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Switchable Photoresponse Mechanisms Implemented in Single van der Waals Semiconductor/Metal Heterostructure
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ACS Nano

Cite this: ACS Nano 2022, 16, 1, 568–576
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https://doi.org/10.1021/acsnano.1c07661
Published January 5, 2022

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Abstract

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van der Waals (vdW) heterostructures based on two-dimensional (2D) semiconducting materials have been extensively studied for functional applications, and most of the reported devices work with sole mechanism. The emerging metallic 2D materials provide us new options for building functional vdW heterostructures via rational band engineering design. Here, we investigate the vdW semiconductor/metal heterostructure built with 2D semiconducting InSe and metallic 1T-phase NbTe2, whose electron affinity χInSe and work function ΦNbTe2 almost exactly align. Electrical characterization verifies exceptional diode-like rectification ratio of >103 for the InSe/NbTe2 heterostructure device. Further photocurrent mappings reveal the switchable photoresponse mechanisms of this heterostructure or, in other words, the alternative roles that metallic NbTe2 plays. Specifically, this heterostructure device works in a photovoltaic manner under reverse bias, whereas it turns to phototransistor with InSe channel and NbTe2 electrode under high forward bias. The switchable photoresponse mechanisms originate from the band alignment at the interface, where the band bending could be readily adjusted by the bias voltage. In addition, a conceptual optoelectronic logic gate is proposed based on the exclusive working mechanisms. Finally, the photodetection performance of this heterostructure is represented by an ultrahigh responsivity of ∼84 A/W to 532 nm laser. Our results demonstrate the valuable application of 2D metals in functional devices, as well as the potential of implementing photovoltaic device and phototransistor with single vdW heterostructure.

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Originally published ASAP on January 5, 2022; Figure 3 color scale modified for improved art effect, reposted on January 25, 2022.

Two-dimensional (2D) materials, including graphene, (1) black phosphorus, (2,3) transition metal dichalcogenides (TMDs), and their heterostructures, (4−8) have been extensively studied for optical and optoelectronic applications. (9−13) They possess bandgap ranging from 0 eV to over 2 eV, (14−17) which depends on not only the chemical composition but also the thickness of 2D flakes. The widely tunable bandgap gives rise to functional electronic devices like field effect transistors (FETs) (18,19) and facilitates ultrasensitive detection of the light ranging from visible to near-infrared. (20−22) Beyond single materials, many van der Waals (vdW) heterostructures integrating various materials have been successfully constructed by means of vdW stacking or chemical synthesis, (23−26) as ultraclean surfaces and edges are readily available with 2D materials. (27) In order to accomplish tailored functions and working mechanisms, the heterostructure devices have to be rationally designed based on band engineering. (28−30) Accordingly, it is worth exploiting more candidate materials with distinctive band structures.
Beyond the 2D semiconducting materials with deterministic electron affinity χ and bandgap Eg, the emerging 2D metallic materials, which have solely work function Φ that needs to be considered for band engineering design, (31,32) provide new options for the development of powerful devices. Specifically, group VB metal tellurides (XTe2, X = V, Nb, Ta) have been theoretically calculated to be metallic and experimentally obtained by chemical synthesis. (33−36) Quantitatively, ultrahigh electrical conductivity on the level of 106 S/m was achieved with the synthesized XTe2 flakes. (37) Owing to the high conductivity, NbTe2 was employed as conductive electrodes of WSe2 FET, resulting in lower contact resistance and higher carrier mobility compared with the counterpart using Cr/Au electrodes. (35) In contrast, the chemically synthesized lateral WS2/NbS2 (semiconductor/metal) heterostructure exhibits considerable rectifying effect, (38,39) which is a typical characteristic of diode-like heterojunction devices. The distinct transport behaviors of these vdW semiconductor/metal heterostructures are partly a result of the specific band alignment at the interfaces. Accordingly, it is promising to implement versatile functionalities at certain vdW semiconductor/metal interfaces designed based on rational band engineering.
Here, vertically stacked vdW semiconductor/metal heterostructure is designed and constructed by stacking mechanically exfoliated flakes of semiconducting InSe and metallic 1T-phase NbTe2. The electron affinity of thick InSe flake is extremely close to the work function of 1T-phase NbTe2, leading to exotic band alignment at the interface. The electrical and optoelectronic characterization, especially the photocurrent mappings, indicate that the vdW InSe/NbTe2 interface is switchable between heterojunction and ohmic contact, when the energy barrier at the interface is electrically tuned by bias voltage. Working as a semiconductor/metal heterojunction, it demonstrates exceptional photovoltaic effect with ∼0.41 V open-circuit voltage (VOC) and ∼380 nA short-circuit current (ISC) under 100 μW illumination of 532 nm laser. Switched to InSe phototransistor with ohmic contact to NbTe2 electrode, high photoresponsivity of ∼84 A/W is achieved under 10 nW illumination. These results reveal the versatile roles that 2D metallic materials could play in future optoelectronic applications.

Results and Discussion

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In this study, InSe and NbTe2 are selected because of their band structures. According to the previously published experimental results, thick InSe flakes have a direct bandgap of ∼1.25 eV and electron affinity of ∼4.45 eV, (17,40) which is extremely close to the work function of metallic 1T-phase NbTe2. (41) The actual band structures of the two materials are measured with ultraviolet photoelectron spectroscopy (UPS), and the results are demonstrated in Figure S1. Accordingly, the energy bands of semiconducting InSe and metallic NbTe2 align as the illustration in Figure 1a. Owing to the proximity between the electron affinity of InSe and the work function of NbTe2, the band bending at this semiconductor/metal interface is expected to be highly adjustable. To experimentally investigate the InSe/NbTe2 heterostructure, a FET device was constructed on SiO2/Si substrate, as illustrated in the schematic of Figure 1b. In addition, an Al2O3 layer was deposited for protection of the device. The 300 nm thick SiO2 layer works as a dielectric layer, and the doped Si (G) on backside is used for applying gate voltage Vgate. Two Ti/Au electrodes deposited on InSe and NbTe2 are the source (S) and drain (D) electrodes, respectively. In all the following measurements, the source electrode was grounded, and bias voltage Vds was applied via the drain electrode.

Figure 1

Figure 1. Design and characterization of the InSe/NbTe2 heterostructure. (a) Band diagram between the bulk materials of semiconducting InSe and metallic 1T-phase NbTe2. The key features are determined with the results of UPS measurements shown in Figure S1. EF denotes the Fermi level of pristine bulk InSe, indicating that it is n-doped. (b) Schematic of the InSe/NbTe2 heterostructure device. The upper panel illustrates the crystal structures of InSe and NbTe2. The Al2O3 layer for protection is not shown in the schematic. (c) Optical microscope image of the stacked InSe/NbTe2 heterostructure. The top diagram illustrates the stacking order. (d) AFM characterization of the InSe/NbTe2 heterostructure. (e) Raman spectra of InSe, NbTe2, and InSe/NbTe2 heterostructure. (f) Absorbance of InSe/NbTe2 heterostructure.

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In practice, the fabrication of InSe/NbTe2 heterostructure device started with mechanical exfoliation and vdW stacking of the 2D flakes (the details are described in Methods). (6,42) Briefly, bulk 1T-phase NbTe2 was first exfoliated, and the obtained flakes were transferred to SiO2/Si substrate, followed by transferring exfoliated InSe flakes on top. As InSe has a direct bandgap only for thick flakes and NbTe2 is difficult to be thinned, (17) both InSe and NbTe2 flakes in relatively thick form were used. Optical microscope images of the InSe/NbTe2 heterostructure and fabricated FET device are shown in Figure 1c and Figure S2, and the overlapping region has an area of ∼8 μm × 10 μm. Thickness of the flakes was measured by atomic force microscopy (AFM), and the line profiles in Figure 1d indicate that the InSe and NbTe2 flakes are ∼17 nm and ∼55 nm thick, respectively. The two flakes were further identified by Raman spectra. As shown in Figure 1e, Raman spectra were collected at both the single materials and the heterostructure. In the Raman spectrum of InSe, three peaks are observed at ∼116 cm–1, ∼178 cm–1, and 227 cm–1, attributed to Ag, E2g, and Ag2 phonon modes, respectively. (43,44) The Raman spectrum of NbTe2 flake consists of three prominent peaks (∼84 cm–1, ∼109 cm–1, and ∼158 cm–1) and two weak peaks (∼219 cm–1 and ∼254 cm–1, detailed in Figure S3). These results agree well with the published results of NbTe2 flakes grown by chemical vapor deposition and vapor phase transport. (3645) All the major peaks described above could be found in the Raman spectrum of InSe/NbTe2 heterostructure, indicating the high quality of fabricated vdW heterostructure. Furthermore, photoluminescence (PL) spectra of the flakes were measured, and the results are depicted in Figure S4. The obvious peak centered at photon energy of around 1.25 eV is a typical value of thick InSe flakes, (17) while the metallic NbTe2 does not show any PL peak. The significant darkness of InSe/NbTe2 heterostructure in Figure 1c is unexpected, and it is generally observed in other samples as well (Figure S5a–d). To quantitatively comprehend this observation, the reflection of these 2D materials on SiO2/Si substrate was measured, and the absorbance was calculated with a silver mirror as reference (Figure S5e). According to the calculated absorbance demonstrated in Figure S5f, the InSe and NbTe2 flakes mainly absorb light in the wavelength ranges of ∼600–700 nm and ∼450–600 nm, respectively. It means that InSe and NbTe2 are complementary for the absorption of visible light. Consequently, the absorbance of InSe/NbTe2 heterostructure is universally enhanced in the whole visible range (up to ∼80% at ∼500–550 nm), as depicted in Figure 1f. One reasonable explanation for the high absorbance is the joint absorption effect of the two thick flakes, as observed in another case of thick InSe/Te (40 nm/120 nm) heterostructure, whose overlapping area is significantly dark as well. (46) However, to the best of our knowledge, dark vdW heterostructures are rarely observed, even in the ones built with thick flakes. Therefore, this phenomenon is worth investigating by systematic and meticulous spectroscopic characterization. Despite the unknown cause, the high absorbance is expected to benefit the photodetection with InSe/NbTe2 heterostructure.
The working mechanism of the InSe/NbTe2 heterostructure device could be predicted based on the published works, (33,36,38) where 2D semiconductor/metal heterostructure devices with various materials exhibit distinctive transport behaviors. On the basis of computational study, (33) the work function of metallic NbS2 drops in the bandgap of WS2, whereas the work function of metallic NbTe2 is much lower than the valence band maximum of WSe2. As a result, an energy barrier with considerable height emerges because of band bending at the WS2/NbS2 interface, while it is absent at the WSe2/NbTe2 interface. These band diagrams lead to an apparent rectifying effect in heterogeneous WS2/NbS2 device and remarkably low contact resistance of homogeneous WSe2 FET with NbTe2 electrodes. (35,39) On the basis of these results, the InSe/NbTe2 interface is predicted to be switchable between heterojunction and ohmic contact, as the difference between the electron affinity of InSe and the work function of NbTe2 is so little.
Initially, electrical characterization was conducted to reveal the basic working mechanism of InSe/NbTe2 heterostructure, and the results are depicted in Figure 2. For comparison, the FETs with pure InSe or NbTe2 channel were fabricated and measured as well. The transfer curves shown in Figure 2a, as well as the gate dependent output Ids–Vds curves in Figure S6, suggest that the exfoliated InSe flake is intrinsically n-doped and highly conductive when it is heavily doped by positive gate voltage, and the NbTe2 flake is a metal with conductivity independent of gate voltage. In addition, the linear output Ids–Vds curves indicate ohmic contact between the flakes and Ti/Au electrodes. These results agree well with the published results of semiconducting InSe and metallic NbTe2, indicating the high quality of exfoliated InSe and NbTe2 flakes. In the following, the InSe/NbTe2 heterostructure FET was measured with the configuration illustrated in Figure 1b, and two different bias voltages Vds of 2 V and 1 V were applied. As shown in Figure 2b and Figure S7, the Ids could be considerably modulated by gate voltage, leading to a high current On/Off ratio of nearly 105. The high On current and On/Off ratio were maintained even 2 months after device fabrication (Figure S8). In contrast, the bare device without protective Al2O3 layer shows low On/Off ratio of <103 when it was measured right after fabrication (Figure S9). This comparison clearly indicates the protection effect of Al2O3 layer. Besides, the significantly nonlinear relation between the Ids and Vds at positive gate voltage verifies the existence of Schottky barrier at the InSe/NbTe2 interface. The Schottky barrier could be further confirmed with the output Ids–Vds curves in Figure 2c, which exhibit a common diode-like rectifying effect. The Ids is significantly suppressed when a negative Vds is applied, as the reverse bias increases the height of Schottky barrier. The rectifying effect could be quantitatively assessed by rectification ratio calculated with the Ids at Vds = 2 V and Vds = −2 V. As depicted in Figure 2d, the rectification ratio rises to ∼300 when the gate voltage of Vgate = 80 V is applied, and a higher value of >103 is obtained with another device (Figure S10). Furthermore, the output curve measured at Vgate = 80 V is fitted by the Shockley diode function and shown in Figure 2e, (47) leading to an ideality factor of n = 2.2.

Figure 2

Figure 2. Electrical characterization of the heterostructure device. (a) Transfer curves of the devices with pure InSe or NbTe2 channel. Bias voltages Vds of 2 and 0.1 V were applied in the measurements of InSe and NbTe2 devices, respectively. (b) Transfer curves of the InSe/NbTe2 heterostructure device measured with bias voltage Vds of 2 V and 1 V. (c) Gate voltage dependent output Ids–Vds curves of the InSe/NbTe2 heterostructure device. The inset shows |Ids| on a logarithmic scale. (d) Gate voltage dependent rectification ratio calculated with the results in (c). (e) Fitting of the output Ids–Vds curve measured at Vgate = 80 V by Shockley diode function. (47) Ideality factor of n = 2.2 is extracted from the fitting. (f) Schematic of band bending at the interface between n-doped InSe and metallic NbTe2 under reverse and forward biases when a positive gate voltage is applied. EF denotes the Fermi level of InSe.

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The exceptional rectifying effect could be readily explained with the band diagram illustrated in Figure 2f. When a negative reverse bias is applied to NbTe2, its Fermi level is lifted relative to InSe, resulting in an increased band mismatch according to Figure 1a. The band mismatch induces significant band bending on InSe side, and a relatively high Schottky barrier is built at the InSe/NbTe2 interface. Therefore, the Ids through InSe/NbTe2 heterojunction is dominated by tunneling current. Once a high positive forward bias (e.g., Vds = 2 V) is applied, the Fermi level of NbTe2 is considerably lowered; thus the band bending on InSe side is eliminated. In this case, the majority carriers in InSe could easily diffuse to NbTe2 and lead to a high conductance of the heterostructure channel. Actually, this band bending conflicts with the band diagram determined with UPS (Figure 1a), suggesting that significant n-doping of semiconducting InSe is induced in the device fabrication process, especially the deposition of Al2O3 (see Methods), where 2 nm thick aluminum layer was deposited and oxidized as seeding layer. The strongly adjustable band diagram at InSe/NbTe2 interface makes it distinct from the massively investigated vdW heterostructures built with semiconductors. This could be verified with the following optoelectronic measurements.
Photocurrent mappings are performed to achieve a more profound interpretation of the working mechanisms of InSe/NbTe2 heterostructure device. In order to study the intrinsic heterostructure, the gate voltage was fixed at Vgate = 0 V in the measurements. The device was laterally moved with steps of 0.5 μm, and a 532 nm laser beam with 1 μW power was focused on the plane of heterostructure and swept in the area shown in Figure S2. Source-drain current Ids (measured with the same configuration in Figure 1b) was recorded under various bias voltages Vds of −2 V, 0 V, and 2 V, and the results are depicted in Figure 3a–c. The green, yellow, and white dashed lines in Figure 3a–c outline the positions of InSe, NbTe2, and Ti/Au electrodes. An intuitive observation of the mappings is the bias-dependent positions of peak Ids. The peak Ids is obtained at InSe/NbTe2 stacking area under bias voltage Vds of −2 V and 0 V. In contrast, under high forward bias voltage of Vds = 2 V, the peak Ids is obtained at pure InSe area. This observation could be more clearly and quantitatively illustrated by the line scannings extracted at the white arrows in Figure 3a–c, and the results are shown in Figure 3d. On the basis of bias-dependent peak Ids positions, a conceptual XOR logic gate with truth table summarized in Figure 3e is proposed. Two inputs of this logic gate are Y position of the laser beam (e.g., “0” for Y = 5 μm, “1” for Y = 13 μm) and bias voltage Vds (e.g., “0” for Vds = −2 V, “1” for Vds = +2 V) in Figure 3d, and the output is source−drain current Ids (e.g., “0” for small |Ids| and “1” for large |Ids|). The bias dependent photocurrent generation is also confirmed with the device shown in the inset of Figure S10. The irregular outline of InSe/NbTe2 overlapping area makes it easy to identify the photoresponse area in the whole heterostructure (Figure S11a–c). The mappings of photocurrent under high negative gate voltage (Figure S11d–f) suggest that this phenomenon occurs even when the Fermi level of InSe channel is significantly lowered by negative Vgate, despite the decreased absolute photocurrent. On the basis of the results in Figure S10, this device demonstrates a weak rectifying effect under negative gate voltage of Vgate = −80 V. Therefore, the bias dependent switchable photoresponse mechanisms exist as long as the device shows a rectifying effect or an effective Schottky barrier exists at the heterostructure interface.

Figure 3

Figure 3. Switchable photoresponse mechanisms of InSe/NbTe2 heterostructure. (a–c) Photocurrent mappings in InSe/NbTe2 heterostructure device at various bias voltages Vds of −2, 0, and 2 V. The green, yellow, and white dashed lines illustrate the outlines of InSe, NbTe2, and Ti/Au electrodes, respectively. Scale bars, 5 μm. (d) Line scannings extracted from the mappings at the positions and directions indicated by white arrows in (a)–(c). The blue and brown shades indicate the Y positions of pure InSe and InSe/NbTe2 heterostructure. (e) Truth table of the conceptual XOR logic gate with inputs of laser beam position (Y) and bias voltage (Vds) and output of Ids. (f, g) Schematic of the InSe/NbTe2 heterostructure device switched between the photovoltaic device (f, reverse bias) and phototransistor (g, forward bias). The role of NbTe2 is switched between a heterojunction component and a contact electrode.

