An Ultrahigh‐Rectification‐Ratio WSe2 Homojunction Defined by High‐Efficiency Charge Trapping Effect

Although 2D material van der Waals heterostructures (vdWHs) exhibit many novel properties and applications, 2D homojunctions have unique advantages in interface lattice matching, band continuity, and charge transfer efficiency. However, the rectification performances of 2D homojunction diodes are severely limited by the small junction barrier, mainly due to inefficient charge doping. In this work, an ultrahigh‐rectification‐ratio WSe2 homojunction diode achieved by the semi‐floating gate doping of graphdiyne oxide (GDYO) is reported. Utilizing the WSe2/GDYO direct charge trapping mode can free the inhibition of charge capturing efficiency by removing conventional insulating barriers and thus improve the rectification ratio. The C─C(sp) and oxygen‐containing functional groups in the GDYO layer can provide outstanding charge‐trapping ability due to their unsaturation. Furthermore, the oxygen plasma treatment used for oxidizing graphdiyne (GDY) into GDYO can make GDYO a flatter surface, thus creating strong‐coupling WSe2/GDYO interfaces to improve the charge transfer efficiency and enhance the electrostatic doping effect. Besides, the WSe2/GDYO interfaces are confirmed to possess a higher junction barrier than that of the WSe2/GDY interfaces. This research proposes a brand‐new approach to building p–n junctions via charge trapping and the homojunction diode with a record‐highest rectification ratio of up to 106 is obtained.


Introduction
[3] Until now, much interest and effort have been dedicated to the construction of p-n junctions based on 2D transition metal dichalcogenides (TMDs).2D building blocks bring p-n junctions new possibilities and prospects, both in structures and properties, compared to conventional 3D p-n junctions. [4,5]However, 2D-materials-based heterojunctions gradually expose non-negligible shortcomings, such as discontinuous band alignments, [6][7][8] lattice mismatch, [9,10] and grain boundaries, [11] which are considered to deteriorate the charge transfer and inhibit the rectification ratio. [12]To date, 2D p-n junctions based on a single material owe natural good matchings of chemical and electronic structures and can avoid the above problems. [13]n terms of construction methods for 2D homojunction, thickness engineering, [14] surface modification, [15] split-gate control, [16] and semi-floating gate modulation [17] have been reported.Among them, semi-floating gate modulation works through different electrostatic doping for two parts of the ambipolar channel material, [17][18][19] which has been proved to be more stable and has higher scalability compared to other methods, and will induce no chemical pollution during the fabrication process. [20]Besides, semi-floating gate modulation has the potential to constitute a large-scale cascade circuit compatible with silicon technology, which is important for future applications.However, it takes the charges a large amount of energy to cross the blocking layer, thus limiting the electrostatic doping efficiency and the energy barrier in the depletion region of the homojunction.Previous research often focuses more on the functions of the 2D homojunctions than the modulation of the energy barrier in the depletion region, which is the key to improving the rectification performance of the homojunction.
In this work, we designed and fabricated a 2D WSe 2 in-plane homojunction by the direct charge trapping of 2D graphdiyne oxide (GDYO) which was obtained by performing reactive oxygen treatment on graphdiyne (GDY).The advantage of this WSe 2 /GDYO architecture lies in that direct charge trapping greatly improves the efficiency of charge doping by avoiding the insulating blocking layer.Thus, the barrier in the depletion region of the homojunction is enhanced and the rectification ratio is increased.In addition, as a charge-trapping layer, the 2D GDYO possesses outstanding charge-capturing ability, due to its abundant hybrid states including sp 2 and sp as well as oxygencontaining functional groups.An optimization strategy of mild oxygen plasma treatment was introduced to enhance the performance of the p-n homojunction and a rectification ratio of up to 10 6 was obtained.Further, we performed Density Functional Theory (DFT) calculations to reveal the reason for the charge transfer in WSe 2 /GDYO van der Waals heterostructures (vdWHs).This strategy provides a stable and highly efficient electronic structure control strategy for 2D materials to construct high-performance rectifiers and semiconductor devices.

