Surface Chemistry Control of Colloidal Quantum Dot Band Gap

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C: Physical Processes in Nanomaterials and Nanostructures

Surface Chemistry Control of Colloidal Quantum Dot Band Gap Carlo Giansante J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05124 • Publication Date (Web): 13 Jul 2018 Downloaded from http://pubs.acs.org on July 15, 2018

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The Journal of Physical Chemistry

Surface Chemistry Control of Colloidal Quantum Dot Band Gap

Carlo Giansante NANOTEC-CNR Istituto di Nanotecnologia, via per Arnesano, 73100 Lecce, Italy

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract. Surface chemistry modification of as-synthesized colloidal inorganic semiconductor nanocrystals (QDs), commonly referred to as ligand exchange, is mandatory towards effective QD-based optoelectronic and photocatalytic applications. The widespread recourse to ligand exchange procedures on metal chalcogenide QDs often narrows optical band gap, although little consensus exists on explanation of this experimental evidence. This work attempts at providing a comprehensive description of such phenomenon by exploiting rationally designed thiol ligands at the surface of colloidal PbS QDs, as archetype of material in the strong quantum confinement regime: the thiol(ate)induced QD optical band gap reduction almost linearly scales with inorganic core surface-to-volume ratio and mainly depends on the sulfur binding atom, which is here suggested to contribute occupied 3p orbitals to the valence band edge of the QDs. As opposed to QD models based on the analogy with core/shell heterostructures, the indecomposable character of ligand/core adducts (the colloidal QDs themselves) arises.

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INTRODUCTION. Surface effects are ubiquitous at the nanoscale, as surface-to-volume ratio inherently increases with decreasing material size. Colloidal inorganic nanocrystals, which can show more than half of their atoms at the surface, may therefore be regarded as soluble frameworks to conveniently study average surface effects in solution phase; inorganic semiconductors in the quantum confinement regime (QDs) do indeed constitute remarkable examples as their size-dependent optical properties can be very sensitive to surface chemical modification. The widespread recourse to post-synthesis strategies to replace bulky, electrically insulating ligands at the QD surface coming from the synthetic procedures requires the elucidation of the role of surface species on the optoelectronic properties of the QDs. The observation of optical band gap reduction in metal chalcogenide (CdS, CdSe, and PbS, prevalently) QDs upon ligand exchange with sulfur-based species (sulfides,1,2 metal-sulfide complexes,3 thiols,4-6 dithiocarbamates,7,8 dithioates9) is indeed an early, common evidence for the pivotal role exerted by surface species on QD optical properties. Two main explanations to such experimental evidence have appeared in the literature accounting for ligands that may induce local modification of the QD (i) electric field and (ii) potential energy landscape. Indeed, ligands are commonly conceived as (i) dielectric shell10 or as (ii) potential energy barrier11 that may affect exciton confinement. Organic molecules yields relevant first exciton red shift (up to several hundred meV), generally larger than that induced by inorganic species, and are thus suitable for fundamental studies: here, thiol(ate) derivatives (shown in Scheme 1) are exploited as replacing ligands with tunable intrinsic electric dipole moments and ionization potentials, but with analogous binding group for the QD surface,12-14 in order to describe and explain ligand-induced optical band gap reduction of colloidal PbS QDs, as archetype of material in the strong quantum confinement regime.15 It is here demonstrated that the origins of ligand-induced QD optical bandgap reduction mainly reside in the sulfur binding group, which is suggested to contribute occupied orbitals to the QD valence band edge with an almost linear dependence on the surface-to-volume ratio; whereas ligand-induced local modification of the electric field and/or the potential energy landscape of colloidal QDs are shown as not adequate to comprehensively explain experimental evidences.

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EXPERIMENTAL SECTION. Materials. All chemicals were of the highest purity available unless otherwise noted and were used as received; complete list appears in the Supporting Information. Material synthesis. Synthesis of colloidal PbS QDs and Pb complexes was performed in a three-neck flask connected to a standard Schlenk line setup under oxygen- and water-free conditions; detailed synthetic and purification procedures appear in the Supporting Information. Ligand Exchange Procedure. Post-synthesis QD surface chemical modification was performed in a closed system (e.g., a vial) by adding µL aliquots of mM solutions of the replacing ligands (Scheme 1) to µM solutions of the QDs in chlorinated solvents. Details appear in the Supporting Information. Material characterization. Spectrophotometric titration experiments were performed in quartz cuvettes with 1 cm path length and recorded with Varian Cary 5000 UV-Vis-NIR and Perkin Elmer Lambda 1050 spectrophotometers; 1