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The bias dependent peak photocurrent positions imply switchable photodetection mechanisms of InSe/NbTe2 heterostructure, attributed to the band bending tuned by bias voltage Vds. Essentially, the excited photocarriers are mainly generated in InSe. Under reverse bias, a high Schottky barrier is built, and the device turns to heterojunction (Figure 2f). Thus, the heterostructure responds to light illumination as a photovoltaic device (Figure 3f). In other words, only the electron–hole pairs generated at InSe/NbTe2 stacking area could be efficiently separated by the built-in electric field and contribute to photocurrent (Figure 3a). However, the energy barrier is significantly lowered or even vanishes when a large forward bias is applied (Figure 2f). Consequently, the device works as a phototransistor with homogeneous InSe channel and NbTe2 conductive electrode (Figure 3g), and the thick InSe channel with direct bandgap could absorb light illumination with high efficiency. (17) Given the above, the InSe/NbTe2 heterostructure device could be readily switched between heterojunction photovoltaic device and InSe phototransistor with NbTe2 contact electrode, indicating the rationality of band engineering design in Figure 1a.
Finally, photodetection performance of the switchable InSe/NbTe2 heterostructure device is systemically investigated with 532 nm laser illumination. As illustrated in the schematic of Figure 4a, the photoresponse mechanism is subject to bias-dependent band bending at the interface (Figure 3f,g). For a phototransistor working under bias voltage Vds = 2 V, Ids is raised when the laser power increases from 10 nW to 100 μW (Figure 4b). Notably, Ids at Vgate = −80 V increases significantly compared with the values at Vgate = 80 V. This phenomenon could be explained by the change of Fermi level EF of InSe. EF is significantly declined, and the density of carriers is extremely low under gate voltage of Vgate = −80 V. Thus even the photocarriers induced by a low-power illumination could lead EF to rise considerably. Therefore Ids changes a lot as the illumination power is increased. The carrier density is intrinsically high under gate voltage Vgate = 80 V. Thus the photocarriers finitely change the band diagram or contribute to the Ids dominated by the thermionic current. Properties of the InSe/NbTe2 heterostructure working as a photovoltaic device are revealed by the Ids–Vds curves measured under laser illumination with various power, and the results are shown in Figure 4c and Figure S12. The large ISC (Figure 4d) and high VOC (Figure 4e) are characteristics of a photovoltaic device. The highest VOC in Figure 4e is ∼0.41 V, and large ISC of ∼380 nA (Figure S13) is obtained with the device demonstrated in Figure S10, indicating the excellent performance of InSe/NbTe2 photovoltaic device. As demonstrated in Figure S14, the InSe/NbTe2 heterostructure device works with higher response speed in photovoltaic mode and lower response speed in phototransistor mode, and the ultimate response time should be less than 10 ms. Besides, the photocurrent Iph and corresponding photoresponsivity are calculated (Figure 4f) based on the Ids–Vds curves measured in dark condition (Figure 2c), and a high photoresponsivity of ∼12 A/W is achieved with the InSe phototransistor when a bias voltage of Vds = 2 V is applied. All the results in Figure 4 are measured with the laser spot centered at stacking InSe/NbTe2 area. Yet, the highest photocurrent in the phototransistor should be obtained when the pure InSe is illuminated under positive bias, according to Figure 3c. Therefore, the photoresponse was additionally measured with 10 nW illumination centered at pure InSe, where peak Ids is obtained in Figure 3c. On the basis of output Ids–Vds curves demonstrated in Figure S15, a remarkably high photoresponsivity of ∼84 A/W is achieved. This photoresponsivity is superior to many of the published works of InSe-based photodetectors, as compared in Table S1. The exceptional photodetection performance affirms the rationality of the band engineering design discussed in Figure 1, indicating a promising strategy for developing powerful vdW devices. As summarized in Table S2, the performance of this InSe phototransistor with metallic NbTe2 electrode is comparable with or even better than the InSe devices with contact electrodes of other materials.

Figure 4

Figure 4. Overall photodetection performance of InSe/NbTe2 heterostructure device. (a) Schematic of the switchable InSe/NbTe2 heterostructure for the detection of 532 nm laser. Working mechanism of this device depends on bias-dependent band bending at the interface. (b) Transfer curves of InSe/NbTe2 phototransistor illuminated by 532 nm laser beam with gradient power. (c) Ids–Vds curves of the InSe/NbTe2 heterostructure device under light illumination. (d, e) Short-circuit current ISC (d) and open-circuit voltage VOC (e) extracted from the Ids-Vds curves in (c). The results are fitted by ISC ∝ Powerα and VOC ∝ ln(Power). (f) Photocurrent Iph and photoresponsivity calculated with the data in (c).

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Conclusions

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Switchable photoresponse mechanisms have been implemented in single van der Waals semiconductor/metal heterostructure. Its working mechanism could be readily switched by the bias voltage that tunes band bending at the semiconductor/metal interface. The electrical characterization demonstrates a significant rectifying effect, indicating an inherent diode-like heterojunction. Further systematic optoelectronic measurements results indicate that reverse bias switches the heterostructure to a photovoltaic device, and alternatively, large forward bias turns it to a phototransistor with InSe channel and NbTe2 electrode. Overall, it exhibits exceptional electronic and optoelectronic performance of >103 rectification ratio and ∼84 A/W photoresponsivity, suggesting that it is a promising candidate for practical applications. The rational band engineering design in this vdW heterostructure and its exceptional device performance indicate a promising strategy for building versatile 2D optoelectronic devices and highlight the functionality of 2D metallic materials.

Methods

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UPS Measurements

Bulk materials of InSe and NbTe2 were used for UPS measurements in high vacuum. The materials were sputtered with Ar+ for cleaning, and a bias voltage of −10 V was applied. The photon energy of the UV light source is 21.22 eV, and the work function of the analyzer is 4.66 eV.

Preparation and Characterization of Two-Dimensional Flakes

The InSe and NbTe2 flakes were mechanically exfoliated from bulk materials (2D Semiconductors). In the following, NbTe2 flakes were transferred to silicon substrates covered with 300 nm thick SiO2, and the InSe flakes supported by polydimethylsiloxane (PDMS) were stacked on top. Raman spectra were collected with WITec micro-Raman system, and a 532 nm laser was used for excitation. The reflection of silver mirror, InSe, NbTe2, and InSe/NbTe2 heterostructures was measured with SNOM system (WITec alpha300). The absorbance of the materials is calculated with the reflection of a silver mirror as reference: absorbance = [(RAg – RX)/RAg] × 100%, where RAg and RX are the reflection of silver mirror and 2D materials, respectively. The PL measurements were also conducted with a SNOM system, and a 532 nm laser with power of ∼1 mW was used for excitation. AFM image of the flakes was collected by Dimension Icon system (Bruker).

Device Fabrication

The patterning and deposition of metal electrodes were accomplished through patterning PMMA photoresist with electron beam lithography (EBL, Vistec EBPG 5000), deposition of Ti/Au (5 nm/100 nm) with electron beam evaporation system (MASA IM-9912), and finally the lift-off process in acetone. For optimization, the device was annealed at 180 °C in high vacuum (∼10–5 mbar) for 2 h (AML-AWB wafer bonding machine). Then a 2 nm thick Al seeding layer was deposited and heated at 130 °C in the air for 2 min. Afterward, 20 nm thick Al2O3 was deposited at 120 °C by atomic layer deposition (Beneq TFS-500), followed by second annealing with the same conditions. Finally, the Ti/Au electrodes were connected to a printed circuit board (PCB) by wire bonding. The fabrication of bare device was terminated after the first annealing.

Electrical and Optoelectronic Measurements

Keithley 2401 and Keithley 2400 were used for applying bias voltage Vds and gate voltage Vgate, and the drain-source current Ids was measured with Keithley 2401. Data acquisition was accomplished with a customized LabVIEW program. The PCB with fabricated device was fixed on the lateral movement stage of the SNOM system, which was precisely controlled by the LabVIEW program. In the optoelectronic test, a 532 nm laser with adjustable power was illuminated on the device through a 20× objective (NA = 0.4).

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsnano.1c07661.

  • Materials characterization with Raman, PL, UPS, and absorption spectra, output Ids–Vds curves and transfer curves of additional devices, photocurrent mapping at negative gate voltage, short-circuit current at 100 μW illumination, measurement of response time, and comparison of various InSe-based photodetectors (PDF)

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Author Information

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  • Corresponding Authors
  • Authors
    • Xiaoqi Cui - Department of Electronics and Nanoengineering, Aalto University, Espoo FI-02150, Finland
    • Hoon Hahn Yoon - Department of Electronics and Nanoengineering, Aalto University, Espoo FI-02150, Finland;  Orcidhttps://orcid.org/0000-0002-4081-3343
    • Susobhan Das - Department of Electronics and Nanoengineering, Aalto University, Espoo FI-02150, Finland
    • MD Gius Uddin - Department of Electronics and Nanoengineering, Aalto University, Espoo FI-02150, Finland
    • Luojun Du - Department of Electronics and Nanoengineering, Aalto University, Espoo FI-02150, Finland;  Orcidhttps://orcid.org/0000-0002-7875-3817
    • Diao Li - Department of Electronics and Nanoengineering, Aalto University, Espoo FI-02150, Finland
  • Author Contributions

    M.D. and Z.S. conceived the project. M.D. fabricated the 2D heterostructure devices and characterized the materials with the help of S.D., M.G.U., and L.D. M.D., X.C., H.H.Y., and D.L. carried out the electrical and optoelectronic measurements. M.D. and Z.S. analyzed the data and wrote the manuscript with input from all authors. All authors reviewed the manuscript.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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We thank the help of Juoko Lahtinen for UPS measurements. We acknowledge the provision of facilities by Aalto University at OtaNano, Micronova Nanofabrication Centre, and the funding from the Academy of Finland (Grants 340932, 295777, 312297, 314810, 333982, 336144, 333099, and 336818), the Academy of Finland Flagship Programme (Grant 320167, PREIN), the European Union’s Horizon 2020 Research and Innovation Program (Grant Agreements 820423, S2QUIP; 965124, FEMTOCHIP), and ERC (Grant 834742).

References

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This article references 47 other publications.