Device Architecture and Charge Capturing Behavior of GDYO
The technology routine to replace the conventional semi-floating gate (SFG) architecture with a charge-trapping layer is shown in Figure 1a.In the new structure, electrons from WSe 2 can be captured into GDYO without overcoming the energy barrier caused by the conventional insulating layer, thus having the potential to improve the doping efficiency greatly.Figure 1b presents a false-color scanning electron microscopy (SEM) image of the WSe 2 /GDYO vdWHs floating gate (FG) FET, WSe 2 /GDYO vd-WHs SFG FET, and WSe 2 FET.Atomic force microscopy (AFM) characterization indicates that WSe 2 includes about ten layers (Figure S1a, Supporting Information).Our GDY was synthesized on the surface of copper via a cross-coupling reaction using hexaethynylbenzene. [21][24] Graphdiyne nanosheet after the oxygen plasma treatment is defined as graphdiyne oxide (GDYO).The optical microscopy images of the GDYO on copper and Si are displayed in Figure S1b,c (Supporting Information).According to AFM and SEM images in Figure S1d,e (Supporting Information), the obtained GDY film is flat with a large area of over 100 μm, and the average thickness is 15 nm.The 2D crystals were confirmed by Raman spectroscopy (Figure 1c), suggesting that oxygen plasma treatment didn't hurt the GDY structure seriously. [25,26]s a typical 2D ambipolar semiconductor, WSe 2 can be modulated between p-type and n-type.As indicated in Figure S2a (Supporting Information), our WSe 2 exhibits n-type electrondominated transport.And the SiO 2 /Si substrate can n-dope WSe 2 in nature.Besides, the atomically thin WSe 2 nanosheet exhibits good gate-tunable properties (Figure S2b, Supporting Information).The hysteresis window of WSe 2 FET is ≈15 V under the amplitude of the swept gate voltage 80 V (Figure 1d) while the hysteresis window of the WSe 2 /GDYO vdWHs FG FET is to be ≈136 V (Figure 1e).The significant increase in hysteresis can be attributed to the charge-capturing ability of GDYO.Under positive gate voltage, GDYO captures electrons from WSe 2 , leading to the generation of a substantial number of holes in the channel and exhibiting p-type transport characteristics.This result of charge capturing phenomenon of GDYO is similar to previous research, which revealed that GDYO can capture charges in vd-WHs and thus be utilized to tune the electron transport in the coupling material. [19,27,28]

