg, 7.09 mmol) in MeOH (18.75 ml) was prepared and stirred at 35 °C until complete dissolution. For a typical phase transfer, the thoroughly purified PbX solutions in hexane (0.3 ml) were further diluted with hexane (0.7 ml) and the MUA phase transfer solution (1.5 ml) was added, resulting in a cloudy biphasic system, which was shaken overnight on a laboratory shaker. Subsequently, the mixture was centrifuged (2500 rcf for 10 min), the biphasic supernatant was discarded, and the precipitated PbX NPLs were redispersed in 2 ml of alkaline water (0.1 M KOH) using a laboratory vortex mixer. To remove any excess ligands, the aqueous 2D PbX solution was thoroughly washed by adding chloroform (2 ml), shaking on a laboratory shaker (10 min), centrifugation (2500 rcf for 10 min), and removal of the organic phase. This process was repeated at least three times, or until the chloroform phase remained completely transparent after shaking, with no white subphase being formed at the chloroform-water interface. The aqueous 2D PbX solutions were stored at 8 °C in a refrigerator.

Transmission electron microscopy (TEM)

For TEM, colloidal 2D PbX NPL solutions were drop-cast onto carbon-coated Cu grids (300 mesh) acquired from Quantifoil. TEM images were obtained using a FEI Tecnai G2 F20 transmission electron microscope equipped with a field emission gun operating at 200 kV.

Visible-NIR PL and absorbance spectroscopy

Samples for optical spectroscopy were prepared by diluting the colloidal 2D PbX solutions in TCE for lipophilic fQDs and NPLs and D₂O for phase transferred aqueous samples (optical density below 0.2 at 500 nm) in quartz cuvettes (quartz glass high performance QS 200–2500 nm with an optical path length of 1 cm from Hellma). Vis-NIR PL spectra were collected using an Edinburgh FLS1000 PL spectrometer equipped with a 450 W ozone free Xe arc lamp for excitation (450 nm for all samples). PbS NPL PL was monitored using a photon counting photomultiplier tube 980 detector from Edinburgh Instruments; PbSe fQD and PbTe NPL NIR PL was monitored using a liquid N₂ cooled InGaAs 1650 photomultiplier tube detector from Edinburgh Instruments. Absolute PLQYs were determined using an integrating sphere. For this, scattering at 450 nm and the PL of TCE/D₂O and the fQDs and NPLs were measured separately, accounting for the difference in sensitivity of both detectors with a correction factor. Visible-NIR absorbance spectra were collected using a double beam Cary 5000 spectrophotometer from Agilent Technologies equipped with a tungsten halogen (visible) and deuterium arc (ultraviolet) lamp and a PbSmart NIR detector for monitoring.

X-ray photoelectron spectroscopy (XPS)

Samples for XPS analysis were prepared by drop-casting the colloidal NPL solutions onto silicon wafers (<001> surface, p-doped with B) from Plano and drying in vacuo overnight. XPS data were obtained on a PHI 5000 VersaProbe III from ULVAC-PHI using an aluminum X-ray source (Al K_α = 1486.6 eV) operating at 24.4 W with a beam diameter of 100 μm. Survey spectra were measured with a pass energy of 224 eV; high-resolution spectra were acquired with a pass energy of 27 eV. Charging effects were accounted for by setting the C 1s peak of sp³ adventitious carbon to 284.8 eV. The background was fitted by a classic Shirley background; all components were fitted by symmetric Voigt functions.

Nuclear magnetic resonance (NMR)

Samples for NMR analysis were prepared by diluting each specimen in D₂O (typically 600 μl) in an NMR tube. NMR measurements were conducted using a Bruker Avance III HDX 400 instrument with a frequency of 400 MHz. Spectra were analyzed using the Bruker TopSpin 4.2.0 software. The residual solvent peak of D₂O/H₂O (4.79 ppm) was used as an internal reference for the ¹H spectra.

