# Adjusting the Energy Levels of HgTe and InAs Nanocrystals with Alkali Ions

Dario Mastrippolito, Majed Ibrahim, Oleksandra Yeromina, Mariarosa

Cavallo, Giorgia Strobbia, Erwan Bossavit, Jiho Roh, Marco Paye, Adrien

Khalili, Yoann Prado, et al.

► To cite this version:

Dario Mastrippolito, Majed Ibrahim, Oleksandra Yeromina, Mariarosa Cavallo, Giorgia Strobbia, et al.. Adjusting the Energy Levels of HgTe and InAs Nanocrystals with Alkali Ions. Journal of Physical Chemistry C, inPress, 10.1021/acs.jpcc.5c06879 . hal-05430624

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Adjusting the Energy Levels of HgTe and InAs Nanocrystals with
Alkali Ions

Dario Mastrippolito¹,², Majed Ibrahim³, Oleksandra Yeromina³, Mariarosa Cavallo¹, Giorgia Strobbia¹, Erwan Bossavit¹,², Jiho Roh¹, Marco Paye¹, Adrien Khalili¹, Yoann Prado¹, James K. Utterback¹, José Avila², Sandrine Ithurria⁴, Mathieu G. Silly², Debora Pierucci¹, Peter Reiss³, Emmanuel Lhuillier¹*

¹Sorbonne Université, CNRS, Institut des NanoSciences de Paris, 4 place Jussieu, 75005 Paris,
France.

²Synchrotron SOLEIL, L'Orme des Merisiers, Départementale 128, 91190 Saint-Aubin, France.

³Univ. Grenoble Alpes, CEA, CNRS, IRIG-SyMMES, STEP, 38000 Grenoble, France

⁴Laboratoire de Physique et d'Etude des Matériaux, ESPCI, PSL Research University, Sorbonne Université, CNRS, 10 rue Vauquelin, 75005 Paris, France.

**Abstract:** The photodiode stack is the most effective design geometry for integrating colloidal nanocrystals (NCs) into optoelectronic devices dedicated to light emission and detection. Traditional designs rely on determining the absolute energy band alignment, followed by selecting suitable materials to transport charges (with energy levels resonant to the active material's bands). Because of this method's inherent limitations, we propose to explore an alternative approach where alkali metals are used to tune the absolute energy levels of the optically active layer. We illustrate this concept using lithium and caesium deposition onto narrow band gap NC films (i.e., HgTe and InAs), which are relevant materials for infrared optoelectronics. Our results show that work function shifts up to 0.9 eV can be achieved and that smaller alkalis are more effective at generating this shift. However, different behaviors are observed for HgTe and InAs. In the case of II-VI materials, the alkali acts as a pure dipole (i.e., no shift in the core level), and the film behaves as a bulk effective medium (i.e., no evidence of alkali intercalation). For III-V NCs, the alkali plays a dual role as both a dipole and a redox agent, making the alkali's effect dependent on the film's surface-to-volume ratio and the size of the alkali.

**Keywords:** nanocrystals, alkali, doping, X-ray photoemission, work function.

*To whom correspondence should be sent: el@insp.upmc.fr
INTRODUCTION

Colloidal nanocrystals (NCs)¹ have been established as relevant building blocks for photonics and optoelectronics.²⁻⁴ However, the latter in particular raises serious challenges, because both optical properties and charge conduction must be managed simultaneously. For a long time, the low mobility of NC films was considered a major bottleneck. However, substantial progress in ligand-exchange procedures has brought carrier mobility to acceptable levels,⁵,⁶ though simultaneously opening new opportunities for device design.⁷

While this kind of ligand exchange improves mobility,⁸,⁹ it also determines the band alignment¹⁰⁻¹² and doping of the material.¹³ This becomes a clear challenge when designing the carrier-density landscape of devices, in particular for vertical diode geometries, which is the most successful device architecture in NC-based optoelectronics. In such devices, the optically active layer is sandwiched between charge-transport layers to form a *p*-(*i*)–*n* junction. Their rational design generally relies on determining the absolute band alignments on an energy scale using methods such as electrochemistry or X-ray/UV photoemission. The hole (respectively electron) transport layer is then chosen so that its valence (respectively conduction) band is quasi-resonant with that of the active layer, while additional filtering of minority carriers can be achieved by carefully selecting the transport-layer band gap. In practice, such conditions can be difficult to realize. For example, oxide layers that are often used for this purpose can be difficult to deposit under mild conditions (e.g., thermal evaporation) due to their high melting points. Their deposition is instead realized by sputtering, which can induce damage in NC films.