H Nuclear Magnetic Resonance (NMR) spectra were recorded in CDCl3 with a Bruker AV400 spectrometer

operating at 400 MHz; Fourier Transform Infrared (FTIR) spectra were recorded in the 4000-400 cm-1 spectral range using a Jasco FT/IR 6300 spectrophotometer apparatus operating in transmission mode at a resolution of 1 cm-1; Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) measurements were performed with a Varian 720-ES spectrometer; Transmission Electron Microscopy (TEM) images were recorded with a Jeol Jem 1011 microscope operated at an accelerating voltage of 100 kV. Details are reported in the Supporting Information.

RESULTS AND DISCUSSION. QD surface chemistry modification. Ligand exchange is performed in solution phase by simple addition of aliquots of thiol ligand solutions to as-synthesized PbS QDs, thus permitting to neglect eventual red shift due to inter-QD excitonic/electronic coupling that may result upon chemically modifying the QD surface in solid phase and to control QD concentration avoiding phase transfer or filtering that may hinder quantitative comparisons.7-10,16 Ligands used in this work are shown in Scheme 1: a widely used, simple ligand as p-methylbenzenethiolate (ArS¯, as triethylammonium ionic couple) is conceived as framework to introduce electron-donating and withdrawing substituents (such as for p-aminobenzenethiolate and p-trifluoromethylbenzenethiolate; D-ArS¯ and A-ArS¯, respectively) and to allow comparison with sterically-encumbered (o-dimethylbenzenethiolate; SE-ArS¯) and

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saturated (1-butanethiol; AlSH, unlikely depronotonated by amines) analogs. Addition of equimolar amount of ArS¯ is found as sufficient to quantitatively displace oleate ligands coming from the synthetic procedure (as demonstrated by 1H-NMR and FTIR spectra shown in Figure S1).

Scheme 1. Depiction of the replacing ligand framework at the QD surface conceived as constituted by a binding group and a pendant moiety, further consisting of a backbone bearing functional substituents. The ligand library used in this work is presented with corresponding acronyms.

Experimental ligand-induced QD optical bandgap reduction. Size-dependence of QD first exciton peak bathochromic shift upon addition of ArS¯ is shown in Figure 1a. The modified absorption spectra do not show any light scattering ascribable to aggregation, with negligible extinction of the incident light at energies below the first excitonic peak; in addition, the spectral changes are constant with time and reach plateau at given ligand to QD molar ratio beyond which the absorption spectra do not appreciably change (plots of the spectrophotometric titrations are shown in Figure S2), suggesting that the QD surface is no longer accessible to extra added ligands; such a behavior is not compatible with aggregation, which would be highly experiment dependent (in this regard, five different titration experiments are shown in Figure S3 permitting to evaluate the reproducibility of such ligandinduced spectral changes).9,16-18 At first sight, it clearly appears that larger optical band gap reduction occurs for

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smaller QDs, in analogy with the concomitant broadband optical absorption enhancement that was previously reported for this ligand/core system.18 Ligand-dependence of the QD optical band gap reduction is presented in Figure 1b. Upon addition of ArS¯ to PbS QDs with 2.9 nm diameter, the bathochromic shift saturates at about 90 meV, with concomitant line narrowing and molar absorption coefficient increase. The introduction of an electron-donating substituent (as the amino group in DArS¯) leads to slightly larger red shift (of about 100 meV), together with a significantly larger increase of molar absorption coefficient (close to about 20 % compared to ArS¯); whereas the presence of an electron-withdrawing group (as the trifluoromethyl group in A-ArS¯) leads to spectral changes analogous, or slightly smaller, to those induced by ArS¯. Upon removing conjugation (as for the aliphatic thiol, AlSH), the extent of red shift and line narrowing is negligibly affected, whereas the enhanced absorption is reduced by about 20 % compared to ArS¯. Butanethiol is here conceived as saturated analog of ArS¯, although triethylamine is expected to be unable to deprotonate butanethiol as it is much weaker acid than aromatic thiols.12-14 In principle, neutral and anionic ligands (thiols and thiolates, respectively, also conceivable as L- and X-type ligands albeit the latter are formally charge neutral) can be expected to show different affinities for neutral (stoichiometric, {100}) and charged (metal-rich, {111}) facets of the PbS QDs; plot of the spectrophotometric titrations leading to the spectra shown in Figure 1b does not suggest that eventually different Hammett constants and binding motifs of the ligands may markedly affect the energy of the resulting QD optical band gap (see Figures S4 and S5). The size- and ligand-dependent QD optical band gap reduction are resumed in Figure 1c clearly showing that larger optical band gap reduction occurs for smaller QDs and with similar extent for all the investigated thiol(ate) ligands, unless sterically encumbered (the two methyl groups in orto position in SE-ArS¯ prevent binding at the QD surface, albeit the affinity for Pb cation can be expected as similar to other thiols).

