
==== Front
iScience
iScience
iScience
2589-0042
Elsevier

S2589-0042(24)02044-3
10.1016/j.isci.2024.110819
110819
Article
Mechanism of local electric oxidation on two-dimensional MoS2 for resistive memory application
Dong Hui 1
Mu Junzheng 3
Peng Jinfeng 3
Zheng Xuejun zhengxuejun@xtu.edu.cn
1∗
Chu Liang chuliang@hdu.edu.cn
24∗∗
1 School of Electro-mechanical Engineering, Guangdong University of Technology, Guangzhou 510006, China
2 School of Electronics and Information & Institute of Carbon Neutrality and New Energy, Hangzhou Dianzi University, Hangzhou 310018, China
3 School of Mechanical Engineering & Engineering Research Center of Complex Tracks Processing Technology and Equipment of MoE & Key Laboratory of Welding Robot and Application Technology of Hunan Province, Xiangtan University, Xiangtan 411105, China
∗ Corresponding author zhengxuejun@xtu.edu.cn
∗∗ Corresponding author chuliang@hdu.edu.cn
4 Lead contact

31 8 2024
18 10 2024
31 8 2024
27 10 1108197 2 2024
6 7 2024
8 8 2024
© 2024 The Author(s)
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Summary

The manipulation and mechanism of two-dimensional (2D) transition metal dichalcogenides (TMDs) by external electric field are significant to the photoelectric properties. Herein, the 2D MoS2 nanosheets were oxidized to form MoS2-MoO3 local heterojunctions by an electric field, applied in multistable memristors for the proposal of NanoQR code. A modified thermal oxidation model was derived to reveal the mechanism of local electric oxidation on 2D MoS2. From current-voltage curves, the barrier height of the MoS2 device showed an increase of 0.39 eV due to local oxidation after applying voltage for 480 s. Based on density-functional theory, the increase of barrier height was calculated as 0.38 eV between MoS2-MoS2 and MoS2-MoO3 supercells. The 2D MoS2-MoO3 local heterojunctions were further applied as multistable memory storage at the nanoscale. The findings suggest a novel strategy for controlling local electric oxidation on 2D TMDs to manipulate the properties for the application of photoelectric memory nanodevices.

Graphical abstract

Highlights

• MoS2-MoO3 local heterojunctions are constructed via local electric oxidation

• Novel thermal oxidation model is proposed to understand local electric oxidation

• Multi-state memory storage is realized by local heterojunctions at the nanoscale

• Photoelectric cooperative nano quick response code are suggested

Physics; Materials science; Materials application

Subject areas

Physics
Materials science
Materials application
Published: August 31, 2024
==== Body
pmcIntroduction

Two-dimensional (2D) transition metal dichalcogenides (TMDs, e.g., WSe2, WS2, and MoTe2) have been considered of great interest due to their excellent photoelectric properties of the unique valley-polarized optical response, fast photoresponse speed and high absorption for photoelectric applications.1,2 However, oxidation has been generally regarded as the primary fault for the long-term stability of TMD-based nanodevices.3 Particularly, the stability of mono and a few-layer 2D TMDs is of vital importance due to the layer-dependent properties.4 For example, the MoO3 surface causes a large band offset with the buried MoS2 due to the different band structures from density-functional theory (DFT).5 In an oxygen (air) environment, the surface of 2D TMDs would be oxidized under an electric field, and their devices are severely degraded in terms of current level, carrier concentration, and carrier mobility.6,7 In order to avoid oxidation, the oxygen-free environment must be selected to measure the long-term stable TMD-based devices in the laboratory,8 and the device packaging is a prerequisite to realize long-term operation with high stability in practical applications.9

