Dark-current reduction accompanied photocurrent enhancement in p-type MnO quantum-dot decorated n-type 2D-MoS₂-based photodetector

Incorporating heterojunction architecture into photodetectors (PDs) based on two-dimensional (2D)-transition metal dichalcogenides (TMDCs) can efficiently enhance their photoresponse characteristics. In recent years, 2D molybdenum disulfide (MoS₂) has attracted great attention in the field of optoelectronics, due to its superior electrical, optical, and mechanical characteristics.^1–4 Its layer-dependent physical properties^5–7 such as the bandgap, work function, and energy dispersion relation, in particular, have increased the likelihood of obtaining an ideal platform for photodetectors, thus overcoming the limits of graphene, such as zero-bandgap and optical transmittance.^8,9 Findings yielded by earlier studies based on single- or few-layered MoS₂ have demonstrated superior photodetection characteristics (e.g., a photoresponsivity of 880 A/W, a photo-gain of 1840, and a detectivity of ∼10¹² Jones).^10–13 Hybrid MoS₂ materials have also been studied, aiming to efficiently use their excellent 2D characteristics, such as charge transfer and light-matter interactions.⁵ For that purpose, formation of heterojunctions with various nanomaterials, such as carbon nanotubes,¹⁴ graphene,¹⁵ and perovskite thin films,¹⁶ was attempted. While such structures have the potential to markedly enhance absorption and quantum efficiency properties, some critical issues must be addressed before they can be used as a platform for developing hybrid 2D-MoS₂ photodetectors. In general, highly crystalline single- or few-layered 2D-MoS₂ induces a high dark current^11,12 by which an extremely small photocurrent generated by a few photons can be veiled or distorted. Therefore, suppression in the dark current along with the increase in the photocurrent is a prerequisite for photoresponse enhancement (high signal-to-noise ratio), allowing devices to operate at a low light detection limit.

To modulate the dark current, several approaches, such as gate biasing,^10,12,17 molecular physisorption gating,¹⁸ surface modification by plasma ions,¹⁹ and plasmonic hot-electron doping,²⁰ have been adopted to date. However, all these methods suffer from certain drawbacks; for example, gate biasing requires a third terminal electrode, molecular physisorption is not permanent, and surface modification and hot-electron doping yield unstable results. In addition, 2D-MoS₂ typically occurs in nature as an n-type material although it can exhibit p-type and/or ambipolar properties depending on the metal contact and doping.^21,22 The aforementioned issues can be overcome by constructing the so-called straddling p–n junction between the n-type 2D-MoS₂ and p-type compound semiconducting materials (CSMs), resulting in a higher photoresponse in photodetector devices (compared to MoS₂ Schottky devices) due to a sufficient electron-hole separation via depletion layer.^19,23

The best p-type material for the ideal straddling p‐n heterojunction can be fabricated from CSMs.^24–30 Currently, the fabrication methods of conventional CSM-based devices are expensive and time-consuming and require high-vacuum processes.³¹ Moreover, the p-type properties of conventional wide bandgap CSMs such as AlN, ZnO, Ga₂O₃, and Al-rich AlGaN are inadequate because the p-type property of such CSMs is neither readily induced nor stable.^32–36 Thus, very recently, this motivated us to discover p-type wide bandgap CSMs based on manganese oxide (MnO) quantum dots (QDs). Solution-processed p-type CSM-QDs featuring superior 0D properties can be very promising candidates for heterojunction formation with TMDCs due to low-cost synthesis and/or doping and easy integration onto a large area of versatile substrates.^37–39 However, first, the crystallinity of CSM-QDs must be preserved during synthesis and fabrication to ensure stable p-type properties. Second, CSM-QDs need to have a homogeneous volume or size that must be dispersed without aggregation. Finally, to uniformly disperse CSM-QDs and stably deliver photo-signals, a stable 2D material as a heterojunction counterpart is necessary. However, p‐n junction (or heterojunction) devices based on n-type 2D TMDCs hybridized with p-type (or n-type) solution-processed QDs have not been investigated yet.

In this work, we demonstrated that the aforementioned issues can be overcome by constructing a p‐n junction between the n-type 2D-MoS₂ and p-type QDs. These preconditions were satisfied by producing cost-effective p‐n junction devices based on 2D TMDC/0D solution-processed QDs. The laser ablated MnO QDs in liquid were spray-coated on an exfoliated 2D-MoS₂ substrate through N₂-assisted spraying method. The photodetectors with the heterojunction structure between the p-MnO QDs and the exfoliated 2D n-MoS₂ exhibited a reduced dark current and an increased photocurrent, resulting in enhanced photoresponse characteristics.