Therefore, it is of the utmost interest to move away from this paradigm. Instead of identifying layers that match the optically active region, it is possible to directly shift the energy levels of the active materials. The presence of a dipole at the material's surface can induce such a shift, which can be used to form a *pn* junction or reduce the magnitude of the Schottky barrier at the interface with metal. In NCs, this has been explored both at the NC surface via ligand exchange¹³ and at the device level by introducing self-assembled monolayers at the interfaces within the diode stack.¹⁴,¹⁵ For thin-film semiconductors, alkali deposition can also induce dipoles,¹⁶⁻¹⁹ that tune the material's work function (WF). Such effects are typically used in photocathode design to ease the electron extraction.²⁰ Recently, the concept was tested on colloidal NCs²¹: potassium (K) deposition was shown to induce WF shifts up to ≈1 eV, while leaving core levels, and thus carrier density, mostly unchanged. The vacuum-level shift follows Eq. (1)¹³,²²:

$$ \Delta E = \frac{eN_{\mu}}{\varepsilon_0 \varepsilon_r} (1) $$

where *e* is the elementary charge, *N* the surface density of dipoles, *μ* the dipole moment, *ε*₀ the vacuum permittivity, and *ε*<sub>r</sub> the relative permittivity. Since varying the atomic radius is expected to modulate both the dipole magnitude and its density, it is therefore of great interest to expand the original work on potassium²¹ to other alkali metals.

We choose to focus here on infrared NCs. Their lower maturity compared to visible NCs, which have reached industrial and commercial status, motivates the development of new strategies to tune electronic structures. More specifically, we focus on two types of NCs. First HgTe²³, a mature system, offering unique spectral tunability²⁴ from the visible to the THz range, and is relatively robust against oxidation.²⁵ Second, InAs,²⁶⁻³² often considered a heavy-metal-free alternative to lead- and mercury-based NCs for short-wave infrared optoelectronics³³,³⁴. Although promising, InAs is more sensitive to oxidation, and its syntheses that involve a strong reduction of arsenic²⁹ may yield surfaces that are less redox-defined than those of II–VI compounds. Using X-ray photoemission spectroscopy, we investigate how the electronic structure of nanocrystals with various sizes is affected by lithium (Li) and caesium (Cs) deposition.
# METHODS

**Chemicals:** Mercury chloride ($HgCl_2$, Sigma-Aldrich, 99%), Tellurium powder (Te, Alfa-aesar, 99.99%), Indium(I) chloride ($InCl_3$, 99.995%; Fisher Scientific). Tris(dimethylamino)arsine (DMAAs, 99%; STREM), trioctylphosphine (TOP) (thermofisher scientific, 90%), Tri-n-butylphosphine (TBP, 99%, anhydrous; Merck.), oleylamine (OLA, Acros, 80-90% for HgTe and OLA, ≥98%; Merck for InAs), dodecanethiol (DDT, Sigma-Aldrich, 98%), ethanol absolute (VWR), hexane (VWR, 99%), toluene (VWR, 99.5%), chlorobenzene (VWR), methanol (VWR, >98%), N,N dimethylformamide (DMF, VWR), 2-mercaptoethanol (MPOH, Merck, >99%). tetrachloroethylene (TCE, VWR). All chemicals are used without further purification, except oleylamine, which is centrifuged before use in the case of HgTe NCs.

**1 M TOP:Te precursor:** 6.35 g of Te powder is mixed with 50 mL of TOP in a three-neck flask. The flask is kept under vacuum at room temperature for 5 min before the temperature is raised to 100 °C. Degassing is conducted at this temperature for 20 min. Then, the atmosphere is switched to $N_2$ and the temperature is raised to 275 °C. The solution is stirred until a clear orange coloration is obtained. The flask is afterward cooled down to room temperature, and the color turns to yellow. Finally, this solution is transferred to a nitrogen-filled glove box for storage.

**Synthesis of HgTe with band-edge at 6000 cm⁻¹:** synthesis is inspired by Ref.³⁵ In a 100 mL three-neck flask, 540 mg of $HgCl_2$ (2 mmol) and 50 mL of oleylamine are degassed under vacuum at 110 °C for 1h. Then, the atmosphere is switched to $N_2$, and the temperature stabilizes at 58 °C. Meanwhile, 2 mL of TOP:Te (1 M) is extracted from the glove box and mixed with 8 mL of oleylamine. Then, the TOP:Te solution is quickly injected. After 3 min, 10 mL of a mixture of 10% DDT in toluene is injected, and a water bath is used to quickly decrease the temperature. The content of the flask is split over 3 centrifuge tubes. MeOH is added, and the solution is sonicated for 3 min. After centrifugation, the formed pellets are redispersed in one centrifuge tube with toluene. The solution is precipitated a second time with absolute EtOH and sonicated for 3 min. Again, the formed pellet is redispersed in toluene. Then, the NCs are centrifuged in pure toluene to get rid of the lamellar phase. The solid phase is discarded.