  1. 1
    Bonaccorso, F.; Sun, Z.; Hasan, T.; Ferrari, A. C. Graphene Photonics and Optoelectronics. Nat. Photonics 2010, 4, 611– 622,  DOI: 10.1038/nphoton.2010.186
    Google Scholar
    1
    Graphene photonics and optoelectronics
    Bonaccorso, F.; Sun, Z.; Hasan, T.; Ferrari, A. C.
    Nature Photonics (2010), 4 (9), 611-622CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
    The richness of optical and electronic properties of graphene attracts enormous interest. Graphene has high mobility and optical transparency, in addn. to flexibility, robustness and environmental stability. So far, the main focus has been on fundamental physics and electronic devices. However, we believe its true potential lies in photonics and optoelectronics, where the combination of its unique optical and electronic properties can be fully exploited, even in the absence of a bandgap, and the linear dispersion of the Dirac electrons enables ultrawideband tunability. The rise of graphene in photonics and optoelectronics is shown by several recent results, ranging from solar cells and light-emitting devices to touch screens, photodetectors and ultrafast lasers. Here we review the state-of-the-art in this emerging field.
  2. 2
    Hu, G.; Albrow-Owen, T.; Jin, X.; Ali, A.; Hu, Y.; Howe, R. C. T.; Shehzad, K.; Yang, Z.; Zhu, X.; Woodward, R. I.; Wu, T.-C.; Jussila, H.; Wu, J.-B.; Peng, P.; Tan, P.-H.; Sun, Z.; Kelleher, E. J. R.; Zhang, M.; Xu, Y.; Hasan, T. Black Phosphorus Ink Formulation for Inkjet Printing of Optoelectronics and Photonics. Nat. Commun. 2017, 8, 278,  DOI: 10.1038/s41467-017-00358-1
    Google Scholar
    2
    Black phosphorus ink formulation for inkjet printing of optoelectronics and photonics
    Hu Guohua; Albrow-Owen Tom; Howe Richard C T; Yang Zongyin; Wu Tien-Chun; Hasan Tawfique; Jin Xinxin; Hu Yuwei; Zhu Xuekun; Zhang Meng; Ali Ayaz; Shehzad Khurram; Xu Yang; Woodward Robert I; Kelleher Edmund J R; Jussila Henri; Sun Zhipei; Wu Jiang-Bin; Tan Ping-Heng; Peng Peng; Peng Peng; Zhang Meng
    Nature communications (2017), 8 (1), 278 ISSN:.
    Black phosphorus is a two-dimensional material of great interest, in part because of its high carrier mobility and thickness dependent direct bandgap. However, its instability under ambient conditions limits material deposition options for device fabrication. Here we show a black phosphorus ink that can be reliably inkjet printed, enabling scalable development of optoelectronic and photonic devices. Our binder-free ink suppresses coffee ring formation through induced recirculating Marangoni flow, and supports excellent consistency (< 2% variation) and spatial uniformity (< 3.4% variation), without substrate pre-treatment. Due to rapid ink drying (< 10 s at < 60 °C), printing causes minimal oxidation. Following encapsulation, the printed black phosphorus is stable against long-term (> 30 days) oxidation. We demonstrate printed black phosphorus as a passive switch for ultrafast lasers, stable against intense irradiation, and as a visible to near-infrared photodetector with high responsivities. Our work highlights the promise of this material as a functional ink platform for printed devices.Atomically thin black phosphorus shows promise for optoelectronics and photonics, yet its instability under environmental conditions and the lack of well-established large-area synthesis protocols hinder its applications. Here, the authors demonstrate a stable black phosphorus ink suitable for printed ultrafast lasers and photodetectors.
  3. 3
    Xia, F.; Wang, H.; Hwang, J. C. M.; Neto, A. H. C.; Yang, L. Black Phosphorus and Its Isoelectronic Materials. Nat. Rev. Phys. 2019, 1, 306– 317,  DOI: 10.1038/s42254-019-0043-5
    Google Scholar
    3
    Black phosphorus and its isoelectronic materials
    Xia, Fengnian; Wang, Han; Hwang, James C. M.; Castro Neto, A. H.; Yang, Li
    Nature Reviews Physics (2019), 1 (5), 306-317CODEN: NRPACZ; ISSN:2522-5820. (Nature Research)
    Abstr.: The family of 2D and layered materials has been expanding rapidly for more than a decade. Within this large family of hundreds of materials, black phosphorus and its isoelectronic group IV monochalcogenides have a unique place. These puckered materials have distinctive cryst. symmetries and exhibit various exciting properties, such as high carrier mobility, strong IR responsivity, widely tunable bandgap, in-plane anisotropy and spontaneous elec. polarization. Here, we review their basic properties, highlight new electronic and photonic device concepts and novel phys. phenomena and discuss future directions.
  4. 4
    Mak, K. F.; Shan, J. Photonics and Optoelectronics of 2D Semiconductor Transition Metal Dichalcogenides. Nat. Photonics 2016, 10, 216– 226,  DOI: 10.1038/nphoton.2015.282
    Google Scholar
    4
    Photonics and optoelectronics of 2D semiconductor transition metal dichalcogenides
    Mak, Kin Fai; Shan, Jie
    Nature Photonics (2016), 10 (4), 216-226CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
    Recent advances in the development of atomically thin layers of van der Waals bonded solids have opened up new possibilities for the exploration of 2D physics as well as for materials for applications. Among them, semiconductor transition metal dichalcogenides, MX2 (M = Mo, W; X = S, Se), have bandgaps in the near-IR to the visible region, in contrast to the zero bandgap of graphene. In the monolayer limit, these materials have been shown to possess direct bandgaps, a property well suited for photonics and optoelectronics applications. Here, we review the electronic and optical properties and the recent progress in applications of 2D semiconductor transition metal dichalcogenides with emphasis on strong excitonic effects, and spin- and valley-dependent properties.
  5. 5
    Li, L.; Guo, Y.; Sun, Y.; Yang, L.; Qin, L.; Guan, S.; Wang, J.; Qiu, X.; Li, H.; Shang, Y.; Fang, Y. A General Method for the Chemical Synthesis of Large-Scale, Seamless Transition Metal Dichalcogenide Electronics. Adv. Mater. 2018, 30, 1706215,  DOI: 10.1002/adma.201706215
    Google Scholar
    There is no corresponding record for this reference.
  6. 6
    Liu, Y.; Weiss, N. O.; Duan, X.; Cheng, H.-C.; Huang, Y.; Duan, X. van der Waals Heterostructures and Devices. Nat. Rev. Mater. 2016, 1, 16042,  DOI: 10.1038/natrevmats.2016.42
    Google Scholar
    6
    Van der Waals heterostructures and devices
    Liu, Yuan; Weiss, Nathan O.; Duan, Xidong; Cheng, Hung-Chieh; Huang, Yu; Duan, Xiangfeng
    Nature Reviews Materials (2016), 1 (9), 16042CODEN: NRMADL; ISSN:2058-8437. (Nature Publishing Group)
    Two-dimensional layered materials (2DLMs) have been a central focus of materials research since the discovery of graphene just over a decade ago. Each layer in 2DLMs consists of a covalently bonded, dangling-bond-free lattice and is weakly bound to neighboring layers by van der Waals interactions. This makes it feasible to isolate, mix and match highly disparate at. layers to create a wide range of van der Waals heterostructures (vdWHs) without the constraints of lattice matching and processing compatibility. Exploiting the novel properties in these vdWHs with diverse layering of metals, semiconductors or insulators, new designs of electronic devices emerge, including tunnelling transistors, barristors and flexible electronics, as well as optoelectronic devices, including photodetectors, photovoltaics and light-emitting devices with unprecedented characteristics or unique functionalities. We review the recent progress and challenges, and offer our perspective on the exploration of 2DLM-based vdWHs for future application in electronics and optoelectronics.
  7. 7
    Xue, H.; Wang, Y.; Dai, Y.; Kim, W.; Jussila, H.; Qi, M.; Susoma, J.; Ren, Z.; Dai, Q.; Zhao, J.; Halonen, K.; Lipsanen, H.; Wang, X.; Gan, X.; Sun, Z. A MoSe2/WSe2 Heterojunction-Based Photodetector at Telecommunication Wavelengths. Adv. Funct. Mater. 2018, 28, 1804388,  DOI: 10.1002/adfm.201804388
    Google Scholar
    There is no corresponding record for this reference.
  8. 8
    Xue, H.; Dai, Y.; Kim, W.; Wang, Y.; Bai, X.; Qi, M.; Halonen, K.; Lipsanen, H.; Sun, Z. High Photoresponsivity and Broadband Photodetection with a Band-Engineered WSe2/SnSe2 Heterostructure. Nanoscale 2019, 11, 3240– 3247,  DOI: 10.1039/C8NR09248F
    Google Scholar
    8
    High photoresponsivity and broadband photodetection with a band-engineered WSe2/SnSe2 heterostructure
    Xue, Hui; Dai, Yunyun; Kim, Wonjae; Wang, Yadong; Bai, Xueyin; Qi, Mei; Halonen, Kari; Lipsanen, Harri; Sun, Zhipei
    Nanoscale (2019), 11 (7), 3240-3247CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    Van der Waals (vdW) heterostructures formed by stacking different two-dimensional layered materials have been demonstrated as a promising platform for next-generation photonic and optoelectronic devices due to their tailorable band-engineering properties. Here, we report a high photoresponsivity and broadband photodetector based on a WSe2/SnSe2 heterostructure. By properly biasing the heterostructure, its band structure changes from near-broken band alignment to type-III band alignment which enables high photoresponsivity from visible to telecommunication wavelengths. The highest photoresponsivity and detectivity at 532 nm are ∼588 A W-1 and 4.4 × 1010 Jones and those at 1550 nm are ∼80 A W-1 and 1.4 × 1010 Jones, which are superior to those of the current state-of-the-art layered transition metal dichalcogenides based photodetectors under similar measurement conditions. Our work not only provides a new method for designing high-performance broadband photodetectors but also enables a deep understanding of the band engineering technol. in the vdW heterostructures possible for other applications, such as modulators and lasers.
  9. 9
    Sun, Z.; Hasan, T.; Torrisi, F.; Popa, D.; Privitera, G.; Wang, F.; Bonaccorso, F.; Basko, D. M.; Ferrari, A. C. Graphene Mode-Locked Ultrafast Laser. ACS Nano 2010, 4, 803– 810,  DOI: 10.1021/nn901703e
    Google Scholar
    9
    Graphene Mode-Locked Ultrafast Laser
    Sun, Zhipei; Hasan, Tawfique; Torrisi, Felice; Popa, Daniel; Privitera, Giulia; Wang, Fengqiu; Bonaccorso, Francesco; Basko, Denis M.; Ferrari, Andrea C.
    ACS Nano (2010), 4 (2), 803-810CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Graphene is at the center of a significant research effort. Near-ballistic transport at room temp. and high mobility make it a potential material for nanoelectronics. Its electronic and mech. properties are also ideal for micro- and nanomech. systems, thin-film transistors, and transparent and conductive composites and electrodes. Here we exploit the optoelectronic properties of graphene to realize an ultrafast laser. A graphene-polymer composite is fabricated using wet-chem. techniques. Pauli blocking following intense illumination results in saturable absorption, independent of wavelength. This is used to passively mode-lock an erbium-doped fiber laser working at 1559 nm, with a 5.24 nm spectral bandwidth and ∼460 fs pulse duration, paving the way to graphene-based photonics.
  10. 10
    Sun, Z.; Martinez, A.; Wang, F. Optical Modulators with 2D Layered Materials. Nat. Photonics 2016, 10, 227– 238,  DOI: 10.1038/nphoton.2016.15
    Google Scholar
    10
    Optical modulators with 2D layered materials
    Sun, Zhipei; Martinez, Amos; Wang, Feng
    Nature Photonics (2016), 10 (4), 227-238CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
    Light modulation is an essential operation in photonics and optoelectronics. With existing and emerging technologies increasingly demanding compact, efficient, fast and broadband optical modulators, high-performance light modulation solns. are becoming indispensable. The recent realization that 2D layered materials could modulate light with superior performance has prompted intense research and significant advances, paving the way for realistic applications. In this Review, we cover the state of the art of optical modulators based on 2D materials, including graphene, transition metal dichalcogenides and black phosphorus. We discuss recent advances employing hybrid structures, such as 2D heterostructures, plasmonic structures, and silicon and fiber integrated structures. We also take a look at the future perspectives and discuss the potential of yet relatively unexplored mechanisms, such as magneto-optic and acousto-optic modulation.
  11. 11
    Dai, M.; Chen, H.; Feng, R.; Feng, W.; Hu, Y.; Yang, H.; Liu, G.; Chen, X.; Zhang, J.; Xu, C.-Y.; Hu, P. A Dual-Band Multilayer InSe Self-Powered Photodetector with High Performance Induced by Surface Plasmon Resonance and Asymmetric Schottky Junction. ACS Nano 2018, 12, 8739– 8747,  DOI: 10.1021/acsnano.8b04931
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    11
    A Dual-Band Multilayer InSe Self-Powered Photodetector with High Performance Induced by Surface Plasmon Resonance and Asymmetric Schottky Junction
    Dai, Mingjin; Chen, Hongyu; Feng, Rui; Feng, Wei; Hu, Yunxia; Yang, Huihui; Liu, Guangbo; Chen, Xiaoshuang; Zhang, Jia; Xu, Cheng-Yan; Hu, PingAn
    ACS Nano (2018), 12 (8), 8739-8747CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    A dual-band self-powered photodetector (SPPD) with high sensitivity is realized by a facile combination of InSe Schottky diode and Au plasmonic nanoparticle (NP) arrays. Comparing with pristine InSe devices, InSe/Au photodetectors possess an addnl. capability of photodetection in visible to near-IR (NIR) region. This intriguing phenomenon is attributed to the wavelength selective enhancement of pristine responsivities by hybridized quadrupole plasmons resonance of Au NPs. It is worth pointing out that the max. of enhancement ratio in responsivity reaches up to ∼1200% at a wavelength of 685 nm. Owing to a large Schottky barrier difference formed between active layer and 2 asym. electrodes, the responsivities of dual-band InSe/Au photodetector could reach up to 369 and 244 mA/W at λ of 365 and 685 nm under zero bias voltage, resp. This work would provide an addnl. opportunity for developing multifunctional photodetectors with high performance based on 2-dimensional materials, upgrading their capacity of photodetection in a complex environment.
  12. 12
    Koppens, F. H. L.; Mueller, T.; Avouris, P.; Ferrari, A. C.; Vitiello, M. S.; Polini, M. Photodetectors Based on Graphene, Other Two-Dimensional Materials and Hybrid Systems. Nat. Nanotechnol. 2014, 9, 780– 793,  DOI: 10.1038/nnano.2014.215
    Google Scholar
    12
    Photodetectors based on graphene, other two-dimensional materials and hybrid systems
    Koppens, F. H. L.; Mueller, T.; Avouris, Ph.; Ferrari, A. C.; Vitiello, M. S.; Polini, M.
    Nature Nanotechnology (2014), 9 (10), 780-793CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
    A review. Graphene and other 2-dimensional materials, such as transition metal dichalcogenides, have rapidly established themselves as intriguing building blocks for optoelectronic applications, with a strong focus on various photodetection platforms. The versatility of these material systems enables their application in areas including ultrafast and ultrasensitive detection of light in the UV, visible, IR and terahertz frequency ranges. These detectors can be integrated with other photonic components based on the same material, as well as with silicon photonic and electronic technologies. Here, the authors provide an overview and evaluation of state-of-the-art photodetectors based on graphene, other 2-dimensional materials, and hybrid systems based on the combination of different 2-dimensional crystals or of 2-dimensional crystals and other (nano)materials, such as plasmonic nanoparticles, semiconductors, quantum dots, or their integration with (silicon) waveguides.
  13. 13
    Li, X.; Zhu, M.; Du, M.; Lv, Z.; Zhang, L.; Li, Y.; Yang, Y.; Yang, T.; Li, X.; Wang, K.; Zhu, H.; Fang, Y. High Detectivity Graphene-Silicon Heterojunction Photodetector. Small 2016, 12, 595– 601,  DOI: 10.1002/smll.201502336
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    13
    High Detectivity Graphene-Silicon Heterojunction Photodetector
    Li, Xinming; Zhu, Miao; Du, Mingde; Lv, Zheng; Zhang, Li; Li, Yuanchang; Yang, Yao; Yang, Tingting; Li, Xiao; Wang, Kunlin; Zhu, Hongwei; Fang, Ying
    Small (2016), 12 (5), 595-601CODEN: SMALBC; ISSN:1613-6810. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A graphene/n-type Si (n-Si) heterojunction exhibited strong rectifying behavior and high photoresponsivity, which can be used for the development of high-performance photodetectors. Graphene/n-Si heterojunction photodetectors reported previously suffer from relatively low specific detectivity due to large dark current. By introducing a thin interfacial oxide layer, the dark current of graphene/n-Si heterojunction was reduced by 2 orders of magnitude at zero bias. At room temp., the graphene/n-Si photodetector with interfacial oxide exhibits a specific detectivity ≤5.77 × 1013 cm Hz1/2 W-1 at peak λ = 890 nm in vacuum, which is highest reported detectivity at room temp. for planar graphene/Si heterojunction photodetectors. The improved graphene/n-Si heterojunction photodetectors possess high responsivity of 0.73 A W-1 and high photo-to-dark current ratio of ∼107. The current noise spectral d. of the graphene/n-Si photodetector was characterized under ambient and vacuum conditions, which shows that the dark current can be further suppressed in vacuum. Graphene/Si heterojunction with interfacial oxide is promising for the development of high detectivity photodetectors.
  14. 14
    Zhao, M.; Xia, W.; Wang, Y.; Luo, M.; Tian, Z.; Guo, Y.; Hu, W.; Xue, J. Nb2SiTe4: A Stable Narrow-Gap Two-Dimensional Material with Ambipolar Transport and Mid-Infrared Response. ACS Nano 2019, 13, 10705– 10710,  DOI: 10.1021/acsnano.9b05080
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    14
    Nb2SiTe4: A Stable Narrow-Gap Two-Dimensional Material with Ambipolar Transport and Mid-Infrared Response
    Zhao, Mingxing; Xia, Wei; Wang, Yang; Luo, Man; Tian, Zhen; Guo, Yanfeng; Hu, Weida; Xue, Jiamin
    ACS Nano (2019), 13 (9), 10705-10710CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Two-dimensional (2D) materials with narrow band gaps (∼0.3 eV) are of great importance for realizing ambipolar transistors and mid-IR (MIR) detections. However, most of the 2D materials studied to date have band gaps that are too large. A few of the materials with suitable band gaps are not stable under ambient conditions. In this study, the layered Nb2SiTe4 is shown to be a stable 2D material with a band gap of 0.39 eV. Field-effect transistors based on few-layer Nb2SiTe4 show ambipolar transport with a similar magnitude of electron and hole current and a high charge-carrier mobility of ∼100 cm2 V-1 s-1 at room temp. Optoelectronic measurements of the devices show clear response to an MIR wavelength of 3.1 μm with a high responsivity of ∼0.66 AW-1. These results establish Nb2SiTe4 as a good candidate for ambipolar devices and MIR detection.
  15. 15
    Li, L.; Kim, J.; Jin, C.; Ye, G. J.; Qiu, D. Y.; da Jornada, F. H.; Shi, Z.; Chen, L.; Zhang, Z.; Yang, F.; Watanabe, K.; Taniguchi, T.; Ren, W.; Louie, S. G.; Chen, X. H.; Zhang, Y.; Wang, F. Direct Observation of the Layer-Dependent Electronic Structure in Phosphorene. Nat. Nanotechnol. 2017, 12, 21– 25,  DOI: 10.1038/nnano.2016.171
    Google Scholar
    15
    Direct observation of the layer-dependent electronic structure in phosphorene
    Li, Likai; Kim, Jonghwan; Jin, Chenhao; Ye, Guo Jun; Qiu, Diana Y.; da Jornada, Felipe H.; Shi, Zhiwen; Chen, Long; Zhang, Zuocheng; Yang, Fangyuan; Watanabe, Kenji; Taniguchi, Takashi; Ren, Wencai; Louie, Steven G.; Chen, Xian Hui; Zhang, Yuanbo; Wang, Feng
    Nature Nanotechnology (2017), 12 (1), 21-25CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
    Phosphorene, a single at. layer of black phosphorus, has recently emerged as a new two-dimensional (2D) material that holds promise for electronic and photonic technologies. Here, we exptl. demonstrate that the electronic structure of few-layer phosphorene varies significantly with the no. of layers, in good agreement with theor. predictions. The interband optical transitions cover a wide, technol. important spectral range from the visible to the mid-IR. In addn., we observe strong photoluminescence in few-layer phosphorene at energies that closely match the absorption edge, indicating that they are direct bandgap semiconductors. The strongly layer-dependent electronic structure of phosphorene, in combination with its high elec. mobility, gives it distinct advantages over other 2D materials in electronic and opto-electronic applications.
  16. 16
    Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F. Atomically Thin MoS2: A New Direct-Gap Semiconductor. Phys. Rev. Lett. 2010, 105, 136805,  DOI: 10.1103/PhysRevLett.105.136805
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    16
    Atomically Thin MoS2. A New Direct-Gap Semiconductor
    Mak, Kin Fai; Lee, Changgu; Hone, James; Shan, Jie; Heinz, Tony F.
    Physical Review Letters (2010), 105 (13), 136805/1-136805/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    The electronic properties of ultrathin crystals of MoS2 consisting of N = 1, 2,...,6 S-Mo-S monolayers were investigated by optical spectroscopy. Through characterization by absorption, photoluminescence, and photocond. spectroscopy, we trace the effect of quantum confinement on the material's electronic structure. With decreasing thickness, the indirect band gap, which lies below the direct gap in the bulk material, shifts upwards in energy by >0.6 eV. This leads to a crossover to a direct-gap material in the limit of the single monolayer. Unlike the bulk material, the MoS2 monolayer emits light strongly. The freestanding monolayer exhibits an increase in luminescence quantum efficiency by more than a factor of 104 compared with the bulk material.
  17. 17