Effect of Oxygen Plasma Treatment on GDYO Charge-Trapping Ability
To further understand the reason why the GDYO adopted in this paper shows superior charge-trapping ability, we studied and compared the surface morphology, composition, structure, and electrical characteristics of the GDY before and after mild oxygen plasma treatment.To assess the effect of plasma on surface treatment, GDY before and after oxygen plasma treatment are both wet-transferred onto SiO 2 /Si substrate with the Cu substrate dissolved in FeCl 3 solution, and then vacuum annealed (the vacuum value amounts to 10 −5 mbar) at 300 °C for 5 h.As depicted in Figure 2a,d, and Figure S3 (Supporting Information), the surface of GDY before oxygen plasma treatment is quite crude owing to the free rotation of alkyne-aryl single bonds in the GDY synthetic process [25] and the inheritance of the crude surface feature of unpolished Cu substrate.The R max of GDY before oxygen plasma treatment is even higher than 10 3 nm, and R a and R q are ≈10 2 nm.In contrast, R max of GDY after oxygen plasma treatment is below 10 3 nm, and R a and R q are much lower than that of GDY before oxygen plasma treatment, indicating oxygen plasma treatment can remove the impurity together with large particles and smooth the surface. [29,30]Auger spectroscopy (AES) was also leveraged to characterize the element distribution of the GDY sheet along the thickness (Figure 2b,e) and found that the GDY sheet before oxygen plasma treatment is sometimes thicker than 2000 nm, containing over 90% of carbon and ≈4% of oxygen (Figure S4a-d, Supporting Information).The oxygen plasma treatment reduced the thickness to ≈15 nm and also injected nearly 50% of oxygen into the whole film, which spread evenly across the film.
X-ray photoelectric spectroscopy (XPS) spectrum of GDY and GDYO was investigated and analyzed, and the deconvolution of C 1s peak resolved four subpeaks at 285, 286, 288, and 290 eV in Figure 2c, which could be assigned to C─C, C≡C, C─O, and C═O, respectively. [21,27]Figure 2f shows the XPS spectrum of GDYO, deconvoluted subpeak positions of C═C, C≡C, C─O, and C═O are close to that of GDY, indicating that oxygen plasma treatment does not hurt the chemical structure of GDY. [31,32]A little increase in the binding energy of GDYO can be observed compared to that of GDY and this suggested that more oxygen-containing groups (C─O and C═O) are introduced. [33]It is believed C─O and C═O are mostly electrophilic groups and thus O element acts as a hole dopant to GDY and improves the electron trapping ability of GDY.Besides, the Raman spectra of GDY and GDYO both display four prominent peaks around 1374. 3, 1563.4,1922.7, and   2182.2 cm −1 (Figure 2g,h).The peak at 1374.3 cm −1 corresponds to the breathing vibration of sp 2 carbon domains in aromatic rings (D-band), while the peak at 1563.7 cm −1 originates from the first-order scattering of the E 2g mode for in-phase stretching vibration of sp 2 carbon lattice in aromatic rings (G-band).The intensity ratio I D /I G can reflect the graphitization degree and defect concentration, which increases from I D /I G = 0.71 for GDY to I D /I G = 0.93 for GDYO, indicating mild oxygen plasma treatment brings defects and other charge trapping centers. [34,35]The peak at 2182.2 cm −1 is attributed to the vibration of conjugated diyne links and the alkyne bonds are also active charge-attracting centers. [36,37]or the sake of comparing the charge-trapping ability of GDY and GDYO, we designed and fabricated FG FETs based on WSe 2 /GDY vdWHs and WSe 2 /GDYO vdWHs.Then we measured and statistically analyzed the electrical performances of these devices.Figure 2i describes the versus V g of the FG FETs to compare the memory characteristics of WSe 2 /GDY vdWHs FG FET and WSe 2 /GDYO vdWHs FG FET.As mentioned above, the hysteresis window stems from the shift of threshold voltage and it can reflect the number of charges trapped in GDY (GDYO), which means a larger hysteresis window arising from a larger number of electrons transferred to GDY (GDYO) under positive V g and a larger number of holes transferred to GDY (GDYO) under negative V g .When V ds = 2 V, at V g = 20, 30, 40, 50, 60, 70, and 80 V, the hysteresis windows of WSe 2 /GDYO vdWHs FG FET are all larger than that of the counterparts, suggesting that under the same electric field, more charges are captured into GDYO (the original transfer hysteresis shown in Figure S5a,b, Supporting Information).This signifies van der Waals (vdW) coupling and more adequate charge transfer was formed in WSe 2 /GDYO SFG FET.We owe this to the smoother surface and electrophilic oxygen-containing groups of GDYO.The charge retention time of GDY and GDYO was also compared in Figure S5c,d (Supporting Information) and we can deduce that GDYO can hold electrons for a longer time than GDY.

