Chemically and Electronically Active Metal Ions on InAs Quantum Dots for Infrared Detectors

Colloidal InAs quantum dots (QDs) are emerging candidates for NIR-SWIR optoelectronic applications because of their excellent electrical and optical properties. However, the synthesis of InAs QDs, which demands a strongly reducing atmosphere or highly reactive precursors, is di�cult because of their strong covalent bonding nature and lack of group 15 precursors. While the co-reduction method with commercially available arsenic precursors enables the facile synthesis of InAs QDs, it causes a broad size distribution, requiring a subsequent size-selection process. In this study, we introduce zinc metal ions in the form of a coordination complex during the co-reduction reaction of indium and arsenic precursors. Zn ions can chemically passivate the surface of InAs QDs, allowing the promotion of size focusing and removal of surface defects. When the InAs QDs are integrated into infrared photodiodes as IR absorbers, the surface-attached Zn ions can electrically modulate the energy level and carrier concentration. The infrared photodiodes with InAs:Zn QD layers exhibit two orders of magnitude lower dark current and about twice faster photo-response than those using bare InAs QDs.

Version of Record: A version of this preprint was published at NPG Asia Materials on May 12th, 2023.See the published version at https://doi.org/10.1038/s41427-023-00477-w.

Introduction
Indium arsenide (InAs), one of III-V compound semiconductors, has a small carrier effective mass, direct bandgap, and small exciton binding energy, which are attractive properties for state-of-the-art electronic and optoelectronic applications. 1,27][8] To achieve high-performance QD-based NIR or SWIR photodetectors, it is important to obtain highly monodisperse InAs QDs as well as to control the surface defects, which can enhance the charge transport properties in the QD lm. 9 However, the synthesis of InAs QDs is not as simple as that of ionic compound semiconductor nanocrystals because of the strong covalent bonding between indium and arsenic and the lack of accessible arsenic precursors. 10,11Previous studies using continuous injection and seeded growth methods have focused on controlling the reactivity of highly reactive silylated or germylated arsenic precursors, such as tris-trimethylsilylarsine and tris-trimethylgermyl arsine, which are di cult to handle owing to their toxicity and pyrophoricity. 12,13Meanwhile, the co-reduction method with commercially available arsenic precursors, such as arsenic chloride and tris-dimethylaminoarsine, could be another facile approach for InAs QD synthesis. 14,15A simple heating process, along with the simultaneous reduction of indium and arsenic precursors may be suitable for large-scale production; however, it generally requires a subsequent size-selection process because it produces a broad size distribution of the resultant QDs.In addition, similar to other nanoscale III-V semiconductors, the surface of InAs QDs is prone to oxidation. 11,16During the synthesis procedure, oxidative defects must be eliminated so that the QDs can have homogeneous optoelectronic properties.
Recent studies have revealed that additional ions, including metals, chalcogens, and halogens, can play critical roles in controlling the structural and optical properties of III-V QDs. 16,17These ions can be introduced into the QD solution during either the synthesis or post-synthesis process.In particular, the effects of Zn addition during the synthesis of indium phosphide (InP) QDs, which presumably possess similar surface characteristics to those of InAs, have been widely studied.The addition of Zn precursors to InP QDs can improve size uniformity and consequently reduce the bandwidth in the absorption and emission spectra of InP QDs. 18The Zn ions incorporated into the InP lattice on the InP surface improve the photoluminescence properties by reducing the surface defect states 19,20 or alleviating lattice mismatches between the core and shell. 21,22Based on these results, transition metal ions can be employed as reaction additives during QD synthesis, resulting in e cient surface passivation, enhanced optical properties, and improved size uniformity of III-V QDs. 23 the case of InAs QDs, the incorporation of additional ions has been conducted mainly through postsynthesis treatments.Post-treatment with cadmium oleate can change the electrical properties of InAs QD lms and protect the surface of InAs QDs from oxidation.24 Hydrogen sul de vapor treatment of InAs QD lms can also tune the electrical properties and improve their stability in air.25 These additional ions are chemically adsorbed on the surface of the QDs and act as electrical dopants.However, there have been insu cient methodological studies to improve the properties of InAs QDs during the synthesis process.The addition of Zn precursors during the seeded growth of InAs/ZnSe core/shell QDs can improve the emission properties; however, the mechanism of the reaction between Zn and the InAs core has not been elucidated.26,27 In this work, we propose a synthetic strategy using zinc chloride-based reaction additives for the synthesis of InAs QDs, which can improve the monodispersity of InAs QDs and simultaneously enhance their optoelectronic properties by removing surface oxidative species.We introduce Zn precursors in the form of a coordination complex into the indium and arsenic reaction mixture, and those appear to be indirectly involved in the reaction between In and As precursors.The Zn ions eventually passivate the surface of the resultant InAs QDs, allowing them to improve colloidal stability and facilitate the fabrication process for follow-up devices.As the surface Zn ions also affect the electrical properties and energy level of InAs QD lms, IR photodiodes incorporating the InAs with Zn QD assembly have two orders lower dark current than bare InAs QDs and e ciently detect NIR/SWIR light at 1050 nm.