**Synthesis of HgTe with band-edge at 4000 cm⁻¹:** In a 100 mL three-neck flask, 540 mg of $HgCl_2$ (2 mmol) and 50 mL of oleylamine are degassed under vacuum at 110 °C for 1h. Then, the atmosphere is switched to $N_2$, and the temperature stabilizes at 82 °C. Meanwhile, 2 mL of TOP:Te (1 M) is extracted from the glove box and mixed with 8 mL of oleylamine. Then, the TOP:Te solution is quickly injected. After 3 min, 10 mL of a mixture of 10% DDT in toluene is injected, and a water bath is used to quickly decrease the temperature. The content of the flask is split over 3 centrifuge tubes. MeOH is added, and the solution is sonicated for 3 min. After centrifugation, the formed pellets are redispersed in one centrifuge tube with toluene. The solution is precipitated a second time with absolute EtOH and sonicated for 3 min. Again, the formed pellet is redispersed in toluene. Then, the NCs are centrifuged in pure toluene to get rid of the lamellar phase. The solid phase is discarded.

**Synthesis of HgTe with band-edge at 3000 cm⁻¹:** In a 100mL three-neck flask, 543 mg of $HgCl_2$ and 50 mL of oleylamine were degassed under vacuum at 110°C. Meanwhile, 2 mL of TOP:Te (1 M) was extracted from the glove box and mixed with 8 mL of oleylamine. After the flask atmosphere was switched to $N_2$ and the temperature stabilized at 110°C, the TOP:Te solution was quickly injected. After 3 mins, 10 mL of a mixture of DDT in toluene (10% of DDT) was injected, and a water bath was used to quickly decrease the temperature. The content of the flask was split over 3 centrifuge tubes, and MeOH was added. After centrifugation, the formed pellet was redispersed in
one centrifuge tube with chloroform. The solution was precipitated a second time using ethanol. The formed pellet was redispersed in chlorobenzene, this solvent offers a higher colloidal stability compared to toluene for large HgTe NCs. At this step, the nanocrystals were centrifuged in pure toluene to get rid of the lamellar phase. The solid phase was discarded. The solution is filtered with a 0.2 µm filter.

**HgTe ink preparation:** 30 mg of HgCl₂ are dissolved in a solution of 18 mL of DMF and 2 mL of mercaptoethanol to form an exchange solution. 500 µL of HgTe NC solution is mixed with 500 µL of the exchange solution. 5 mL of hexane and 500 µL of DMF are added to the HgTe solution. The top phase is then removed, and the solution is washed two more times, adding 5 mL of hexane and removing the top phase each time. At each step, the solution is stirred with a vortex and an ultrasonic bath. Finally, 1.5 mL of toluene is added, and the solution is centrifuged for 3 min at 6000 rpm to precipitate the nanoparticles. The supernatant is discarded, and the formed pellet is dried using nitrogen flux, then redispersed in 300 µL of DMF to achieve, during spin-coating, a target thickness of approximately 50 nm. The resulting solution is referred to as 'ink'. Before ink deposition, a drop of DMF is spin-coated on the gold-coated (80 nm) silicon substrate to improve wettability.

**Synthesis of InAs NCs:** 30 µL of tris(dimethylamino)arsine (DMAAs) in 0.4 mL of degassed OLA were heated at 50°C for 10 minutes. InCl₃ (0.5 mmol, 3.1 eq), 0.3 mL TBP, and 7.2 mL of OLA were loaded into a 50 mL three-neck round-bottom flask equipped with a magnetic stirring bar and reflux condenser. The mixture was degassed under vacuum at 50°C for one hour and then heated to 240°C under argon. Once the reaction mixture reached 240°C, 0.4 mL of arsenic precursor was rapidly injected under vigorous stirring. After one hour, the reaction was stopped by cooling down the flask to room temperature. This synthesis yields around 3.2 nm InAs QDs with an excitonic peak centered at 940 nm. For the larger, 5.7 nm InAs CQDs with a 1200 nm peak position, the synthesis was performed at a higher temperature, 300°C instead of 240°C. To extract the NCs from the reaction mixture, 0.3 mL of the crude colloidal solution were mixed with 0.3 mL of anhydrous chloroform, 1 mL of anhydrous methyl acetate, and 1 mL of anhydrous acetone. The resulting suspension was centrifuged (8000 rpm, 2 min). The precipitate was dispersed in 0.5 mL of toluene for further use.

**III-V NCs thin film preparation:** Inside a glove box, a few µL of the as-synthesized InAs solution are drop-cast onto a gold-coated (80 nm) silicon substrate and left to dry. The sample is then immersed in methanol, which serves to strip the ligands and minimize charging during XPS measurements. All the synthesis, cleaning, and film preparation are conducted in air-free conditions, and then brought to the synchrotron in a nitrogen-filled hermetic vial. The only moment where films are exposed to air is the 2-minute transfer from the vial to the load lock of the XPS setup.