The chemical reaction occurs more rapidly with increasing temperature because more activated molecules collide vigorously, which increases the probability of bond cleavage and rearrangement.10,11 In the low dimensional systems, there are nanoscale hot spots, which are difficult to dissipate heat and easily cause the device failure from overheating.12,13 The thermal oxidation of TMDs has been investigated by in-situ Raman spectra over a wide range of temperatures.14 The WS2 monolayer will be oxidized when it is exposed to the ambient condition with the presence of light.15 The in-situ-grown WS2 monolayer on suspended graphene can resist oxidation because of the screening effect from the conductive graphene substrate, while the WS2 on the SiO2 substrate would be easily oxidized. In other words, the controlled layer-by-layer oxidation on TMDs could be significant in constructing 2D heterojunctions for advanced photoelectric devices. Previously, the whole surface of MoTe2 nanosheets was oxidized to MoOx with atomic-level accuracy by varying ozone exposure time, which were further integrated into complementary metal oxide semiconductor (CMOS) devices.16 Notably, the electric field can accelerate the charge carriers to generate more Joule energy, which can also induce chemical reactions. Thus, the local oxidation of WS2 on the SiO2 substrate can be created by a local electric field using a conductive atomic force microscope (AFM).17 In previous studies, the local oxidation on the TMD surface has been observed with an AFM tip. However, the oxidation mechanism has been needed to deeply analyze for guiding possible practical applications.18,19

To date, memory devices are desired to simultaneously meet the demands of high storage density, non-volatility, fast operation, and low power consumption. To achieve high storage density, there are typically two paths in principle: minimizing effective cells and developing multistate storage.20,21 The multistate memory technique can increase the storage density beyond Von Neumann's architecture.22 The storage cell miniaturization can increase the density of memory, accompanied by the decrease of cost per bit at the nanoscale.23 Memristor has been regarded as the most potential non-volatile resistance memory because of the simple structure, fast read and write, high integration, and nanoscale.24 In recent years, it is expected to realize the multi-state resistance properties based on photo-responsive semiconductors via the cooperative regulation of both light and electric fields.

Herein, the photoelectric properties of 2D MoS2 nanosheets were manipulated to form MoS2-MoO3 local heterostructures by local electic oxidation from AFM tip, which can be further applied in light-assisted multistate memristors for NanoQR code information. The local electric field from the AFM tip can simultaneously construct and tune memories at the nanoscale. Based on the finite-element method (FEM) simulation, a modified thermal oxidation model was proposed by introducing Ohmic dissipation to describe the dependence of oxidation thickness on the applied voltage with loading time. The current-voltage (I-V) and current-time (I-T) characteristics were measured by the applied voltage for the different loading time intervals. The experimental barrier height difference between MoS2-MoS2 and MoS2-MoO3 was calculated from the I-V curves through Fowler–Nordheim tunneling (FNT) theory, which consists with the value by DFT based on the atomic supercell models. The different barrier height decline is caused by the formation of MoO3 with different thicknesses under the local applied voltage, which can represent different light-assisted multistate memristors for NanoQR code information application. Thus, the utilization of the local electric oxidation of 2D TMDs (usually detrimental) provides a new principle and novel way for the design of photoelectric nanodevices.

Results and discussion

The 2D MoS2-based nanodevice with AFM-top electrode was schematically depicted in Figure 1A. The MoS2-based nanodevice was constructed as Au tip/MoS2 nanosheet/Pt substrate, and then the voltage was applied. It is notable that under an electric field in air condition, the MoO3 local surfaces are formed on the MoS2 nanosheet to construct 2D heterostructures. The local thickness of the nanosheet increases under the applied voltage due to the formation of MoO3 local surface. The oxidation aroused by the local electrical field would change the barrier height, which has a great effect on the photoelectric performance of the constructed nanodevices. On one hand, the relative thicknesses of MoS2-MoO3 heterostructures can be changed by applying voltage with different loading times (Figure 1B). The oxidation thickness increases with the applied voltage and loading time, which leads to different memory states. Importantly, the precisely controlled local 2D heterojunction memristors can realize information confidential storage, because of the 2D bar code characteristics at the nanoscale. On the other hand, every local MoS2-MoO3 heterostructure can realize the characteristic of resistive random access memory. Because the effective contact is the effective emission area at the injecting electrode of the Au tip and the corresponding barrier height refers to the barrier between the Fermi level of the Au tip and the bottom of the conduction band MoS2 or MoO3-MoS2 according to Fowler-Nordheim tunneling theory (Figure 1C).25 Thus, we proposed information NanoQR code with strong encryption for the first time, in which every cell is a distinctive multi-state photoelectric memristor.Figure 1 Schematic diagram of device structure and NanoQR code

(A) 2D MoS2-based nanodevice with C-AFM top electrode, where the local electric oxidation can construct local MoS2-MoO3 heterostructures.

