Light-triggered interfacial charge transfer and enhanced photodetection in CdSe/ZnS quantum dots/MoS 2 mixed-dimensional phototransistors Opto-Electronic Advances

Mix-dimensional van der Waals heterostructures (vdWHs) have inspired worldwide interests and efforts in the field of ad-vanced electronics and optoelectronics. The fundamental understanding of interfacial charge transfer is of vital import-ance for guiding the design of functional optoelectronic applications. In this work, type-II 0D-2D CdSe/ZnS quantum dots/MoS 2 vdWHs are designed to study the light-triggered interfacial charge behaviors and enhanced optoelectronic performances. From spectral measurements in both steady and transient states, the phenomena of suppressed photoluminescence (PL) emissions, shifted Raman signals and changed PL lifetimes provide strong evidences of efficient charge transfer at the 0D-2D interface. A series of spectral evolutions of heterostructures with various QDs overlapping concentrations at different laser powers are analyzed in details, which clarifies the dynamic competition between exciton and trion during an efficient doping of 3.9×10 13 cm −2 . The enhanced photoresponses (1.57×10 4 A·W –1 ) and detectivities (2.86×10 11 Jones) in 0D/2D phototransistors further demonstrate that the light-induced charge transfer is still a feasible way to optimize the performance of optoelectronic devices. These results are expected to inspire the basic understanding of interfacial physics at 0D/2D interfaces, and shed the light on promoting the development of mixed-dimensional optoelectronic devices in the near future. X, CG et al. Light-triggered interfacial charge transfer and enhanced photodetection in CdSe/ZnS quantum dots/MoS 2 mixed-dimensional phototransistors. Opto-Electron Adv 4 , 210017 (2021).


Introduction
Low-dimensional heterostructures are extremely important members in various semiconductor devices, including photodetectors, phototransistors and light-emitting beyond traditional silicon-based integrated circuits 3,4 . Such 2D/2D vdWHs show unique properties, including high carrier mobility, atomic-scale depletion region and tunable light-matter interaction, which are stemming from the designed energy band alignment and the ultrafast interfacial charge transfer 5−7 . However, the interlayer coulomb interaction and the carrier scattering are dominant in such atomically-thin layers, which have great influences on the performance of optical behaviors and optoelectronic devices 8 . Besides, these kinds of devices still suffer from performance limitations due to their weak light absorption and pristine atom defects 9 .
Several strategies have been devoted to tackle these problems, such as surface chemical modification 10,11 , nanophotonic structures integration 12,13 , mixed-dimensional vdWHs design 14 , etc. The reduced dimension of semiconductors provides large specific surface areas and abundant interfaces, which means, TMDs flakes integrated with one-dimensional (1D) nanowires, especially with 0D quantum dots (QDs), could perform as promising architectures for high-performance optoelectronic devices 15−18 . To further achieve the high optoelectronic performance of mixed-dimensional vdWHs device, the integration of 2D materials with high carrier mobility and 1D or 0D materials with large light-sensitive absorption should be designed and fabricated. The reduced dimension creates abundant interfaces providing alternative channels to generate highly-efficient charge doping at the interface of 0D/2D semiconductors 19,20 . 0D QDs possess high quantum efficiencies, strong absorption coefficients, and low-cost processings, which are usually utilized to construct mixed-dimensional devices exhibiting broadband photodetection, excellent stability and remarkable photoresponsivity 21 .
Molybdenum disulfides (MoS 2 ) monolayers are typical TMD semiconductors with band energy of 1.83 eV in visible spectral range, which usually show room temperature mobility of 0.1-10 cm 2 ·V −1 ·s −1 , photoresponsivity of <10 A·W -1 and on-off ratios of 10 6 -10 8 22,23 . Perovskite QDs, such as CH 3 NH 3 PbBr 3 and CsPbBr 3 , have recently been studied to build mixed-dimensional heterostructures with MoS 2 flakes showing enhanced optoelectronic responses 24−26 . But they face challenges in realizing airstable heterostructures 27 . Besides, the design of heterostructure type among type-I and type-II is crucial, which decides carrier transfer behaviors and photoresponse enhancements 25,28 . Colloidal QDs and their core-shell QDs are stable semiconductors, which have been applied on MoS 2 layers to realize high-performance photodetectors 29,30 . Although these 0D/2D conceptual devices show outstanding performances in phototransistors and photodetectors, the charge transfer at the mixed-dimensional interfaces with complex ultrafast processes remains poorly understood 31−33 . Moreover, the light-controlled charge transfer processes in 0D/2D vd-WHs have less been investigated, which may stimulate the design and creativity of photosensitive transistors.