**X-ray photoemission measurements (XPS):** For photoemission spectroscopy, measurements were conducted at the TEMPO beamline of synchrotron SOLEIL.³⁶ Samples were introduced in the preparation chamber and degassed until a vacuum below 10⁻⁹ mbar was reached. Then, samples were introduced into the analysis chamber. The signal was acquired by a MBS A-1 photoelectron analyzer, with acquisitions performed at a constant pass energy of 50 eV within the detector. Photon energy of 150 eV was used for valence band acquisition, low binding energy core levels (BE <120 eV), while 700 eV photon energy was employed for the analysis of the core levels at higher BE. The zero binding energy (BE) reference (i.e., the Fermi level) was taken at the leading edge of a clean Au film. Core levels were first acquired at 700 eV, after which the photon energy was switched to 150 eV to maximize the surface contribution and thus the signal from the valence band and from the cut off of the secondary electron (i.e., the WF). After each deposition step, a series of spectra
was collected while keeping the energy constant. This procedure allows us to exclude any shift
coming from calibration issues. Once evaporation were completed, the photon energy was
switched back to 700 eV for high-energy core level acquisition. The acquired data were processed
by subtracting a Shirley/linear background and using pseudo-Voigt functions (20% weighted
Lorentzian contribution) for peak fitting.

**Alkali deposition procedure:** The evaporation of alkali was performed in the UHV analysis chamber ($P_{\text{chamber}}=5\times10^{-10}$ mbar) by heating an alkali metal dispenser (supplied by SAES). A current of 3 A was applied to each dispenser for an entire day to degas it and remove potential contamination, particularly oxygen species. For the alkali evaporation, a current of 5.1 A was used for the Cs evaporation, while a current of 7.2 A was used for the Li evaporation. After each evaporation, the dispenser was maintained at 3 A to keep it clean and free from contaminants. Multiple depositions were carried out, and after each deposition, the alkali coverage was assessed by comparing the relative intensities of its specific alkali core level with that of a core level of the underlying material with a similar binding energy. Specifically, for the Li evaporation, we track the evolution of the Li 1s in relation to the Te 4d (for the HgTe) or the As 3d (for the InAs). Conversely, for the Cs evaporation, we track the evolution of the Cs 4d in relation to the Hg 4f (for the HgTe) or the As 3d (for the InAs).

RESULTS AND DISCUSSION

We start by growing a series of HgTe NCs, whose sizes are chosen to match short-wave, extended short-wave, and mid-wave infrared detection. For the sake of simplicity, samples will later be identified by their cut-off wavenumber. HgTe 4k, therefore, refers to a sample with a cut-off wavenumber of 4000 cm⁻¹ (or 2.5 µm cut-off wavelength), see **Figure 1a**. After synthesis, the pristine solutions are ligand exchanged using a hybrid strategy combining inorganic (HgCl₂) and organic (mercaptoethanol) ligands. By using short ligands, the deposited films are rendered more conductive, matching the surface chemistry used for the device and limiting the possible photocharging induced by the photoemission experiment. Additionally, the films are deposited on a gold-coated silicon substrate to minimize the photocharging.

We then conduct photoemission spectroscopy with a synchrotron source. This type of source offers
a tunable photon energy to probe different depths (i.e., being more surface or bulk sensitive
depending on the chosen photon energy).
<figure><img src="image_2.png" /><figcaption>**Figure 1** Electronic spectra of HgTe NCs upon Li exposure. a. Absorption spectra from HgTe NCs with different sizes. Their names refer to their cut-off wavenumber given in cm⁻¹. b. X-ray photoemission survey spectra acquired at 700 eV for HgTe 4k before and after Li exposure. c. X-ray photoemission survey spectra covering the range of both Te 4d and Li 1s core levels, acquired at 150 eV for HgTe 4k before and after various Li evaporation. d. X-ray photoemission spectra of the secondary electron cut off, acquired at 150 eV, for HgTe 4k before and after various durations of Li exposure. The green arrow shows the trend of the shift as the amount of Li increases.</figcaption></figure>