(B) Thickness of local MoS2-MoO3 heterostructures can be tuned by various voltages at different times. The heterostructure array can be used as NanoQR code.

(C) Schematic diagram of the barrier height of 2D MoS2-based nanodevice with C-AFM top electrode.

X-ray diffraction (XRD) pattern of the MoS2 nanosheet on Pt/SiO2/Si substrate is recorded as Figure 2A. The strongest peak at 40° can be indexed to the Pt phase (JCPDS No. 04–0802), and the others could be assigned to the hexagonal MoS2 phase with the lattice parameters of a = b = 0.31612 nm and c = 1.22985 nm (JCPDS No. 37–1492). No impurity phases can be detected in the pattern, indicating the pure MoS2 phase. Energy dispersive X-ray (EDX, installed on SEM equipment) spectroscopy further confirmed the pure MoS2 initially (Figure S1). AFM technology was carried out to illustrate the prepared MoS2 nanosheet in Figure 2B. There are obvious hierarchical structures with thicknesses of 6 and 11 nm, indicating the multilayer nanosheet. The HRTEM image of the MoS2 nanosheet indicates a single-crystal characteristic in Figure 2C. The face angle of 60° shows the hexagonally symmetric lattice structure of the MoS2 nanosheet. The lattice spacing is estimated as 0.27 nm, which corresponds to the (100) or (010) plane of MoS2. The intensity profile further conformed to the lattice spacing of 0.27 nm in Figure 2D. The fast Fourier transformation (FFT) pattern are plotted in Figure 2E, which also indicates the single-crystalline characteristics with the hexagon indexed to the (010) and (110) planes of hexagonal-phase MoS2.Figure 2 Characterization of MoS2 nanosheet

(A) XRD pattern, (B) AFM image, (C) HRTEM, (D) intensity profile extracted from (C), (E) corresponding FFT image.

The points marked as “1,” “2,” and “3” were selected to apply voltage on the MoS2 nanosheet with the pristine height of ∼11 nm, and their AFM images at the pristine states are indicated in Figures 3A–3C, respectively. The voltages of 3, 4, and 5 V were applied for 30 s at the “1,” “2,” and “3” points, and the corresponding AFM images are illustrated in Figures 3D–3F, respectively. Obviously, the thicknesses of “1,” “2,” and “3” points increase remarkably after applying the voltages. The cross-sections before and after applying voltage are indicated in Figures 3G–3I. The thicknesses increased from the pristine height of 11.0–25.1, 31.4, and 68.7 nm, and the corresponding height variations are 14.1, 20.4, and 57.7 nm for the “1,” “2,” and “3” points, respectively. The height variations of the devices under 3, 4, and 5 V for 30 s are 1, 2, and 1 nm, respectively from Figure 3. As compared with height variations of 14.1, 20.4, and 57.7 nm, the oxidation error is located at the error of 10%. Similarly, 0.1, 0.3, and 0.5 V with the loading time interval of 480 s were applied at the points “4,” “5,” and “6.” The pristine height is 6 nm as shown in Figures S2A–S2C, and the thickness variations are found to be 13.2, 19.8, and 24.9 nm, respectively. The relationship between oxidation thickness and rate with time and voltage is summarized in Table 1. The oxidation thickness increases with applied voltage at a certain time of 30 and 480 s and the oxidation rate is also in in proportion with voltage independence of position, which indicates that the voltage could be utilized as a simple method to control oxidation thickness and rate precisely at the nanoscale. The local thickness increase can be ascribed to local oxidation reaction under the electric field.Figure 3 AFM images of MoS2 nanosheets before and after applying voltage

(A–C) Points “1,” “2,” and “3” at the pristine states and (D–F) after applying a voltage of 3, 4, and 5 V with a time interval of 30 s.

(G–I) The corresponding cross-sections with different increasing heights.