Femtosecond laser ablation in liquid (FLAL) was carried out to synthesize MnO QDs. A Mn₂O₃ sputtering target (99.9% purity, ALB-Materials, Inc.) was immersed in a quartz glass beaker with ethanol (C₂H₅OH). The target surface was ablated by using a Ti:sapphire (Coherent-Mira 900) femtosecond (fs) laser with a pulse width and repetition rate of 150 fs and 76 MHz, respectively, at a wavelength of 800 nm.^37–39 The beaker was placed on a rotating stage to ensure uniform laser ablation. The ablation duration was ∼1 h at a constant laser power of 1.7 W. A 200 nm-thick silicon oxide (SiO₂)/Si (a heavily doped p-type material) substrate was prepared by transferring the exfoliated MoS₂ on it. After photolithography, metal deposition, and lift-off process, Ti (∼3 nm)/Au (∼70 nm) interdigitated electrodes (IDEs) with a channel length and width of 5 μm and 640 μm were constructed on the exfoliated MoS₂/SiO₂ substrate. Thermal annealing was carried out for 1 h at 100 °C in vacuum to reduce contact resistance at the interface between the IDEs and 2D-MoS₂. The prepared MnO QD solution was spray-coated on the MoS₂/SiO₂ sample by using a nitrogen-assisted spray gun (Fig. 1). During spray-coating, the substrate temperature was maintained at 90 °C by keeping it on a hot plate to rapidly vaporize ethanol, thus minimizing MnO QD aggregation. Note that the spraying time was optimized by monitoring a two-probe electrical resistance to avoid the formation of a complete MnO QD film and/or network throughout the channel.

High-resolution transmission electron microscopy (HR-TEM) and scanning TEM (STEM) measurements were conducted (Titan 60–300 microscope, FEI Corp.) using an electron beam energy of 300 keV. To confirm the material composition, energy dispersive x-ray spectroscopy (EDX) was carried out using the STEM system. X-ray photoelectron spectroscopy (XPS) was performed in a Kratos Axis Supra DLD spectrometer, which generates a monochromatic Al K_α x-ray signal (hν = 1486.6 eV) under an operating power of 150 W and a vacuum of 1 × 10⁻⁹ mbar. Absorbance was measured at room temperature using a UV-Vis Varian Cary 5000 spectrophotometer. Room temperature micro-photoluminescence spectra were excited by a 325 nm He–Cd laser. Both micro-photoluminescence (PL) and Raman measurements were performed using a Horiba Aramis micro-Raman spectrometer. The topography and roughness of the MnO QD film were confirmed using a Bruker atomic force microscopy (AFM) system (MultiMode 8-HR AFM). PD devices were probed and connected to a Keithley 2400S Source Meter, using electrical feedthrough. Changes in electrical resistance were monitored using LabVIEW I−V software. PDs were illuminated by a solar simulator for generating air mass (AM) 1.5 standard solar spectral light, and a monochromator (FLS 980, Edinburgh Instruments) was employed for modulating the light wavelength from 260 nm to 700 nm. The irradiation time was accurately controlled, using an automatic shutter system. To modulate the power density (P_d), neutral-density (ND) filters with different transmittances (40, 25, 15, 10, 1, and 0.1%) were placed underneath the solar simulator shutter emitting a standard calibration P_d of 100 mW/cm². Detectivity (D^*), defined as the capability of distinguishing weak signals from noise, was calculated based on the following expression:

where A is the channel area, e denotes the electron charge, and I_dark is the dark current, while R_ph is the photoresponsivity, which is defined as the ratio of I_ph to the incident light power.

Figure 1(a) shows the fabrication process of the MnO QD-decorated 2D-MoS₂ device adopted in the present study. The MnO QDs were synthesized via FLAL, as it is an easy and highly reproducible technique for fabricating well-dispersed QDs without the aid of any surfactant material. More details about the FLAL process have been reported in our previous studies.^37–39 The HR-TEM image [Fig. 1(b)] shows the crystalline MnO QDs with a diameter range < 7 nm.³⁸ A HR-STEM image of an individual MnO QD shown in the inset of Fig. 1(b) indicates that the QD lattice spacing corresponds to the (200) and (111) atomic planes, respectively.⁴⁰ EDX maps [Fig. 1(c)] confirm that the Mn and O atoms were homogeneously distributed within individual MnO QDs [the inset shows the corresponding high-angle annular dark-field (HAADF)-STEM image]. The XPS spectra shown in Figs. 1(d) and S1 (supplementary material) reveal that MnO as a major phase accounted for 81.5% of the QD composition, while the minor phases are MnOOH (12%) and Mn₂O₃ (6.5%), which is in line with the previous findings.³⁸ 