In the survey spectra of the HgTe NCs (Figure 1b), the expected Hg and Te core levels are clearly visible, along with signals from gold (substrate) and carbon (ligands). Despite a preparation of the sample in air and the absence of an annealing step, no oxygen contribution is detected, as the O 1s peak at binding energy (BE) = 532 eV is absent. The spectra remain unchanged before and after Li deposition, indicating that the HgTe material is neither damaged (same Hg:Te ratio), nor alloyed (Li intercalation), nor oxidized (no appearance of the O 1s state, nor higher binding energy contribution in the Hg 4f or Te3d states), and that the Li coverage is sufficiently limited not to screen the signal from the underlying material. At this energy (700 eV), the Li 1s state cross section is weak (7.8 kbarn),³⁷ and we cannot see any signature of its deposition. However, under more surface-sensitive conditions (hv=150 eV), Li deposition becomes evident (cross section is 0.58 Mbarn, x74 compared to its 700 eV value), see Figure 1c, with Li 1s intensity increasing with the deposition time. To estimate the amount of deposited material, spectra are acquired over a window including a core level from the NCs (Te 4d in this case; Hg 4f for Cs deposition), see Figure 1c. Therefore, by normalizing the area of the peak by its atomic cross-section, we can determine the atomic ratio over the probed thickness with good approximation, given the close binding energies of the two peaks, see the first note of the SI.³⁸,³⁹
As anticipated from the survey spectrum, the core levels remain unaffected (see Figure S1) by Li deposition. The absence of a rigid shift in the spectra indicates that the carrier density of the material remains unchanged. On the other hand, the WF is clearly reduced, as shown in **Figure 1d**. Such a behavior drastically contrasts with the one observed when conducting ligand exchange, where both the vacuum level and core levels (and so does the carrier density) are shifted. With ligand exchange the ligand dipole is strongly coupled in the near field to the NC electronic structure and shift the whole spectrum, see Figure S4. Conversely, with alkali the coupling is weak and only the alkali gets polarized tuning the film WF, but leaving NC unchanged.

Also note that the lack of change in the core level not only allows for the exclusion of a charge transfer, but also a possible intercalation of the Li, likely prevents by the remaining ligands, which would have led to a change in the environment of the atom from the inorganic core.

The same decrease in the WF is observed for other nanocrystals (HgTe 6k and HgTe 3k), as depicted in Figure S2. The WF remains nevertheless higher than that of pure Li (2.2 eV), confirming that the amount of Li remains limited. This conclusion is further supported by the clear observation of the NCs core levels after the alkali deposition, despite the ≈1 nm probing depth of the XPS analysis (considering the photon energies of 150 eV and 700 eV used)⁴⁰. Therefore, we are not forming a metallic layer atop the film, but rather a discontinuous coating that tunes the WF of the underneath film.

<figure><img src="image_3.png" /><figcaption>**Figure 2.** Electronic spectra of HgTe NCs upon Cs exposure a. X-ray photoemission survey spectra covering the range of both Hg 4f and Cs 4d core levels, acquired at 150 eV for HgTe 4k before and after various Cs evaporation. b. X-ray photoemission spectra of the secondary electron cut off, acquired at 150 eV for HgTe 4k before and after various Cs evaporation. Green arrows show the trend of the shift as the amount of Cs increases.</figcaption></figure>

When the NC film is functionalized with Cs (Figure 2), the material remains undamaged (i.e., same survey spectrum and same core levels, see Figure S3), and a similar decrease in the WF is observed, see Figure 2b. The main difference appears in the region showing both Cs and Hg core levels: the Cs peak intensity increases, and a decrease in the Hg signal is now observed, see Figure 2a. This effect arises because, compared to Li deposition, achieving a similar WF reduction with Cs requires a Cs-to-HgTe ratio roughly one order of magnitude higher, so the Cs atoms deposited atop the NCs induce greater screening of the underlying layer.
**Figure 3** summarizes the impact of alkali deposition on HgTe NCs. For Li deposition, the shift in WF can reach up to 0.8 eV, see **Figure 3a**, and seems to saturate beyond that. The shift follows the same trend across all NC sizes, and therefore across all band gaps, as expected from equation (1), which is independent of the material's gap. The similarity in the trends also indicates that the shift is not sensitive to the surface-to-volume ratio of the film, suggesting that alkali functionalization occurs primarily at the surface of the film and that the NC array acts as an effective medium for the alkali deposition.

We then compared the efficiency of the different alkali metals in modifying the WF, including data from Ref.²¹, relative to K deposition on HgTe NCs. A clear trend appears: the smaller the alkali, the stronger the WF shift for a given atomic ratio, see **Figure 3b**. According to equation (1), if a full monolayer of alkali is deposited, we should expect a scaling of the energy shift with the alkali radius R. The dipole term μ is proportional to R and the atom density term N is proportional to R⁻², leading to an R⁻¹ scaling of the energy shift. Since the Li atomic radius is 60% of the Cs one, Li is expected to induce a shift 66% larger for the same alkali/HgTe ratio. However, we observe a difference of energy shift close to an order of magnitude between the two alkalis, suggesting that the effective charge involved in the dipole term depends on the alkali species. It is worth noticing that these trends of smaller alkali being more efficient to shift the work function at given coverage are contrasting with what has been observed for bulk Ge,⁴¹ where an opposite trend is seen (Cs is the most efficient) or with 2D metal dichalcogenides, where K, with its intermediate size, generates the largest shift.⁴² In addition to a difference of dipole magnitude, this may suggests that the small alkali can diffuse within the voids of the film, thereby increasing the effective interaction area between the NC and the alkali. However, the ligands prevent the direct coupling between the alkali and the inorganic surface.