Synthesis of InAs QDs.
InAs QDs were synthesized via a modi ed co-reduction method using As precursor with oleylamine.InCl 3 (1 mmol) and AsCl 3 (5 mmol) were dissolved in 20 ml of pre-degassed OLAM by stirring overnight at ~ 60°C in a glovebox.ZnCl 2 (10 mmol) was dissolved in TOP (10 ml of TOP) and stirred for two days at 80°C in a glovebox.For the reducing agents, the preparation process was the same as in the previous method.A 2M solution of super-hydride (LiEt 3 BH in THF) was in pre-degassed dioctyl ether was prepared.
Before synthesis, 10 ml of 0.05 M indium precursor, 1 ml of 0.25 M As precursor, and a certain ratio of zinc chloride complex precursor were mixed in a three-necked ask under an inert atmosphere.After a 1.25 ml of 2 M super-hydride was injected into the reaction ask, the mixture turned dark and bubbles were generated.The reaction ask was then heated to the reaction temperature at 3°C/min and maintained at that temperature for 15 min.After growth, the reaction was quenched quickly and the product was transferred to a glovebox without air exposure for puri cation.For puri cation, 30 ml of toluene were added to the reaction mixture, which was then transferred into two centrifuge tubes.The dispersion was then centrifuged at 5000 rpm for 5 min.Acetonitrile (12 ml) was then added to each supernatant with subsequent centrifugation.The aggregates were then dissolved in 5 ml of toluene and centrifuged at 5000 rpm for 5 min after adding ethanol to the solution until it turned turbid.Finally, the aggregates were re-dispersed in octane after centrifugation.
An ITO glass substrate was cleaned in an ultrasonic bath for 30 min, rst with acetone, then isopropyl alcohol.The substrate was then treated with an ultraviolet (UV) ozone cleaner for 15 min.After cleaning, ZnO nanoparticles, synthesized by a sol-gel process using zinc acetate dihydrate and methyl alcohol, were spin-coated onto the ITO substrate at 2000 rpm for 30 s and annealed in air over 300°C for 40 min.InAs QDs redispersed in octane (100 mg/ml) were spin-coated onto the substrate and annealed at 200°C for 30 min in a glovebox.MoO 3 and Au layers were deposited at thicknesses of 10 and 120 nm, respectively, using a thermal evaporator.All the devices were encapsulated using cover glasses with UVcured epoxy.