As reported in our recent published work, the p-type property of the MnO QDs has been confirmed by studying the field-effect transistor (FET) characteristics.³⁸ Theoretically, hybridizing n-type 2D-MoS₂ (hereafter, denoted simply as MoS₂) by p-type MnO QDs forms a p‐n junction, inducing charge transfer due to a work function difference that results in a “straddling heterojunction” at the interface. Figure 2(a) shows the Raman spectra of both bare MoS₂ and MnO QD-decorated MoS₂ (MnO QD/MoS₂), measured at a laser excitation wavelength of 473 nm. Two MoS₂ characteristic peaks of E_2g and A_1g, originating from in-plane and out-of-plane vibration mode, respectively,⁴¹ are clearly visible in the spectra. E_2g and A_1g of the bare MoS₂ were located at 383.44 cm⁻¹ and 408.41 cm⁻¹, respectively, which indicates the existence of multi-layered MoS₂. After decoration with MnO QDs, a notable change in the A_1g/E_2g intensity ratio from 1.77 to 1.87 occurred, which can imply charge transfer through the heterojunction interface.^42–44 

The PL spectra of the samples (without IDEs) shown in Fig. 2(b) and Fig. S2 provide further evidence supporting the charge transfer. Typical A^* exciton peaks were found at 1.831 eV and 1.824 eV for bare MoS₂ and MnO QD/MoS₂, respectively,¹ while the PL intensity of MnO QD/MoS₂ emission increased by ∼50% (Fig. S2 shows the PL spectra for the other five sets). In the pertinent literature, high PL intensities were ascribed to positively charged excitons or trions in MoS₂, as QDs provide more carriers.^17,18,45 However, PL has been performed on materials but not on devices, implying that the generated carrier density is enhanced for MnO QD/MoS₂ samples, as the carriers are generated from the two materials (electrons from the n-type and holes from the p-type material), and resulting in a higher carrier generation rate compared to a bare MoS₂, ultimately yielding a higher PL intensity. As there is no external electric field separating the electron−hole pairs, they recombine radiatively.^46–48 

Figure 3(a) displays a schematic diagram of the MnO QD/MoS₂ photodetector (PD) structure, whereas the image shown in Fig. 2(b) confirms the presence of MnO QD/MoS₂ sample between the Au IDEs, with a channel length of 5 μm. The MoS₂ flakes used for PDs ranged in thickness from 5 nm to 20 nm, as confirmed by AFM. A root mean square roughness of individual MoS₂ flakes for the PDs was less than 0.6 nm, as shown in Fig. 3(c). The energy diagrams obtained before and after hybridizing MoS₂ by MnO QDs are shown in Figs. 3(d) and 3(e). Recently, we have found the work function and the Fermi level of these p-MnO QDs using Kelvin probe measurements, (the findings and detailed discussions have been published elsewhere).³⁸ The work function of the MnO QD film was estimated to be at ∼4.87 V, while its bandgap energy, estimated from the absorption spectra of colloidal MnO QDs in ethanol, is ∼5 eV (Fig. S3, supplementary material). The work function and bandgap energy of a 6 nm thick MoS₂ were ∼4.8 eV and 1.3 eV, respectively. When the two materials are attached after spray-coating the QDs on MoS₂ [Fig. 3(e)], their respective Fermi energies are aligned and carriers are transferred to the MnO QDs, thereby decreasing the electron conductivity of MoS₂. As the valence band (VB) maximum of the MnO QDs is at the lower energy level than the VB of MoS₂, a cusp discontinuity can be created at the VB junction.

Under illumination, three representative absorption paths (denoted as λ₁, λ₂, and λ₃) can be activated to generate electron−hole pairs. λ₁ and λ₂ are the major photogeneration paths corresponding to the intrinsic bandgap energies of MoS₂ and MnO QDs, respectively. The photogeneration frequency through λ₂ is high enough due to the sufficient hole density around the VB of MnO QDs. Consequently, photogenerated electrons at the conduction band (CB) of MnO QDs can be favorably supplied into MoS₂ by a high junction potential. An alternative absorption path, denoted as λ₃, can exist since the recombination is prone to occur around the VB cusp with many trap states.

Figure 4 shows the performance of both bare MoS₂ and MnO QD/MoS₂ PDs. The dark current (I_dark) and photocurrent (I_ph) before and after MnO QD decoration were analyzed under irradiation at different voltages, which varied from −1 V to 1 V in 20 mV increments [Fig. 4(a)]. I_dark of the MnO QD/MoS₂ PD (e.g., 5 nA) was much smaller than that of the bare MoS₂ PD (e.g., 10 nA) at every voltage. In contrast, under the power density (P_d, defined as the incident light power per unit projected area) of 100 mW/cm², I_ph of the MnO QD/MoS₂ PD (e.g., 52 nA) was much higher than that of the bare MoS₂ PD (e.g., 32 nA). Figure 4(b) exhibits the cyclic photoresponse of both PDs under the 5 s ON/10 s OFF irradiation cycle. At 0.2 V, both PDs were subjected to different power densities (corresponding to the P_d values of 100, 40, 25, 15, 10, and 1 mW/cm²). As indicated in Fig. 4(a), the MnO QD/MoS₂ PD produced higher I_ph at every P_d, which demonstrates that an enhanced photoresponse was obtained by the PD incorporating the straddling p‐n junction. Photoresponse efficiency was evaluated by calculating R_ph. Under 100 mW/cm², the MnO QD/MoS₂ PD showed twofold higher R_ph [Fig. 4(c)]. At 1 mW/cm², R_ph of the MnO QD/MoS₂ PD surpassed 1 A/W, while no I_ph was observed in the bare MoS₂ PD. Considering a long channel length of 5 μm, which is generally unfavorable for efficient photogeneration by recombination and/or trapping, the achieved I_ph increase and the I_dark reduction are notable.