<figure><img src="image_4.png" /><figcaption>**Figure 3 Size and alkali effect on HgTe work function.** a. Variation of the HgTe NCs thin film's WF for three sizes of NCs as a function of the deposited Li dose. b. Variation of the HgTe 4k NCs thin film's WF as a function of the alkali dose for three alkalies (Li, K, and Cs).</figcaption></figure>

Having established the effect of alkali on HgTe II-VI NCs, we now turn to the impact of the alkali functionalization on III-V NCs. We focus specifically on core-only InAs NCs of two sizes, corresponding to band edges at 940 nm and 1200 nm, as indicated by the absorption spectra in **Figure 4a**. The survey spectrum (**Figure 4b**) of the pristine material exhibits features from In and
As, and signals from gold, oxygen, carbon, and chloride, which originate from the substrate, partial material oxidation, ligand, and residual indium precursor, respectively.

The In 3d core level (Figure 4e) displays a doublet with an asymmetric shape, suggesting the presence of two contributions. The main one (BE=444.7 eV) relates to indium bound to arsenic, while the second one (BE=445.2 eV) arises from oxidized indium (see Figure S7).<sup>43</sup> For As 3d (Figure 4f), the presence of native oxide is even more pronounced, with a clear contribution at higher binding energy. As visible in Figure S7, the decomposition of the spectrum actually leads to three contributions (BE=41 eV, 41.8 eV, and 43.9 eV) that we attribute to As<sup>3-</sup> (i.e., bound to In), As<sup>0</sup>, and As<sup>3</sup>, respectively.<sup>43</sup>

<figure><img src="image_5.png" /><figcaption>**Figure 4** Electronic spectra of InAs NCs upon Li exposure
a. Absorption spectra from InAs NCs with different sizes. b. X-ray photoemission survey spectra acquired at 700 eV for InAs (940 nm band edge) before and after Li exposure. c. X-ray photoemission survey spectra covering the range of both As 3d and Li 1s core levels, acquired at 150 eV for InAs (940 nm band edge) before and after various Li evaporation. d. X-ray photoemission spectra of the secondary electron cut off, acquired at 150 eV for InAs (940 nm band edge) before and after various Li evaporation. e. green arrow shows the trend of the shift as the amount of Li increases. f. (resp. f) In 3d (resp. As 3d) core level acquired at 700 eV for InAs (940 nm band edge) before and after Li exposure.</figcaption></figure>

Similar to HgTe, no signature of the Li deposition (Figure 4b) is detected at high-photon energy (hv=700 eV). Under more surface-sensitive conditions, however (hv=150 eV, see Figure 4c), the increase of the Li 1s core level intensity with deposition time becomes evident. Unlike the case of HgTe, the core-level spectra of InAs are clearly affected by the Li deposition, with the relative weight of the different contributions being modified by the process. For arsenic, the As 3d shows a clear change in the weight of its different contributions (Figure 4c and f), while for indium, the In 3d peak becomes narrower (Figure 4e). This effect is even more pronounced upon Cs deposition (Figure S5) or for Li deposition conducted on larger InAs NCs (Figure S6 and S7). Therefore, the indium
tends to be reduced upon Li deposition, while As becomes more oxidized. The exact ratio of reduction and oxidation seems nevertheless to depend on both particle size and alkali species. The underlying mechanism remains unclear. While a partial charge transfer can occur, a side mechanism, such as a change of oxidation induced by local heating from the hot alkali dispenser during deposition, may also be at play.

This behavior comes in addition to the reduction of the WF (Figure 4d and S5) that is also observed for InAs, both using Li and Cs (Figure 5b) and for both NC sizes (Figure 5a). This suggests that alkali atoms do not behave as pure surface dipoles but also tune the particles' redox state. The combination of both phenomena means that the WF shift now depends on the particle size (Figure 5a). This feature can be attributed to the change in the degree of oxidation since the pure dipole effect was shown to be independent of the surface-to-volume ratio in the case of HgTe, as pointed out in Figure 3a. For Li, the maximum shift of the WF appears to saturate at a lower value (<0.6 eV) than for HgTe, for which a shift of up to 0.8 eV can be achieved. A second consequence is that the maximum WF shift is now larger for Cs than for Li, though at low doses, Li remains more effective than Cs, see Figure 5b.