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Figure 1. a) As-recorded absorption spectra of as-synthesized PbS QDs (dashed lines) and upon addition of ArS¯ at plateau (solid lines). Spectra of QDs with different diameters are vertically offset for clarity; the asterisk, *, indicates omitted spectral region showing ligand absorption features; spectra in linear energy scale are shown as Figure S6. b) Molar absorption coefficients of PbS QDs with diameter of 2.9 nm upon addition of the thiol(ate) ligands shown in Scheme 1. c) Plot of the size–dependent QD first exciton peak bathochromic shift observed upon addition of the five thiol(ate) ligands to QD solutions.

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On the origins of ligand-induced QD optical bandgap reduction. Data shown in Figure 1 suggest the following. The red shift of the first exciton peak is confirmed as general to PbS QDs upon thiol(ate) addition; the extent of such spectral shift largely depends on effective QD surface coordination and is indeed directly proportional to the number of bound ligands, whereas minor for sterically hindered replacing ligands (as o-dimethylbenzenethiolate). The red shift is larger the smaller the QDs and varies between about 5 and 120 meV. As the PbS QDs here investigated are in a size range that implies strong quantum confinement (with diameters between about 2 and 7 nm), size-dependence can be expected to cease for QD sizes exceeding charge carrier Bohr radii in the corresponding bulk material (which is of about 20 nm for PbS). The extent of red shift appears as almost insensitive to the ligand pendant moiety of the thiol(ate) ligands, thus mainly related to the sulfur binding atom. The pendant moiety instead exerts its effects mostly on the absorption coefficient: conjugated ligands do indeed increase absorption coefficient at the first exciton peak (and at all wavelengths18), with further contribution from electron-donor substituents. Presented experimental evidences point to a red shift of the QD first exciton peak prevalently due to the sulfur binding atom of the thiol(ate) replacing ligands, with contribution rather linearly proportional to the inorganic core −1

surface-to-volume ratio ( d QD ): this is clearly shown in Figure 2.

Figure 2. Plot of the first exciton peak red shift observed upon addition of ArS¯ to solutions of colloidal PbS QDs as a function of the QD surface-to-volume ratio; solid line is the linear regression of experimental data. Circles represent mean values, vertical

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error bars account for two standard deviations of uncertainty on the QD first exciton peak, and horizontal error bars account for one standard deviation of uncertainty on QD diameter.

In order to take into account that smallest cluster ( ∆E g

max

∆Eg can be expected to tend to 0 for bulk PbS and to a finite value for the

, here tentatively estimated from the optical absorption onset of Pb-oleate and Pb-(tert-

butyl)benzenethiolate complexes shown in the Supporting Information ), a logistic function with only one dimensionless parameter is used and reported as Supporting Information (equation S1 and Figure S7). On the basis of PbS electronic structure,21 it is thus suggested that thiol(ate) ligands do contribute occupied 3p orbitals, localized on sulfur lone pair, mainly to the valence band edge of PbS QDs, which is prevalently constituted by 3p orbitals of the chalogenide anions –and by 6s Pb orbitals–; as band gap informs on the relative positions of the band energies, the concomitant effect of QD surface coordination by thiol(ate) species on the conduction band edge (which is prevalently constituted by unoccupied 6p orbitals of the metal cations) is here arbitrarily neglected. Within this framework, carboxylate and phosphonate ligands are instead expected to contribute 2p orbitals of the binding oxygen atoms that may lie deeper in the valence band with minor impact on the optical band gap, as also observed for cynnamate derivatives at the PbS QD surface.22 The suggested 3p orbital contribution is directly proportional to the number of ligand orbitals over the total number of QD orbitals as inferred by the almost linear surface-to-volume dependence of the first exciton bathochromic shift (shown in Figure 2). Comprehensive discussion of ligand-induced optical band gap reduction, however, have to include concomitant electron-phonon coupling,22 together with broadband optical absorption enhancement and shift of relevant band energies compared to vacuum, which are instead largely ligand pendant moiety dependent.9,18,23-25 This may lead to a thorough description of the electronic structure of colloidal QDs. Comparison with previous models: thiol ligands as sulfur monolayer. The relaxation of exciton confinement inherent to the band gap reduction could be qualitatively regarded as analogous to the QD growth by addition of a sulfur monolayer coming from thiol ligands to the metal-rich PbS core. Such an apparent increase of the QD diameter ( ∆d QD ) can be evaluated as an excitonic diameter increase by using an empirical relationship between QD bandgap and size (an inverse second order polynomial;26 details on calculations are given as Supporting Information). The apparent QD diameter increase upon exchange with ArS− ligands shows non-monotonic size-