DFT Calculations Affirming the Origination of GDYO Charge Capturing Ability
To further understand the charge-trapping mechanism of GDY, a DFT calculation was executed.The optimized lattice constant of the GDY, GDYO, and WSe 2 are 9.632 Å × 9.632 Å, 19.364 Å × 19.385 Å and 3.314 Å × 3.314 Å, respectively (3D models in Figure S6a-c, Supporting Information).To study the charge density distribution of the GDY and GDYO, the isosurfaces of the charge density model (side and top views in Figure S6d,e, Supporting Information) are extracted.As indicated in Figure 3a(I), charges are distributed around the nearest C atoms to form the C─C (sp 2 , sp) bonds and the charge density around the C─C(sp) is much higher than that around C─C(sp 2 ) bonds, thus it is deduced that the charge-trapping ability of GDY comes mainly from the C─C(sp) bonds.The charge density model of GDYO is constructed and the ratio of carbon-oxygen atoms is 18:8 according to the XPS result in Figure 2f.From Figure 3a(II), charge density around oxygen-containing functional groups is even higher in contrast to C─C(sp) bonds and this explains why the charge trapping capability of GDYO is stronger than GDY.To simulate the WSe 2 /GDY vdWHs, the 1 × 1 × 1 GDY unicell and 3 × 3 × 1 WSe 2 supercell were used (Figure S7a, Supporting Information).The lattice mismatch for the GDY and WSe 2 are ≈1.6% and 1.5%, To build WSe 2 /GDYO vdWHs, the 1 × 1 × 1 GDYO unicell and 6 × 6 × 1 WSe 2 supercell were used (Figure S7b, Supporting Information).The lattice mismatch for the GDYO and WSe 2 are ≈1.2% and 1.2%, respectively.The equilibrium distance between the WSe 2 monolayer and GDY (GDYO) layer is calculated to be 3.347 Å.
To explore the electron coupling at the WSe 2 /GDY and WSe 2 /GDYO interface and characterize the interfacial electronic structure, 3D charge density difference plots are constructed by subtracting the calculated electronic charge of WSe 2 /GDY (WSe 2 /GDYO) vdWHs from that of the independent WSe 2 and GDY (GDYO) monolayers as shown in Figure 3b.The figures show that the charge density is redistributed by forming electron-hole puddles in the WSe 2 /GDY (WSe 2 /GDYO) vdWH.The formation of the electron-hole puddle is understood as the charge-capturing alkyne bonds and oxygencontaining functional groups driving the charge transfer from WSe 2 to GDY and GDYO. [38]It is found that in WSe 2 /GDY vdWHs, the charge quantity transferred is 0.165e, and in WSe 2 /GDYO vdWHs the correspondent amount is 0.174e.This indicates that in a similar vdWHs system, GDYO exhibits stronger charge-capturing capability for a single unicell in theory.
Further, as displayed in Figure 3c, the work functions (W F s) of GDY, GDYO, and WSe 2 were calculated according to Equation (1) below: where the  and the E F are the electrostatic potentials of vacuum and valence band maximum (VBM).W F of GDY, GDYO, and WSe 2 was calculated to be 5.45, 5.48, and 4.97 eV, respectively.The value of W F can represent the ability of a specific material to capture electrons and electrons can escape from low-W F materials more easily compared to high-W F materials. [39]As expected, there emerges a small increase in the work function for GDYO compared to GDY and we owe this to the introduction of the -O groups in the structure.When forming the heterostructures, the vacuum levels align so electrons in WSe 2 with lower W F will transfer to the GDY (or GDYO) with higher W F .Under the same V g and V ds , the amount of transferred charge only depends on the difference of W F between the two vdWHs materials.It is thus easily understood why the charge-capturing ability of GDYO is much stronger than GDY.In WSe 2 /GDY vdWHs, most electrons transfer from the -S atom layer to the alkyne bond as depicted in the plane-averaged electron-density differences in Figure 3b(I).Nevertheless, in WSe 2 /GDYO vdWHs, most electrons transfer from the -Se atom layer to the -O layer as well as the alkyne bond as depicted in Figure 3b(II).