<figure><img src="image_6.png" /><figcaption>**Figure 5 Size and Alkali effect on InAs work function.** a. Variation of the InAs NCs thin film's WF for two sizes of NCs as a function of the deposited Li dose. b. Variation of the InAs NCs with 940 nm band edge WF as a function of the alkali dose for two alkalis.</figcaption></figure>

It is worth stressing that this behavior (i.e., modification of core levels) was not observed for the III-V core-shell made of InAs/ZnSe,²¹ though functionalization was made with K. This suggests that the shell provides the additional benefit (beyond confining the wavefunction) of stabilizing the optically active material's redox state.

## CONCLUSION

To summarize, our work demonstrates that alkali deposition is an effective method to tune the absolute energy band alignment of NC films. Shifts up to 0.9 eV can be obtained, with smaller alkali metals requiring less material to induce a similar shift. In the case of HgTe, the alkali behaves as a pure dipole, tuning the properties of the film through surface functionalization without significant diffusion into the film. This result contrasts with the behavior observed for InAs NCs, where the alkali is also redox active (i.e., it induces a change in the relative weight of the different redox states). This
dual behavior reduces the efficiency of the work function reduction and makes the behavior both size- and alkali-dependent. The next challenge will be to move from demonstrating the effect to exploiting it as a tool for improving carrier extraction and injection within diode stacks.

## DATA AVAILABILITY

The data that support the findings of this study are available from the corresponding author upon request.

## ACKNOWLEDGMENTS

Authors thank the European Union's Horizon research and innovation program under grant agreement 101135704 (HortiQD project) and ERC grant AQDtive (grant n°101086358). We are also grateful to French National Research Agency (ANR) through the grants Bright (ANR-21-CE24-0012-02), MixDferro (ANR-21-CE09-0029), Quicktera (ANR-22-CE09-0018), Operatwist (ANR-22-CE09-0037-01), E-map (ANR-23-CE50-0025), DIRAC (ANR-24-ASM1-0001), camIR (ANR-24-CE42-2757), Piquant (ANR-24-CE09-0786), nanomirage (ANR-25-CE. We acknowledge the use of clean-room facilities from the "Centrale de Proximité Paris-Centre" and support from Renatech+ for micro and nanofabrication. This project has also received financial support from the CNRS through the MITI interdisciplinary programs (project WITHIN) and Region Ile de France through Sesame project INSIDE.

## COMPETING INTEREST

The authors declare no competing interests.

## Supporting Information

Supporting Information includes the following. (i) Quantitative analysis to determine atomic ratio, (ii) Effect of Li deposition on HgTe NCs, (iii) Effect of Cs deposition on HgTe NCs, (iv) Effect of Cs deposition on InAs NCs, and (v) Effect of Li deposition on large InAs NCs.

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TOC Graphic

<figure><img src="image_7.png" /></figure>
Adjusting the Energy Levels of HgTe and InAs Nanocrystals with Alkali Ions

Dario Mastrippolito¹,², Majed Ibrahim³, Oleksandra Yeromina³, Mariarosa Cavallo¹, Giorgia Strobbia¹, Erwan Bossavit¹,², Jiho Roh¹, Marco Paye¹, Adrien Khalili¹, Yoann Prado¹, James K. Utterback¹, José Avila², Sandrine Ithurria⁴, Mathieu G. Silly², Debora Pierucci¹, Peter Reiss³, Emmanuel Lhuillier¹*

¹Sorbonne Université, CNRS, Institut des NanoSciences de Paris, 4 place Jussieu, 75005 Paris,
France.

²Synchrotron SOLEIL, L'Orme des Merisiers, Départementale 128, 91190 Saint-Aubin, France.

³Univ. Grenoble Alpes, CEA, CNRS, IRIG-SyMMES, STEP, 38000 Grenoble, France

⁴Laboratoire de Physique et d'Etude des Matériaux, ESPCI, PSL Research University, Sorbonne Université, CNRS, 10 rue Vauquelin, 75005 Paris, France.

Table of contents

<table><tr><td>1.</td><td>Quantitative analysis to determine the atomic ratio</td><td>2</td></tr><tr><td>2.</td><td>Effect of Li deposition on HgTe NCs</td><td>3</td></tr><tr><td>3.</td><td>Effect of Cs deposition on HgTe NCs</td><td>4</td></tr><tr><td>4.</td><td>Effect of Cs deposition on InAs NCs</td><td>5</td></tr><tr><td>5.</td><td>Effect of Li deposition on large InAs NCs</td><td>6</td></tr><tr><td>•</td><td>REFERENCES</td><td>7</td></tr></table>

<footnote>*To whom correspondence should be sent: el@insp.upmc.fr</footnote>
# 1. Quantitative analysis to determine the atomic ratio

In order to calculate the atomic concentration of Te and alkali in our samples, the following equation (1) is used¹⁻³:

$$C_A = \frac{I_A}{F_A} \qquad (1)$$

with $I_A$ is the photoemission intensity for a specific orbital from element A and $F_A$ is the relative sensitivity factor of the corresponding element A = Hg/Te or alkali:

$$F_A = \sigma_A(h\nu) \cdot T(Ek_A) \cdot \lambda(Ek_A) \qquad (2)$$

where:

* $\sigma_A(h\nu)$ is the element, orbital and photon energy (h\nu) specific photoemission cross section⁴.

