Quantum dots (QDs) show excellent optical properties, such as a high extinction coefficient, tunable colors, and superior photostability. However, the transport properties of QDs, such as carrier mobility, are quite limited, which hinder optoelectronic applications. On the other hand, carbon nanotubes (CNTs) generally have high carrier mobility and thermal stability with a weak optical response. These features inspire us to couple QDs with CNTs to achieve improved optoelectronics. We take infrared HgTe QDs and multi-walled CNTs as examples. With appropriate coupling between QD and CNT matrices, carrier mobility could reach 34.6–54.1 cm2/Vs in the nanocomposite, a 1000-fold increase compared with the reference. The nanocomposite benefits external quantum efficiency up to 12 500% and detectivity 1012 Jones on the 2500 nm infrared photodetectors. The CNT matrix also helps relaxing thermally generated carriers, improving the photodetector thermal stability. We also demonstrate that the device maintains high detectivity at a high operating temperature.

Quantum dots (QDs) are of interest for optoelectronic devices1 due to their solution processing, low-cost fabrication, size-tunable bandgap, and high extinction coefficient. However, the carrier mobility of QDs is usually poor (<0.1 cm2/Vs), limiting the emerging applications2 of QD photodetection, emission, lasers, and sensing. So far, surface ligand exchange3–5 has been proved to be an effective method to improve the carrier mobility of QDs. The short-chain ligands not only reduce Coulomb charging energies and interparticle spacing6 but also increase electric coupling in QD solids, which has been successfully applied in multiple QD groups.7–9 For example, Zhao et al.10 introduced a hydrazine-free hybrid ligand exchange process to improve carrier transport in III–V QD thin films and realize InAs QD field-effect transistors (FETs) with an electron mobility >5 cm2/Vs. In 2022, Yang et al.11 developed a ligand-engineering approach with a record electron mobility of up to 18.4 cm2/Vs. Still, further improving the carrier mobility of QDs to achieve practical optoelectronics is still an arduous task.

Compared with 0-dimensional QDs, 1-dimensional materials, such as carbon nanotubes (CNTs),12,13 generally have an extraordinary carrier mobility exceeding 105 cm2/Vs.14,15 Meanwhile, CNTs are an excellent holder for light collection and charge transmission components with a large surface area, strong mechanical flexibility, and high conductivity due to their unique electron conduction effect. However, the thin atomic layer thicknesses cause weak optical absorption, where CNTs have no obvious absorption at certain wavelengths, such as infrared.

Is it possible to develop nanocomposites combining multi-dimensional materials such as QDs and CNTs to maintain both excellent optical and electrical properties? There has been some work realizing the combination of CNTs and ultraviolet or visible active QDs, such as ZnO, CdS, and CdSe,16,17 by means of bond coupling,18, in situ growth,19 insulating silicon shell wrapping CNTs,20 etc. Effective charge injection from photoexcited QDs to CNTs was observed, which provides an opportunity to improve the photoconversion efficiency of QD-based nanohybrid solar cells. In recent decades, research21–23 has shown that PbS QD/CNT nanoarchitecture contributes to near-infrared (NIR) power conversion efficiency. Most of these nanoarchitectures are based on a two terminal photodiode or three terminal phototransistor device structure since the CNT is strong p type. Adding CNTs enhances carrier separation/transport capabilities. Ka et al. used pulsed laser deposition to decorate double-walled CNTs with PbS-QDs.24,25 The double-walled CNT/PbS-QD was integrated into microfabricated photodetectors, exhibiting a high responsivity and fast response. In 2023, Han et al. demonstrated that a combination of a PbS QD photodiode and a CNT film field-effect transistor (FET) could achieve a transistorized 950 nm NIR photodetector with a photosensitive gate.26 These works showed the potential of QD/CNT composites as the basis for future optoelectronics in visible or near-infrared regions.