In this work, we design 0D/2D mix-dimensional vd-WHs (CdSe/ZnS QD/MoS 2 ) to investigate the lighttriggered charge transfer processes at the nanoscale interface. The steady-state and transient-state optical measurements of colloidal QDs, MoS 2 monolayers and heterostructures are systematically investigated, showing tunable PL spectra and ultrafast charge transfer stemmed from a type-II band alignment. In a three-energy-level model, an effective charge transfer from QDs to MoS 2 monolayers can be verified from the weight ratio of exciton spectra, which has been estimated as 3.9×10 13 cm −2 . The competition of QD exciton, MoS 2 exciton and trion can be further modulated in a light-triggered n-type doping process. Moreover, these power-sensitive charge transfer processes at the interface help to improve the performance of optoelectronic devices based on TMDs with large responsivities and detectivities. These results are expected to provide the fundamental understanding of light-triggered charge transfer in 0D/2D mixed-dimensional vdWHs, which will contributes to the designing of high-performance optoelectronic devices in the near future.

Materials and methods
Material preparation MoS 2 monolayers were synthesized by chemical vapor deposition method. A quartz boat was filled with molybdenum oxide (MoO 3 ) powder and then located in the center of a furnace. The boat was covered by a cleaned Si/SiO 2 substrate. Another ceramic boat filled with sulfur (S) powder was placed at the upstream region of the furnace in a lower temperature zone. The temperature was raised to 830 °C for 10 min to grow MoS 2 crystals. Meanwhile, the S powder was heated to 300 °C by a heating belt, and the sulfur vapor flowed into the furnace by the carrier gas of Ar. Finally, the furnace was gradually cooled down to room temperature. During the whole process, the ultrahigh-purity Ar gas was held at 75 sccm under atmospheric pressure. Colloidal CdSe/ZnS QDs are commercially available. The heterostructures were fabricated in a spin-coating process, where dilute solutions of 0.025 mg/L QDs can be spin-coated onto the MoS 2 monolayer at a rotation speed of 2700 rpm for 1 min.

Optial characterization
The optical properties were measured by a home-built μ-PL system. An iHR550 Raman spectrometer from Horiba was utilized to measure the PL and Raman spectra with 300 g·mm −1 and 1200 g·mm −1 gratings, respectively. A Ti: Sapphire laser at 400 nm (100 fs, 80 MHz) was used as the excitation source for transient-state measurements, which was generated by an 800 nm laser from a mode-locked oscillator (Tsunami 3941-X1BB, Spectra-Physics) positioned after a BBO crystal. Timeresolved PL (TRPL) is measured by a streak camera (Hasmamatsu Universal, C10910) with a resolution of 3 ps for short-time range measurements (~200 ps). A 532 nm solid-state laser was also induced to excite samples to obtain the steady-state spectra. The objective lens is 50× magnifications, and the diameter of the laser spot is ~2 μm.