    Bandurin, D. A.; Tyurnina, A. V.; Yu, G. L.; Mishchenko, A.; Zólyomi, V.; Morozov, S. V.; Kumar, R. K.; Gorbachev, R. V.; Kudrynskyi, Z. R.; Pezzini, S.; Kovalyuk, Z. D.; Zeitler, U.; Novoselov, K. S.; Patanè, A.; Eaves, L.; Grigorieva, I. V.; Fal’ko, V. I.; Geim, A. K.; Cao, Y. High Electron Mobility, Quantum Hall Effect and Anomalous Optical Response in Atomically Thin InSe. Nat. Nanotechnol. 2017, 12, 223– 227,  DOI: 10.1038/nnano.2016.242
    Google Scholar
    17
    High electron mobility, quantum Hall effect and anomalous optical response in atomically thin InSe
    Bandurin, Denis A.; Tyurnina, Anastasia V.; Yu, Geliang L.; Mishchenko, Artem; Zolyomi, Viktor; Morozov, Sergey V.; Kumar, Roshan Krishna; Gorbachev, Roman V.; Kudrynskyi, Zakhar R.; Pezzini, Sergio; Kovalyuk, Zakhar D.; Zeitler, Uli; Novoselov, Konstantin S.; Patane, Amalia; Eaves, Laurence; Grigorieva, Irina V.; Fal'ko, Vladimir I.; Geim, Andre K.; Cao, Yang
    Nature Nanotechnology (2017), 12 (3), 223-227CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
    A decade of intense research on two-dimensional (2D) at. crystals has revealed that their properties can differ greatly from those of the parent compd. These differences are governed by changes in the band structure due to quantum confinement and are most profound if the underlying lattice symmetry changes. Here we report a high-quality 2D electron gas in few-layer InSe encapsulated in hexagonal boron nitride under an inert atm. Carrier mobilities are found to exceed 103 cm2 V-1 s-1 and 104 cm2 V-1 s-1 at room and liq.-helium temps., resp., allowing the observation of the fully developed quantum Hall effect. The conduction electrons occupy a single 2D subband and have a small effective mass. Photoluminescence spectroscopy reveals that the bandgap increases by more than 0.5 eV with decreasing the thickness from bulk to bilayer InSe. The band-edge optical response vanishes in monolayer InSe, which is attributed to the monolayer's mirror-plane symmetry. Encapsulated 2D InSe expands the family of graphene-like semiconductors and, in terms of quality, is competitive with atomically thin dichalcogenides and black phosphorus.
  18. 18
    Chhowalla, M.; Jena, D.; Zhang, H. Two-Dimensional Semiconductors for Transistors. Nat. Rev. Mater. 2016, 1, 16052,  DOI: 10.1038/natrevmats.2016.52
    Google Scholar
    18
    Two-dimensional semiconductors for transistors
    Chhowalla, Manish; Jena, Debdeep; Zhang, Hua
    Nature Reviews Materials (2016), 1 (11), 16052CODEN: NRMADL; ISSN:2058-8437. (Nature Publishing Group)
    In the quest for higher performance, the dimensions of field-effect transistors (FETs) continue to decrease. However, the redn. in size of FETs comprising 3D semiconductors is limited by the rate at which heat, generated from static power, is dissipated. The increase in static power and the leakage of current between the source and drain electrodes that causes this increase, are referred to as short-channel effects. In FETs with channels made from 2D semiconductors, leakage current is almost eliminated because all electrons are confined in atomically thin channels and, hence, are uniformly influenced by the gate voltage. In this Review, we provide a math. framework to evaluate the performance of FETs and describe the challenges for improving the performances of short-channel FETs in relation to the properties of 2D materials, including graphene, transition metal dichalcogenides, phosphorene and silicene. We also describe tunnelling FETs that possess extremely low-power switching behavior and explain how they can be realized using heterostructures of 2D semiconductors.
  19. 19
    Wu, P.; Ameen, T.; Zhang, H.; Bendersky, L. A.; Ilatikhameneh, H.; Klimeck, G.; Rahman, R.; Davydov, A. V.; Appenzeller, J. Complementary Black Phosphorus Tunneling Field-Effect Transistors. ACS Nano 2019, 13, 377– 385,  DOI: 10.1021/acsnano.8b06441
    Google Scholar
    19
    Complementary black phosphorus tunneling field-effect transistors
    Wu, Peng; Ameen, Tarek; Zhang, Huairuo; Bendersky, Leonid A.; Ilatikhameneh, Hesameddin; Klimeck, Gerhard; Rahman, Rajib; Davydov, Albert V.; Appenzeller, Joerg
    ACS Nano (2019), 13 (1), 377-385CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Band-to-band tunneling field-effect transistors (TFETs) have emerged as promising candidates for low-power integration circuits beyond conventional metal-oxide-semiconductor field-effect transistors (MOSFETs) and have been demonstrated to overcome the thermionic limit, which results intrinsically in sub-threshold swings of at least 60 mV/dec at room temp. Here, we demonstrate complementary TFETs based on few-layer black phosphorus, in which multiple top gates create electrostatic doping in the source and drain regions. By elec. tuning the doping types and levels in the source and drain regions, the device can be reconfigured to allow for TFET or MOSFET operation and can be tuned to be n-type or p-type. Owing to the proper choice of materials and careful engineering of device structures, record-high current densities have been achieved in 2D TFETs. Full-band atomistic quantum transport simulations of the fabricated devices agree quant. with the current-voltage measurements, which gives credibility to the promising simulation results of ultrascaled phosphorene TFETs. Using atomistic simulations, we project substantial improvements in the performance of the fabricated TFETs when channel thicknesses and oxide thicknesses are scaled down.
  20. 20
    Huang, L.; Dong, B.; Guo, X.; Chang, Y.; Chen, N.; Huang, X.; Liao, W.; Zhu, C.; Wang, H.; Lee, C.; Ang, K.-W. Waveguide-Integrated Black Phosphorus Photodetector for Mid-Infrared Applications. ACS Nano 2019, 13, 913– 921,  DOI: 10.1021/acsnano.8b08758
    Google Scholar
    20
    Waveguide-Integrated Black Phosphorus Photodetector for Mid-Infrared Applications
    Huang, Li; Dong, Bowei; Guo, Xin; Chang, Yuhua; Chen, Nan; Huang, Xin; Liao, Wugang; Zhu, Chunxiang; Wang, Hong; Lee, Chengkuo; Ang, Kah-Wee
    ACS Nano (2019), 13 (1), 913-921CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    An integration of Si-on-insulator (SOI) waveguides with black P (BP) photodetectors was realized. When operating near BP's cutoff wavelength where absorption is weak, the light-BP interaction is enhanced by exploiting the optical confinement in the Si waveguide and grating structure to overcome the limitation of absorption length constrained by the BP thickness. Devices with different BP crystal orientation and thickness are compared in terms of their responsivity and noise equiv. power (NEP). Spectral photoresponse at 3.68-4.03 μm was studied. Power-dependent responsivity and gate-tunable photocurrent were studied. At a bias of 1 V, the BP photodetector achieved a responsivity of 23 A/W at 3.68 μm and 2 A/W at 4 μm and a NEP <1 nW/Hz1/2 at room temp. The integration of passive Si photonics and active BP photodetector is envisaged to offer a potential pathway toward the realization of integrated on-chip systems for MIR sensing applications.
  21. 21
    Rao, G.; Wang, X.; Wang, Y.; Wangyang, P.; Yan, C.; Chu, J.; Xue, L.; Gong, C.; Huang, J.; Xiong, J.; Li, Y. Two-Dimensional Heterostructure Promoted Infrared Photodetection Devices. InfoMat 2019, 1, 272– 288,  DOI: 10.1002/inf2.12018
    Google Scholar
    21
    Two-dimensional heterostructure promoted infrared photodetection devices
    Rao, Gaofeng; Wang, Xuepeng; Wang, Yang; Wangyang, Peihua; Yan, Chaoyi; Chu, Junwei; Xue, Lanxin; Gong, Chuanhui; Huang, Jianwen; Xiong, Jie; Li, Yanrong
    InfoMat (2019), 1 (3), 272-288CODEN: INFOHH; ISSN:2567-3165. (John Wiley & Sons Australia, Ltd.)
    It is a rapidly developed subject in expanding the fundamental properties and application of two-dimensional (2D) materials. The weak van der Waals interaction in 2D materials inspired researchers to explore 2D heterostructures (2DHs) based broadband photodetectors in the far-IR (IR) and middle-IR regions with high response and high detectivity. This review focuses on the strategy and motivation of designing 2DHs based high-performance IR photodetectors, which provides a wide view of this field and new expectation for advanced photodetectors. First, the photocarriers' generation mechanism and frequently employed device structures are presented. Then, the 2DHs are divided into semimetal/semiconductor 2DHs, semiconductor/semiconductor 2DHs, and multidimensional semi-2DHs; the advantages, motivation, mechanism, recent progress, and outlook are discussed. Finally, the challenges for next-generation photodetectors are described for this rapidly developing field.
  22. 22
    Buscema, M.; Island, J. O.; Groenendijk, D. J.; Blanter, S. I.; Steele, G. A.; van der Zant, H. S. J.; Castellanos-Gomez, A. Photocurrent Generation with Two-Dimensional van der Waals Semiconductors. Chem. Soc. Rev. 2015, 44, 3691– 3718,  DOI: 10.1039/C5CS00106D
    Google Scholar
    22
    Photocurrent generation with two-dimensional van der Waals semiconductors
    Buscema, Michele; Island, Joshua O.; Groenendijk, Dirk J.; Blanter, Sofya I.; Steele, Gary A.; van der Zant, Herre S. J.; Castellanos-Gomez, Andres
    Chemical Society Reviews (2015), 44 (11), 3691-3718CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    Two-dimensional (2D) materials have attracted a great deal of interest in recent years. This family of materials allows for the realization of versatile electronic devices and holds promise for next-generation (opto)electronics. Their electronic properties strongly depend on the no. of layers, making them interesting from a fundamental standpoint. For electronic applications, semiconducting 2D materials benefit from sizable mobilities and large on/off ratios, due to the large modulation achievable via the gate field-effect. Moreover, being mech. strong and flexible, these materials can withstand large strain (>10%) before rupture, making them interesting for strain engineering and flexible devices. Even in their single layer form, semiconducting 2D materials have demonstrated efficient light absorption, enabling large responsivity in photodetectors. Therefore, semiconducting layered 2D materials are strong candidates for optoelectronic applications, esp. for photodetection. Here, we review the state-of-the-art in photodetectors based on semiconducting 2D materials, focusing on the transition metal dichalcogenides, novel van der Waals materials, black phosphorus, and heterostructures.
  23. 23
    Geim, A. K.; Grigorieva, I. V. van der Waals Heterostructures. Nature 2013, 499, 419– 425,  DOI: 10.1038/nature12385
    Google Scholar
    23
    Van der Waals heterostructures
    Geim, A. K.; Grigorieva, I. V.
    Nature (London, United Kingdom) (2013), 499 (7459), 419-425CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
    A review. Research on graphene and other two-dimensional at. crystals is intense and is likely to remain one of the leading topics in condensed matter physics and materials science for many years. Looking beyond this field, isolated at. planes can also be reassembled into designer heterostructures made layer by layer in a precisely chosen sequence. The first, already remarkably complex, such heterostructures (often referred to as van der Waals') have recently been fabricated and investigated, revealing unusual properties and new phenomena. Here we review this emerging research area and identify possible future directions. With steady improvement in fabrication techniques and using graphene's springboard, van der Waals heterostructures should develop into a large field of their own.
  24. 24
    Yang, T.; Zheng, B.; Wang, Z.; Xu, T.; Pan, C.; Zou, J.; Zhang, X.; Qi, Z.; Liu, H.; Feng, Y.; Hu, W.; Miao, F.; Sun, L.; Duan, X.; Pan, A. van der Waals Epitaxial Growth and Optoelectronics of Large-Scale WSe2/SnS2 Vertical Bilayer p-n Junctions. Nat. Commun. 2017, 8, 1906,  DOI: 10.1038/s41467-017-02093-z
    Google Scholar
    24
    Van der Waals epitaxial growth and optoelectronics of large-scale WSe2/SnS2 vertical bilayer p-n junctions
    Yang Tiefeng; Zheng Biyuan; Zou Juan; Zhang Xuehong; Qi Zhaoyang; Liu Hongjun; Feng Yexin; Pan Anlian; Wang Zhen; Hu Weida; Xu Tao; Sun Litao; Pan Chen; Miao Feng; Duan Xiangfeng
    Nature communications (2017), 8 (1), 1906 ISSN:.
    High-quality two-dimensional atomic layered p-n heterostructures are essential for high-performance integrated optoelectronics. The studies to date have been largely limited to exfoliated and restacked flakes, and the controlled growth of such heterostructures remains a significant challenge. Here we report the direct van der Waals epitaxial growth of large-scale WSe2/SnS2 vertical bilayer p-n junctions on SiO2/Si substrates, with the lateral sizes reaching up to millimeter scale. Multi-electrode field-effect transistors have been integrated on a single heterostructure bilayer. Electrical transport measurements indicate that the field-effect transistors of the junction show an ultra-low off-state leakage current of 10(-14) A and a highest on-off ratio of up to 10(7). Optoelectronic characterizations show prominent photoresponse, with a fast response time of 500 μs, faster than all the directly grown vertical 2D heterostructures. The direct growth of high-quality van der Waals junctions marks an important step toward high-performance integrated optoelectronic devices and systems.
  25. 25
    Bai, X.; Li, S.; Das, S.; Du, L.; Dai, Y.; Yao, L.; Raju, R.; Du, M.; Lipsanen, H.; Sun, Z. Single-Step Chemical Vapour Deposition of Anti-Pyramid MoS2/WS2 Vertical Heterostructures. Nanoscale 2021, 13, 4537– 4542,  DOI: 10.1039/D0NR08281C
    Google Scholar
    25
    Single-step chemical vapour deposition of anti-pyramid MoS2/WS2 vertical heterostructures
    Bai, Xueyin; Li, Shisheng; Das, Susobhan; Du, Luojun; Dai, Yunyun; Yao, Lide; Raju, Ramesh; Du, Mingde; Lipsanen, Harri; Sun, Zhipei
    Nanoscale (2021), 13 (8), 4537-4542CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    Van der Waals heterostructures are the fundamental building blocks of electronic and optoelectronic devices. Here we report that, through a single-step chem. vapor deposition (CVD) process, high-quality vertical bilayer MoS2/WS2 heterostructures with a grain size up to ~ 60μm can be synthesized from molten salt precursors, Na2MoO4 and Na2WO4. Instead of normal pyramid vertical heterostructures grown by CVD, this method synthesizes an anti-pyramid MoS2/WS2 structure, which is characterized by Raman, photoluminescence and second harmonic generation microscopy. Our facile CVD strategy for synthesizing anti-pyramid structures unveils a new synthesis route for the products of two-dimensional heterostructures and their devices for application.
  26. 26
    Du, M.; Du, L.; Wei, N.; Liu, W.; Bai, X.; Sun, Z. Dual-Gated Mono-Bilayer Graphene Junctions. Nanoscale Adv. 2021, 3, 399– 406,  DOI: 10.1039/D0NA00547A
    Google Scholar
    26
    Dual-gated mono-bilayer graphene junctions
    Du, Mingde; Du, Luojun; Wei, Nan; Liu, Wei; Bai, Xueyin; Sun, Zhipei
    Nanoscale Advances (2021), 3 (2), 399-406CODEN: NAADAI; ISSN:2516-0230. (Royal Society of Chemistry)
    A lateral junction with an atomically sharp interface is extensively studied in fundamental research and plays a key role in the development of electronics, photonics and optoelectronics. Here, we demonstrate an elec. tunable lateral junction at atomically sharp interfaces between dual-gated mono- and bilayer graphene. The transport properties of the mono-bilayer graphene interface are systematically investigated with Ids-Vds curves and transfer curves, which are measured with bias voltage Vds applied in opposite directions across the asym. mono-bilayer interface. Nearly 30% difference between the output Ids-Vds curves of graphene channels measured at opposite Vds directions is obsd. Furthermore, the measured transfer curves confirm that the conductance difference of graphene channels greatly depends on the doping level, which is detd. by dual-gating. The Vds direction dependent conductance difference indicates the existence of a gate tunable junction in the mono-bilayer graphene channel, due to different band structures of monolayer graphene with zero bandgap and bilayer graphene with a bandgap opened by dual-gating. Simulation of the Ids-Vds curves based on a new numerical model validates the gate tunable junction at the mono-bilayer graphene interface from another point of view. The dual-gated mono-bilayer graphene junction and new protocol for Ids-Vds curve simulation pave a possible way for functional applications of graphene in next-generation electronics.
  27. 27
    Liu, Y.; Huang, Y.; Duan, X. van der Waals Integration before and Beyond Two-Dimensional Materials. Nature 2019, 567, 323– 333,  DOI: 10.1038/s41586-019-1013-x
    Google Scholar
    27
    Van der Waals integration before and beyond two-dimensional materials
    Liu, Yuan; Huang, Yu; Duan, Xiangfeng
    Nature (London, United Kingdom) (2019), 567 (7748), 323-333CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
    A review. Material integration strategies, such as epitaxial growth, usually involve strong chem. bonds and are typically limited to materials with strict structure matching and processing compatibility. Van der Waals integration, in which pre-fabricated building blocks are phys. assembled together through weak van der Waals interactions, offers an alternative bond-free integration strategy without lattice and processing limitations, as exemplified by 2-dimensional van der Waals heterostructures. Here, the authors review the development, challenges and opportunities of this emerging approach, generalizing it for flexible integration of diverse material systems beyond two dimensions, and discuss its potential for creating artificial heterostructures or superlattices beyond the reach of existing materials.
  28. 28
    Xu, Y.; Cheng, C.; Du, S.; Yang, J.; Yu, B.; Luo, J.; Yin, W.; Li, E.; Dong, S.; Ye, P.; Duan, X. Contacts between Two- and Three-Dimensional Materials: Ohmic, Schottky, and p-n Heterojunctions. ACS Nano 2016, 10, 4895– 4919,  DOI: 10.1021/acsnano.6b01842
    Google Scholar
    28
    Contacts between Two- and Three-Dimensional Materials: Ohmic, Schottky, and p-n Heterojunctions
    Xu, Yang; Cheng, Cheng; Du, Sichao; Yang, Jianyi; Yu, Bin; Luo, Jack; Yin, Wenyan; Li, Erping; Dong, Shurong; Ye, Peide; Duan, Xiangfeng
    ACS Nano (2016), 10 (5), 4895-4919CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    A review. After a decade of intensive research on two-dimensional (2D) materials inspired by the discovery of graphene, the field of 2D electronics has reached a stage with booming materials and device architectures. However, the efficient integration of 2D functional layers with three-dimensional (3D) systems remains a significant challenge, limiting device performance and circuit design. In this review, the exptl. efforts in interfacing 2D layers with 3D materials is investigated and the properties of the heterojunctions formed between them analyzed. The contact resistivity of metal on graphene and related 2D materials deserves special attention, while the Schottky junctions formed between metal/2D semiconductor or graphene/3D semiconductor call for careful reconsideration of the phys. models describing the junction behavior. The combination of 2D and 3D semiconductors presents a form of p-n junctions that have just marked their debut. For each type of the heterojunctions, the potential applications are reviewed briefly.
  29. 29
    Wei, W.; Yang, S.; Wang, G.; Zhang, T.; Pan, W.; Cai, Z.; Yang, Y.; Zheng, L.; He, P.; Wang, L.; Baktash, A.; Zhang, Q.; Liu, L.; Wang, Y.; Ding, G.; Kang, Z.; Yakobson, B. I.; Searles, D. J.; Yuan, Q. Bandgap Engineering of Two-Dimensional C3N Bilayers. Nat. Electron. 2021, 4, 486– 494,  DOI: 10.1038/s41928-021-00602-z
    Google Scholar
    29
    Bandgap engineering of two-dimensional C3N bilayers
    Wei, Wenya; Yang, Siwei; Wang, Gang; Zhang, Teng; Pan, Wei; Cai, Zenghua; Yang, Yucheng; Zheng, Li; He, Peng; Wang, Lei; Baktash, Ardeshir; Zhang, Quanzhen; Liu, Liwei; Wang, Yeliang; Ding, Guqiao; Kang, Zhenhui; Yakobson, Boris I.; Searles, Debra J.; Yuan, Qinghong
    Nature Electronics (2021), 4 (7), 486-494CODEN: NEALB3; ISSN:2520-1131. (Nature Portfolio)
    Abstr.: Carbon materials such as graphene are of potential use in the development of electronic devices because of properties such as high mech. strength and elec. and thermal cond. However, tech. challenges, including difficulties in generating and modulating bandgaps, have limited the application of such materials. Here we show that the bandgaps of bilayers of two-dimensional C3N can be engineered by controlling the stacking order or applying an elec. field. AA' stacked C3N bilayers are found to have a smaller bandgap (0.30 eV) than AB' stacked bilayers (0.89 eV), and both bandgaps are lower than that of monolayer C3N (1.23 eV). The larger bandgap redn. obsd. in AA' stacked bilayers, compared with AB' stacked bilayers, is attributed to the greater pz-orbital overlap. By applying an elec. field of ∼1.4 V nm-1, a bandgap modulation of around 0.6 eV can be achieved in the AB' structure. We also show that the C3N bilayers can offer controllable on/off ratios, high carrier mobilities and photoelec. detection capabilities.
  30. 30
    Frisenda, R.; Molina-Mendoza, A. J.; Mueller, T.; Castellanos-Gomez, A.; van der Zant, H. S. J. Atomically Thin p-n Junctions Based on Two-Dimensional Materials. Chem. Soc. Rev. 2018, 47, 3339– 3358,  DOI: 10.1039/C7CS00880E
    Google Scholar
    30
    Atomically thin p-n junctions based on two-dimensional materials
    Frisenda, Riccardo; Molina-Mendoza, Aday J.; Mueller, Thomas; Castellanos-Gomez, Andres; van der Zant, Herre S. J.