In this work, we expand the QD/CNT composite wavelength to 2500 nm infrared. Improving high operation temperature performance is usually tough for infrared photodetectors due to the small infrared energy gap. The high carrier mobility and thermal stability of CNT benefit thermal carrier relaxations. We demonstrate that strong electric coupling is obtained between HgTe QDs and CNTs with proper short-chain ligands, where the mobility could achieve as high as 54.1 cm2/Vs while maintaining the optical response, which is three orders of magnitude higher compared with the reference sample. More importantly, the doping type could be adjusted with different QD-to-CNT ratios. When QDs and CNTs are mixed in a ratio of 1:1 by weight, the nanocomposite is almost intrinsic.

The coupling energy is estimated at 10 meV between QDs and CNTs from the transport property, which is strong enough for high carrier mobility but not obviously broadening the optical energy bandgap. With a simple photoconductor device structure, we demonstrate high responsivity up to 252 A/W at 2500 nm infrared with 1 V applied bias. The EQE exceeds 12 500%. The high internal gain comes from the ratio between the long carrier lifetime and the short transit time. We also demonstrate that the CNT matrix helps relax thermal carries in the nanocomposites. The nanocomposite photodetector maintains 100-times higher detectivity compared with a pure QD photodetector at a high operating temperature of 345 K.

Carrier mobility is considered one of the critical parameters to determine responsivity and detectivity in HgTe QD infrared photodetectors.27,28 Here, we propose a strategy for improving carrier mobility by mixing CNTs and HgTe QDs. Figure 1(a) illustrates the conception. Figure S1 illustrates the process. Figure 1(b) shows the absorption spectra of pure CNTs, pure QDs, and the nanocomposites with different QD-to-CNT ratios from 1:1 to 1:4 by weight. The cutoff wavenumber of the HgTe QD films is about 4200 cm−1. No obvious band edge in the infrared wavelength is observed on CNTs. In Fig. S2, there is absorption around 3400 and 1630 cm−1 from CNTs, attributed to the stretching vibrations of hydroxyl (–OH) and C–C groups, respectively.29,30 The nanocomposites with different QD-to-CNT ratios all show an obvious band edge following pure QDs. Figure 1(c) shows the photoluminescence (PL) spectra of pure QDs, CNTs, and the nanocomposites, respectively. The pure QDs exhibit an intense infrared emission peak at 3740 cm−1 under an 808 nm excitation. The CNTs show no obvious infrared photoluminescence, while the nanocomposites exhibit an infrared luminescence peak from 3800 to 4000 cm−1, red-shifting with a higher CNT concentration. The red-shifts in the luminescence peak may come from the QD surrounding changes due to CNT attachments. The Raman spectrum of CNTs is shown in Fig. S2, where the peak of the graphite band at 1590 cm−1 is associated with the vibration of sp2-bonded carbon atoms in a 2D hexagonal lattice, usually used to recognize well-ordered degree. The peak that appeared at 1340 cm−1 is the defect band, which is related to the disordered defect structure.31 

FIG. 1.

Nanocomposite characterization. (a) Schematic diagram of the nanocomposite. (b) and (c) FT-IR spectra and the normalized PL spectra of HgTe QDs, CNTs, and the nanocomposites, respectively. (d)–(f) TEM images of HgTe QDs, CNTs, and the nanocomposites, respectively.

FIG. 1.

Nanocomposite characterization. (a) Schematic diagram of the nanocomposite. (b) and (c) FT-IR spectra and the normalized PL spectra of HgTe QDs, CNTs, and the nanocomposites, respectively. (d)–(f) TEM images of HgTe QDs, CNTs, and the nanocomposites, respectively.