Device fabrication and measurement
The metal electrodes of MoS 2 and heterostructure devices were fabricated in a typical E-beam lithography (EBL) process, followed by e-beam evaporation of Au/Cr (100 nm/5 nm) electrodes. Then, samples were annealed in vacuum for 3 h at ≈180 °C to improve the contact conductance. The surface ligands of the coated QDs were purified by ethyl acetate and hexane to facilitate charge transport. Finally, the devices were annealed on a hot plate at 50 °C for 10 min to evaporate the solvent for electrical measurements. The electrical and optoelectronic properties of the as-fabricated devices were performed in a vacuum Lake Shore Probe Station combined with an Agilent B1500A semiconductor analyzer at room temperature. The structure and morphology of as-prepared QD/MoS 2 heterostructures can be confirmed by transmission electron microscopy (TEM) and atomic force microscopy (AFM). Figure 1(b) shows the high-resolution TEM image of MoS 2 monolayer. Our MoS 2 samples are of high-quality that there are no defects or substitute atoms observed from measurements. The inset shows the corresponding selected area electron diffraction (SAED) pattern. The pattern only presents one set of six-fold symmetry diffraction spots, which is stemming from the hexagonal atom arrangement of MoS 2 flakes. The enlarged high-resolution TEM (HRTEM) image is shown in Fig. 1(c), which is derived from the marked squares in Fig. 1(b). The interplanar spacing is measured to be 2.78 Å corresponding to (100) planes of hexagonal MoS 2 . AFM topography of MoS 2 monolayer shows a clear triangular shape, and the thickness is determined to be 7.5 Å (1 Å=10 −10 m).

Results and discussion
After the wetting transfer process, QD/MoS 2 heterostructures can be successfully transferred onto the Cu grid. Figure 1 . Notably, the wrinkles or the edges of MoS 2 monolayer are clearly observed as orange arrows point, which confirms that the as-transferred films are intact. From an enlarged area of HRTEM image of QD/MoS 2 heterostructure ( Fig. 1(g)), Moiré patterns can be observed at the edge of QDs, which is caused by overlapping lattices between MoS 2 and QDs. The scale bar is 2 nm. The corresponding SAED image shows two different sets of arranged diffraction patterns, as shown in the inset of Fig. 1(g). The orange circles show a six-fold symmetry diffraction spots from MoS 2 , while the red circles exhibit a parallelogram configuration from CdSe/ZnS QDs.
The optical and spectral properties have been investigated among CdSe/ZnS QDs, MoS 2 monolayers and their heterostructures. Figure 2(a) shows the UV-visible absorption spectrum of QDs solution, where two dominant absorption peaks are observed at around 450 nm and 620 nm. The PL spectra of QDs solution (0.2 mg·L −1 ) and QDs film on SiO 2 /Si substrate show Lorentzian lineshapes, where the peak center is 650 nm with a full width at half maximum (FWHM) of 45 nm. Although the excitation conditions are different, the PL spectrum of QDs on substrate exhibits a slightly-stronger spectral broadening than QDs in solutions. It is because of the change of the dielectric environment of exciton emissions. Figure 2(b) shows the Raman spectra of MoS 2 monolayer (red curve) and QD/MoS 2 heterostructures (purple curve). Two typical Raman peaks of MoS 2 monolayer are observed at 384.2 cm −1 and 404.3 cm −1 , which correspond to the E 1 2g and A 1 g vibrational modes, respectively. The difference between these modes in the as-synthesized MoS 2 monolayer is approximately 20.1 cm −1 , which is slightly larger than the mechanically exfoliated monolayer MoS 2 because of the crystalline strain 34,35 . While in heterostructures, the Raman peaks of E 1 2g and A 1 g modes blue-shift lightly, which are arising from the strain force and the charge transfer at the interface of heterostructures. The peak intensity of heterostructures shows a great attenuation, which is due to the blocked signals by the overlapped QDs layer. Insets show two optical images of MoS 2 monolayer (left) and QD/MoS 2 heterostructures (right), and scale bars are 6 μm.