Table 1 Relationship between oxidation thickness and rate with time and voltage

Time (s)	Voltage(V)	Oxidation thickness(nm)	Oxidation rate (nm/s)	
30	3	25.1	0.84	
30	4	31.4	1.05	
30	5	68.7	2.29	
480	0.1	13.2	0.03	
480	0.3	19.8	0.04	
480	0.5	24.9	0.05	

To further understand the morphological changes in the MoS2 vertical heterostructure, a high-resolution transmission electron microscope (HRTEM) and Energy dispersive spectrometer (EDS) are used to characterize its structural distortion under the applied voltage of 3 V. After the formation of four hillocks set by the cyclic electrical field on the surface of the MoS2 heterostructure (Figure 4A), a focused ion beam (FIB) was performed to cut along the red line to obtain the cross-section (Figure 4B) for TEM characterization. An energy dispersive spectrometer (EDS) was used to characterize the respective I and II regions marked by a yellow rectangle, and the corresponding results were summarized in Figure 4C. Oxygen only occupied less than five in region I while rising up to 90% in region II after voltage was applied, indicating that the oxidation process was induced by the AFM tip under the electric field. More efforts have been made with HRTEM and related fast Fourier transform to elucidate the structural modifications of the heterostructure after voltage application. As shown in Figures 4D and 4F, the single-crystal MoS2 has a d-spacing of 6.3 Å, corresponding to the (002) plane. However, it is difficult to distinguish the crystal structures of the molybdenum oxide layer due to its amorphization (Figures 4D and 4E). In conclusion, the formation of a molybdenum oxide layer under the local electric field was further confirmed by both EDS and HRTEM analyses.Figure 4 MoS2-MoO3 heterostructure characterization

(A) AFM image of four hillocks formed on the surface of MoS2.

(B) Cross-sectional TEM image processed along with the position and direction of the red line in (A) by the FIB.

(C) Atomic fraction of Regin I of MoS2 and regin II of MoOx marked by the yellow rectangle in (B). HRTEM image of the cross-section of MoOx/MoS2 heterostructure.

(D) and the corresponding fast Fourier transform pattern of MoOx (E) and MoS2 in (F).

In order to further confirm the local oxidation reaction, Raman spectra were carried out to investigate the MoS2 nanosheet before and after applying a local voltage of 5 V for 30 s (Figure 5A). There are the peaks of E12g at 384 cm−1 and A1g at 407 cm−1 with several peaks of the weak intensity for the second-order and combination phonon modes. The primary Raman active modes (E12g and A1g) are corresponded to MoS2.26,27 After applying the voltage, the peak intensity of E12g and A1g is found to be similar to the initial states.28 The enlarged peaks of (i), (ii), and (iii) at the Raman shifts (Figure 5B) show the two main peaks appear at 820 cm−1 (i) and at 954 cm−1 (ii), indicating the formation of MoO329 and MoO3−x.30 One peak (iii) with very weak intensity appears at 820 cm−1, which indicates the oxygen chemisorption on the surface of the MoS2 nanosheet due to the active surface defects at a pristine state. After applying the voltage, the peak intensity at the Raman shift (820 cm−1) of the local MoS2 nanosheet is much larger than that of the original MoS2 nanosheet, and the former peak area fitted after baseline subtraction is found to be 371% larger than that of the latter. The increased peak “i” and additional peak “ii” confirm the formed local oxide surface on the MoS2 nanosheet under the applied voltage.Figure 5 MoS2-MoO3 heterostructure construction with thickness increasing after voltage loading

(A) Raman spectra of MoS2 nanosheet at the pristine state and after the applied voltage and (B) the enlarged peaks of (i), (ii), and (iii) from Raman spectra. The experimental thickness variation Δd and the maximum temperature simulated by the FEM method as a function of the applied voltage (C) at the points “1,” “2,” and “3” in Figure 3 and (D) at the points “4,” “5,” and “6” in Figure S2.