Close modal

Figures 1(d)1(f) show the TEM images of QDs, CNTs, and the nanocomposites, respectively. To achieve a strong attachment between HgTe QDs and CNTs, proper ligands and solvents should be chosen. CNTs are usually well dissolved in polar solvents, while QDs are usually dissolved in non-polar solvents caped with long chain organic ligands. Ligand exchange with phase transfer is necessary to achieve good bonding between the two components. In our case, CNTs are dissolved in polar N, N-dimethylformamide (DMF). HgTe QDs are prepared with mercury (II) salts and trioctylphosphine telluride (TOPTe) in long-chain amine solvents, which are non-polar. The surface ligand modification is vital.32 Here, to transfer HgTe QDs from a non-polar solvent to DMF, 2-mercaptoethanol (2-Me) is used for liquid phase ligand exchange. On the one hand, the thiol group could strongly bond to Hg on the QD surface. On the other hand, the oxhydryl group could not only improve the QD stability in the polar solvent but also form a hydrogen bond with CNT. However, the HgTe QDs synthesized from the TOPTe precursor usually precipitate without long chain 1-dodecanethiol (DDT). We notice that the coupling between CNTs and QDs is poor when there are DDT ligands, as shown in Fig. S3, where QD aggregation would form. Fortunately, we find that the 2-Me and precipitated QD mixture could directly re-dissolve in DMF without forming strong aggregation, as shown in Fig. 1(d). Then, QD–CNT mixtures show good coupling, as shown in Fig. 1(f).

The strong coupling between CNTs and QDs is also determined by transport characterization, as shown in Fig. 2. The carrier mobility is measured by FET, where the substrate is a 300 nm SiO2/Si wafer. A thin ∼100 nm coating layer of polymethyl methacrylate (PMMA) is spin coated on the top of the device. The mobility is then extracted by μFET=LW×Ci×VDdIDdVG, where W is the channel width (0.5 cm), L is the channel length (0.3 cm), Ci is the capacitance per unit area (1.15 × 10−4 F/m2), VD is the applied voltage (1 V), ID is the source–drain current, and VG is the gate voltage.

FIG. 2.

FET measurement. (a) and (b) FET transfer characteristics (at 300 K) on pure CNTs and QDs, respectively. (c)–(e) FET transfer characteristics (at 300 K) of the nanocomposites with different QD-to-CNT ratios. (f) Drift mobility as a function of temperature on pure CNTs (blue), QD:CNT = 1:1 with strong coupling (brown), QD:CNT = 1:1 with weak coupling (green), and pure QDs (orange), respectively.

FIG. 2.

FET measurement. (a) and (b) FET transfer characteristics (at 300 K) on pure CNTs and QDs, respectively. (c)–(e) FET transfer characteristics (at 300 K) of the nanocomposites with different QD-to-CNT ratios. (f) Drift mobility as a function of temperature on pure CNTs (blue), QD:CNT = 1:1 with strong coupling (brown), QD:CNT = 1:1 with weak coupling (green), and pure QDs (orange), respectively.

Close modal

As shown in Figs. 2(a) and 2(b), pure CNT is a strong p type with a hole mobility ∼184 ± 0.1 cm2/Vs at 300 K, while pure QDs are of strong n type with an electron mobility ∼0.037 ± 0.007 cm2/Vs. The hysteresis in QDs is usually large at room temperature, as shown in Ref. 33. Figures 2(c)2(e) show transfer curves on nanocomposites with QD-to-CNT ratios of 1:4, 1:2, and 1:1, with hole mobilities of 56.1 ± 0.2, 46.5 ± 2.1, and 34.6 ± 2.9 cm2/Vs, respectively. With a higher CNT component, the transfer curve follows the typical features of pure CNT, where the n type channel is not turned on. With a higher QD component, the hysteresis would increase, although it would be much smaller compared with pure QDs. With QD:CNT = 1, the transfer curve is ambipolar, indicating the nanocomposite is near intrinsic, with an electron mobility of 28.5 ± 1.2 cm2/Vs such as hole mobility. The ambipolar character of the nanocomposite is not due to a bulk heterojunction. Because if such a bulk heterojunction were formed, the electron mobility would be close to that of HgTe CQDs. The only coupling mechanism is a charge transfer from the low mobility HgTe CQD solid to the high mobility CNT matrix.