The absorption spectrum of MoS 2 monolayer is measured as shown in Fig. 2(c), which is derived from the differential reflectance spectra between MoS 2 /substrate and substrate under white light illuminations. Two absorption peaks are observed to be located at approximately 622 nm and 659 nm, which correspond to direct excitonic transitions at K points in the Brillouin zone of MoS 2 . Correspondingly, the PL spectrum of MoS 2 monolayer presents two featured peaks at 630 nm and 678 nm, respectively. In 0D/2D vdWHs, PL spectra show two peaks at 650 nm and 677 nm 14,36 . The peak intensity of exciton A decreases much, because charge transfer behaviors occurring at the interface suppress the radiative recombination. The PL spectra are excited by a 532 nm laser at 0.3 mW, and the detected samples are heterostructures with 0.025 mg·L −1 QDs.  PL intensity mapping has been performed to show spectral change in MoS 2 monolayer and heterostructures. Figure 2(d) and 2(e) show the images of PL mapping of MoS 2 monolayer at 665−750 nm and 640−660 nm, respectively, which are consistent with PL peaks of QDs and exciton A of MoS 2 . It is obviously observed that the PL intensity is dominant at the spectral range of exciton A. Images of PL intensity mapping of vdWHs are shown in Fig. 2(f) (665−750 nm) and Fig. 2(g) (640−660 nm). In comparison to Fig. 2(d), the PL intensity of heterostructure in Fig. 2(f) decreases a little owing to the interfacial charge transfer, and the intensity distribution shows slight inhomogeneity. While at the detection wavelength of 640−660 nm, the intensity of heterostructure is slightly stronger than that of MoS 2 monolayer, where the intensity enhancement is mainly from the high-efficiency emission of QDs. Besides, a few QDs islands distributed on the substrate can also be observed. The laser powers are controlled at 0.3 mW for all the mapping measurements, and all the scale bars in Fig.  2 Time-resolved photoluminescence (TRPL) spectroscopy was carried out to further study and understand the spectral quenching caused by charge transfer. Fluor- escence lifetime detection were obtained with 0.4 mg L −1 QD/MoS 2 samples excited by a pulsed laser (400 nm, 80 MHz), at an excitation power of 0.3 mW. The signals were subsequently filtered by 650/50 nm bandpass filters, which can collect the full PL spectral range of QDs while keeping away from the main PL peak of MoS 2 monolayer. The PL lifetimes for the two samples in Fig. 2(h) can be fitted and estimated using a biexponential decay model ( ). The decay processes are considered to be consisted of fast decay τ 1 and slow decay τ 2 . The fast decay is related to the trap-assisted recombination in QDs or carrier extraction by the layers in heterostructures, while the slow decay is related to radiative recombination 37,38 . By analyzing the components of two decay processes, the average lifetime of QDs and QD/MoS 2 heterostructures are determined to be 498.6 ps and 98.9 ps, respectively. The average lifetime of QD/MoS 2 heterostructure decreases much compared to QDs, indicating that considerable charge transfer can occur at the 0D/2D interface. Notably, the QDs in heterostructures exhibit a larger proportion of the fast decay than that exhibited by QDs alone, which confirms the trap-assisted recombination or carrier extraction is induced and dominant in QD/MoS 2 heterostructures due   to the charge transfer process (Supporting Information S3). Figure 2(i) shows the energy band diagram of QD/MoS 2 heterostructures. The conduction band and valence band of MoS 2 monolayer are reported to be 4.25 eV and 6.1 eV in previous work, respectively 26 . While for QDs, the conductance and valence bands are reported to be 3.2−3.4 eV and 5.3−5.5 eV, respectively 39,40 . The energy band structure of QD/MoS 2 heterostructure is determined to be a type-Ⅱ band alignment. The built-in field is realized due to the interfacial charge transfer. With the excitation of incident laser, electron-hole pairs are mainly generated in the highly light-absorbing QDs layer, and then separated by the built-in field. As a result, electrons drift to the MoS 2 layer while holes are left in QDs.
To estimate the charge doping efficiency in heterostructures at various concentrations, the analysis of spectral evolution are performed to demonstrate the contribution of both exciton and trion in the PL spectra. Figure 3(a) shows the competition between exciton and trion emissions when the QD concentration is increased. At low concentrations, the exciton components are dominated in the total spectra, while the weight of exciton decreases as the concentration increases. All the components are fitted using Lorenze line-shaped peaks (Support-ing Information S4).