The thickness variation can be defined as a function of the applied voltage, as shown in Figure 5C for points “1,” “2,” and “3” and Figure 5D for points“4,” “5,” and “6,” Obviously, the thickness variations at the points “1,” “2,” and “3” increase with the applied voltage. Thus, it can be concluded that the thickness variations from the AFM profiles in Figures 2 and S2 are identified as the thickness of MoO3 based on Raman spectra analysis in Figure 5B. The thermal oxidation model has already been successfully utilized to describe the thickness variation of the oxidation surface under the thermal field.31 Thus the Joule heating as the thermal field can be introduced to describe the oxidation thickness variation. The formation mechanism of molybdenum oxide could be illustrated through the chemical equation: 2MoS2+7O2 → 2MoO3+4SO2. The chemical bonds of Mo-S were broken, and chemical bonds of Mo-O and S-O recombined under the Joule heating induced by voltage, therefore molybdenum oxide was formed with the chemical bonds of Mo-S destruction and Mo-O recombination. The oxidation thickness shows a linear relationship with the loading time according to the thermal oxidation model31:(Equation 1) Δd=k×(t+τ)

where t is the loading time interval, τ is the time constant, and k is the oxidation rate of linearity. The influence of temperature on the chemical reaction rate is always interpreted in terms of the Arrhenius equation, and k can be expressed as the following equation32:(Equation 2) k=Aexp(−EGT)

where A is the pre-exponential factor, E is the activation energy, T is the temperature, and G is the gas constant.

To explore the temperature on the oxidation rate of linearity, FEM simulation was carried out to obtain the temperature distribution induced by the Joule heating, as shown in Figure 3 for the points “1,” “2,” “3” under 3, 4, 5 V, respectively, and Figure S3 for the points “4,” “5,” “6” under 0.1, 0.3, 0.5 V, respectively. The maximum temperature occurs at the applied voltage points on the local surface of MoS2 nanosheets, which are summarized as the function of the applied voltage in Figures 5C and 5D. The maximum temperature of the local oxide surface can be fitted by the Ohmic dissipation model as depicted by the red lines33:(Equation 3) T=T0+α−V2R

where α is the thermal resistance, V is the applied voltage, T0 is the room temperature, and R is the resistance. The relationship between the thickness variation Δd and the applied voltage V can be expressed from the Equations 1, 2, and 3 as the following equation:(Equation 4) Δd=Aexp(−EG(T0+αV2R))×(t+τ)

The modified thermal oxidation model can fit the relationship between Δd and V at the different loading time intervals, which are marked by the black dashed lines in Figures 5C and 5D for 30 s and 480 s. Obviously, the experimental thickness variation is in good agreement with the modified model.10 Therefore, the oxidation thickness variation on the MoS2 nanosheet under the local applied voltage is dominated by the thermal oxidation due to the Joule heating. Herein, it is the first time to illuminate the mechanism on tuning atomic 2D heterojunction using the local electric field, which can offer novel guidelines to control the local oxidation with atomic precision.

The I-V curves at the applied voltage of 0.5 V with different loading time intervals are demonstrated in Figure 6A for the MoS2 nanosheet at point “6” in Figure S2C. The effective contact area Aeff is estimated as ∼7.04 × 10−16 m2 according to the AFM tip with a diameter of 30 nm from Figure S5, and thus the ln(I/V2) versus 1/V curves can be fitted by Equation 6 in the methodology, as shown in the inset of Figure 6A. There is a strong linear relationship between ln(I/V2) and 1/V, which indicates that the tunneling current through ultrathin MoS2 and MoS2-MoO3 heterostructure can be explained by the FNT model. The barrier heights are fitted as 0.51 eV (ϕe1) for the MoS2 nanosheet with a thickness of 6 nm, and 0.9 eV (ϕe2) for the MoS2-MoO3 heterostructure with a thickness of 31 nm. Therefore, the barrier height difference (Δϕe = ϕe1−ϕe2) is 0.39 eV. The formation of MoO3 induced by the applied voltage leads to an increase in barrier height. The vacuum level E01 and the conduction band minimum Ec1 for MoS2, and E02 and Ec2 for MoS2-MoO3 heterostructure can be obtained by the DFT method.34 And the variation of Schottky barrier height through Equations 3 and 4 can be explained by comparing with the experimental and simulation differences of the barrier height Δϕe. In order to calculate the variation of Schottky barrier height under the applied voltage, the atomic models of MoS2-MoS2 homostructure and MoS2-MoO3 heterostructure were constructed for the DFT calculation (Figure 6B). The MoS2-MoS2 homostructure and MoS2-MoO3 heterostructure represent the MoS2 nanosheet in a pristine state and applied voltage state, respectively.Figure 6 Barrier height variation after constructing MoS2-MoO3 heterostructure

(A) I-V curves of MoS2 nanosheet at the point “6” with the different loading time intervals and ln(I/V2) versus 1/V curves fitted by the FNT model for 6 and 31 nm in the inset.