The nanocomposites show a three-order magnitude increase in mobility compared to pure QDs, attributed to the strong coupling between QDs and CNTs. Figure S4 shows the poor coupling case, although the mobility could be improved, with a magnitude of 0.1–1 cm2/Vs. This may be explained by the QD aggregation domain mobility as a function of temperature, which helps to figure out the carrier transport mechanism, as shown in Fig. 2(f). Mobility on pure CNT shows weak temperature dependence from 345 to 80 K. The mobility of pure QD solids also shows weak temperature dependence, which would first increase and then decrease while cooling from 345 to 80 K. However, both nanocomposites, with strong coupling or with weak coupling, show strong temperature dependence in the low temperature region at 165 K. We believe that in low temperature region, the lower thermal activation energy limits the carrier transport between the QD and CNT interfaces. Figure S5 shows the mobility with reversed temperature, showing a clear exponential dependence in the strongly coupled nanocomponents below 160 K, with the activation energy Ea = 80 meV fit with exp(–Ea/kT), k Boltzmann constant, and T temperature. The activation energy is less than 20% compared with the QD energy gap. The small-polaron Marcus electron transfer model might give a more clear explanation. In this model, μ(T) mobility as a function of temperature is
μ(T)=e(d+l)22π6J24πλkBT3expλ+ΔG24λkBT,
where J is the electronic transfer integral, λ is the reorganization energy, and ΔG is the energy disorder.
The reorganization energy is due to the polarization of the material. An estimate, as provided,34 is
λ=e4πε01r12(r+l)1εM1εst,
where e is the elementary charge of 1.6 × 10−19 C, ɛM is the optical dielectric constant of the matrix surrounding QDs, and ɛst is its static dielectric constant. The reorganization energy λ is about 5 mV in our case. Then the energy disorder ΔG is estimated at 35 meV. The electronic transfer integral J means the coupling energy, which is about 10 meV. The J value is about ten times larger compared with pure HgTe QD solids.33 

Figure 3 shows the photodetection characterization. Figure 3(a) shows the schematic diagram of the photoconductor. About 500 nm nanocomposite solids are deposited on an interdigital gold electrode with a gap of Lgap = 10 µm, followed by a thin PMMA layer for protection. Figures 3(b) and 3(c) show IV curves on the nanocomposite photodetector and pure QD photodetector at 300 and 345 K operating temperatures, respectively. At 300 K, the nanocomposite photodetector has a photocurrent density of ∼0.5 A/cm2 and a dark current density of 0.073 A/cm2 at 1 V bias; the value is almost two orders higher compared with a pure QD photodetector under the same condition, which is 6 and 1.2 mA/cm2, respectively. The photocurrent-to-dark current ratio is 7 and 5 in the nanocomposite and pure QD photodetectors, respectively. The high carrier mobility of CNT helps to amplify the current in the nanocomposite, while a proper QD-to-CNT ratio maintains the near intrinsic doping, benefiting the photocurrent-to-dark current ratio. While the operating temperature increases from 300 to 345 K, the advantage of the nanocomposite photodetector becomes more obvious, whose photocurrent-to-dark current ratio remains the same while that of the pure QDs exhibits a fivefold decrease. The decrease in photocurrent-to-dark current ratio in a pure QD photodetector comes from both reduced photocurrent and increased dark current, attributed to stronger phonon scattering and increased thermally activated carriers at higher temperatures. The high carrier mobility and thermal stability of the nanocomposite somehow suppress the effect. As a result, the nanocomposite photodetector shows decently high operating temperature performance. Still, if the temperature was further increased to 360 K, the photocurrent could decrease while the dark current increases. Figure 3(d) shows the spectral response at 300 and 345 K, with a small shift of 20 cm−1. The inset graph shows the temporal response within several milliseconds.

FIG. 3.

Photodetection. (a) A schematic diagram of the nanocomposite photodetector. (b) The nanocomposite photodetector’s dark current and photocurrent under 873 K blackbody radiation at 300 and 345 K operating temperatures, respectively. (c) The pure QD photodetector’s dark current and photocurrent under 873 K blackbody radiation, operating temperature at 300 and 345 K, respectively. (d) Spectral response at 300 and 345 K operating temperatures, with the inserted graph showing the temporal response. (e) and (f) Responsivity and detectivity of nanocomposite and pure QD photodetectors as a function of temperature.