The three-energy-levels model is shown in Fig. 3(b) to illustrate the competition between exciton and trion. Exciton is excited from the ground state to exciton energy level under the laser excitation (G). In an assumption that the trion is generated from the exciton in a non-radiative channel (K tr (n)). Both the exciton (Γ ex = 0.002 ps −1 ) and trion (Γ tr = 0.02 ps −1 ) in MoS 2 monolayer generate PL signals in radiative channels. Then, the rate equations for the population of exciton (N A ) and trion (N A' ) can be expressed as where K tr (n) is the formation rate of trion that relates with the QDs concentration (n), and K tr (0) = 0.5 ps −1 is the formation rate of trion for the pure MoS 2 film 41 . The minimum k tr (δ) can be obtained as 0.02 ps −1 as reported before. α is a parameter that reflects physical adsorption probability of QDs (Supporting Information S5) 42 .    total PL intensity and the exciton intensity attenuate greatly as the concentration increases. Because the charge transfer electrons become abundant with high QDs concentrations, the recombination process of electron-hole pairs is deeply suppressed. However, the intensity of trion is almost unchanged in the doping process, which is due to the Pauli blocking effect and manybody interactions that the luminescence of the trion is easily saturated at low doping levels 43,44 . The weight ratio of exciton in total PL (I A /I Total ) at different concentrations can be obtained from Fig. 3(c), where the ratio gradually decreases from 0.81 to 0.48 when the concentration increases, as shown in Fig. 3(d). Based on the mass action law, the charge density can be estimated from the weight ratio of exciton. Figure 3(e) shows the dependence of charge density with the weight ratio. To further study the modulation ability of light-induced charge transfer and competitive exciton emissions, the laser-power dependent spectra of QDs, MoS 2 and heterostructures with 0.025 mg L −1 QDs have been systematically investigated. Figure 4(a) and 4(b) show power-dependent PL spectra of MoS 2 and QDs, respectively. When laser powers are increased from 65 to 670 μW, the intensity enhancement factors are determined as P=I/I 0 , where I and I 0 are the intensities detected at high and low laser powers, respectively. P are detected as 8.4 for MoS 2 exciton A and 7 for trion, respectively (Supporting Information S7). While for QDs, the intensity enhancement factor is detected as 20, and peak positions are stably appearing at 650 nm. Figure 4(c) shows the PL spectral evolution of QD/MoS 2 heterostructures with increasing laser powers ranging from 65 to 670 μW. There is an obvious competition between QD exciton (green), MoS 2 exciton A (blue) and trion (purple). At low power densities (<120 μW), MoS 2 exciton and trion emissions are dominant resulting in a higher PL intensity larger than QDs. When laser powers are further increased, all components of exciton  and trion get stronger, but the increasement speed behaves differently. It can be observed that the component of QDs exciton increases much fast, which gets stronger and becomes comparable to the component of MoS 2 exciton at about 120 μW (Supporting Information S8). The evolution of peak intensity of exciton and trion with laser powers has been plotted in Fig. 4(d). Both xaxis and y-axis are presented in log scales. The relationship of PL intensity (I) and laser power can be approximately written as I=(P) m , where m denotes exponent. For QD exciton, m decreases from 1.28 to 1.15 (QDs exciton in heterostructures). The value of m obtained from exciton A of MoS 2 decreases from 0.9 to 0.87 (exciton A in heterostructures). The difference of PL sensitive-response gives rise to the competition and spectral evolution at various light powers. Figure 4(e) shows the schematics to illustrate the light-controlled spectral change. The blue rectangular and dotted orange rectangular show the areas of MoS 2 flakes and QDs films, respectively. In a QD/MoS 2 heterostructure with low QD density (as shown as the overlapping region), the number of MoS 2 excitons excited at low laser powers is larger than QDs excitons. However, when the laser powers are increased to a high level, more QD excitons are generated, which are comparable to the contribution of MoS 2 excitons, or even larger than that. The light-controlled spectral evolution shows the potential ability of tunable light-matter interactions in TMDs, which helps to influence the performance of optoelectronic devices.
The photo-induced charge behaviors are crucial to improve the performance of optoelectronic devices. The phototransistors of pristine MoS 2 and vdWHs are systematically investigated, and their characteristics of photocurrents are measured to investigate the influence of interfacial charge doping. Figure 5(a) shows the schematic view of the device, where MoS 2 monolayer served as a channel in the mix-dimensional FET (field-effect transistor), and Au/Cr (100 nm/5 nm) metal served as source/drain contacts on the back-gate Si/SiO 2 substrate. Under the light illumination, electrons from QDs are transferred into MoS 2 monolayer resulting in an efficient charge doping. The dilute solution of QDs can be uniformly spin-coated onto the MoS 2 monolayer to realize the mixed-dimensional vdWHs device.