Results and discussion

Phase transfer of 2D PbX fQDs and NPLs

Colloidal 2D PbX (X = S, Se, Te) NPLs and fQDs were synthesized from lead oleate and thiourea, selenourea, or aminophosphine telluride, respectively, using methods described previously by our group.^12,14,15,17Scheme 1 illustrates the general ligand exchange procedure used for the phase transfer (see Fig. S1 for photographs of the phase transfer process). Two-fold deprotonated 11-mercaptoundecanoic acid (dissolved with KOH in water) is used to replace the native oleic acid (and octylamine) ligands bound to the as-synthesized fQD and NPL surface. MUA coordinates to 2D PbX mainly via the sulfur, with the unbound carboxylate function ensuring colloidal stability in the aqueous solution. We note that it is crucial to thoroughly precipitate and redisperse the organic pristine PbX fQDs and NPLs so that unreacted precursors or excess ligands are absent prior to adding the phase transfer solution (MUA and KOH in MeOH). This prevents the formation of large quantities of potassium oleate.

Fig. 1 shows TEM images of PbS NPLs, PbSe fQDs, and PbTe NPLs before (a–c) and after (d–f) the phase transfer. The as-synthesized 2D PbS NPLs, in line with previous reports, exhibit a considerably larger lateral size of (20.2 ± 2.4) × (6.6 ± 1.7) nm² (a) compared to PbSe fQDs ((4.3 ± 0.8) × (3.2 ± 0.5) nm² (b)) and PbTe NPLs ((5.5 ± 1.5) × (3.9 ± 1.1) nm² (c)) and show the biggest aspect ratio (3.1 : 1), closely resembling a rectangular shape (see Fig. S2 for lateral size histograms of all samples shown). For organic PbS NPLs we further observe stacking of vertically aligned NPLs in the TEM images (see Fig. S3), consistent with previous reports. Laterally smaller and slightly more irregularly shaped PbSe fQDs and PbTe NPLs do not show stacking. Recently, we have assessed the PbX fQDs and NPLs thickness directly via scanning tunnelling microscopy¹⁶ as well as by X-ray scattering experiments¹⁷ and from stacking in TEM.^15,19 We find values ranging from 0.6–2 nm, which corresponds to a monolayer (1 ML) to a few atomic monolayers. After the phase transfer the average lateral sizes of 2D PbX NCs exhibit only minor changes ((19.3 ± 3.1) × (6.4 ± 1.2) nm² (d), (4.4 ± 0.8) × (3.3 ± 0.6) nm² (e), and (5.6 ± 1.0) × (3.8 ± 0.8) nm² (f)). The cubic rock salt crystalline structure (space group Fm3̄m) remains visible from the lattice fringes in TEM (see Fig. 1 and Fig. S4 for examples at higher magnification) as well as the reflexes in the FFT patterns shown in the associated insets in Fig. 1 (characteristic (200) lattice spacings of 2.9 Å (PbS), 3.1 Å (PbSe), and 3.2 Å (PbTe); see Fig. S4 additionally). The minor size changes can be attributed to surface self-reconstruction of the materials and coincidental size separation due to the additional centrifugation steps during the phase transfer and subsequent purification.

Fig. 1 TEM images of organic PbS (a), PbSe (b), and PbTe (c) NPLs, as well as aqueous PbS (d), PbSe (e), and PbTe (f) NPLs (exemplary NPLs are highlighted for visual aid), exhibiting average lateral sizes ranging from (4.3 ± 0.8) × (3.2 ± 0.5) to (20.2 ± 2.4) × (6.6 ± 1.7) nm² with PbS NPLs generally being the largest. Notably, there is no apparent significant change in the lateral size of the NPLs after the phase transfer to water. All of the presented lead chalcogenide NPLs exhibit the cubic rock salt crystal structure, as indicated by the FFT patterns shown in the insets in each panel, which are composed of the characteristic (200) diffraction peaks (corresponding to 2.9 Å (PbS), 3.1 Å (PbSe), and 3.2 Å (PbTe)).