Characterizations
For the ultraviolet-visible NIR (UV-vis-NIR) absorption measurements, the synthesized InAs QDs were dispersed in TCE.Absorption spectra were obtained using a SHIMADZU UV-2600 UV-vis-NIR spectrometer (200-1400 nm).XRD measurements were performed on a miniFlex 600 diffractometer (RIGAKU) with Cu Kα (40 kV and 15mA) in scanning mode.InAs QDs dispersed in hexane were drop-cast on an XRD glass holder (RIGAKU).The sample for TEM imaging was dissolved in hexane and drop-cast onto a carbon-coated 300 mesh copper TEM grid.TEM images were captured using a JEOLJEM-2010 operating at 200 kV.ICP-OES measurements were performed using an ICAP 7000 SERIES instrument (Thermo Fisher Scienti c).UPS measurements were performed on an XPS-Theta Probe (Thermo Fisher Scienti c).InAs QDs dispersed in octane were drop-cast on 1.0 cm 1.0 cm silicon substrates and baked at 200 ℃ for 30 min in a glove box.During the UPS measurements, a helium discharge source (He 1α = 21.22 eV) was used, and the samples were kept at high pressure.The work function was calculated from the equation: For the bandwidth measurement, a frequency modulated 808 nm laser diode using a function generator (HP 33120A) and a lock-in ampli er (SR830) are used.Noise spectral density was measured by SR570 low-noise preampli er and Advantest R9221B.The dark current and the responsivity of photodiode devices were characterized using a source-meter (Keithley 2602) and function generator (Agilent 33220A) under the illumination of 1050 nm laser diode.

Results And Discussion
For the synthesis of InAs QDs (Fig. 1a), we employed a modi ed co-reduction method that included commercial indium and arsenic chloride precursors dissolved in oleylamine. 28The co-reduction method has the advantages of simplicity and facile scalable production of QDs. 29 However, the resultant QDs have a wide size distribution, and thus a size-selective precipitation process is required to select the appropriate size of QDs. 2,14,28Fig. 1b shows the absorption spectra of conventional InAs QDs synthesized at 300°C using the standard co-reduction method before and after the size selection process.Before size selection (black line), the absorption spectrum has a featureless broad peak with a tail, indicating heterodispersity of the InAs QDs.After removing relatively small and large InAs particles, the absorbance ratio between the excitonic peak and the valley (peak-valley ratio), as a parameter to con rm the uniformity of the QD size and shape, was 0.87, indicating a narrower size distribution of InAs QDs. 30 In this regard, the InAs nucleation process does not seem to be completely suppressed but continues to occur even in growth periods.
To address this problem in the co-reduction method, we employed reaction additives to modify the nucleation and growth process, improving the dispersity and uniformity without a follow-up size-selection process.The broad size distribution is attributed to surface-reaction-controlled growth due to strong covalent bonding between the In and As ions, which impedes the surface reaction on the InAs nuclei. 31In our synthesis method, In and As precursors are chemically reduced immediately after injecting the reducing agent and diffuse to the surface of the InAs nuclei as the reaction temperature increases.The slow surface reaction of In and As precursors determines the growth rate of InAs QDs; therefore, we need to consider reaction additives to alter the reaction path and improve the surface reaction rate.
Based on the previous studies on the synthesis of InP QDs that incorporate Zn, zinc chloride could be an effective additive for changing the precursor reactivity and passivating the surface of III-V QDs.Because Zn and Cl ions are reactive, they aggressively replace the native surface ligands of InAs nucleates.Instead, we used a zinc chloride-based coordination complex prepared by mixing a 1:2 molar ratio of zinc chloride and tri-n-octylphosphine (ZnCl 2 -TOP). 33We synthesized InAs along with the ZnCl 2 -TOP precursor before injecting the reducing agent, while keeping the other reaction conditions unchanged (Fig. 1a).In Fig. 1c, regardless of the follow-up process, the absorption spectra of InAs QDs synthesized with the ZnCl 2 -TOP precursor (referred as to InAs:Zn QDs) show distinct excitonic peaks and have a similar peak-valley ratio value of 0.78.The addition of the ZnCl 2 -TOP precursor improved the size uniformity of the InAs QDs without altering their optical properties.A recent study reported the effect of ZnCl 2 on the synthesis of InAs QDs.However, the complex form of the Zn precursor that we used is more stable than ZnCl 2 , even at temperatures above 200°C where the InAs nuclei are formed.In addition, the direct use of ZnCl 2 may induce the formation of Zn ions, yielding ZnAs or Zn 3 As 2 particles.We believe that the ZnCl 2 -TOP complex can effectively facilitate the nucleation and growth process of InAs, even at high temperatures.