(B) Calculated electrostatic potential as a function of the position along z axis of MoS2-MoS2 homostructure and MoS2-MoO3 heterostructure. Band structures for (C) MoS2 homostructure and (D) MoS2-MoO3 heterostructure calculated by DFT.

(E) Energy level change of MoS2 and MoS2-MoO3. The work function of gold (WAu) is 5.15 eV.35

The electrostatic potentials of the MoS2-MoS2 and MoS2-MoO3 supercells are calculated as the functions of the distance along the z axis of the thickness (Figure 6B). As for the MoS2-MoS2 and MoS2-MoO3 supercells, the vacuum levels E01 and E02 are extracted as 5.48 and 4.73 eV, respectively. The band structures of the MoS2-MoS2 homostructure and MoS2-MoO3 heterostructure are presented in Figures 6C and 6D to obtain the corresponding Ec1 and Ec2, respectively. The conduction band minimum of Ec1 and Ec2 are extracted as 0.87 and 0.50 eV, respectively. The energy level difference between MoS2 and MoS2-MoO3 supercells is schematically depicted in Figure 6E. Thus, it can be compared to the barrier height variation caused by the formation of MoO3 under the locally applied voltage. The Schottky barrier heights of ϕ1 and ϕ2 are calculated as 0.54 and 0.92 eV for Au/MoS2 and Au/MoS2-MoO3 heterostructure, respectively. Therefore, the simulation difference of barrier height Δϕ (0.38 eV) approaches the experimental result Δϕe (0.39 eV) from the inset of Figure 6A. Both the experimental and theoretical results confirm that the increase in height in the Schottky barrier of the MoS2 nanosheet is caused by the formation of MoO3 at the local applied voltage. The study can be helpful in understanding the degradation mechanism of the electrical performance, which will provide the safe working voltage and time of 2D material devices at the atomic scale.

In order to investigate the effect of voltage and loading time interval on the resistance states, we further carried current versus voltage (I−V) curves without and with light illustration after applying a voltage of 0.1 V at loading time intervals (Figures 7A–7E). All I–V curves show the bipolar conductive behaviors after the formation of conductive paths in the vertical direction. For the MoS2 device (Figure 7A), under the 0 to 2 V forward sweep, the memristor was first located at a high-resistance state (HRS) and then switched to a low-resistance state (LRS) during the sweep. Notably, the set voltages move to smaller under light irradiation because of the additional photocurrent, indicating that light can be utilized to regulate operating voltages (Figures 7A–7E).Figure 7 Photoelectric properties with voltage and loading time interval on the resistance states

I−V curves of the MoS2-based nanodevice with time intervals of 0 s (A), after applied voltage of 0.1 V (B–E) and 3 V (F–I) under dark and light irritation with time intervals of 120, 240, 360, 480 s.

The asymmetric curve can be observed on the negative X axis, where the current variation is stronger than that on the positive half-axis, because of the different work functions of the top and bottom electrodes. The set voltages increase from 1.3 to 2.2 and 2.6 V with electric oxidation time from 0 s to 480 s under the bias of 3 V in Figures 7F–7I. The hysteresis tends to expand with the increase of voltage and time, which is caused by the increase of barrier height due to the formation of molybdenum oxide. According to the FNT theory, fewer electrons are not prone to tune the barrier as the barrier increases and thus the resistance increases together with hysteresis and tends to expand. Thus, storage can be realized by applying different voltage and loading time intervals, and each local MoS2-MoO3 heterostructure can realize the characteristics of the multiple-state photoelectric memristor.