Optical properties of aqueous 2D PbX fQDs and NPLs

Fig. 2 displays normalized absorbance (Abs) and NIR PL spectra of PbX fQDs and NPLs before (organic; dotted line) and after (aqueous; solid line) the transfer to aqueous solution. The optical characteristics (absorbance features, PL maxima, and fwhm of the PL) of the samples are summarized in Table 1.

Fig. 2 Vis-NIR absorbance and PL spectra of PbS NPLs (top), PbSe fQDs (middle) and PbTe NPLs (bottom) before (dotted) and after (straight line) phase transfer. 2D PbX NCs exhibit weekly expressed excitonic absorbance in the range from 584 and 990 nm as well as NIR PL from 724 to 1184 nm.

Table 1 Excitonic absorbance features (λ_Abs, determined by the 2^nd derivative of the absorbance), PL maxima (λ_PL), and fwhm of the PL of organic and aqueous PbS NPLs, PbSe fQDs, and PbTe NPLs

Sample	λ Abs (nm)	λ PL (nm)	PL fwhm (meV)
org. PbS NPLs	584	731	264
aq. PbS NPLs	592	724	221
org. PbSe fQDs	787	929	233
aq. PbSe fQDs	870	1023	212
org. PbTe NPLs	990	1111	155
aq. PbTe NPLs	989	1184	167

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Before phase transfer all three 2D PbX NC samples exhibit NIR PL at positions (with associated fwhm) that are in good agreement with our previous reports and are representative of the respective material system.^12,14,17,19 PLQY values (org. PbS: 1%, org. PbSe: 23%, and org. PbTe: 4%) are similarly, albeit slightly lower for PbS and PbTe NPLs. We attribute this to the relatively harsh purification required to remove excess oleate and oleic acid before the phase transfer, which is in turn accompanied by ligand stripping, exposing surface traps. The purification step is particularly demanding for PbS and PbTe NPLs, since their syntheses require a large amount of oleic acid and they are generally more labile to chemical changes than PbSe fQDs.

After the phase transfer all three 2D PbX NCs retain sizeable telecommunication band NIR PL. The PL of PbSe fQDs and PbTe NPLs shows bathochromic shifts from 929 nm to 1023 nm and 1111 nm to 1184 nm, respectively. This observation has previously been attributed to a reduction of quantum and dielectric confinement, caused by the addition of a quasi-sulfide layer by the MUA-ligand³⁸ and a dielectric surrounding change due to the water transfer.^28,39 However, for the PbS NPLs we find a slight blue-shift in the absorbance (731 nm to 724 nm). It is assumed that the dielectric surrounding effects are counteracted by size narrowing/exclusion caused by the rigorous washing in the PbS NPL case. This is also reflected in the small decrease in average lateral size, from (20.2 ± 2.4) × (6.6 ± 1.7) nm² down to (19.3 ± 3.1) × (6.4 ± 1.2) nm². In addition, PbS NPLs have the largest lateral size and PbS exhibits the smallest exciton Bohr radius (a_B,PbS = 20 nm),⁴⁰ making it the system with the smallest confinement and minor changes in the chemical environment are expected to influence PbS NPLs less than the extremely confined PbSe fQDs (a_B,PbSe = 46 nm)⁴⁰ and PbTe NPLs (a_B,PbTe ≈ 82 nm).⁴¹

After the phase transfer, the PbX NPLs and fQDS exhibit a PLQY of 1% for PbS NPLs, 15% for PbSe fQDs and 2% for PbTe NPLs. This corresponds to retaining 65% of the initial PLQY in organic solution for PbSe fQDs (a decrease from 23% to 15%) and 50% for PbTe NPLs (from 4% down to 2%), which is in good agreement with the relative PLQY decrease observed during the phase transfer of CdSe NPLs to water (∼50% loss).^24,35 These results render aqueous 2D PbSe fQDs in particular efficient NIR emitters in aqueous solution. For example, in comparison with well-established zero-dimensional PbSe QDs at these wavelengths, e.g. Hyun et al. reported “at least 10%” PLQY for aqueous PbS and PbSe QDs,²⁷ and Lin et al. measured 3.8 to 10% PLQY for ligand exchanged PbS, PbSe, and PbTe NCs in N-methyl-formamide.³¹ To further improve the PLQY of (aqueous phase) 2D lead chalcogenides, shelling with e.g. CdS, resulting in type-I heterostructures, is under investigation currently.^6,42,43