We theorize an indirect role of ZnCl 2 -TOP precursors in the formation of InAs.Pnictogen elements, such as P, As, and Sb, are known to form Zintl phases, which are polyanionic clusters. 34,35Homoatomic units, such as [As 7 ] 3− , often form coordination compounds with transition metals, such as Zn, Cd, and Cs.For increase the reaction rate of As with In ions.Reaction between the Zn and As precursors can be con rmed by the formation of zinc-arsenic particles.During a similar co-reduction reaction with only Zn and As precursors in the presence of a reducing agent, Zn-As particles were formed at elevated temperatures (Figure S1a).The indirect band gap of the zinc arsenide particles exhibits a broad absorption spectrum in the 700-800 nm range (Figure S1b), and the XRD pattern of the particles indicates low crystallinity, presumably owing to mixed growth of Zn 3 As 2 and ZnAs 2 (Figure S1c).
To elucidate the effect of the ZnCl 2 -TOP precursor on the resultant InAs structure, we synthesized InAs QDs with different molar ratios of Zn and In/As precursors at a reaction temperature of 300°C.Figure 2 shows the UV-vis-NIR absorption spectra, photoluminescence (PL) spectra, and XRD patterns of the InAs QDs using various concentrations of the ZnCl 2 -TOP precursor.As the Zn/In molar ratio increases from 0 to 2 the absorption spectra show a deep valley with a sharp excitonic peak, corresponding to a change in the peak-valley ratio from 0.93 to 0.74 without the size-selection process (Fig. 2a).With increasing amounts of Zn precursor, a slight blue-shift in the absorption spectra from 936 nm to 926 nm is observed.However, the small shift does not indicate a signi cant size change, rather indicates the disappearance of relatively large QDs.The deep valley in the absorption curve also reveals that the fraction of relatively small InAs QDs diminished.Size focusing was observed when the reaction temperature was increased to 330°C to obtain large InAs QDs.(Figure S2).
In addition to the size focusing effect, the ZnCl 2 -TOP precursor also in uences the luminescence properties of InAs QDs, as shown in the PL spectra (Fig. 2b).While distinct infrared light emission could be obtained from both InAs and InAs:Zn QDs, the InAs:Zn QDs exhibited a higher peak intensity and narrower emission bandwidth than the bare InAs QDs.The narrow emission spectra of InAs:Zn QDs is attributed to their uniform size and is affected by the reduced trap states.The trap emission ranging from 1100 nm to 1400 nm decreased as the amount of ZnCl 2 -TOP precursor increased.There are two possible causes for this phenomenon: surface trap states, such as In and As dangling bonds are passivated, and/or the oxidative species that are unavoidably formed on the surface are removed by the precursors.
The changes in the surface states as well as the chemical analysis will be discussed in a later section.
Crystal structure analysis can reveal if the InAs QDs synthesized with ZnCl 2 -TOP precursors are alloyed.
In the X-ray diffraction (XRD) measurements, all InAs QDs synthesized at 300 and 330°C with various mole ratios of added Zn precursors had a zinc-blende crystal structure with peaks at 25.54°, 42.33°, and 50.1° corresponding to the (111), (220), and (311) planes, respectively (Fig. 2c and Figure S3).If an InZnAs alloy had formed, the XRD patterns would have a broad shape or shift at a higher angle.However, the lattice parameters and crystal structures did not change, even when the molar amount of added Zn precursors was twice as high as that of In.To con rm the elemental composition, inductively coupled plasma optical emission spectrometry (ICP-OES) was performed on puri ed InAs and InAs:Zn QDs.As the amount of the ZnCl 2 -TOP precursor increased, the detected Zn:In ratio increased (Fig. 2d).However, the molar amount of Zn detected in InAs:Zn QDs was much less than that in the ZnCl 2 -TOP precursor used for the synthesis, unlike in a previous study on InP QD synthesis using a Zn precursor (Table S1).We believe that the Zn ions are located on the surface of the InAs QDs, or if not, diffused out to the surface by a self-puri cation process. 36An estimate based on the amount of Zn detected indicates that approximately 8-10 Zn atoms are adsorbed on a single InAs QD.The Zn ions possibly passivate As dangling bonds to improve chemical and colloidal stability.These results indicate that Zn ions with relatively low reactivity surrounds the InAs surface and affect the growth rate of InAs QDs, thereby narrowing the size distribution.