    Chemical Society Reviews (2018), 47 (9), 3339-3358CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    A review. Recent research in two-dimensional (2D) materials has boosted a renovated interest in the p-n junction, one of the oldest elec. components which can be used in electronics and optoelectronics. 2D materials offer remarkable flexibility to design novel p-n junction device architectures, not possible with conventional bulk semiconductors. In this Review we thoroughly describe the different 2D p-n junction geometries studied so far, focusing on vertical (out-of-plane) and lateral (in-plane) 2D junctions and on mixed-dimensional junctions. We discuss the assembly methods developed to fabricate 2D p-n junctions making a distinction between top-down and bottom-up approaches. We also revise the literature studying the different applications of these atomically thin p-n junctions in electronic and optoelectronic devices. We discuss expts. on 2D p-n junctions used as current rectifiers, photodetectors, solar cells and light emitting devices. The important electronics and optoelectronics parameters of the discussed devices are listed in a table to facilitate their comparison. We conclude the Review with a crit. discussion about the future outlook and challenges of this incipient research field.
  31. 31
    Han, G. H.; Duong, D. L.; Keum, D. H.; Yun, S. J.; Lee, Y. H. van der Waals Metallic Transition Metal Dichalcogenides. Chem. Rev. 2018, 118, 6297– 6336,  DOI: 10.1021/acs.chemrev.7b00618
    Google Scholar
    31
    van der Waals metallic transition metal dichalcogenides
    Han, Gang Hee; Duong, Dinh Loc; Keum, Dong Hoon; Yun, Seok Joon; Lee, Young Hee
    Chemical Reviews (Washington, DC, United States) (2018), 118 (13), 6297-6336CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    Transition metal dichalcogenides are layered materials which are composed of transition metals and chalcogens of the group VIA in a 1:2 ratio. These layered materials have been extensively investigated over synthesis and optical and elec. properties for several decades. It can be insulators, semiconductors, or metals revealing all types of condensed matter properties from a magnetic lattice distorted to superconducting characteristics. Some of these also feature the topol. manner. Instead of covering the semiconducting properties of transition metal dichalcogenides, which have been extensively revisited and reviewed elsewhere, here we present the structures of metallic transition metal dichalcogenides and their synthetic approaches for not only high-quality wafer-scale samples using conventional methods (e.g., chem. vapor transport, chem. vapor deposition) but also local small areas by a modification of the materials using Li intercalation, electron beam irradn., light illumination, pressures, and strains. Some representative band structures of metallic transition metal dichalcogenides and their strong layer-dependence are reviewed and updated, both in theor. calcns. and expts. In addn., we discuss the phys. properties of metallic transition metal dichalcogenides such as periodic lattice distortion, magnetoresistance, supercond., topol. insulator, and Weyl semimetal. Approaches to overcome current challenges related to these materials are also proposed.
  32. 32
    Wang, B.; Xia, W.; Li, S.; Wang, K.; Yang, S. A.; Guo, Y.; Xue, J. One-Dimensional Metal Embedded in Two-Dimensional Semiconductor in Nb2Six-1Te4. ACS Nano 2021, 15, 7149– 7154,  DOI: 10.1021/acsnano.1c00320
    Google Scholar
    32
    One-Dimensional Metal Embedded in Two-Dimensional Semiconductor in Nb2Six-1Te4
    Wang, Binbin; Xia, Wei; Li, Si; Wang, Kang; Yang, Shengyuan A.; Guo, Yanfeng; Xue, Jiamin
    ACS Nano (2021), 15 (4), 7149-7154CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    The ternary van der Waals material Nb2Six-1Te4 demonstrates many interesting properties as the content of Si is changed, ranging from metallic Nb3SiTe6 (x = 5/3) to narrow-gap semiconductor Nb2SiTe4 (x = 2) and with the emergence of 1-dimensional Dirac fermion excitations in between. An in-depth understanding of their properties with different stoichiometry is important. Here the authors use scanning tunneling microscopy and spectroscopy to reveal that Nb2Six-1Te4 is a system with spontaneously developed and self-aligned 1-dimensional metallic chains embedded in a 2-dimensional semiconductor. Electron quasiparticles form 1- and 2-dimensional standing waves side by side. This special microscopic structure results in strong transport anisotropy. Along the chain direction the material behaves like a metal, while perpendicular to the chain direction, it behaves like a semiconductor. These findings provide an important basis for further study of this intriguing system.
  33. 33
    Lebègue, S.; Björkman, T.; Klintenberg, M.; Nieminen, R. M.; Eriksson, O. Two-Dimensional Materials from Data Filtering and Ab Initio Calculations. Phys. Rev. X 2013, 3, 031002,  DOI: 10.1103/PhysRevX.3.031002
    Google Scholar
    33
    Two-dimensional materials from data filtering and Ab initio calculations
    Lebegue, S.; Bjorkman, T.; Klintenberg, M.; Nieminen, R. M.; Eriksson, O.
    Physical Review X (2013), 3 (3), 031002CODEN: PRXHAE; ISSN:2160-3308. (American Physical Society)
    Progress in materials science depends on the ability to discover new materials and to obtain and understand their properties. This has recently become particularly apparent for compds. with reduced dimensionality, which often display unexpected phys. and chem. properties, making them very attractive for applications in electronics, graphene being so far the most noteworthy example. Here, we report some previously unknown two-dimensional materials and their electronic structure by data mining among crystal structures listed in the International Crystallog. Structural Database, combined with d.-functional-theory calcns. As a result, we propose to explore the synthesis of a large group of two-dimensional materials, with properties suggestive of applications in nanoscale devices and anticipate further studies of electronic and magnetic phenomena in low-dimensional systems.
  34. 34
    Zhang, C.; Gong, C.; Nie, Y.; Min, K.-A.; Liang, C.; Oh, Y. J.; Zhang, H.; Wang, W.; Hong, S.; Colombo, L.; Wallace, R. M.; Cho, K. Systematic Study of Electronic Structure and Band Alignment of Monolayer Transition Metal Dichalcogenides in van der Waals Heterostructures. 2D Mater. 2017, 4, 015026,  DOI: 10.1088/2053-1583/4/1/015026
    Google Scholar
    34
    Systematic study of electronic structure and band alignment of monolayer transition metal dichalcogenides in Van der Waals heterostructures
    Zhang, Chenxi; Gong, Cheng; Nie, Yifan; Min, Kyung-Ah; Liang, Chaoping; Oh, Young Jun; Zhang, Hengji; Wang, Weihua; Hong, Suklyun; Colombo, Luigi; Wallace, Robert M.; Cho, Kyeongjae
    2D Materials (2017), 4 (1), 015026/1-015026/10CODEN: DMATB7; ISSN:2053-1583. (IOP Publishing Ltd.)
    Two-dimensional transition metal dichalcogenides (TMDs) are promising low-dimensional materials which can produce diverse electronic properties and band alignment in van der Waals heterostructures. Systematic d. functional theory (DFT) calcns. are performed for 24 different TMDmonolayers and their bilayer heterostacks. DFT calcns. show that monolayer TMDs can behave as semiconducting, metallic or semimetallic depending on their structures; we also calcd. the band alignment of the TMDs to predict their alignment in van der Waals heterostacks. We have applied the charge equilibration model (CEM) to obtain a quant. formula predicting the highest occupied state of any type of bilayerTMDheterostacks (552 pairs for 24 TMDs). The CEM predicted values agree quite well with the selected DFT simulation results. The quant. prediction of the band alignment in the TMD heterostructures can provide an insightful guidance to the development of TMD-based devices.
  35. 35
    Wu, R.; Tao, Q.; Dang, W.; Liu, Y.; Li, B.; Li, J.; Zhao, B.; Zhang, Z.; Ma, H.; Sun, G.; Duan, X.; Duan, X. van der Waals Epitaxial Growth of Atomically Thin 2D Metals on Dangling-Bond-Free WSe2 and WS2. Adv. Funct. Mater. 2019, 29, 1806611,  DOI: 10.1002/adfm.201806611
    Google Scholar
    There is no corresponding record for this reference.
  36. 36
    Li, J.; Zhao, B.; Chen, P.; Wu, R.; Li, B.; Xia, Q.; Guo, G.; Luo, J.; Zang, K.; Zhang, Z.; Ma, H.; Sun, G.; Duan, X.; Duan, X. Synthesis of Ultrathin Metallic MTe2 (M = V, Nb, Ta) Single-Crystalline Nanoplates. Adv. Mater. 2018, 30, 1801043,  DOI: 10.1002/adma.201801043
    Google Scholar
    There is no corresponding record for this reference.
  37. 37
    Zhang, Z.; Niu, J.; Yang, P.; Gong, Y.; Ji, Q.; Shi, J.; Fang, Q.; Jiang, S.; Li, H.; Zhou, X.; Gu, L.; Wu, X.; Zhang, Y. van der Waals Epitaxial Growth of 2D Metallic Vanadium Diselenide Single Crystals and Their Extra-High Electrical Conductivity. Adv. Mater. 2017, 29, 1702359,  DOI: 10.1002/adma.201702359
    Google Scholar
    There is no corresponding record for this reference.
  38. 38
    Zhao, S.; Hotta, T.; Koretsune, T.; Watanabe, K.; Taniguchi, T.; Sugawara, K.; Takahashi, T.; Shinohara, H.; Kitaura, R. Two-Dimensional Metallic NbS2: Growth, Optical Identification and Transport Properties. 2D Mater. 2016, 3, 025027,  DOI: 10.1088/2053-1583/3/2/025027
    Google Scholar
    38
    Two-dimensional metallic NbS2: growth, optical identification and transport properties
    Zhao, Sihan; Hotta, Takato; Koretsune, Takashi; Watanabe, Kenji; Taniguchi, Takashi; Sugawara, Katsuaki; Takahashi, Takashi; Shinohara, Hisanori; Kitaura, Ryo
    2D Materials (2016), 3 (2), 025027/1-025027/9CODEN: DMATB7; ISSN:2053-1583. (IOP Publishing Ltd.)
    Progress on researches of two-dimensional (2D) metals strongly relies on development of the growth technique. Studies on prepn. of 2D metals have so far been limited, and this is in stark contrast to the situation of 2D semiconductors, where various layered semiconductors, including MoS2,WS2, MoSe2, WSe2, have been isolated in its monolayer form. In this work, we have developed a facile method to prep. 2D metallic transition metal dichalcogenides (TMDCs) by chem. vapor deposition (CVD) method, where direct growth of few-layered NbS2 (3R phase) on atomically flat hexagonal boron nitride (hBN) has been demonstrated. Structural characterization of the so-grown NbS2 was performed with at. force microscopy, optical microscopy, electron microscopy and optical spectroscopy, revealing that the utilization of hBN as growth substrates is a key factor for the first successful CVD growth of 2D metallic TMDCs with large single-domain size (several μm). Elec. transport measurements have clearly shown that NbS2 at. layers down to few-layer-thickness are metal. The current study opens up a new synthetic route for controllable growth of 2D layered metallic materials, which is of great importance in study of rich physics in 2D metals, as well as in search for novel 2D superconductors.
  39. 39
    Zhang, Y.; Yin, L.; Chu, J.; Shifa, T. A.; Xia, J.; Wang, F.; Wen, Y.; Zhan, X.; Wang, Z.; He, J. Edge-Epitaxial Growth of 2D NbS2-WS2 Lateral Metal-Semiconductor Heterostructures. Adv. Mater. 2018, 30, 1803665,  DOI: 10.1002/adma.201803665
    Google Scholar
    There is no corresponding record for this reference.
  40. 40
    Man, M. K. L.; Margiolakis, A.; Deckoff-Jones, S.; Harada, T.; Wong, E. L.; Krishna, M. B. M.; Madéo, J.; Winchester, A.; Lei, S.; Vajtai, R.; Ajayan, P. M.; Dani, K. M. Imaging the Motion of Electrons across Semiconductor Heterojunctions. Nat. Nanotechnol. 2017, 12, 36– 40,  DOI: 10.1038/nnano.2016.183
    Google Scholar
    40
    Imaging the motion of electrons across semiconductor heterojunctions
    Man, Michael K. L.; Margiolakis, Athanasios; Deckoff-Jones, Skylar; Harada, Takaaki; Wong, E. Laine; Krishna, M. Bala Murali; Madeo, Julien; Winchester, Andrew; Lei, Sidong; Vajtai, Robert; Ajayan, Pulickel M.; Dani, Keshav M.
    Nature Nanotechnology (2017), 12 (1), 36-40CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
    Technol. progress since the late twentieth century has centered on semiconductor devices, such as transistors, diodes and solar cells. At the heart of these devices is the internal motion of electrons through semiconductor materials due to applied elec. fields or by the excitation of photocarriers. Imaging the motion of these electrons would provide unprecedented insight into this important phenomenon, but requires high spatial and temporal resoln. Current studies of electron dynamics in semiconductors are generally limited by the spatial resoln. of optical probes, or by the temporal resoln. of electronic probes. Here, by combining femtosecond pump-probe techniques with spectroscopic photoemission electron microscopy, we imaged the motion of photoexcited electrons from high-energy to low-energy states in a type-II 2D InSe/GaAs heterostructure. At the instant of photoexcitation, energy-resolved photoelectron images revealed a highly non-equil. distribution of photocarriers in space and energy. Thereafter, in response to the out-of-equil. photocarriers, we obsd. the spatial redistribution of charges, thus forming internal elec. fields, bending the semiconductor bands, and finally impeding further charge transfer. By assembling images taken at different time-delays, we produced a movie lasting a few trillionths of a second of the electron-transfer process in the photoexcited type-II heterostructure-a fundamental phenomenon in semiconductor devices such as solar cells. Quant. anal. and theor. modeling of spatial variations in the movie provide insight into future solar cells, 2D materials and other semiconductor devices.
  41. 41
    Majidi, L.; Yasaei, P.; Warburton, R. E.; Fuladi, S.; Cavin, J.; Hu, X.; Hemmat, Z.; Cho, S. B.; Abbasi, P.; Vörös, M.; Cheng, L.; Sayahpour, B.; Bolotin, I. L.; Zapol, P.; Greeley, J.; Klie, R. F.; Mishra, R.; Khalili-Araghi, F.; Curtiss, L. A.; Salehi-Khojin, A. New Class of Electrocatalysts Based on 2D Transition Metal Dichalcogenides in Ionic Liquid. Adv. Mater. 2019, 31, 1804453,  DOI: 10.1002/adma.201804453
    Google Scholar
    There is no corresponding record for this reference.
  42. 42
    Zhou, X.; Hu, X.; Yu, J.; Liu, S.; Shu, Z.; Zhang, Q.; Li, H.; Ma, Y.; Xu, H.; Zhai, T. 2D Layered Material-Based van der Waals Heterostructures for Optoelectronics. Adv. Funct. Mater. 2018, 28, 1706587,  DOI: 10.1002/adfm.201706587
    Google Scholar
    There is no corresponding record for this reference.
  43. 43
    Yan, Y.; Li, S.; Du, J.; Yang, H.; Wang, X.; Song, X.; Li, L.; Li, X.; Xia, C.; Liu, Y.; Li, J.; Wei, Z. Reversible Half Wave Rectifier Based on 2D InSe/GeSe Heterostructure with near-Broken Band Alignment. Adv. Sci. 2021, 8, 1903252,  DOI: 10.1002/advs.201903252
    Google Scholar
    43
    Reversible Half Wave Rectifier Based on 2D InSe/GeSe Heterostructure with Near-Broken Band Alignment
    Yan, Yong; Li, Shasha; Du, Juan; Yang, Huai; Wang, Xiaoting; Song, Xiaohui; Li, Lixia; Li, Xueping; Xia, Congxin; Liu, Yufang; Li, Jingbo; Wei, Zhongming
    Advanced Science (Weinheim, Germany) (2021), 8 (4), 1903252CODEN: ASDCCF; ISSN:2198-3844. (Wiley-VCH Verlag GmbH & Co. KGaA)
    2D van der Waals heterostructures (vdWHs) offer tremendous opportunities in designing multifunctional electronic devices. Due to the ultrathin nature of 2D materials, the gate-induced change in charge d. makes amplitude control possible, creating a new programmable unilateral rectifier. The study of 2D vdWHs-based reversible unilateral rectifier is lacking, although it can give rise to a new degree of freedom for modulating the output state. Here, a InSe/GeSe vdWH-FET is constructed as a gate-controllable half wave rectifier. The device exhibits stepless adjustment from forward to backward rectifying performance, leading to multiple operation states of output level. Near-broken band alignment in the InSe/GeSe vdWH-FET is a crucial feature for high-performance reversible rectifier, which is shown to have backward and forward rectification ratio of 1:38 and 963:1, resp. Being further explored as a new bridge rectifier, the InSe/GeSe device has great potential in future gate-controllable a.c./d.c. converter. These results indicate that 2D vdWHs with near-broken band alignment can offer a pathway to simplify the commutating circuit and regulating speed circuit.
  44. 44
    Lei, S.; Ge, L.; Najmaei, S.; George, A.; Kappera, R.; Lou, J.; Chhowalla, M.; Yamaguchi, H.; Gupta, G.; Vajtai, R.; Mohite, A. D.; Ajayan, P. M. Evolution of the Electronic Band Structure and Efficient Photo-Detection in Atomic Layers of InSe. ACS Nano 2014, 8, 1263– 1272,  DOI: 10.1021/nn405036u
    Google Scholar
    44
    Evolution of the Electronic Band Structure and Efficient Photo-Detection in Atomic Layers of InSe
    Lei, Sidong; Ge, Liehui; Najmaei, Sina; George, Antony; Kappera, Rajesh; Lou, Jun; Chhowalla, Manish; Yamaguchi, Hisato; Gupta, Gautam; Vajtai, Robert; Mohite, Aditya D.; Ajayan, Pulickel M.
    ACS Nano (2014), 8 (2), 1263-1272CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Atomic layers of two-dimensional (2D) materials have recently been the focus of extensive research. This follows from the footsteps of graphene, which has shown great potential for ultrathin optoelectronic devices. In this paper, we present a comprehensive study on the synthesis, characterization, and thin film photodetector application of at. layers of InSe. Correlation between resonance Raman spectroscopy and photocond. measurements allows us to systematically track the evolution of the electronic band structure of 2D InSe as its thickness approaches few at. layers. Anal. of photocond. spectra suggests that few-layered InSe has an indirect band gap of 1.4 eV, which is 200 meV higher than bulk InSe due to the suppressed interlayer electron orbital coupling. Temp.-dependent photocurrent measurements reveal that the suppressed interlayer interaction also results in more localized pz-like orbitals, and these orbitals couple strongly with the in-plane E' and E'' phonons. Finally, we measured a strong photoresponse of 34.7 mA/W and fast response time of 488 μs for a few layered InSe, suggesting that it is a good material for thin film optoelectronic applications.
  45. 45
    Erdogan, H.; Kirby, R. D. Raman Spectrum and Lattice Dynamics of NbTe2. Solid State Commun. 1989, 70, 713– 715,  DOI: 10.1016/0038-1098(89)90987-3
    Google Scholar
    45
    Raman spectrum and lattice dynamics of niobium ditelluride
    Erdogan, Hasan; Kirby, Roger D.
    Solid State Communications (1989), 70 (7), 713-15CODEN: SSCOA4; ISSN:0038-1098.
    Raman scattering measurements on the layered-structure compd. NbTe2 are reported. No evidences for a phase transition is found for 80-420 K. A group-theor. anal. of the crystal structure predicts 51 optic phonon branches, with 24 of these being Raman-active. The obsd. Raman spectrum was interpreted in terms of the phonon branches of the simpler CdI2 structure.
  46. 46
    Qin, F.; Gao, F.; Dai, M.; Hu, Y.; Yu, M.; Wang, L.; Feng, W.; Li, B.; Hu, P. Multilayer InSe-Te van der Waals Heterostructures with an Ultrahigh Rectification Ratio and Ultrasensitive Photoresponse. ACS Appl. Mater. Interfaces 2020, 12, 37313– 37319,  DOI: 10.1021/acsami.0c08461
    Google Scholar
    46
    Multilayer InSe-Te van der Waals Heterostructures with an Ultrahigh Rectification Ratio and Ultrasensitive Photoresponse
    Qin, Fanglu; Gao, Feng; Dai, Mingjin; Hu, Yunxia; Yu, Miaomiao; Wang, Lifeng; Feng, Wei; Li, Bin; Hu, PingAn
    ACS Applied Materials & Interfaces (2020), 12 (33), 37313-37319CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    Multilayer van der Waals (vdWs) semiconductors have promising applications in high-performance optoelectronic devices. However, photoconductive photodetectors based on layered semiconductors often suffer from sizeable dark currents and high external driving bias voltages. Here, we report vertical van der Waals heterostructures (vdWHs) consisting of multilayer indium selenide (InSe) and tellurium (Te). The multilayer InSe-Te vdWH device shows a record high forward rectification ratio greater than 107 at room temp. The vdWH device achieves an ultrasensitive and broadband photoresponse photodetector with an ultrahigh photo/dark current ratio over 104 and a high detectivity of 1013 Jones under visible light illumination with weak incident power. Moreover, the vdWH device has a photovoltaic effect and can function as a self-powered photodetector (SPPD). The SPPD is also ultrasensitive to a broadband spectrum ranging from 300 to 1000 nm and is capable of detecting weak light signals. This work offers an opportunity to develop next-generation electronic and optoelectronic devices based on multilayer vdWs materials.
  47. 47
    Baugher, B. W. H.; Churchill, H. O. H.; Yang, Y.; Jarillo-Herrero, P. Optoelectronic Devices Based on Electrically Tunable p-n Diodes in a Monolayer Dichalcogenide. Nat. Nanotechnol. 2014, 9, 262– 267,  DOI: 10.1038/nnano.2014.25
    Google Scholar
    47
    Optoelectronic devices based on electrically tunable p-n diodes in a monolayer dichalcogenide
    Baugher, Britton W. H.; Churchill, Hugh O. H.; Yang, Yafang; Jarillo-Herrero, Pablo
    Nature Nanotechnology (2014), 9 (4), 262-267CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
    The p-n junction is the functional element of many electronic and optoelectronic devices, including diodes, bipolar transistors, photodetectors, light-emitting diodes, and solar cells. In conventional p-n junctions, the adjacent p- and n-type regions of a semiconductor are formed by chem. doping. Ambipolar semiconductors, such as C nanotubes, nanowires and org. mols., allow for p-n junctions to be configured and modified by electrostatic gating. This elec. control enables a single device to have multiple functionalities. Here, we report ambipolar monolayer WSe2 devices in which two local gates are used to define a p-n junction within the WSe2 sheet. With these elec. tunable p-n junctions, we demonstrate both p-n and n-p diodes with ideality factors better than 2. Under optical excitation, the diodes demonstrate a photodetection responsivity of 210 mA W-1 and photovoltaic power generation with a peak external quantum efficiency of 0.2%, promising values for a nearly transparent monolayer material in a lateral device geometry. Finally, we demonstrate a light-emitting diode based on monolayer WSe2. These devices provide a building block for ultrathin, flexible and nearly transparent optoelectronic and electronic applications based on ambipolar dichalcogenide materials.