FIG. 3.

Photodetection. (a) A schematic diagram of the nanocomposite photodetector. (b) The nanocomposite photodetector’s dark current and photocurrent under 873 K blackbody radiation at 300 and 345 K operating temperatures, respectively. (c) The pure QD photodetector’s dark current and photocurrent under 873 K blackbody radiation, operating temperature at 300 and 345 K, respectively. (d) Spectral response at 300 and 345 K operating temperatures, with the inserted graph showing the temporal response. (e) and (f) Responsivity and detectivity of nanocomposite and pure QD photodetectors as a function of temperature.

Close modal

Here, we note the photocurrent is measured with 873 K blackbody radiation, where the power density P is 20 µW/mm2. The responsivity is calculated as R=IphP. The responsivity is 252 and 3 A/W with a 1 V small bias on the nanocomposite and pure QD photodetectors. The EQE reaches as high as 12 500% in the nanocomposite photodetector due to the high internal gain from the ratio between the carrier lifetime and short transit time. The transit time, τt=Lgap2μV, is 33 ns for a 30 cm2/Vs high mobility sample at 1 V bias. Then the carrier lifetime could be estimated as μs in the nanocomposite. The detectivity D* is AIphInP, and A = 0.5 mm2 is the conductor area. In is the current spectral density measured using a SR760 spectrum analyzer, which exhibits a flat noise of 18 pA Hz−0.5 for the nanocomposite and 4.6 pA Hz−0.5 for the pure QD photodetector at 300 K. The D* is 1012 Jones and 4 × 1010 Jones for the nanocomposite and pure QD photodetector at 300 K, respectively. Figures 3(e) and 3(f) show the responsivity and detectivity as a function of temperature on the nanocomposite and pure QD photodetector, respectively. The nanocomposite photodetector shows higher thermal stability up to 345 K. Further increasing the operating temperature would quickly quench the photoresponse.

An imaging demonstration of the HgTe QD–CNT photoconductor is shown in Fig. 4, with a single-pixel scanning imaging system shown in Fig. 4(a). The infrared image maps the reflected/scattered light from the tungsten lamp. A lens is scanned to move the projected image over the HgTe QD–CNT photoconductive detectors using an X–Y dual-axis mobile platform. The chemical solvents of IPA, TCE, and water in glass vials are the imaging objects, as shown in Fig. 4(b). All liquids are transparent in the visible wavelength, while the absorption vibration of C–H and O–H chemical bonds in the infrared range is different, causing these four chemicals to appear different on gray scales, and opaque silicon becomes almost transparent in the short-wave infrared range in Fig. 4(c). Figures 4(d) and 4(e) show the images of a 400 °C soldering iron behind the silicon wafer captured using a visible camera and a nanocomposite photodetector, reflecting thermal information. Figures 4(f) and 4(g) show the images of the artificial and fresh leaves captured using a visible camera and a nanocomposite photodetector. The shape of the two leaves is almost identical, which makes it difficult to distinguish their authenticity at visible wavelengths. Due to water absorption in the infrared from the fresh leaf, it appears dark, while the artificial leaf is transparent in the short-wave infrared wavelength. This is why both the apple and the pear in Fig. 4(a) look dark, even though they have different appearances. It demonstrates that the nanocomposite photodetector could open vast opportunities in chemical detection, wafer inspection, and advanced anti-counterfeiting applications.

FIG. 4.

Infrared imaging with the HgTe QD–CNT photoconductive detectors. (a) Illustration of the infrared imaging system of the HgTe QD–CNT photoconductive detectors. A series of infrared images are captured using the HgTe QD–CNT photoconductive detectors. (b) and (c) Visible and infrared images of isopropanol (IPA), tetrachloroethylene (TCE), and water. In front of the vials, a silicon wafer is placed. (d) and (e) Visible and infrared images of a 400 °C soldering iron behind a silicon wafer. (f) and (g) Visible and infrared images of artificial and fresh leaves.