To further reveal the resistance regulated by voltage and loading time interval, HRS and LRS together with voltage and loading time are summarized in Table S1 and Figure S7. For HRS, the resistance increases from 1.2×103 to 3.1×103 and 4.8 × 103 MΩ under dark, while from 1.1×103 to 2.2×103 and 3.4 × 103 MΩ, respectively, after applying voltage of 0.1 and 3 V with loading time from 0 to 480 s. For LRS, the resistance increases from 20.3 to 28.9 MΩ and 18.7 to 27.5 MΩ under dark, while from 18.2 to 27.7 MΩ, and 17.4 to 27.1 MΩ under light, after respectively applying a voltage of 0.1 and 3 V with loading time from 0 to 480 s. The array of local MoS2-MoO3 heterostructures can induce the proposal of NanoQR code with strong encryption, consisting of a patterned multi-state photoelectric memristor as Figure 1C. The on/off ratios of our devices were estimated as 220 (Figure S8), closer to those of TMD heterojunctions of 140.36,37,38,39 Thus, our work would provide guidelines for memory prototype of 2D MoS2 photoelectric nanodevices via local electric oxidation with precisely tuning the multiple resistance at atomic scale in the information field.

Conclusions

To summarize, the 2D MoS2-MoO3 local heterojunctions were constructed by local electric oxidation, which provides a new memory prototype for multistable storage. A modified thermal oxidation model was suggested to describe the relationships between the oxidation thickness and voltage, and the formation of molybdenum oxide induced by the Joule heating was identified by Raman spectra. The increase in the Schottky barrier height was caused by the formation of molybdenum oxide after the applied voltage. Multistable storage can be precisely regulated by controlling voltages and loading time together with light irradiation. This work would provide a novel design to manipulate the multistable memristors of 2D-based nanodevices and enrich thermal oxidation theory to understand the mechanism of control of oxidation thickness at the atomic level through local electric oxidation.

Resource availability

Lead contact

Further information and requests for resources and materials should be directed to and will be fulfilled by the lead contact, Pro. Liang Chu (chuliang@hdu.edu.cn).

Materials availability

The study did not generate new unique materials. The readers can buy the chemicals to remake the materials as mentioned in the text.

Data and code availability

• Data: All data reported in this article will be shared by the lead contact upon request.

• Code: This article does not report the original code.

• Any additional information required to reanalyze the data reported in this article is available from the lead contact upon request.

• All data supporting the findings of this study are available within the article and its supplemental information or from the corresponding authors upon reasonable request.

Acknowledgments

This work was supported by the 10.13039/501100001809 National Natural Science Foundation of China (52172205 , 11832016 , 51775471 , 11902284 , 11804166 ), the 10.13039/501100004735 Natural Science Foundation of Hunan Province (2021JJ20008 ) and the fellowship of China postdoctoral science foundation (2021M692705 ).

Author contributions

H.D conceived the idea and wrote the article. J.M and J.F.P performed AFM, SEM, TEM, and FEM calculations. X.J.Z and L. C contributed data analyses and provided instructive suggestions. All authors discussed the results and assisted in the preparation of the article.

Declaration of interests

The authors declare that they have no competing interests.

STAR★Methods

Key resources table

REAGENT or RESOURCE	SOURCE	IDENTIFIER	
Chemicals, peptides, and recombinant proteins	
	
Mos2 single crystal	Nanjing MKNANO Technology Co., Ltd	N/A	
Pt substate	Nanjing MKNANO Technology Co., Ltd	N/A	

Experimental model and study participant details

There are no experimental models (animals, human subjects, plants, microbe strains, cell lines, primary cell cultures) used in the study.

Method details

Preparation and characterization

MoS2 single crystal and Pt-coated SiO2/Si(100) substrate were purchased from Nanjing MKNANO Technology Co., Ltd. The MoS2 single-crystal 2D nanosheets were prepared by ultrahigh-pressure and low-temperature exfoliation of our unpublished method and then transferred to the above substrate. The crystalline structure and surface morphology were characterized by X-ray diffraction (XRD, Bruker, D8 Advance Cu-Ka, German), scanning electron microscope (SEM, Hitachi, SU5000, Japan), atomic force microscopy (AFM, Cypher S, Oxford Instruments, USA), and high-resolution transmission electron microscope (HRTEM, Titan, 80–300 kV, USA).