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  8. Xiaoyu Gao, Chenglin Wang, Qianqian Wu, Chunyan Xu, Xitao Guo, Zhengyang Cai, Shaoqing Xiao, Xiaofeng Gu, Haiyan Nan. Enhancement of Electronic and Optoelectronic Performance of the InSe Multilayer by Surface Transfer Engineering. ACS Applied Electronic Materials 2022, 4 (12) , 5867-5874. https://doi.org/10.1021/acsaelm.2c01041
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  36. Li Zhang, Xiaoning Han, Shihao Zhang, Hanyu Wang, Ying Huang, Zhaoqiang Zheng, Nengjie Huo, Wei Gao, Jingbo Li. Gate‐Tunable Photovoltaic Behavior and Polarized Image Sensor Based on All‐2D TaIrTe 4 /MoS 2 Van Der Waals Schottky Diode. Advanced Electronic Materials 2022, 8 (11) https://doi.org/10.1002/aelm.202200551
  37. Hemiao Wang, Yurui Wang, Xin Li, Xiaolian Liu, Xin Zheng, Yueqin Shi, Minxuan Xu, Jian Zhang, Qi Zhang. Self-powered photodetectors based on stacked WSe2/graphene/SnS2 p-g-n heterostructures. Journal of Alloys and Compounds 2022, 920 , 165974. https://doi.org/10.1016/j.jallcom.2022.165974
  38. Hoon Hahn Yoon, Henry A. Fernandez, Fedor Nigmatulin, Weiwei Cai, Zongyin Yang, Hanxiao Cui, Faisal Ahmed, Xiaoqi Cui, Md Gius Uddin, Ethan D. Minot, Harri Lipsanen, Kwanpyo Kim, Pertti Hakonen, Tawfique Hasan, Zhipei Sun. Miniaturized spectrometers with a tunable van der Waals junction. Science 2022, 378 (6617) , 296-299. https://doi.org/10.1126/science.add8544
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  40. Yunlong Xiao, He Zhu, Ke Deng, Peng Wang, Qing Li, Ting He, Tao Zhang, Jinshui Miao, Ning Li, Wei Lu, Ning Dai, Weida Hu. Progress and challenges in blocked impurity band infrared detectors for space-based astronomy. Science China Physics, Mechanics & Astronomy 2022, 65 (8) https://doi.org/10.1007/s11433-022-1906-y
  41. Haowen Liu, Shuren Zhou, Hong Zhang, Lijuan Ye, Yuanqiang Xiong, Peng Yu, Wanjun Li, Xun Yang, Honglin Li, Chunyang Kong. Ultrasensitive fully transparent amorphous Ga 2 O 3 solar-blind deep-ultraviolet photodetector for corona discharge detection. Journal of Physics D: Applied Physics 2022, 55 (30) , 305104. https://doi.org/10.1088/1361-6463/ac6d26
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  43. Tao Zheng, Mengmeng Yang, Yiming Sun, Lixiang Han, Yuan Pan, Qixiao Zhao, Zhaoqiang Zheng, Nengjie Huo, Wei Gao, Jingbo Li. A solution-fabricated tellurium/silicon mixed-dimensional van der Waals heterojunction for self-powered photodetectors. Journal of Materials Chemistry C 2022, 10 (18) , 7283-7293. https://doi.org/10.1039/D2TC00785A
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  • Abstract