* $T(Ek_A)$ is the transmission function of the spectrometer at kinetic energy $Ek_A$, that can be described as :

$$T(Ek_A) \sim Ek_A^\alpha \quad (\alpha \text{ depends on the particular spectrometer used})$$

* $\lambda(Ek_A)$ is the kinetic energy dependent electron mean free path that can be approximated as:

$$\lambda(Ek_A) \sim Ek_A^\beta \quad \text{with } \beta \sim 0.5.5$$

Then if we compare the intensity $I_{Te\ 4d}$ and $I_{Li\ 1s}$ the ratio of Te and Li is therefore:

$$\frac{C_{Te}}{C_{Li}} = \frac{I_{Te\ 4d}}{I_{Li\ 1s}} \cdot \frac{\sigma_{Li\ 1s}(h\nu)}{\sigma_{Te\ 4d}(h\nu)} \cdot \left( \frac{Ek_{Li\ 1s}}{Ek_{Te\ 4d}} \right)^{\alpha+\beta} \qquad (3)$$

Te 4d and Li 1s have close binding energy values. Then using the same photon energy $h\nu = 150$ eV, they present also close kinetic energy values that means $(\frac{Ek_{Li\ 1s}}{Ek_{Te\ 4d}})^{\alpha+\beta} \sim 1$. Then:

$$\frac{C_{Te}}{C_{Li}} \sim \frac{I_{Te\ 4d}}{I_{Li\ 1s}} \cdot \frac{\sigma_{Li\ 1s}(h\nu)}{\sigma_{Te\ 4d}(h\nu)} \qquad (4)$$

That is the case for Te 4d and Li 1s and for the (Hg 4f; Cs 4d) couple.
## 2. Effect of Li deposition on HgTe NCs

<figure><img src="image_8.png" /></figure>

**Figure S 1 Absence of effect on the core levels of HgTe upon Li deposition.** a. (resp. b) Hg 4f (resp. Te 3d) core level acquired at 700 eV for HgTe 4k before and after Li exposure.

<figure><img src="image_9.png" /></figure>

**Figure S 2 Effect of Li deposition on HgTe NCs thin films work function.** a. (resp. b) X-ray photoemission spectra of the secondary electron cut off, acquired at 150 eV for HgTe 6k (resp HgTe 3k) before and after various Li evaporation. The green arrows show the trend of the shift as the amount of Li increases.
### 3. Effect of Cs deposition on HgTe NCs

<figure><img src="image_10.png" /><figcaption>**Figure S 3** Effect of Cs deposition on HgTe NCs electronic structure. a. X-ray photoemission survey spectra acquired at 700 eV for HgTe 4k before and after Cs exposure. b.(resp. c) Hg 4f (resp. Te 3d) core levels acquired at 700 eV for HgTe 4k before and after Cs exposure.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Figure S 4** Effect of alkali deposition and ligand exchange of the electronic structure of HgTe 4k.</figcaption></figure>
4. Effect of Cs deposition on InAs NCs

<figure><img src="image_12.png" /></figure>

**Figure S 5 Electronic spectra of InAs NCs upon Cs exposure** a. (resp. b) In 3d (resp. As 3d) core level acquired at 700 eV for InAs (940 nm band edge) before and after Cs exposure. c. X-ray photoemission survey spectra covering both As 3d and Cs 4d core level range, acquired at 150 eV for InAs (940 nm band edge) before and after various Cs evaporation. d. X-ray photoemission spectra of the secondary electron cut off, acquired at 150 eV for InAs (940 nm band edge) before and after various Cs evaporation.
5. Effect of Li deposition on large InAs NCs

<figure><img src="image_13.png" /><figcaption>**Figure S6** Electronic spectra of InAs NCs (1200 nm) upon Li exposure. a. X-ray photoemission survey spectra acquired at 700 eV for InAs (1200 nm band edge) before and after Li exposure. b. (resp. c) In 3d (resp. As 3d) core level acquired at 700 eV for InAs (1200 nm band edge) before and after Li exposure.</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>**Figure S 7 Decomposition of the electronic spectra of InAs NCs (1200 nm) upon Li exposition.**
a. (resp. b) In 3d core level acquired at 700 eV for InAs (1200 nm band edge) before (resp. after) Li exposure. c. (resp. d) As 3d core level acquired at 700 eV for InAs (1200 nm band edge) before (resp. after) Li exposure.</figcaption></figure>

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