Ligand binding to aqueous 2D PbX fQDs and NPLs

To follow and characterize the lipophilic-to-hydrophilic ligand exchange at the molecular level, we perform X-ray photoelectron spectroscopy (XPS) measurements. Fig. 3 shows the S 2p XPS core-level spectra of organic (a–c) and aqueous (d–f) 2D PbX NCs. The signals primarily consist of the spin–orbit split S 2p_3/2 and 2p_1/2 doublet, with a 2 : 1 area ratio and a doublet separation of 1.2 eV (ref. 44) and minor overlapping non-sulfur contributions by the Se 3p_3/2 and 3d_1/2 doublet^45,46 (separation of 5.3 eV) in case of the PbSe fQDs (Fig. 3b and e) and the Te 4s singlet⁴⁷ in case of aqueous PbTe NPLs (Fig. 3f). The absence of the Te 4s singlet in organic PbTe NPLs suggests that the phase-exchanged PbTe NPLs are more stable under ambient conditions (which is crucial given the aqueous solutions are typically stored outside of the glovebox). They do not decompose by short-term exposure to air, for example during transfer into the vacuum of the XPS. In contrast, the less stable organic PbTe NPLs likely form oxide moieties, with a Te 4s signal consequently shifted to higher energies and originated outside the measured S 2p region compared to the Te–Pb binding motif.

Fig. 3 XPS analysis of the S 2p core level region prior to and after phase transfer of PbS NPLs (a and d), PbSe fQDs (b and e), and PbTe (c and f) NPLs. After the phase transferring to water by the introducing MUA as a ligand, the spectra of all three aqueous 2D PbX NCs show the appearance of RS-Pb (light green) and RSH-Pb (yellow) components, which indicates a combination of X-type bound thiolate and L-type binding of the protonated thiol to the 2D PbX NC surface.

Of the three organic 2D PbX NCs prior to phase transfer, only PbS NPLs (Fig. 3a) exhibit significant S 2p core-level region signals. This can be explained by organic PbSe and PbTe NPLs both not containing sulfur or sulfur-containing ligands. In perfect agreement with values reported by Cao et al. and Malgras et al., the narrow, high-intensity signal contribution at 160.6 and 161.8 eV (S–Pb, light green) is attributed to sulfur bound to lead within the PbS NPL core.^48,49 Additionally, minor contributions of sulfur species with higher oxidation states occur above 163.0 eV (dark grey), which are assigned to the oxidation of sulfur surface atoms due to a lack of protective ligands following the purification required prior to the phase transfer.⁴⁹ The lowest intensity components (Pb*, light grey) was previously assigned to the energy loss from Pb 4f signals.⁴⁹

After the ligand exchange and transfer to water (d–f) S 2p signals rise for all 2D PbX NCs, as the sulfur-containing MUA ligand is introduced. The main component at 160.9 ± 0.3 and 162.1 ± 0.3 eV (RS-Pb and S-Pb, light green) is attributed to deprotonated MUA, which is bound to Pb surface sites via the thiolate function in an X-type manner (see Fig. S5 for a schematic representation of the different ligands binding types).^46,50 For PbS NPLs, these signals coincide with the S–Pb core component and are thus rather indistinguishable. Notably, the X-type binding motif, where thiolate as an anionic one-electron donor forms a covalent bond to a surface metal atom,²³ is the expected one. This motif is commonly depicted in reports on the water transfer of different NCs (also shown in Scheme 1) with mercaptocarboxylic acids (though it is not necessarily probed).^28,35 The assignment of the second major component present for all 2D PbX NCs at 163.2 ± 0.2 and 164.4 ± 0.2 eV (yellow) is less unambiguous in literature, with three different options:

1. Free, unbound thiol (here MUA).⁴⁸

2. “Reverse” X-type binding of mercaptocarboxylic acid by the carboxylate with the free thiol facing the solution.³⁶

3. L-type binding of a protonated thiol as a neutral Lewis base which donates two electrons from the sulfur to form a dative covalent bond.^23,51

Similar to Bagaria et al., we deem the presence of considerable amounts of free, unbound MUA to be unlikely in our case, since the aqueous colloidal solutions were thoroughly precipitated to remove excess MUA (as well as any amine or oleate residues).³⁶ In addition, the drop-casted XPS samples were further rinsed with chloroform prior to being measured. To clarify this and to distinguish between the other two options (see Fig. S5), ¹H NMR measurements were performed and are discussed in Fig. 4. Lastly, it is noteworthy that PbSe fQD and PbTe NPL XPS spectra show no signs of oxidized sulfur species at higher binding energies. This result is consistent with the stability of relatively long-chained thiols towards photooxidation. It underpins that the oxidized sulfur species present in the aqueous PbS NPL sample originates from sulfur atoms in the core that are already present in organic PbS NPLs (see discussion above) and therefore are not to be expected for PbSe fQDs and PbTe NPLs.⁵²

Fig. 4 ¹H NMR spectra of free, deprotonated MUA (a) as well as aqueous PbS NPLs (b), PbSe fQDs (c), and PbTe NPLs (d). The downfield shift of the resonances of free MUA (assigned in panel (e)) compared to MUA in the 2D PbX samples indicates its binding to the 2D PbX surface. The larger shift of the α- and β-protons of the thiolate side ((1) and (3.1)) in comparision to the carboxylate side ((2) and (3.2)) of MUA suggests preferential binding via the sulfur.

Fig. 4 depicts ¹H NMR spectra of deprotonated MUA (phase transfer stock solution (a)), as well as aqueous PbS NPLs (b), PbSe fQDs (c), and PbTe NPLs (d) in D₂O, within the range of 1.45–2.85 ppm. The relevant protons and their respective chemical shifts are labeled in panel (e) and were assigned by applying work of Ristig et al.⁵³ Two-fold unbound deprotonated MUA (as present in the phase transfer stock solution) exhibits two triplet resonances at 2.47 ppm (t, 2H, α_S-CH₂) and 2.10 ppm (t, 2H, α_COO-CH₂) corresponding to the protons in the α-position of the thiolate (1) and the carboxylate (2), respectively. The β-protons appear as two overlapping quintet resonances at approximately 1.53 ppm and 1.50 ppm (m, 4H, β-CH₂). The aliphatic protons, not shown in Fig. 4a, occur as a broad singlet resonance around 1.24 ppm (s, 12H, CH₂), the thiol-proton of partial protonated MUA is expected at a similar resonance (s, 1H, SH), overlapping with the aliphatic protons. The carboxyl-proton (expected above 10 ppm (s, 1H, COOH)) is not present in any of the samples. For discussing the aqueous 2D PbX NC spectra, we focus on the α- and β-protons, all of which are downfield shifted by a similar amount compared to the free MUA for the three lead chalcogenides, which indicates a binding to the 2D PbX surface. It has to be noted that the fQD and NPL spectra do not show resonances of the unbound MUA ligand, which further confirms that the aqueous samples are indeed free of excess ligand (and ruling out free unbound thiol as the origin of the discussed XPS signals at 163.2 ± 0.2 and 164.4 ± 0.2 eV). In detail, the α-protons are shifted by +0.30 ppm to 2.74 (t, 2H, α_S-CH₂, (1)) and +0.07 ppm to 2.17 ppm (t, 2H, α_COO-CH₂, (2)), respectively. These shifts are consistent with those of the β-protons, which diverge in the fQD and NPL samples, resulting in two well separated quintets, with shifts of +0.16 ppm to 1.69 ppm (q, 2H, β_S-CH₂, (3.1)) and +0.04 ppm to 1.54 ppm (q, 2H, β_COO-CH₂, (3.2)), respectively. Notably, the similar shift ratios Δδ(α_S-CH₂)/Δδ(α_COO-CH₂) of 4.29 and Δδ(β_S-CH₂)/Δδ(β_COO-CH₂) of 4.0 underpin the proton assignments. The downfield shifting is associated with binding to the surface because the NC deshields the nuclei of the ligand.^53,54 Consequently, nuclei in closer proximity to the NC surface experience larger deshielding than those farther away. From the larger shifts of the α_S-CH₂ and β_S-CH₂ it is concluded, that MUA is bound to the fQD and NPL surface via the sulfur side. Together with the XPS data it is inferred that the aqueous PbX fQD and NPL surface is covered by a combination of X-type and L-type bound MUA. The NMR analysis further rules out the presence of large quantities of “reversely” X-type bound MUA. Such a configuration would result in different chemical shifts, showing two sets of resonances for the two different orientations. Notably, we do not observe significant NMR line broadening (around 1 Hz for the α_S-proton), which we attribute to solvent–ligand interactions, previously described by De Roo et al.⁵⁵ In brief, the authors conclude that poor ligand solvation is the main cause for NMR line broadening in NC samples and found that water/D₂O produces the narrowest line widths in their study. We assume the same for the highly soluble deprotonated MUA ligands applied here.