To identify the potential factors contributing to the observed R_ph increase in QD/MoS₂ PDs, especially in the UV range, as p-MnO QDs have a wide bandgap, operating in the deep UV spectral region, wavelength-dependent photoresponse was analyzed using monochromatic light, whereby the wavelength was increased from 260 nm to 800 nm in 50 nm steps. Interestingly, at 260 nm, the R_ph value (∼20 A/W) was enhanced for the MnO QD/MoS₂ PD compared to the bare device, as shown in Fig. 4(d). Note that monochromatic light is characterized by different P_d values, as indicated in the inset of Fig. 4(d); hence, it is rational to compare R_ph values obtained for the device with and without the MnO QDs at the same wavelength. R_ph markedly increased at wavelengths below 300 nm, which is sufficient for generating electron−hole pairs from the wide bandgap MnO QDs. Moreover, the detectivity values of both MnO QD/MoS₂ and bare MoS₂ PDs as a function of wavelength are shown in Fig. 4(e). In line with the increasing R_ph trend in MnO QD/MoS₂ PD, the detectivity was markedly enhanced at wavelengths below 300 nm, reaching about 5 × 10¹¹ Jones at λ = 260 nm as the photoresponse in the UV range below 350 nm is expected to be through λ₃, with only a minor contribution from λ₂.³⁸ These finding support our hypothesis that some of the photogenerated carriers are delivered from the MnO QDs (as an additional absorber) to MoS₂ (signal-transducing platform). The depletion layer formed at the p‐n junction aids in the fast separation of photogenerated carriers,¹⁹ thereby reducing the likelihood of recombination and thus improving the total photocurrent and device photo responsivity compared to that of Schottky devices based on bare MoS₂, as shown in Fig. 4. Based on the band diagram shown in Fig. 3(e), as well as the spectroscopic data, we can expect that this p–n junction can contribute to the dark current decrement while increasing the photocurrent of the MnO QD/MoS₂ PD. The observed decrease in the dark current can also be attributed to the MnO QDs working as the electron blocking layer.

The kinetics of photogeneration and transportation through the junction and/or materials can be indirectly estimated by fitting the relationship between the photocurrent and power density based on the power law equation I_ph = α·P^β, where α is a proportionality constant and β is a photosensitivity factor [in the inset of Fig. 4(e)]. The logarithmic-scale sublinear fitting lines yielded a β of 0.87 and 0.85 for the bare MoS₂ PD and the MnO QD/MoS₂ PD, respectively. The slightly reduced β is mainly due to trap states at the junction and the oxygen-related defects⁴ and can indicate that the MnO QD exerts negligible influence on the recombination of the photogenerated carriers.

The calculated response (T_Resp) and recovery (T_Recv) times are shown in Fig. 4(f), which are defined as the time required to reach 90% and attenuate to 90% of the maximum current, respectively, were extracted. For both bare MoS₂ and MnO QD/MoS₂ PDs, T_Resp and T_Recv below 0.5 s were obtained under every P_d. T_Recv values become slightly longer due to the MnO QD decoration, but P_d dependency of either T_Resp or T_Recv could not be confirmed. This slightly longer recovery time can be due to states in the bandgap that are related to the MnO QD surface states.³⁸ In addition, the metal−semiconductor junction can result in different recovery times.^4,49

In conclusion, highly crystalline p-type MnO QDs were fabricated via pulsed femtosecond laser ablation, which is a low-cost, efficient, and environmentally friendly technique. The dark current of MnO QD/MoS₂ PDs was suppressed due to MnO QD decoration, while the photocurrent markedly increased. The heterojunction contributed to the lower electron conductivity of MoS₂ under dark conditions, while electron−hole pair generation was enhanced through the intrinsic absorption path of the p-type MnO QDs. The method described here can be further explored for facile fabrication of heterojunction devices based on other CSM QDs and 2D-TMDCs to endow them with unique optoelectronic properties.

See the supplementary material for the detailed experimental data on XPS, PL, and absorption spectra.