Based on the change in the size distribution without alloying, we can assume that the ZnCl 2 -TOP precursor plays a critical role in the nucleation and growth of InAs QDs.We measured the absorption spectra of aliquots extracted at different reaction temperatures during the synthesis of InAs (Fig. 2e) and InAs:Zn (Fig. 2f).In the case of InAs:Zn QDs, the nuclei begin to form at 200°C, whereas in the case of conventional synthesis of InAs QDs, the absorption shoulder barely appears at 220°C.This means that the ZnCl 2 -TOP precursor readily reacts with the indium and arsenic precursors, allowing the simultaneous formation of nuclei.After the nucleation process, InAs grows as the reaction temperature increases, but the absorption peak continues to shift and broaden, indicating that the size distribution of InAs QDs is poor because the growth rate of most nuclei formed at different temperatures is similar (Fig. 2e). 12,18owever, in the case of InAs:Zn QDs the excitonic peak moves toward a longer wavelength and is simultaneously narrowed and focused (Fig. 2f).This evolution of the absorption spectra supports our hypothesis that the ZnCl 2 -TOP precursor forms Zn-As intermediates and helps the InAs nucleation process as well as size focusing during the growth process.
We analyzed the chemical composition of InAs and InAs:Zn QDs using X-ray photoemission spectroscopy (XPS).First, a clear difference appears in the Zn 2p peaks located at 1045 and 1021.9 eV which is observed only in the InAs:Zn QD samples (Fig. 3a).This also reveals the presence of Zn on the surface of the InAs:Zn QDs.Because phosphorus in the TOP, which is used to form the ZnCl 2 -TOP complex, is not detected, it con rms that TOP acts as a weakly bound ligand or may merely affect the reaction process. 37(Figure S4).In the InAs QDs both with and without Zn precursors, In 3d 3/2 and 3d 5/2 peaks are located at 451.9 and 444.3 eV, respectively.However, the full width at half maximum (FWHM) of the In 3d bands in the InAs:Zn QDs is narrower than those of the bare InAs QDs, indicating the presence of bonds with In other than the In-As bond in the bare InAs QDs (Fig. 3b).The As 3d spectrum has peaks at 41.5 and 40.8 eV, which indicates As 3d 3/2 and 3d 5/2 (Fig. 3c).However, we observe a shoulder peak near 43.8 eV in both the InAs and InAs:Zn samples, presumably corresponding to the unintentional surface oxidation of As even though the synthesis was conducted under air-free conditions. 16,24Notably, the area ratio of the As-O/As-In peaks decreases from 0.48 to 0.05 using the Zn precursor.
The reduced size of the As-O peak indicates that the added Zn precursor not only passivates the surface of InAs nuclei but also prevents surface oxidation, consistent with a previous report for InZnP 38 .If a large amount of Zn reacts with In and As, there would be a change in the binding energy of In or As; however, there is no peak shift for the InAs:Zn QDs.As only 2 at.% of Zn was con rmed by ICP-OES analysis, the unchanged XPS peaks also indicate that Zn is located at the surface of the InAs QDs.The addition of the ZnCl 2 -TOP precursor, which increases the size uniformity and removes surface defects, can consequently improve the optical properties of InAs QDs, which is consistent with the narrow absorption and emission spectra.