Rectifying Performance of WSe 2 Homojunction
Figure 4a demonstrates the configuration of the WSe 2 /GDYO vdWHs semi-floating gate FET, in which only half of the channel WSe 2 is aligned over GDYO.The source electrode is close to the WSe 2 /GDYO vdWHs side and the drain electrode (GND) is next to the WSe 2 side.Transfer characteristics of WSe 2 /GDYO vdWHs SFG FET were measured respectively under V ds = 1 V and V ds = −1 V, which present quite different performances.
As is shown in Figure 4b, positive gate voltage restrains I ds at V ds = −1 V compared to V ds = 1 V and it presents a rectification ratio of nearly 10 4 as the prominent characteristic of a homojunction.Output curves exhibit outstanding rectifying behavior under positive gate voltages.As illustrated in Figure 4c, driven by the electric field, electrons in WSe 2 move to GDYO and are trapped by GDYO, thus holes become the majority carriers of this part, and high-rectification-ratio homojunction forms across WSe 2 [40]   (as depicted in Figure 5a).The rectifying behavior can be modulated by varying the gate voltages and the rectification ratio (RR) can reach 10 6 at V g = 50 V, the highest RR of WSe 2 homojunctions ever reported [17,41,42] (more specific information shown in Table S1, Supporting Information).However, as shown in Figure 4d, output curves of the WSe 2 /GDYO vdWHs SFG FET under negative gate voltages possess good symmetric characteristics.This should be the result of the GDYO charge trapping and the field-effect working together.At this moment, the electric field drives electrons still in WSe 2 or back to WSe 2 , and bottom GDYO canott win versus the electric field.As the gate voltage increases negatively, more electrons are back from GDYO and the WSe 2 poses the initial property of bipolar transport.As a result, the major carrier in the channel changes to the hole (Figure 5b) and the FET would not show a rectifying behavior.Similarly, the rectifying ratios of WSe 2 /GDYO vdWHs SFG FET and WSe 2 /GDY vdWHs SFG FET were also calculated.At each V g , the RR of WSe 2 /GDYO vdWHs FG FET is also about a magnitude higher than that of its correspondent (Figure S2c,d, Supporting Information).
The electron transfer between the WSe 2 and GDYO was investigated by the threshold voltage shift of WSe 2 /GDYO vd-WHs FG FET. Figure 5c depicts the transfer characteristics of the device measured by sweeping from different positive V g to their negative counterparts.As the starting positive voltage increases, more electrons are collected in GDYO, thus more holes are produced in WSe 2 .This is confirmed by the rightshifted turn-on voltage (V th ), and the improved I ds at V g = 0 V. Similarly, when sweeping from negative V g to the positive, as V g starts more negatively, the turn-on voltage shifts left (Figure 5d).This indicates more electrons in GDYO transfer back to WSe 2 and the density of electrons in WSe 2 is increasing.The inset shows the fitting result of the original PL spectrum under zero gate voltage bias.f) Integral area ratio of X T and X E (A(X T )/A(X E )) (the gray curve) and the integral area percentage of X T to fitted PL spectrum (the red curve).X T and X E represent negative exciton peak and free exciton peak, respectively.A(X T ) /A(X E ) and the integral area percentage of X T to fitted PL spectrum can indicate the change of density of electrons in channel WSe 2.
The turn-on voltage shift is determined by the following Equation (2): In which Q is the charges trapped in GDYO and is the capacitance between GDYO and Si control gate. [43]In comparison with conventional SFG FET, our WSe 2 /GDYO vdWHs SFG FET replaced tunnel oxide and floating gate with GDYO film, [17,41] and the direct charge trapping in WSe 2 /GDYO vdWHs SFG FET allows electrons to transfer to GDYO without overcoming the high insulating barrier.It is validated that the more efficient direct charge trapping as well as the outstanding charge trapping ability brought high RR to the SFG FET.
Photoluminescence (PL) spectrometry was also exploited to characterize the change of electron density in WSe 2 while positive V g was applied to the WSe 2 /GDYO vdWHs SFG FET.As depicted in Figure 5e, the PL spectrum of WSe 2 (≈7 nm) contains two peaks without an external electric field, Peak(I) at 774 nm and Peak(II) at 1006 nm, respectively. [44]Peak(I) at 774 nm (≈1.60 eV) is in good agreement with previous reports on WSe 2 and the emission at 1006 nm (≈1.23 eV) arising from the energy level within the bandgap is attributed to the presence of surface defects. [45]We further deconvoluted the PL spectrums into three sub-peaks at 770, 785, and 805 nm, which can be assigned to free exciton emission peak X E , trion emission peak X T , and impurity exciton peak X D , [45] respectively.The emergence of X D in WSe 2 is mostly due to the introduction of O, H, Si, or other impurities in the process of device fabrication.X E is also named as neutral exciton emission peak and X T stems from the combination of a free exciton with a hole in the prerequisite of rich holes accumulated in WSe 2 .Generally, the concentration of most carriers is inversely proportional to the intensity of the PL spectrum.With gradient positive V g exerted on the device, PL intensity quenches, indicating that the majority hole concentration in WSe 2 on GDYO will increase.This result suggests the charge transfer truly exists between WSe 2 and GDYO. [46]Because if there is no interlayer charge transfer, the current and majority hole concentration in WSe 2 on GDYO (Figure 1e) would gradually decrease with V g increase from 0 V, which results in the increase of PL spectral intensity rather than decrease.
It is speculated that the photogenerated holes transfer to GDYO and couple with electrons in GDYO.A blue shift of the PL peak from 774 nm (V g = 0 V) to 768 nm (V g = 35 V) was also observed demonstrating the decrease of electron density in the WSe 2 channel.We further deconvoluted the PL spectrums into three sub-peaks at 770, 785, and 805 nm, which can be assigned to free exciton emission peak X E , negative exciton emission peak X T , and impurity exciton peak X D , respectively.The emergence X D in WSe 2 is mostly due to the introduction of O, H, Si, or other impurities in the process of device fabrication.X E is also named as neutral exciton emission peak and X T stems from the combination of a free exciton with an electron in the prerequisite of rich electrons accumulated in WSe 2 .The negative exciton is metastable, and the binding energy is just 30 meV.We calculated the integral area ratio of A(X T )/A(X E ) and the percentage of A(X T ) to the integral area of the fitted PL spectrum A, as shown in Figure 5f, both of the ratios descend as the V g ascends and the deconvolution of the PL spectrum under each V g is displayed in Figure S8 (Supporting Information).Therefore, negative excitons decrease, suggesting that electrons in WSe 2 are captured by GDYO under positive V g and the charge trapping can be tuned via V g .As long as GDYO and WSe 2 form a good van der Waals heterostructure, charge transfer in SFG FET happens the same as in FG FET and endows the device with different performance.