FIG. 4.

Infrared imaging with the HgTe QD–CNT photoconductive detectors. (a) Illustration of the infrared imaging system of the HgTe QD–CNT photoconductive detectors. A series of infrared images are captured using the HgTe QD–CNT photoconductive detectors. (b) and (c) Visible and infrared images of isopropanol (IPA), tetrachloroethylene (TCE), and water. In front of the vials, a silicon wafer is placed. (d) and (e) Visible and infrared images of a 400 °C soldering iron behind a silicon wafer. (f) and (g) Visible and infrared images of artificial and fresh leaves.

Close modal

Hybridizing HgTe QDs with other material systems has proved to be an effective method for improving infrared optical electronic properties.35–38 In this work, we have designed dual regulation of carrier mobility and polarity behavior on HgTe QD and CNT nanocomposite. FET measurements show that the mobility could reach 28.5–54.1 cm2/Vs while maintaining the optical response, which is three orders of magnitude higher compared with the reference sample. The doping type could be adjusted with different QD-to-CNT ratios. When QDs and CNTs are mixed with a ratio of 1:1 by weight, the nanocomposite photoconductors demonstrate a high responsivity of 252 A/W at 2500 nm infrared with 1 V applied bias at 300 K. The EQE exceeds 12 500%. The strong coupling between QDs and CNTs would be maintained from 165 to 345 K. CNT thermal stability benefits the nanocomposite photodetector performance at high operating temperatures. Ultimately, we demonstrate the nanocomposite photoconductor with applications in infrared imaging.

The supplementary material is available free of charge, including a schematic diagram of the QD–CNT nanocomposite preparation process; CNT characterization; TEM images of nanocomposite with long-chain thiol ligands; FET on nanocomposite with long-chain thiol ligands; mobility as a function of the reciprocal of temperature; the measure of noise of nanocomposites device at different biases; temporal response; the detectivity as a function of the bias voltage; and FET characterization of intrinsic high mobility nanocomposite solids.

This work was funded by the Westlake Institute for Optoelectronics (Grant No. 2024GD003). This work was also funded by the National Natural Science Foundation of China (Grant No. 62105022) and the Beijing National Laboratory for Condensed Matter Physics (Grant No. 2023BNLCMPKF012).

The authors have no conflicts to disclose.

X.X. and H.L. contributed equally to this work. X.X. and M.C. investigated the QD-CNT coupling and device fabrications. H.L. and M.C. investigated the weak coupling case and photodetector applications. All authors contributed to the writing.

Xiaomeng Xue: Formal analysis (equal); Investigation (equal); Writing – original draft (equal); Writing – review & editing (equal). Hongyu Lv: Data curation (equal); Investigation (equal). Yanyan Qiu: Formal analysis (supporting); Investigation (supporting). Qun Hao: Supervision (equal). Menglu Chen: Conceptualization (lead); Data curation (equal); Formal analysis (lead); Funding acquisition (lead); Investigation (lead); Methodology (lead); Project administration (lead); Supervision (lead); Writing – original draft (lead); Writing – review & editing (lead).

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

1. Materials

The multi-walled CNTs (30–50 nm in diameter, 10–20 µm in length, 95% purity) were purchased from Chengdu Organic Chemicals Co., Ltd., Chinese Academy of Sciences. HgCl2 (≥98%), Te (≥99.99%, RG), Tri-n-octylphosphine (97%, Sigma-Aldrich), isopropanol (99.9%, Adamas), and 1,2-ethanedithiol (≥98%) were used. Oleylamine (≥90%, Adamas, RG) was dried under dynamic vacuum at 120 °C for 2 h and stored in a nitrogen glovebox. Preparation of 1M TOPTe precursor: 1.276 g Te was dissolved in 10 ml of tri-n-octylphosphine (TOP) at 80 °C for 24 h.