Additionally, the removal of surface oxidative species by Zn precursors was found to improve the colloidal stability and dispersibility.In low-and high-resolution transmission electron microscopy (TEM) images, we observe differences in the sizes and shapes of InAs QDs with the addition of the ZnCl 2 -TOP precursor.The average diameter of InAs:Zn QDs is 4.35 ± 0.9 nm, slightly smaller than that of InAs QDs (4.7 ± 1.4 nm).As shown in Fig. 4a, the bare InAs QDs appear to be unstable, poorly distributed, and agglomerated, resulting in irregular shapes.However, the narrow size distribution and stable dispersibility of the InAs:Zn QDs are attributed to a clean surface owing to Zn ions acting as etchant for the surface oxide (Fig. 4b). 18As shown in Figure S5, when about 100 particles are measured in each sample, the size distribution of InAs:Zn QDs is narrower than that of InAs QDs.The photographs in Figure S6 show InAs and InAs:Zn QD solutions two months after synthesis.While the bare InAs QDs are aggregated to form gel-like states, InAs QDs treated with the ZnCl 2 -TOP precursor are well dispersed in a colloidal form.In parallel with the XPS and ICP-OES results, it was noticed that the Zn precursors remove the surface oxide and prevent further oxide formation by surface passivation of the InAs QDs, providing a well-dispersed colloid.Based on previous research, the weakly bound native ligand, oleylamine, is not su cient to passivate the surface of III-V QDs; thus, Z-type ligands, such as oleic acid may be required. 2,8,28The ZnCl 2 -TOP complex and decomposed Zn ions can act as Z-type ligands because of their Lewis acidic nature.We believe that the ZnCl 2 -TOP complex can remove the surface oxidative species from InAs QDs and enhance the dispersibility and colloidal stability of InAs QDs, enabling the subsequent solution process for infrared device fabrication.
Notably, the zinc precursors on the InAs surface can tune the electronic states of the InAs QDs.We con rmed the energy band level of the InAs and InAs:Zn QD lms by ultraviolet photoelectron spectroscopy (UPS) along with Tauc plots of the absorption spectra (Figure S7).The InAs:Zn QD lm exhibits a lower Fermi level (i.e., more likely intrinsic) at -4.93 eV with a conduction band minimum (CBM) at -4.36 eV and valence band maximum (VBM) at -5.55 eV, while the bare InAs QD lm is more likely ntype with the CBM, VBM, and work function levels at -4.0, -5.14, and − 4.52 eV, respectively (Fig. 5a).The overall decreased energy levels of the InAs:Zn QDs indicates that the surface Zn ions act as p-type dopants of InAs, electrically inducing the Fermi level to less n-type energy level. 38 con rm the improved optical properties and modi ed electronic structure of InAs:Zn QDs, we investigated the spectral response of QD photodiodes with an inverted structure consisting of ITO/ZnO/QDs/MoO 3 /Au (Fig. 5a).We observed the current density-voltage (J-V) characteristics of two photodiodes with 3 × 3 mm IR active area containing the bare InAs or InAs:Zn QDs under the 1050 nm of infrared light illumination.Because the ligand exchange process may alter the effect of surface-attached Zn, the relatively thin QD lms are fabricated without a ligand exchange process for a proof-of-concept demonstration.(Fig. 5b).In the diode based on InAs:Zn QD lm, the dark current at -1 V is approximately 7 µA, which is more than about two orders of magnitude lower than that of the bare InAs QD-based photodiode.The InAs:Zn QD diode demonstrates more ideal behavior and less ohmic/non-ohmic leakage current than the InAs QD diode, attributed to the removed defect states and reduced recombination centers. 3Given that surface Zn doping shifts the Fermi level of InAs QDs close to the intrinsic level, the overall structure is close to a p-i-n structure along with n-type ZnO and p-type MoOx layers.Figure S8 shows the overall I-V characteristics of InAs QDs and InAs:Zn QDs with an applied bias from − 2 to 2 V at different wavelengths and as a function of optical power density under 1050 nm illumination.Distinctively, the InAs:Zn QDs exhibit a lowered dark current in our device structure.