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    Figure 1

    Figure 1. Design and characterization of the InSe/NbTe2 heterostructure. (a) Band diagram between the bulk materials of semiconducting InSe and metallic 1T-phase NbTe2. The key features are determined with the results of UPS measurements shown in Figure S1. EF denotes the Fermi level of pristine bulk InSe, indicating that it is n-doped. (b) Schematic of the InSe/NbTe2 heterostructure device. The upper panel illustrates the crystal structures of InSe and NbTe2. The Al2O3 layer for protection is not shown in the schematic. (c) Optical microscope image of the stacked InSe/NbTe2 heterostructure. The top diagram illustrates the stacking order. (d) AFM characterization of the InSe/NbTe2 heterostructure. (e) Raman spectra of InSe, NbTe2, and InSe/NbTe2 heterostructure. (f) Absorbance of InSe/NbTe2 heterostructure.

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    Figure 2

    Figure 2. Electrical characterization of the heterostructure device. (a) Transfer curves of the devices with pure InSe or NbTe2 channel. Bias voltages Vds of 2 and 0.1 V were applied in the measurements of InSe and NbTe2 devices, respectively. (b) Transfer curves of the InSe/NbTe2 heterostructure device measured with bias voltage Vds of 2 V and 1 V. (c) Gate voltage dependent output Ids–Vds curves of the InSe/NbTe2 heterostructure device. The inset shows |Ids| on a logarithmic scale. (d) Gate voltage dependent rectification ratio calculated with the results in (c). (e) Fitting of the output Ids–Vds curve measured at Vgate = 80 V by Shockley diode function. (47) Ideality factor of n = 2.2 is extracted from the fitting. (f) Schematic of band bending at the interface between n-doped InSe and metallic NbTe2 under reverse and forward biases when a positive gate voltage is applied. EF denotes the Fermi level of InSe.

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    Figure 3

    Figure 3. Switchable photoresponse mechanisms of InSe/NbTe2 heterostructure. (a–c) Photocurrent mappings in InSe/NbTe2 heterostructure device at various bias voltages Vds of −2, 0, and 2 V. The green, yellow, and white dashed lines illustrate the outlines of InSe, NbTe2, and Ti/Au electrodes, respectively. Scale bars, 5 μm. (d) Line scannings extracted from the mappings at the positions and directions indicated by white arrows in (a)–(c). The blue and brown shades indicate the Y positions of pure InSe and InSe/NbTe2 heterostructure. (e) Truth table of the conceptual XOR logic gate with inputs of laser beam position (Y) and bias voltage (Vds) and output of Ids. (f, g) Schematic of the InSe/NbTe2 heterostructure device switched between the photovoltaic device (f, reverse bias) and phototransistor (g, forward bias). The role of NbTe2 is switched between a heterojunction component and a contact electrode.

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    Figure 4

    Figure 4. Overall photodetection performance of InSe/NbTe2 heterostructure device. (a) Schematic of the switchable InSe/NbTe2 heterostructure for the detection of 532 nm laser. Working mechanism of this device depends on bias-dependent band bending at the interface. (b) Transfer curves of InSe/NbTe2 phototransistor illuminated by 532 nm laser beam with gradient power. (c) Ids–Vds curves of the InSe/NbTe2 heterostructure device under light illumination. (d, e) Short-circuit current ISC (d) and open-circuit voltage VOC (e) extracted from the Ids-Vds curves in (c). The results are fitted by ISC ∝ Powerα and VOC ∝ ln(Power). (f) Photocurrent Iph and photoresponsivity calculated with the data in (c).

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  • References

    This article references 47 other publications.