Construction of MoS2-based nanodevice

The MoS2-based nanodevices were constructed using conductive AFM with architecture of Au tip/MoS2 nanosheet/Pt substrate. The different interval voltages were applied on the local nanodevices. The photoelectric properties of nanodevices were measured under light irradiation, when Au-coated Si tip (PPP-NCHAu) was always grounded. Power consumption is a major concern because of the ever-increasing density of solid-state electronic devices, and it is known that power consumption drops quadratically with decrease of supply voltage.40 Under the high and low supply voltages the power consumption may be differently varied, therefore the high voltages of 3, 4, and 5 V and low voltages of 0.1, 0.3, and 0.5 V were applied at the different points for the loading time interval 30 s and 480 s. The voltages of 0.1–5 V were applied on the MoS2 nanosheets with the loading time intervals of 0–480 s, before the I-V and current-time (I-t) characteristics were measured by sweeping voltage from −3 V to +4 V and applying a voltage of 0.5 V under the dark condition and laser irradiation (660 nm, 2 mW/cm2). The light was only applied for sweeping voltage from −3 V to +4 V as comparison with those under the dark conditions not through the whole measurement process. Raman spectra of MoS2 nanosheet at pristine and after the applied voltage 5 V were carried out using Raman microscope (Raman, InVia Raman Microscope, Renishaw, UK) at a wavelength of 532 nm, so that the phase formation of molybdenum oxide could be identified by the enlarged peaks with main Raman active modes. The oxidation thickness was determined by comparing height profiles of MoS2 nanosheet at pristine and after the applied voltage states.

Analysis on the barrier height

Based on the FNT theory, the equivalent barrier height ϕe could be analyzed by I-V characteristics25:(Equation 5) I(V)=Aeffq3mV28πhϕed2m∗exp(−8π2m∗ϕe32d3hqV)

where ϕe, Aeff, q, m, m∗, d, and h are the barrier height, the effective contact area, electron charge, free electron mass, effective electron mass separation between two electrodes, the distance between two electrodes, and Planck’s constant, respectively. Aeff could be estimated by circular contact area with the diameter of AFM tip. For line fitting of I-V curves, the Equation 1 can be expressed as follows:(Equation 6) lnI(V)V2=lnAeffq3m8πhϕed2m∗−8π2m∗ϕe32d3hqV

ϕe could be obtained by fitting the ln(I/V2) versus 1/V curves through linear fitting method, therefore it can be determined that the barrier height difference Δϕe between MoS2 nanosheet at pristine and after the applied voltages.

DFT calculation method

The electrostatic potentials of MoS2 and MoS2-MoO3 layers were calculated by using Vienna Ab-initio Simulation Package (VASP),41,42 as the stable energy level on the electrostatic potential is generally defined as the vacuum level.43 The difference in Schottky barrier height Δϕ (ϕ1−ϕ2) would be calculated for the heterostructures at pristine states and after the applied voltage, and they are compared with the barrier height difference Δϕe to analyze the variation on tunneling current. The projector-augmented wave method was applied to describe electron-ion interactions,44,45 whereas the exchange-correlation function was treated by the generalized gradient approximation considering Perdew-Burke-Ernzerhof method.46 van der Waals corrections were included by using Grimmes DFT-D2 method as implemented in VASP.47,48 The cutoff energy of the plane wave was set to 400 eV, and the atomic positions were fully relaxed until the maximum force less than 0.01 eV−1 on each atom. K-point samplings of 3 × 3 × 1 and 3 × 3 × 1 were used for MoS2 and MoS2-MoO3 heterostructures, and a vacuum region of 15 Å was introduced to avoid any spurious interactions. Electrostatic potentials and band structures were carried out to study the barrier height difference at the certain points on MoS2 and MoS2-MoO3 supercells. The Schottky barrier height ϕ of electrons is defined as follows34:(Equation 7) ϕ=WAu−χ

(Equation 8) χ=E0−Ec

where WAu is the work function of gold, χ is the electron affinity of the semiconductor, E0 is the vacuum level, and Ec is the minimum conduction band.

Quantification and statistical analysis

This study does not include statistical analysis or quantification.

Additional resources

This work does not include any additional resource.

Supplemental information

Document S1. Figures S1–S8 and Table S1

Supplemental information can be found online at https://doi.org/10.1016/j.isci.2024.110819.
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