To evaluate the optoelectronic performance of the vd-WHs device compared to that of the MoS 2 transistor, we investigated the photo-induced transfer curves ( Fig.  5(b)), as well as the current-voltage characteristics in the dark and under illuminations (Fig. 5(c)). Both pristine MoS 2 and hybrid phototransistors exhibit a typical ntype behavior in the dark. The threshold voltages of pristine MoS 2 and heterostructure phototransistors are obtained as −20 V and −60 V. As shown from the photoinduced transfer curves (I ds −V g ) under the illumination (520 nm), I ds of the photodetector in the dark is increased by ~4.8 times compared to that of the pristine MoS 2 device (7.55×10 −7 A vs 1.56×10 −7 A) at V g =60 V.
The enhanced drain currents (I ds ) under the illumination are obtained as 4.4 times for pure MoS 2 device (from 1.56×10 −7 A to 6.86×10 −7 A) and 11.3 times for QD/MoS 2 heterostructure device (from 7.55×10 −7 A to 8.52×10 −6 A) at V g =60 V, respectively. It can be understood that the strong light absorption of QDs and the effective charge separation at the interface contribute to the amplified photocurrents in vdWHs devices. I ds and V g are the channel current and the gate voltage, respectively.
Furthermore, the photoresponsivity (R=I ph /P eff ) and specific detectivity (D*=RS (1/2) /(2eI d ) 1/2 ) are calculated to investigate the device performance, where I ph is the difference between photocurrent intensity and the darkstate current intensity, P eff is the effective incident laser power (P eff =P in ×S device /S laserspot ), S device is the effective detection area of the device, I d is the dark current, and e is the unit charge. Figure 5(d) and 5(e) show the calculated values of R and D* as a function of P eff , respectively, and the maximum values are detected at V g = 60 V (the D* versus V g is presented in the Supporting Information S11). The calculated R of the hybrid device is enhanced by 21.4 times (from 733 A·W -1 to 1.57×10 4 A·W -1 ) compared to that of the pristine MoS 2 device, which is an outstanding performance compared to that of other TMD-based photodetectors (Supporting Information S12). Furthermore, the calculated R value exponentially increases as the incident power decreases due to the reduced scattering and recombination. The obtained highest D* for the heterostructure device is enhanced by a factor of 9.6 compared to that of the pristine MoS 2 photodetector (from 2.98×10 10 Jones to 2.86×10 11 Jones).
We now discuss the possible mechanisms for improved photoresponsivities in these vdWHs devices. Both the photogating and photoconducting effects can coexist at the nanoscale interface, but the dominant mechanism varies. For the photoconductive effect, the current intensity linearly increases with excitation laser powers (I∝P eff ), while it shows sublinear trend for photogating effect (I∝P eff α and α<1) 48,49 . The values of α at various V g can be calculated and plotted as shown in   increases as the concentration increases. It confirms that the enhancement of light absorption from QDs can generate more photoinduced carriers and improve the detection photoresponsivity of devices.

Conclusions
In summary, we have demonstrated that interfacial charge transfer in 0D/2D mixed-dimensional vdWHs can be tailored and utilized to enhance the optoelectronic performance of phototransistors. A series of morphology characteristics and spectral measurements, including high-resolution TEM, AFM, Raman shift, suppressed fluorescence intensity, and short PL lifetime provide strong evidences to show type-II band alignment induced charge transfer. In a model of three-energy-levels, an effect photo-induced doping can be estimated as 3.9×10 13 cm −2 , which contributes to the improved carrier mobility and enhanced optoelectronic performance of device in comparison with pure MoS 2 device (1.57×10 4 A·W −1 vs 733 A·W −1 , 2.86×10 10 Jones vs 2.98×10 10 Jones @520 nm) at a low power density (≈0.1 nW). Furthermore, the detuning of laser power and back-gate voltage provides an alternative way to manipulate the gain mechanism of device. Therefore, the interfacial control of charge transfer gives more freedoms to optimize the optoelectronic features of mixed-dimensional vdWHs, which may bring more opportunities for the coming atomic-level device integrations.