The combination of X- and L-type bound MUA can address surface sites in a manner similar to the commonly used combination of X- and L-type oleate and octyl- or oleylamine in organic solution. This may be interpreted as one of the reasons for the successful and widespread use of mercaptocarboxylic acids in phase transfer processes involving compositionally and structurally very diverse NC systems.^27,28,35

Conclusions

Up to now, there have been no phase transfer protocols for 2D PbX fQDs and NPLs with telecommunication band NIR PL in aqueous solution. Here, we adopt phase transfer protocols based on MUA to PbS NPLs, PbSe fQDs, and PbTe NPLs, yielding aqueous solutions and retaining their NIR PL. Aqueous PbSe fQDs (post-phase transfer quantum yield of 13%) are competitive with aqueous spherical PbX NCs, making them interesting for further processing into functional nanocomposites. Using XPS and NMR, we conclude on a combined role of X- and L-type binding of MUA to the fQDs and NPLs via the sulfur side (as thiolate and thiol). Our findings advance the understanding of efficient mercaptocarboxylic acid-based nanomaterial water transfer and will help to use the potential of 2D PbX NPLs and fQDs for the incorporation in (quantum) optical applications at technologically relevant wavelengths.

Author contributions

Conceptualization: L. B. and J. L.; funding acquisition: J. L.; investigation: L. B., D. A. R., and M. T. V.; supervision: J. L.; visualization and writing – original draft: L. B.; writing – review and editing: L. B. and J. L. All authors have given approval to the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article has been included as part of the supplementary information (SI). Supplementary information: photographs of the phase transfer procedure; lateral size histograms of organic and aqueous PbS NPLs, PbSe fQDs, and PbTe NPLs; TEM images of stacks of vertically oriented organic PbS NPLs; higher magnification TEM images of 2D PbX NCs; schematic representation of the possible binding motifs of MUA to a PbX NPL surface. See DOI: https://doi.org/10.1039/d5nr03504j.

Acknowledgements

L. B. and J. L. gratefully acknowledge funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany's Excellence Strategy within the Cluster of Excellence PhoenixD (EXC 2122, Project ID 390833453). J. L. is thankful for funding by the Ministry for Science and Culture of the State of Lower Saxony (MWK) for a Stay Inspired: European Excellence for Lower Saxony (Stay 3/22 7633/2022) Grant and for additional funding by an Athene Grant of the University of Tübingen (by the Federal Ministry of Education and Research (BMBF) and the Baden Württemberg Ministry of Science as part of the Excellence Strategy of the German Federal and State Governments).

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