2. QD synthesis and purification

HgTe QD synthesis followed the process reported in Ref. 39. 27.2 mg HgCl2 (0.1 mmol) was dissolved in 4 ml of oleylamine in a 20 ml glass vial at 100 °C for 1 h with stirring in a glovebox. The temperature of the solution was reduced and stabilized at 80 °C. About 100 μL of TOPTe solution was rapidly injected into the HgCl2 solution; the reaction was allowed to proceed for 4 min and then quickly cooled by injection of 4 ml tetrachloroethylene (TCE). After the glass vial was taken out of the glovebox and cooled with running water, the HgTe QDs were placed into the centrifuge tube, and 30 ml of isopropanol (IPA) was added for cleaning, stirred, and put into a centrifuge. After 5000 r/min for 5 min, the supernatant was poured off and the remaining HgTe QD solids were dried with nitrogen and then dissolved in 5 ml of n-hexane.

3. High carrier mobility nanocomposite (HgTe QD–CNT)

HgTe QDs modified mixed-phase ligand exchange method: 5 ml HgTe QDs in n-hexane was mixed with 50 μL of β-mercaptoethanol (β-me) and 15 mg of didodecyldimethylammonium bromide (DDAB). After ultrasound shaking, the QDs were transferred to the N, N-dimethylformamide (DMF), and the supernatant (n-hexane) was removed. For n-type HgTe QDs, 20 mg HgCl2 was added in CQD/DMF. Then, 30 ml of toluene was added for cleaning, stirred, and put into a centrifuge. After 5000 r/min for 5 min, the supernatant was poured off and the remaining HgTe QD solids were dried with nitrogen and then dissolved in 200 μL of DMF.

CNT/DMF: CNTs were dissolved in the polar solvent DMF.

HgTe QD–CNT solution: HgTe QD/DMF and CNT/DMF were mixed in different proportions, and the mixed solution was ultrasonicated for 3 min.

4. Device fabrication

Nanocomposite photodetector: For a nanocomposite photodetector with a QD-to-CNT ratio of 1:1, the device was built upon an Al2O3 substrate and 50 nm of ITO was subsequently grown by magnetron sputtering. The substrate was treated with 3-mercaptopropyl trimethoxysilane (MPTS) for 30 s and rinsed with IPA. Two drops of nanocomposite solution were drop-cast to the substrate, and the dry film was exposed to 10 mM HgCl2 in methanol for 10 s, rinsed with IPA, and dried with nitrogen. The film was then immersed in an EDT/HCl/IPA 1/1/50 (v/v/v) solution for 10 s, rinsed with IPA, and dried with nitrogen. Two layers of the nanocomposite in total were drop-cast following the same process.

FET devices: Silicon wafers with dry thermal oxide (n++ Si/300 nm SiO2) were used as the substrates. The substrate is placed on a table at 60 °C and evenly drop-coated with materials such as pure CNTs and nanocomposites with different QD-to-CNT ratios. The nanocomposite films were then immersed in an EDT/HCl/IPA 1/1/50 (v/v/v) solution for 10 s, rinsed with IPA, and dried with nitrogen.

5. Optical characterization

Absorption: The absorption spectra were collected with a Ying Sa Optical Instruments FOLI20 FT-IR using a ZnSe window.

Photoluminescence: Solids were photoexcited using an 808 nm continuous laser chopped at 100 kHz, and the interference peak signal was collected on the oscilloscope using a homemade Michelson interferometer with an MCT detector and a phase-locked amplifier. The PL spectra would be obtained by the Fast Fourier Transform (FFT).

6. Device characterization

For all photodetectors placed in a cryostat for temperature-dependent characterization, the effective device area is 0.5 mm2, where the 25 pairs of interdigitated evaporated gold electrodes have a finger width of 10 μm, a gap of 10 μm, and a finger length of 1 mm. The IR light uses a blackbody radiation source at 600 °C, and the I–V curves were recorded using the source meter Keithley 2602B. The method for testing FET transfer curves is similar. A photodetector is connected to FT-IR as an external detector for spectral response.

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Supplementary Material