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dependence ( ∆d QD as function of

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d QD is plotted in Figure 3). Albeit data are scattered, conceiving the red shift as

analogous to the addition of a sulfur monolayer appears as simplistic, also considering the size-dependent composition, shape, and ligand coverage of PbS QDs.19,20

Figure 3. Plot of the ArS−-induced first exciton peak red shift as an apparent diameter increase and as corresponding number of added sulfur monolayers; circles represent mean values, vertical error bars account for two standard deviations of uncertainty on the QD first exciton peak energy, and horizontal error bars account for one standard deviation of uncertainty on QD diameter.

Comparison with previous models: ligands as dielectric shell. The experimental data are below discussed within the framework of a common description of ligands at the QD surface mutuated from classical electromagnetism, in which ligands may affect the core electric field with potential impact on first exciton energy. In analogy to core/shell heterostructures, the surface ligands can be regarded as a dielectric shell surrounding the inorganic core, at which dielectric mismatch may induce surface polarization affecting exciton energy. Upon considering a two-state nearly free-charge carrier model within the framework of effective mass approximation,27 such polarization effect can be described as a correction to the energy of the QD first exciton;28-30 the observed bathochromic shift, ∆E g , thus coincides with the difference in the polarization term upon ligand exchange, ∆δ , according to:

∆E g ≡ ∆δ

(1)

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∞ d πe 2 QD with δ = ∑ 2ε c ε 0 d QD l =1 2

and

2l +1

1

Al ∫ [ j0 (πx)] x 2l +2 dx 2

(2)

0

Al ∝ ε c , ε l , ε s

in which

ε c , ε l , and ε s

(3)

refer to the high-frequency dielectric constants of PbS core, ligand shell, and solvent,

respectively, as depicted in the inset of Figure 4. Within this model for colloidal QDs, minor polarization contribution to the first exciton energy is expected upon exchanging oleates for benzenethiolates, as shown by dashed line in Figure 4 (further details on calculation are given in the Supporting Information). Previous explanation for ligand-induced optical bandgap reduction based on the description of ligands at the QD surface as dielectric shell indeed relies on envisaging anomalously large polarization effects.10

Figure 4. Plot of the first exciton peak red shift observed upon addition of ArS¯ to solutions of colloidal PbS QDs (full circles) and optical band gap reduction predicted by polarization model (dashed line; eqs. 1-3). Depiction of the dielectric QD model appears in the inset. Details appear in the Supporting Information.

Comparison with previous models: ligands as potential energy barrier. The experimental evidences are now discussed within the framework of another common description of ligands at the QD surface that is derived from quantum electrodynamics, in which ligands determine the potential energy barrier at core boundaries with

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consequent impact on quantum confinement of charge carriers. Indeed, as further analogy to core/shell heterostructures, colloidal QDs can be described as hybrid heterojunctions, in which the height of the potential energy barrier at the ligand/core interface affects quantum confinement. Upon describing QD first exciton energy, within the effective mass approximation,27 with the particle in a double spherical finite potential well model, the probability of exciton delocalization on the ligand shell can be described as first order perturbation of the ground state energy;31-33 the experimentally observed optical band gap reduction, ∆E g , can be thus related to the different quantum confinement energies of one of the charge carriers (the hole, presumably, ∆E h ), according to:

∆E g ≡ ∆E h

(4)

For such a charge carrier under the action of a force in a symmetric, stepwise potential:

 h2 1 r r r − ∇ ∇ + V (r )ψ (r ) = E hψ (r )  2 m 

 Vc ,   r V (r ) = V (r ) = Vl ,   Vs , 

in which

Vc , Vl , and Vs

0≤r≤ d QD 2 d QD 2

(5)

d QD 2

Surface Chemistry Control of Colloidal Quantum Dot Band Gap

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