Conclusion
In summary, we have constructed a WSe 2 homojunction rectifier that exhibits a record-highest rectification ratio of up to 10 6 .The excellent rectifying behavior is attributed to GDY capturing electrons from WSe 2 under the drive of the gate electric field.Oxygen plasma treatment makes GDYO a flatter surface, thus creating a strong-coupling WSe 2 /GDYO vdW interface to enhance the charge exchange efficiency.DFT calculations confirmed that work functions (W F s) of GDYO, GDY, and WSe 2 are decreasing successively, revealing the internal cause of charge transfer in WSe 2 /GDYO (GDY/WSe 2 ) vdWHs.C─C(sp) and oxygen-containing functional groups as well as the defects are the main charge-trapping centers in GDYO due to their unsaturation.We explored the origination of the charge-trapping ability of GDY (GDYO), and an optimizing route of mild oxygen plasma treatment was proposed and demonstrated.The work enhances our understanding of the properties of the new 2D carbon material GDY and expands its application in electronic devices.

Experimental Section
Wet Transfer of Graphdiyne Film: Graphdiyne nanosheet was grown on the surface of copper foil in the presence of pyridine by a cross-coupling reaction of the monomer of hexaethynylbenzene.Copper serves as a not only catalyst by also a substrate for growing GDY film. [21]Large area and flat graphdiyne films were obtained by soaking in FeCl 3 solution for ≈6 h to remove copper after being covered with PMMA.For a thick film of ≈800 nm, first it went through mild oxygen plasma treatment with the oxygen of ≈50 sccm, power of 20 W, and time of 500 s and be reduced to ≈15 nm.Graphdiyne film clinging to PMMA was then cleaned in deionized water and transferred onto SiO 2 /Si substrate.It was then heated at 200 °C for 5 min and washed in acetone for 10 min to remove PMMA.Finally, a large area of pure GDY (GDYO) film on SiO 2 /Si was obtained.
Fabrication of GDY/WSe 2 vdWHs Devices: After transferring the GDY film onto the SiO 2 /Si substrate, the mechanically exfoliated WSe 2 nanosheet was then transferred onto GDY to build the WSe 2 /GDY vdW heterostructure via the manipulated platform.The electrode patterns were formed by electron beam lithography and development.The thermal evaporation method was used to fabricate Au electrodes (≈70 nm).
Characterizations: The surface topography of the graphdiyne film and devices was characterized by atomic force microscopy (AFM, Bruker Dimension ICON) and scanning electron microscopy (SEM, FEI Nova Nano450).Raman spectrometer (Horiba HR800) was used to analyze the chemical bonds of GDY, GDYO, and WSe 2 as well as the internal excitons.A nano scanning auger system (ULVAC-PHI PHI-700) was leveraged to measure the element distribution along the thickness direction.X-ray photoelectron spectroscopy (XPS, ThermoFisher Thermo Scientific K-Alpha+) was utilized to study the oxygen-containing functional groups.
DFT Calculations: The theoretical calculations were performed by using Vienna Ab initio Simulation Package (VASP) [47,48] with the projector augment wave (PAW) [49,50] method.The Perdew-Burke-Ernzerhof (PBE) of the generalized gradient approximation (GGA) was used for the exchange-correlation function. [51,52]The vdW dispersion forces were described by the DFT-D3 method. [53]The cutoff energy of 500 eV was used for both optimization and self-consistent calculation.