We also tested photodiodes with either InAs or InAs:Zn QD lms after exchanging the native oleylamine with ethanedithiol (EDT) (Figure S9). EDT is a relatively short molecule with S atoms at each end, which can act as donors of InAs.In photocurrent plots, InAs QD photodiodes treated with EDT show metallic characteristics as the EDT leads degenerate n-type InAs (Figure S9a).However, InAs:Zn QDs treated with EDT show much lower dark current and leakage current in the photodiodes than InAs QDs, possibly due to the Zn ions on the surface of InAs QDs (Figure S9b).The dark current for InAs:Zn with EDT treatment is approximately an order of magnitude higher than that of the InAs:Zn photodiode without EDT treatment, suggesting that the improvement and optimization of ligand exchange process is needed.Yet we can con rm that the surface Zn ions as acceptors affect the electronic states of the InAs QDs by adjusting the carrier concentration and Fermi levels and consequently blocking the back injection of carriers to the electrodes.
Based on the frequency response of the photodiodes under NIR laser diode (Figure S10), the 3 dB cutoff frequency of the InAs:Zn QD photodiode exceeds 100 kHz whereas that of the bare InAs QD photodiode is 69 kHz.The fast response is also attributed to the elimination of surface oxidative defects by the Zn precursors.Figure 5c shows the bias-dependent photocurrent density of the InAs:Zn QD photodiodes with different optical power densities at 1050 nm.As the bias increases, the photocurrent increases linearly with the optical power density.The limited absorption of thin QD layers causes a decrease in photocurrent increase at high photon ux; further development and optimization of device fabrication process is still required.However, the InAs:Zn QDs effectively generate photocarriers by absorbing 1050 nm light, and transfer them without severe recombination.Figure 5d shows the noise characteristics of the InAs QD photodiodes with and without Zn precursor.The noise spectral density (NSD) is associated with dark current and is affected by surface traps. 39,40The photodiodes based on InAs QDs have a high NSD value, especially InAs with EDT treatment, due to their high dark current mentioned above.However, the NSD value of InAs:Zn QDs is about ve times lower than that of InAs QDs at 1 kHz since the Zn precursor removes the presence of the trap in the InAs QDs surface.Based on these results, the InAs:Zn QD lm exhibits a low dark current as well as a su ciently high performance compared to the bare InAs QD lm as the removed surface defects reduce trapping of photogenerated carriers.Our synthetic strategy of the ZnCl 2 -TOP complex improves the quality of colloidal InAs QDs and promises opportunities for InAs QDs to be useful for fast and highly sensitive IR detectors.

Conclusion
We report a novel strategy for the synthesis of infrared-active InAs QDs via a zinc-based coordination complex.The proposed ZnCl 2 -TOP reaction additive imparts chemical and electrical effects on InAs QDs.
The Zn precursors injected during the co-reduction reaction can improve the size homogeneity of InAs QDs without additional size-selection process, previously considered essential.Moreover, the addition of the Zn precursor reduces the formation of oxidation on InAs QDs and improve their optical properties as well as chemical/colloidal stability.The surface-attached Zn modulates the electrical properties and energy levels of InAs QD lms.We designed proof-of-concept QD photodiodes without a ligand exchange process to identify the electrical characteristics of InAs:Zn QDs.We con rmed that Zn passivation negates the effect of n-type InAs QDs, resulting in energy levels that are well matched with those of n-type ZnO and p-type MoO 3 layers in an IR photodiode device and improves the electrical properties.The InAs:Zn QD photodiode exhibits two orders of magnitude lower dark current and faster response time than bare InAs QD-based photodiodes without ligand exchange.We anticipate that our unique synthesis approach for InAs QDs with a chemically and electrically active Zn precursor will inspire a new direction for III-V QD-based NIR and SWIR applications.

Supplementary Files
This is a list of supplementary les associated with this preprint.Click to download. SupplementalMaterial.docx example, Zn ions can bridge two [As 7 ] 3− units to yield isolated [Zn(As 7 ) 2 ] 4− anions, which are subsequently stabilized and generally solvated by group I metal ions, such as Li + and Na + cations.These zinc-coordinated Zintl anions can be the intermediate states of As precursors for InAs formation, especially in the strong reducing atmosphere formed by Li(C 2 H 5 ) 3 BH.We believe that the intermediate phases induced by the ZnCl 2 -TOP precursor likely retard the formation of pure arsenic precipitates and

Figure 3 Surface
Figure 3