Figure 1 .
Figure 1.Routine to improve the conventional semi-floating gate technology and architecture of our devices.a) Design and novelties of our charge trapping WSe 2 homojunction compared to conventional semi-floating gate homojunction.b) False-colored SEM image of the device.The white dashed area indicates the WSe 2 via mechanical exfoliation."D" and "S" represent the drain and source electrodes."FG" represents a floating gate with GDYO under the whole channel and "SFG" represents a semi-floating gate with GDYO under half of the channel.c) Raman spectrum of the GDYO/WSe 2 vertical heterostructure.The corresponding Raman peaks of GDYO and WSe 2 are marked by the asterisks with the colors blue and red, respectively.d) Dual-sweeping transfer characteristics of the WSe 2 FET.e) Dual-sweeping transfer characteristics of the WSe 2 /GDYO FG FET.

Figure 2 .
Figure 2. Comparison between GDY before and after oxygen plasma treatment (GDYO).a) AFM image of the GDY on Cu before oxygen plasma treatment.Scale bar, 6 μm.b) AES image of GDY, which shows the variation of the content of elements C, O, Cu, and Si as the sputtering time prolongs.c) XPS spectrum of GDY before oxygen plasma treatment.The binding energy of C─C(sp 2 ), C─C(sp), C─O, and C═O are 285, 286, 288, 290 eV.d) AFM image of the GDYO on Cu.Scale bar, 6 μm.e) AES image of GDYO, which shows the variation of the content of elements C, O, Cu, and Si as the sputtering time prolongs.f) XPS spectrum of GDYO.The binding energy of C─C(sp 2 ), C─C(sp), C─O, and C═O are 285, 286, 289, and 292 eV.Raman spectra of g) GDY and h) GDYO.i) Comparison of the voltage hysteresis ΔV th dependent on the amplitude of the sweeping gate voltage.Here, ΔVth is the variation of threshold voltage between sweeping from positive voltage to corresponding negative voltage and the ΔV th increases linearly with the gate voltage.

Figure 3 .
Figure 3. DFT calculations of the charge transfer in WSe 2 /GDY and WSe 2 /GDYO vdWHs.a) Charge density distribution of GDY (I) and GDYO (II).b) Side view of the 3D charge density difference plot of WSe 2 /GDY vdWHs (I) and WSe 2 /GDYO vdWHs (II).The number of electrons transferred from WSe 2 to GDY and from WSe 2 to GDYO are 0.165 and 0.174e, respectively.c) The electrostatic potential of GDY, GDYO, and WSe 2 , respectively, as a function of distance in the direction of Z.The work function (W F ) is estimated with the equation: W F =  − E F .

Figure 4 .
Figure 4. Charge storage effect of the WSe 2 /GDYO vdWHs and rectification behavior of the WSe 2 homojunction in WSe 2 /GDYO vdWHs SFG FET.a) Schematic diagram of WSe 2 /GDYO vdWHs SFG FET.b) Transfer characteristics of the device.The red and black lines refer to the transfer curve under V ds = 1 V and −1 V, respectively.c) Output curves of the WSe 2 /GDYO vdWHs SFG FET under positive gate voltages.The inset window shows the rectification ratios under positive V g .d) Output curves of the WSe 2 /GDYO vdWHs SFG FET under negative gate voltages.The inset window shows the rectification ratios under negative V g .

Figure 5 .
Figure 5. Mechanism of the WSe 2 homojunction under positive and negative gate voltages.a,b) Schematic diagram of the charge transfer in WSe 2 /GDYO vdWHs SFG FET under a) positive and b) negative gate voltages.The major carrier of the channel is electron in positive and hole in negative, respectively.c) Transfer curves of WSe 2 /GDYO vdWHs FG FET with the sweeping voltage from positive to negative (20 to −20 V, 30 to −30 V, 40 to −40 V, 50 to −50 V, 60 to −60 V, 70 to −70 V, 80 to −80 V, respectively).d) Corresponding curves with the sweeping voltage from negative to positive.e) PL spectra of WSe 2 /GDYO vdWHs FG FET under different positive gate voltages.The inset shows the fitting result of the original PL spectrum under zero gate voltage bias.f) Integral area ratio of X T and X E (A(X T )/A(X E )) (the gray curve) and the integral area percentage of X T to fitted PL spectrum (the red curve).X T and X E represent negative exciton peak and free exciton peak, respectively.A(X T ) /A(X E ) and the integral area percentage of X T to fitted PL spectrum can indicate the change of density of electrons in channel WSe 2.