    1. 1
      Bonaccorso, F.; Sun, Z.; Hasan, T.; Ferrari, A. C. Graphene Photonics and Optoelectronics. Nat. Photonics 2010, 4, 611– 622,  DOI: 10.1038/nphoton.2010.186
      1
      Graphene photonics and optoelectronics
      Bonaccorso, F.; Sun, Z.; Hasan, T.; Ferrari, A. C.
      Nature Photonics (2010), 4 (9), 611-622CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
      The richness of optical and electronic properties of graphene attracts enormous interest. Graphene has high mobility and optical transparency, in addn. to flexibility, robustness and environmental stability. So far, the main focus has been on fundamental physics and electronic devices. However, we believe its true potential lies in photonics and optoelectronics, where the combination of its unique optical and electronic properties can be fully exploited, even in the absence of a bandgap, and the linear dispersion of the Dirac electrons enables ultrawideband tunability. The rise of graphene in photonics and optoelectronics is shown by several recent results, ranging from solar cells and light-emitting devices to touch screens, photodetectors and ultrafast lasers. Here we review the state-of-the-art in this emerging field.
    2. 2
      Hu, G.; Albrow-Owen, T.; Jin, X.; Ali, A.; Hu, Y.; Howe, R. C. T.; Shehzad, K.; Yang, Z.; Zhu, X.; Woodward, R. I.; Wu, T.-C.; Jussila, H.; Wu, J.-B.; Peng, P.; Tan, P.-H.; Sun, Z.; Kelleher, E. J. R.; Zhang, M.; Xu, Y.; Hasan, T. Black Phosphorus Ink Formulation for Inkjet Printing of Optoelectronics and Photonics. Nat. Commun. 2017, 8, 278,  DOI: 10.1038/s41467-017-00358-1
      2
      Black phosphorus ink formulation for inkjet printing of optoelectronics and photonics
      Hu Guohua; Albrow-Owen Tom; Howe Richard C T; Yang Zongyin; Wu Tien-Chun; Hasan Tawfique; Jin Xinxin; Hu Yuwei; Zhu Xuekun; Zhang Meng; Ali Ayaz; Shehzad Khurram; Xu Yang; Woodward Robert I; Kelleher Edmund J R; Jussila Henri; Sun Zhipei; Wu Jiang-Bin; Tan Ping-Heng; Peng Peng; Peng Peng; Zhang Meng
      Nature communications (2017), 8 (1), 278 ISSN:.
      Black phosphorus is a two-dimensional material of great interest, in part because of its high carrier mobility and thickness dependent direct bandgap. However, its instability under ambient conditions limits material deposition options for device fabrication. Here we show a black phosphorus ink that can be reliably inkjet printed, enabling scalable development of optoelectronic and photonic devices. Our binder-free ink suppresses coffee ring formation through induced recirculating Marangoni flow, and supports excellent consistency (< 2% variation) and spatial uniformity (< 3.4% variation), without substrate pre-treatment. Due to rapid ink drying (< 10 s at < 60 °C), printing causes minimal oxidation. Following encapsulation, the printed black phosphorus is stable against long-term (> 30 days) oxidation. We demonstrate printed black phosphorus as a passive switch for ultrafast lasers, stable against intense irradiation, and as a visible to near-infrared photodetector with high responsivities. Our work highlights the promise of this material as a functional ink platform for printed devices.Atomically thin black phosphorus shows promise for optoelectronics and photonics, yet its instability under environmental conditions and the lack of well-established large-area synthesis protocols hinder its applications. Here, the authors demonstrate a stable black phosphorus ink suitable for printed ultrafast lasers and photodetectors.
    3. 3
      Xia, F.; Wang, H.; Hwang, J. C. M.; Neto, A. H. C.; Yang, L. Black Phosphorus and Its Isoelectronic Materials. Nat. Rev. Phys. 2019, 1, 306– 317,  DOI: 10.1038/s42254-019-0043-5
      3
      Black phosphorus and its isoelectronic materials
      Xia, Fengnian; Wang, Han; Hwang, James C. M.; Castro Neto, A. H.; Yang, Li
      Nature Reviews Physics (2019), 1 (5), 306-317CODEN: NRPACZ; ISSN:2522-5820. (Nature Research)
      Abstr.: The family of 2D and layered materials has been expanding rapidly for more than a decade. Within this large family of hundreds of materials, black phosphorus and its isoelectronic group IV monochalcogenides have a unique place. These puckered materials have distinctive cryst. symmetries and exhibit various exciting properties, such as high carrier mobility, strong IR responsivity, widely tunable bandgap, in-plane anisotropy and spontaneous elec. polarization. Here, we review their basic properties, highlight new electronic and photonic device concepts and novel phys. phenomena and discuss future directions.
    4. 4
      Mak, K. F.; Shan, J. Photonics and Optoelectronics of 2D Semiconductor Transition Metal Dichalcogenides. Nat. Photonics 2016, 10, 216– 226,  DOI: 10.1038/nphoton.2015.282
      4
      Photonics and optoelectronics of 2D semiconductor transition metal dichalcogenides
      Mak, Kin Fai; Shan, Jie
      Nature Photonics (2016), 10 (4), 216-226CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
      Recent advances in the development of atomically thin layers of van der Waals bonded solids have opened up new possibilities for the exploration of 2D physics as well as for materials for applications. Among them, semiconductor transition metal dichalcogenides, MX2 (M = Mo, W; X = S, Se), have bandgaps in the near-IR to the visible region, in contrast to the zero bandgap of graphene. In the monolayer limit, these materials have been shown to possess direct bandgaps, a property well suited for photonics and optoelectronics applications. Here, we review the electronic and optical properties and the recent progress in applications of 2D semiconductor transition metal dichalcogenides with emphasis on strong excitonic effects, and spin- and valley-dependent properties.
    5. 5
      Li, L.; Guo, Y.; Sun, Y.; Yang, L.; Qin, L.; Guan, S.; Wang, J.; Qiu, X.; Li, H.; Shang, Y.; Fang, Y. A General Method for the Chemical Synthesis of Large-Scale, Seamless Transition Metal Dichalcogenide Electronics. Adv. Mater. 2018, 30, 1706215,  DOI: 10.1002/adma.201706215
      There is no corresponding record for this reference.
    6. 6
      Liu, Y.; Weiss, N. O.; Duan, X.; Cheng, H.-C.; Huang, Y.; Duan, X. van der Waals Heterostructures and Devices. Nat. Rev. Mater. 2016, 1, 16042,  DOI: 10.1038/natrevmats.2016.42
      6
      Van der Waals heterostructures and devices
      Liu, Yuan; Weiss, Nathan O.; Duan, Xidong; Cheng, Hung-Chieh; Huang, Yu; Duan, Xiangfeng
      Nature Reviews Materials (2016), 1 (9), 16042CODEN: NRMADL; ISSN:2058-8437. (Nature Publishing Group)
      Two-dimensional layered materials (2DLMs) have been a central focus of materials research since the discovery of graphene just over a decade ago. Each layer in 2DLMs consists of a covalently bonded, dangling-bond-free lattice and is weakly bound to neighboring layers by van der Waals interactions. This makes it feasible to isolate, mix and match highly disparate at. layers to create a wide range of van der Waals heterostructures (vdWHs) without the constraints of lattice matching and processing compatibility. Exploiting the novel properties in these vdWHs with diverse layering of metals, semiconductors or insulators, new designs of electronic devices emerge, including tunnelling transistors, barristors and flexible electronics, as well as optoelectronic devices, including photodetectors, photovoltaics and light-emitting devices with unprecedented characteristics or unique functionalities. We review the recent progress and challenges, and offer our perspective on the exploration of 2DLM-based vdWHs for future application in electronics and optoelectronics.
    7. 7
      Xue, H.; Wang, Y.; Dai, Y.; Kim, W.; Jussila, H.; Qi, M.; Susoma, J.; Ren, Z.; Dai, Q.; Zhao, J.; Halonen, K.; Lipsanen, H.; Wang, X.; Gan, X.; Sun, Z. A MoSe2/WSe2 Heterojunction-Based Photodetector at Telecommunication Wavelengths. Adv. Funct. Mater. 2018, 28, 1804388,  DOI: 10.1002/adfm.201804388
      There is no corresponding record for this reference.
    8. 8
      Xue, H.; Dai, Y.; Kim, W.; Wang, Y.; Bai, X.; Qi, M.; Halonen, K.; Lipsanen, H.; Sun, Z. High Photoresponsivity and Broadband Photodetection with a Band-Engineered WSe2/SnSe2 Heterostructure. Nanoscale 2019, 11, 3240– 3247,  DOI: 10.1039/C8NR09248F
      8
      High photoresponsivity and broadband photodetection with a band-engineered WSe2/SnSe2 heterostructure
      Xue, Hui; Dai, Yunyun; Kim, Wonjae; Wang, Yadong; Bai, Xueyin; Qi, Mei; Halonen, Kari; Lipsanen, Harri; Sun, Zhipei
      Nanoscale (2019), 11 (7), 3240-3247CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      Van der Waals (vdW) heterostructures formed by stacking different two-dimensional layered materials have been demonstrated as a promising platform for next-generation photonic and optoelectronic devices due to their tailorable band-engineering properties. Here, we report a high photoresponsivity and broadband photodetector based on a WSe2/SnSe2 heterostructure. By properly biasing the heterostructure, its band structure changes from near-broken band alignment to type-III band alignment which enables high photoresponsivity from visible to telecommunication wavelengths. The highest photoresponsivity and detectivity at 532 nm are ∼588 A W-1 and 4.4 × 1010 Jones and those at 1550 nm are ∼80 A W-1 and 1.4 × 1010 Jones, which are superior to those of the current state-of-the-art layered transition metal dichalcogenides based photodetectors under similar measurement conditions. Our work not only provides a new method for designing high-performance broadband photodetectors but also enables a deep understanding of the band engineering technol. in the vdW heterostructures possible for other applications, such as modulators and lasers.
    9. 9
      Sun, Z.; Hasan, T.; Torrisi, F.; Popa, D.; Privitera, G.; Wang, F.; Bonaccorso, F.; Basko, D. M.; Ferrari, A. C. Graphene Mode-Locked Ultrafast Laser. ACS Nano 2010, 4, 803– 810,  DOI: 10.1021/nn901703e
      9
      Graphene Mode-Locked Ultrafast Laser
      Sun, Zhipei; Hasan, Tawfique; Torrisi, Felice; Popa, Daniel; Privitera, Giulia; Wang, Fengqiu; Bonaccorso, Francesco; Basko, Denis M.; Ferrari, Andrea C.
      ACS Nano (2010), 4 (2), 803-810CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Graphene is at the center of a significant research effort. Near-ballistic transport at room temp. and high mobility make it a potential material for nanoelectronics. Its electronic and mech. properties are also ideal for micro- and nanomech. systems, thin-film transistors, and transparent and conductive composites and electrodes. Here we exploit the optoelectronic properties of graphene to realize an ultrafast laser. A graphene-polymer composite is fabricated using wet-chem. techniques. Pauli blocking following intense illumination results in saturable absorption, independent of wavelength. This is used to passively mode-lock an erbium-doped fiber laser working at 1559 nm, with a 5.24 nm spectral bandwidth and ∼460 fs pulse duration, paving the way to graphene-based photonics.
    10. 10
      Sun, Z.; Martinez, A.; Wang, F. Optical Modulators with 2D Layered Materials. Nat. Photonics 2016, 10, 227– 238,  DOI: 10.1038/nphoton.2016.15
      10
      Optical modulators with 2D layered materials
      Sun, Zhipei; Martinez, Amos; Wang, Feng
      Nature Photonics (2016), 10 (4), 227-238CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
      Light modulation is an essential operation in photonics and optoelectronics. With existing and emerging technologies increasingly demanding compact, efficient, fast and broadband optical modulators, high-performance light modulation solns. are becoming indispensable. The recent realization that 2D layered materials could modulate light with superior performance has prompted intense research and significant advances, paving the way for realistic applications. In this Review, we cover the state of the art of optical modulators based on 2D materials, including graphene, transition metal dichalcogenides and black phosphorus. We discuss recent advances employing hybrid structures, such as 2D heterostructures, plasmonic structures, and silicon and fiber integrated structures. We also take a look at the future perspectives and discuss the potential of yet relatively unexplored mechanisms, such as magneto-optic and acousto-optic modulation.
    11. 11
      Dai, M.; Chen, H.; Feng, R.; Feng, W.; Hu, Y.; Yang, H.; Liu, G.; Chen, X.; Zhang, J.; Xu, C.-Y.; Hu, P. A Dual-Band Multilayer InSe Self-Powered Photodetector with High Performance Induced by Surface Plasmon Resonance and Asymmetric Schottky Junction. ACS Nano 2018, 12, 8739– 8747,  DOI: 10.1021/acsnano.8b04931
      11
      A Dual-Band Multilayer InSe Self-Powered Photodetector with High Performance Induced by Surface Plasmon Resonance and Asymmetric Schottky Junction
      Dai, Mingjin; Chen, Hongyu; Feng, Rui; Feng, Wei; Hu, Yunxia; Yang, Huihui; Liu, Guangbo; Chen, Xiaoshuang; Zhang, Jia; Xu, Cheng-Yan; Hu, PingAn
      ACS Nano (2018), 12 (8), 8739-8747CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      A dual-band self-powered photodetector (SPPD) with high sensitivity is realized by a facile combination of InSe Schottky diode and Au plasmonic nanoparticle (NP) arrays. Comparing with pristine InSe devices, InSe/Au photodetectors possess an addnl. capability of photodetection in visible to near-IR (NIR) region. This intriguing phenomenon is attributed to the wavelength selective enhancement of pristine responsivities by hybridized quadrupole plasmons resonance of Au NPs. It is worth pointing out that the max. of enhancement ratio in responsivity reaches up to ∼1200% at a wavelength of 685 nm. Owing to a large Schottky barrier difference formed between active layer and 2 asym. electrodes, the responsivities of dual-band InSe/Au photodetector could reach up to 369 and 244 mA/W at λ of 365 and 685 nm under zero bias voltage, resp. This work would provide an addnl. opportunity for developing multifunctional photodetectors with high performance based on 2-dimensional materials, upgrading their capacity of photodetection in a complex environment.
    12. 12
      Koppens, F. H. L.; Mueller, T.; Avouris, P.; Ferrari, A. C.; Vitiello, M. S.; Polini, M. Photodetectors Based on Graphene, Other Two-Dimensional Materials and Hybrid Systems. Nat. Nanotechnol. 2014, 9, 780– 793,  DOI: 10.1038/nnano.2014.215
      12
      Photodetectors based on graphene, other two-dimensional materials and hybrid systems
      Koppens, F. H. L.; Mueller, T.; Avouris, Ph.; Ferrari, A. C.; Vitiello, M. S.; Polini, M.
      Nature Nanotechnology (2014), 9 (10), 780-793CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
      A review. Graphene and other 2-dimensional materials, such as transition metal dichalcogenides, have rapidly established themselves as intriguing building blocks for optoelectronic applications, with a strong focus on various photodetection platforms. The versatility of these material systems enables their application in areas including ultrafast and ultrasensitive detection of light in the UV, visible, IR and terahertz frequency ranges. These detectors can be integrated with other photonic components based on the same material, as well as with silicon photonic and electronic technologies. Here, the authors provide an overview and evaluation of state-of-the-art photodetectors based on graphene, other 2-dimensional materials, and hybrid systems based on the combination of different 2-dimensional crystals or of 2-dimensional crystals and other (nano)materials, such as plasmonic nanoparticles, semiconductors, quantum dots, or their integration with (silicon) waveguides.
    13. 13
      Li, X.; Zhu, M.; Du, M.; Lv, Z.; Zhang, L.; Li, Y.; Yang, Y.; Yang, T.; Li, X.; Wang, K.; Zhu, H.; Fang, Y. High Detectivity Graphene-Silicon Heterojunction Photodetector. Small 2016, 12, 595– 601,  DOI: 10.1002/smll.201502336
      13
      High Detectivity Graphene-Silicon Heterojunction Photodetector
      Li, Xinming; Zhu, Miao; Du, Mingde; Lv, Zheng; Zhang, Li; Li, Yuanchang; Yang, Yao; Yang, Tingting; Li, Xiao; Wang, Kunlin; Zhu, Hongwei; Fang, Ying
      Small (2016), 12 (5), 595-601CODEN: SMALBC; ISSN:1613-6810. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A graphene/n-type Si (n-Si) heterojunction exhibited strong rectifying behavior and high photoresponsivity, which can be used for the development of high-performance photodetectors. Graphene/n-Si heterojunction photodetectors reported previously suffer from relatively low specific detectivity due to large dark current. By introducing a thin interfacial oxide layer, the dark current of graphene/n-Si heterojunction was reduced by 2 orders of magnitude at zero bias. At room temp., the graphene/n-Si photodetector with interfacial oxide exhibits a specific detectivity ≤5.77 × 1013 cm Hz1/2 W-1 at peak λ = 890 nm in vacuum, which is highest reported detectivity at room temp. for planar graphene/Si heterojunction photodetectors. The improved graphene/n-Si heterojunction photodetectors possess high responsivity of 0.73 A W-1 and high photo-to-dark current ratio of ∼107. The current noise spectral d. of the graphene/n-Si photodetector was characterized under ambient and vacuum conditions, which shows that the dark current can be further suppressed in vacuum. Graphene/Si heterojunction with interfacial oxide is promising for the development of high detectivity photodetectors.
    14. 14
      Zhao, M.; Xia, W.; Wang, Y.; Luo, M.; Tian, Z.; Guo, Y.; Hu, W.; Xue, J. Nb2SiTe4: A Stable Narrow-Gap Two-Dimensional Material with Ambipolar Transport and Mid-Infrared Response. ACS Nano 2019, 13, 10705– 10710,  DOI: 10.1021/acsnano.9b05080
      14
      Nb2SiTe4: A Stable Narrow-Gap Two-Dimensional Material with Ambipolar Transport and Mid-Infrared Response
      Zhao, Mingxing; Xia, Wei; Wang, Yang; Luo, Man; Tian, Zhen; Guo, Yanfeng; Hu, Weida; Xue, Jiamin
      ACS Nano (2019), 13 (9), 10705-10710CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Two-dimensional (2D) materials with narrow band gaps (∼0.3 eV) are of great importance for realizing ambipolar transistors and mid-IR (MIR) detections. However, most of the 2D materials studied to date have band gaps that are too large. A few of the materials with suitable band gaps are not stable under ambient conditions. In this study, the layered Nb2SiTe4 is shown to be a stable 2D material with a band gap of 0.39 eV. Field-effect transistors based on few-layer Nb2SiTe4 show ambipolar transport with a similar magnitude of electron and hole current and a high charge-carrier mobility of ∼100 cm2 V-1 s-1 at room temp. Optoelectronic measurements of the devices show clear response to an MIR wavelength of 3.1 μm with a high responsivity of ∼0.66 AW-1. These results establish Nb2SiTe4 as a good candidate for ambipolar devices and MIR detection.
    15. 15
      Li, L.; Kim, J.; Jin, C.; Ye, G. J.; Qiu, D. Y.; da Jornada, F. H.; Shi, Z.; Chen, L.; Zhang, Z.; Yang, F.; Watanabe, K.; Taniguchi, T.; Ren, W.; Louie, S. G.; Chen, X. H.; Zhang, Y.; Wang, F. Direct Observation of the Layer-Dependent Electronic Structure in Phosphorene. Nat. Nanotechnol. 2017, 12, 21– 25,  DOI: 10.1038/nnano.2016.171
      15
      Direct observation of the layer-dependent electronic structure in phosphorene
      Li, Likai; Kim, Jonghwan; Jin, Chenhao; Ye, Guo Jun; Qiu, Diana Y.; da Jornada, Felipe H.; Shi, Zhiwen; Chen, Long; Zhang, Zuocheng; Yang, Fangyuan; Watanabe, Kenji; Taniguchi, Takashi; Ren, Wencai; Louie, Steven G.; Chen, Xian Hui; Zhang, Yuanbo; Wang, Feng
      Nature Nanotechnology (2017), 12 (1), 21-25CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
      Phosphorene, a single at. layer of black phosphorus, has recently emerged as a new two-dimensional (2D) material that holds promise for electronic and photonic technologies. Here, we exptl. demonstrate that the electronic structure of few-layer phosphorene varies significantly with the no. of layers, in good agreement with theor. predictions. The interband optical transitions cover a wide, technol. important spectral range from the visible to the mid-IR. In addn., we observe strong photoluminescence in few-layer phosphorene at energies that closely match the absorption edge, indicating that they are direct bandgap semiconductors. The strongly layer-dependent electronic structure of phosphorene, in combination with its high elec. mobility, gives it distinct advantages over other 2D materials in electronic and opto-electronic applications.
    16. 16
      Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F. Atomically Thin MoS2: A New Direct-Gap Semiconductor. Phys. Rev. Lett. 2010, 105, 136805,  DOI: 10.1103/PhysRevLett.105.136805
      16
      Atomically Thin MoS2. A New Direct-Gap Semiconductor
      Mak, Kin Fai; Lee, Changgu; Hone, James; Shan, Jie; Heinz, Tony F.
      Physical Review Letters (2010), 105 (13), 136805/1-136805/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      The electronic properties of ultrathin crystals of MoS2 consisting of N = 1, 2,...,6 S-Mo-S monolayers were investigated by optical spectroscopy. Through characterization by absorption, photoluminescence, and photocond. spectroscopy, we trace the effect of quantum confinement on the material's electronic structure. With decreasing thickness, the indirect band gap, which lies below the direct gap in the bulk material, shifts upwards in energy by >0.6 eV. This leads to a crossover to a direct-gap material in the limit of the single monolayer. Unlike the bulk material, the MoS2 monolayer emits light strongly. The freestanding monolayer exhibits an increase in luminescence quantum efficiency by more than a factor of 104 compared with the bulk material.
    17. 17
      Bandurin, D. A.; Tyurnina, A. V.; Yu, G. L.; Mishchenko, A.; Zólyomi, V.; Morozov, S. V.; Kumar, R. K.; Gorbachev, R. V.; Kudrynskyi, Z. R.; Pezzini, S.; Kovalyuk, Z. D.; Zeitler, U.; Novoselov, K. S.; Patanè, A.; Eaves, L.; Grigorieva, I. V.; Fal’ko, V. I.; Geim, A. K.; Cao, Y. High Electron Mobility, Quantum Hall Effect and Anomalous Optical Response in Atomically Thin InSe. Nat. Nanotechnol. 2017, 12, 223– 227,  DOI: 10.1038/nnano.2016.242
      17
      High electron mobility, quantum Hall effect and anomalous optical response in atomically thin InSe
      Bandurin, Denis A.; Tyurnina, Anastasia V.; Yu, Geliang L.; Mishchenko, Artem; Zolyomi, Viktor; Morozov, Sergey V.; Kumar, Roshan Krishna; Gorbachev, Roman V.; Kudrynskyi, Zakhar R.; Pezzini, Sergio; Kovalyuk, Zakhar D.; Zeitler, Uli; Novoselov, Konstantin S.; Patane, Amalia; Eaves, Laurence; Grigorieva, Irina V.; Fal'ko, Vladimir I.; Geim, Andre K.; Cao, Yang
      Nature Nanotechnology (2017), 12 (3), 223-227CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
      A decade of intense research on two-dimensional (2D) at. crystals has revealed that their properties can differ greatly from those of the parent compd. These differences are governed by changes in the band structure due to quantum confinement and are most profound if the underlying lattice symmetry changes. Here we report a high-quality 2D electron gas in few-layer InSe encapsulated in hexagonal boron nitride under an inert atm. Carrier mobilities are found to exceed 103 cm2 V-1 s-1 and 104 cm2 V-1 s-1 at room and liq.-helium temps., resp., allowing the observation of the fully developed quantum Hall effect. The conduction electrons occupy a single 2D subband and have a small effective mass. Photoluminescence spectroscopy reveals that the bandgap increases by more than 0.5 eV with decreasing the thickness from bulk to bilayer InSe. The band-edge optical response vanishes in monolayer InSe, which is attributed to the monolayer's mirror-plane symmetry. Encapsulated 2D InSe expands the family of graphene-like semiconductors and, in terms of quality, is competitive with atomically thin dichalcogenides and black phosphorus.
    18. 18
      Chhowalla, M.; Jena, D.; Zhang, H. Two-Dimensional Semiconductors for Transistors. Nat. Rev. Mater. 2016, 1, 16052,  DOI: 10.1038/natrevmats.2016.52
      18
      Two-dimensional semiconductors for transistors
      Chhowalla, Manish; Jena, Debdeep; Zhang, Hua
      Nature Reviews Materials (2016), 1 (11), 16052CODEN: NRMADL; ISSN:2058-8437. (Nature Publishing Group)
      In the quest for higher performance, the dimensions of field-effect transistors (FETs) continue to decrease. However, the redn. in size of FETs comprising 3D semiconductors is limited by the rate at which heat, generated from static power, is dissipated. The increase in static power and the leakage of current between the source and drain electrodes that causes this increase, are referred to as short-channel effects. In FETs with channels made from 2D semiconductors, leakage current is almost eliminated because all electrons are confined in atomically thin channels and, hence, are uniformly influenced by the gate voltage. In this Review, we provide a math. framework to evaluate the performance of FETs and describe the challenges for improving the performances of short-channel FETs in relation to the properties of 2D materials, including graphene, transition metal dichalcogenides, phosphorene and silicene. We also describe tunnelling FETs that possess extremely low-power switching behavior and explain how they can be realized using heterostructures of 2D semiconductors.
    19. 19
      Wu, P.; Ameen, T.; Zhang, H.; Bendersky, L. A.; Ilatikhameneh, H.; Klimeck, G.; Rahman, R.; Davydov, A. V.; Appenzeller, J. Complementary Black Phosphorus Tunneling Field-Effect Transistors. ACS Nano 2019, 13, 377– 385,  DOI: 10.1021/acsnano.8b06441
      19
      Complementary black phosphorus tunneling field-effect transistors
      Wu, Peng; Ameen, Tarek; Zhang, Huairuo; Bendersky, Leonid A.; Ilatikhameneh, Hesameddin; Klimeck, Gerhard; Rahman, Rajib; Davydov, Albert V.; Appenzeller, Joerg
      ACS Nano (2019), 13 (1), 377-385CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Band-to-band tunneling field-effect transistors (TFETs) have emerged as promising candidates for low-power integration circuits beyond conventional metal-oxide-semiconductor field-effect transistors (MOSFETs) and have been demonstrated to overcome the thermionic limit, which results intrinsically in sub-threshold swings of at least 60 mV/dec at room temp. Here, we demonstrate complementary TFETs based on few-layer black phosphorus, in which multiple top gates create electrostatic doping in the source and drain regions. By elec. tuning the doping types and levels in the source and drain regions, the device can be reconfigured to allow for TFET or MOSFET operation and can be tuned to be n-type or p-type. Owing to the proper choice of materials and careful engineering of device structures, record-high current densities have been achieved in 2D TFETs. Full-band atomistic quantum transport simulations of the fabricated devices agree quant. with the current-voltage measurements, which gives credibility to the promising simulation results of ultrascaled phosphorene TFETs. Using atomistic simulations, we project substantial improvements in the performance of the fabricated TFETs when channel thicknesses and oxide thicknesses are scaled down.
    20. 20
      Huang, L.; Dong, B.; Guo, X.; Chang, Y.; Chen, N.; Huang, X.; Liao, W.; Zhu, C.; Wang, H.; Lee, C.; Ang, K.-W. Waveguide-Integrated Black Phosphorus Photodetector for Mid-Infrared Applications. ACS Nano 2019, 13, 913– 921,  DOI: 10.1021/acsnano.8b08758
      20
      Waveguide-Integrated Black Phosphorus Photodetector for Mid-Infrared Applications
      Huang, Li; Dong, Bowei; Guo, Xin; Chang, Yuhua; Chen, Nan; Huang, Xin; Liao, Wugang; Zhu, Chunxiang; Wang, Hong; Lee, Chengkuo; Ang, Kah-Wee
      ACS Nano (2019), 13 (1), 913-921CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      An integration of Si-on-insulator (SOI) waveguides with black P (BP) photodetectors was realized. When operating near BP's cutoff wavelength where absorption is weak, the light-BP interaction is enhanced by exploiting the optical confinement in the Si waveguide and grating structure to overcome the limitation of absorption length constrained by the BP thickness. Devices with different BP crystal orientation and thickness are compared in terms of their responsivity and noise equiv. power (NEP). Spectral photoresponse at 3.68-4.03 μm was studied. Power-dependent responsivity and gate-tunable photocurrent were studied. At a bias of 1 V, the BP photodetector achieved a responsivity of 23 A/W at 3.68 μm and 2 A/W at 4 μm and a NEP <1 nW/Hz1/2 at room temp. The integration of passive Si photonics and active BP photodetector is envisaged to offer a potential pathway toward the realization of integrated on-chip systems for MIR sensing applications.
    21. 21
      Rao, G.; Wang, X.; Wang, Y.; Wangyang, P.; Yan, C.; Chu, J.; Xue, L.; Gong, C.; Huang, J.; Xiong, J.; Li, Y. Two-Dimensional Heterostructure Promoted Infrared Photodetection Devices. InfoMat 2019, 1, 272– 288,  DOI: 10.1002/inf2.12018
      21
      Two-dimensional heterostructure promoted infrared photodetection devices
      Rao, Gaofeng; Wang, Xuepeng; Wang, Yang; Wangyang, Peihua; Yan, Chaoyi; Chu, Junwei; Xue, Lanxin; Gong, Chuanhui; Huang, Jianwen; Xiong, Jie; Li, Yanrong
      InfoMat (2019), 1 (3), 272-288CODEN: INFOHH; ISSN:2567-3165. (John Wiley & Sons Australia, Ltd.)
      It is a rapidly developed subject in expanding the fundamental properties and application of two-dimensional (2D) materials. The weak van der Waals interaction in 2D materials inspired researchers to explore 2D heterostructures (2DHs) based broadband photodetectors in the far-IR (IR) and middle-IR regions with high response and high detectivity. This review focuses on the strategy and motivation of designing 2DHs based high-performance IR photodetectors, which provides a wide view of this field and new expectation for advanced photodetectors. First, the photocarriers' generation mechanism and frequently employed device structures are presented. Then, the 2DHs are divided into semimetal/semiconductor 2DHs, semiconductor/semiconductor 2DHs, and multidimensional semi-2DHs; the advantages, motivation, mechanism, recent progress, and outlook are discussed. Finally, the challenges for next-generation photodetectors are described for this rapidly developing field.
    22. 22
      Buscema, M.; Island, J. O.; Groenendijk, D. J.; Blanter, S. I.; Steele, G. A.; van der Zant, H. S. J.; Castellanos-Gomez, A. Photocurrent Generation with Two-Dimensional van der Waals Semiconductors. Chem. Soc. Rev. 2015, 44, 3691– 3718,  DOI: 10.1039/C5CS00106D
      22
      Photocurrent generation with two-dimensional van der Waals semiconductors
      Buscema, Michele; Island, Joshua O.; Groenendijk, Dirk J.; Blanter, Sofya I.; Steele, Gary A.; van der Zant, Herre S. J.; Castellanos-Gomez, Andres
      Chemical Society Reviews (2015), 44 (11), 3691-3718CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      Two-dimensional (2D) materials have attracted a great deal of interest in recent years. This family of materials allows for the realization of versatile electronic devices and holds promise for next-generation (opto)electronics. Their electronic properties strongly depend on the no. of layers, making them interesting from a fundamental standpoint. For electronic applications, semiconducting 2D materials benefit from sizable mobilities and large on/off ratios, due to the large modulation achievable via the gate field-effect. Moreover, being mech. strong and flexible, these materials can withstand large strain (>10%) before rupture, making them interesting for strain engineering and flexible devices. Even in their single layer form, semiconducting 2D materials have demonstrated efficient light absorption, enabling large responsivity in photodetectors. Therefore, semiconducting layered 2D materials are strong candidates for optoelectronic applications, esp. for photodetection. Here, we review the state-of-the-art in photodetectors based on semiconducting 2D materials, focusing on the transition metal dichalcogenides, novel van der Waals materials, black phosphorus, and heterostructures.
    23. 23
      Geim, A. K.; Grigorieva, I. V. van der Waals Heterostructures. Nature 2013, 499, 419– 425,  DOI: 10.1038/nature12385
      23
      Van der Waals heterostructures
      Geim, A. K.; Grigorieva, I. V.
      Nature (London, United Kingdom) (2013), 499 (7459), 419-425CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
      A review. Research on graphene and other two-dimensional at. crystals is intense and is likely to remain one of the leading topics in condensed matter physics and materials science for many years. Looking beyond this field, isolated at. planes can also be reassembled into designer heterostructures made layer by layer in a precisely chosen sequence. The first, already remarkably complex, such heterostructures (often referred to as van der Waals') have recently been fabricated and investigated, revealing unusual properties and new phenomena. Here we review this emerging research area and identify possible future directions. With steady improvement in fabrication techniques and using graphene's springboard, van der Waals heterostructures should develop into a large field of their own.
    24. 24
      Yang, T.; Zheng, B.; Wang, Z.; Xu, T.; Pan, C.; Zou, J.; Zhang, X.; Qi, Z.; Liu, H.; Feng, Y.; Hu, W.; Miao, F.; Sun, L.; Duan, X.; Pan, A. van der Waals Epitaxial Growth and Optoelectronics of Large-Scale WSe2/SnS2 Vertical Bilayer p-n Junctions. Nat. Commun. 2017, 8, 1906,  DOI: 10.1038/s41467-017-02093-z
      24
      Van der Waals epitaxial growth and optoelectronics of large-scale WSe2/SnS2 vertical bilayer p-n junctions
      Yang Tiefeng; Zheng Biyuan; Zou Juan; Zhang Xuehong; Qi Zhaoyang; Liu Hongjun; Feng Yexin; Pan Anlian; Wang Zhen; Hu Weida; Xu Tao; Sun Litao; Pan Chen; Miao Feng; Duan Xiangfeng
      Nature communications (2017), 8 (1), 1906 ISSN:.
      High-quality two-dimensional atomic layered p-n heterostructures are essential for high-performance integrated optoelectronics. The studies to date have been largely limited to exfoliated and restacked flakes, and the controlled growth of such heterostructures remains a significant challenge. Here we report the direct van der Waals epitaxial growth of large-scale WSe2/SnS2 vertical bilayer p-n junctions on SiO2/Si substrates, with the lateral sizes reaching up to millimeter scale. Multi-electrode field-effect transistors have been integrated on a single heterostructure bilayer. Electrical transport measurements indicate that the field-effect transistors of the junction show an ultra-low off-state leakage current of 10(-14) A and a highest on-off ratio of up to 10(7). Optoelectronic characterizations show prominent photoresponse, with a fast response time of 500 μs, faster than all the directly grown vertical 2D heterostructures. The direct growth of high-quality van der Waals junctions marks an important step toward high-performance integrated optoelectronic devices and systems.
    25. 25
      Bai, X.; Li, S.; Das, S.; Du, L.; Dai, Y.; Yao, L.; Raju, R.; Du, M.; Lipsanen, H.; Sun, Z. Single-Step Chemical Vapour Deposition of Anti-Pyramid MoS2/WS2 Vertical Heterostructures. Nanoscale 2021, 13, 4537– 4542,  DOI: 10.1039/D0NR08281C
      25
      Single-step chemical vapour deposition of anti-pyramid MoS2/WS2 vertical heterostructures
      Bai, Xueyin; Li, Shisheng; Das, Susobhan; Du, Luojun; Dai, Yunyun; Yao, Lide; Raju, Ramesh; Du, Mingde; Lipsanen, Harri; Sun, Zhipei
      Nanoscale (2021), 13 (8), 4537-4542CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      Van der Waals heterostructures are the fundamental building blocks of electronic and optoelectronic devices. Here we report that, through a single-step chem. vapor deposition (CVD) process, high-quality vertical bilayer MoS2/WS2 heterostructures with a grain size up to ~ 60μm can be synthesized from molten salt precursors, Na2MoO4 and Na2WO4. Instead of normal pyramid vertical heterostructures grown by CVD, this method synthesizes an anti-pyramid MoS2/WS2 structure, which is characterized by Raman, photoluminescence and second harmonic generation microscopy. Our facile CVD strategy for synthesizing anti-pyramid structures unveils a new synthesis route for the products of two-dimensional heterostructures and their devices for application.
    26. 26
      Du, M.; Du, L.; Wei, N.; Liu, W.; Bai, X.; Sun, Z. Dual-Gated Mono-Bilayer Graphene Junctions. Nanoscale Adv. 2021, 3, 399– 406,  DOI: 10.1039/D0NA00547A
      26
      Dual-gated mono-bilayer graphene junctions
      Du, Mingde; Du, Luojun; Wei, Nan; Liu, Wei; Bai, Xueyin; Sun, Zhipei
      Nanoscale Advances (2021), 3 (2), 399-406CODEN: NAADAI; ISSN:2516-0230. (Royal Society of Chemistry)
      A lateral junction with an atomically sharp interface is extensively studied in fundamental research and plays a key role in the development of electronics, photonics and optoelectronics. Here, we demonstrate an elec. tunable lateral junction at atomically sharp interfaces between dual-gated mono- and bilayer graphene. The transport properties of the mono-bilayer graphene interface are systematically investigated with Ids-Vds curves and transfer curves, which are measured with bias voltage Vds applied in opposite directions across the asym. mono-bilayer interface. Nearly 30% difference between the output Ids-Vds curves of graphene channels measured at opposite Vds directions is obsd. Furthermore, the measured transfer curves confirm that the conductance difference of graphene channels greatly depends on the doping level, which is detd. by dual-gating. The Vds direction dependent conductance difference indicates the existence of a gate tunable junction in the mono-bilayer graphene channel, due to different band structures of monolayer graphene with zero bandgap and bilayer graphene with a bandgap opened by dual-gating. Simulation of the Ids-Vds curves based on a new numerical model validates the gate tunable junction at the mono-bilayer graphene interface from another point of view. The dual-gated mono-bilayer graphene junction and new protocol for Ids-Vds curve simulation pave a possible way for functional applications of graphene in next-generation electronics.
    27. 27
      Liu, Y.; Huang, Y.; Duan, X. van der Waals Integration before and Beyond Two-Dimensional Materials. Nature 2019, 567, 323– 333,  DOI: 10.1038/s41586-019-1013-x
      27
      Van der Waals integration before and beyond two-dimensional materials
      Liu, Yuan; Huang, Yu; Duan, Xiangfeng
      Nature (London, United Kingdom) (2019), 567 (7748), 323-333CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
      A review. Material integration strategies, such as epitaxial growth, usually involve strong chem. bonds and are typically limited to materials with strict structure matching and processing compatibility. Van der Waals integration, in which pre-fabricated building blocks are phys. assembled together through weak van der Waals interactions, offers an alternative bond-free integration strategy without lattice and processing limitations, as exemplified by 2-dimensional van der Waals heterostructures. Here, the authors review the development, challenges and opportunities of this emerging approach, generalizing it for flexible integration of diverse material systems beyond two dimensions, and discuss its potential for creating artificial heterostructures or superlattices beyond the reach of existing materials.
    28. 28
      Xu, Y.; Cheng, C.; Du, S.; Yang, J.; Yu, B.; Luo, J.; Yin, W.; Li, E.; Dong, S.; Ye, P.; Duan, X. Contacts between Two- and Three-Dimensional Materials: Ohmic, Schottky, and p-n Heterojunctions. ACS Nano 2016, 10, 4895– 4919,  DOI: 10.1021/acsnano.6b01842
      28
      Contacts between Two- and Three-Dimensional Materials: Ohmic, Schottky, and p-n Heterojunctions
      Xu, Yang; Cheng, Cheng; Du, Sichao; Yang, Jianyi; Yu, Bin; Luo, Jack; Yin, Wenyan; Li, Erping; Dong, Shurong; Ye, Peide; Duan, Xiangfeng
      ACS Nano (2016), 10 (5), 4895-4919CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      A review. After a decade of intensive research on two-dimensional (2D) materials inspired by the discovery of graphene, the field of 2D electronics has reached a stage with booming materials and device architectures. However, the efficient integration of 2D functional layers with three-dimensional (3D) systems remains a significant challenge, limiting device performance and circuit design. In this review, the exptl. efforts in interfacing 2D layers with 3D materials is investigated and the properties of the heterojunctions formed between them analyzed. The contact resistivity of metal on graphene and related 2D materials deserves special attention, while the Schottky junctions formed between metal/2D semiconductor or graphene/3D semiconductor call for careful reconsideration of the phys. models describing the junction behavior. The combination of 2D and 3D semiconductors presents a form of p-n junctions that have just marked their debut. For each type of the heterojunctions, the potential applications are reviewed briefly.
    29. 29
      Wei, W.; Yang, S.; Wang, G.; Zhang, T.; Pan, W.; Cai, Z.; Yang, Y.; Zheng, L.; He, P.; Wang, L.; Baktash, A.; Zhang, Q.; Liu, L.; Wang, Y.; Ding, G.; Kang, Z.; Yakobson, B. I.; Searles, D. J.; Yuan, Q. Bandgap Engineering of Two-Dimensional C3N Bilayers. Nat. Electron. 2021, 4, 486– 494,  DOI: 10.1038/s41928-021-00602-z
      29
      Bandgap engineering of two-dimensional C3N bilayers
      Wei, Wenya; Yang, Siwei; Wang, Gang; Zhang, Teng; Pan, Wei; Cai, Zenghua; Yang, Yucheng; Zheng, Li; He, Peng; Wang, Lei; Baktash, Ardeshir; Zhang, Quanzhen; Liu, Liwei; Wang, Yeliang; Ding, Guqiao; Kang, Zhenhui; Yakobson, Boris I.; Searles, Debra J.; Yuan, Qinghong
      Nature Electronics (2021), 4 (7), 486-494CODEN: NEALB3; ISSN:2520-1131. (Nature Portfolio)
      Abstr.: Carbon materials such as graphene are of potential use in the development of electronic devices because of properties such as high mech. strength and elec. and thermal cond. However, tech. challenges, including difficulties in generating and modulating bandgaps, have limited the application of such materials. Here we show that the bandgaps of bilayers of two-dimensional C3N can be engineered by controlling the stacking order or applying an elec. field. AA' stacked C3N bilayers are found to have a smaller bandgap (0.30 eV) than AB' stacked bilayers (0.89 eV), and both bandgaps are lower than that of monolayer C3N (1.23 eV). The larger bandgap redn. obsd. in AA' stacked bilayers, compared with AB' stacked bilayers, is attributed to the greater pz-orbital overlap. By applying an elec. field of ∼1.4 V nm-1, a bandgap modulation of around 0.6 eV can be achieved in the AB' structure. We also show that the C3N bilayers can offer controllable on/off ratios, high carrier mobilities and photoelec. detection capabilities.
    30. 30
      Frisenda, R.; Molina-Mendoza, A. J.; Mueller, T.; Castellanos-Gomez, A.; van der Zant, H. S. J. Atomically Thin p-n Junctions Based on Two-Dimensional Materials. Chem. Soc. Rev. 2018, 47, 3339– 3358,  DOI: 10.1039/C7CS00880E
      30
      Atomically thin p-n junctions based on two-dimensional materials
      Frisenda, Riccardo; Molina-Mendoza, Aday J.; Mueller, Thomas; Castellanos-Gomez, Andres; van der Zant, Herre S. J.
      Chemical Society Reviews (2018), 47 (9), 3339-3358CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      A review. Recent research in two-dimensional (2D) materials has boosted a renovated interest in the p-n junction, one of the oldest elec. components which can be used in electronics and optoelectronics. 2D materials offer remarkable flexibility to design novel p-n junction device architectures, not possible with conventional bulk semiconductors. In this Review we thoroughly describe the different 2D p-n junction geometries studied so far, focusing on vertical (out-of-plane) and lateral (in-plane) 2D junctions and on mixed-dimensional junctions. We discuss the assembly methods developed to fabricate 2D p-n junctions making a distinction between top-down and bottom-up approaches. We also revise the literature studying the different applications of these atomically thin p-n junctions in electronic and optoelectronic devices. We discuss expts. on 2D p-n junctions used as current rectifiers, photodetectors, solar cells and light emitting devices. The important electronics and optoelectronics parameters of the discussed devices are listed in a table to facilitate their comparison. We conclude the Review with a crit. discussion about the future outlook and challenges of this incipient research field.
    31. 31
      Han, G. H.; Duong, D. L.; Keum, D. H.; Yun, S. J.; Lee, Y. H. van der Waals Metallic Transition Metal Dichalcogenides. Chem. Rev. 2018, 118, 6297– 6336,  DOI: 10.1021/acs.chemrev.7b00618
      31
      van der Waals metallic transition metal dichalcogenides
      Han, Gang Hee; Duong, Dinh Loc; Keum, Dong Hoon; Yun, Seok Joon; Lee, Young Hee
      Chemical Reviews (Washington, DC, United States) (2018), 118 (13), 6297-6336CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      Transition metal dichalcogenides are layered materials which are composed of transition metals and chalcogens of the group VIA in a 1:2 ratio. These layered materials have been extensively investigated over synthesis and optical and elec. properties for several decades. It can be insulators, semiconductors, or metals revealing all types of condensed matter properties from a magnetic lattice distorted to superconducting characteristics. Some of these also feature the topol. manner. Instead of covering the semiconducting properties of transition metal dichalcogenides, which have been extensively revisited and reviewed elsewhere, here we present the structures of metallic transition metal dichalcogenides and their synthetic approaches for not only high-quality wafer-scale samples using conventional methods (e.g., chem. vapor transport, chem. vapor deposition) but also local small areas by a modification of the materials using Li intercalation, electron beam irradn., light illumination, pressures, and strains. Some representative band structures of metallic transition metal dichalcogenides and their strong layer-dependence are reviewed and updated, both in theor. calcns. and expts. In addn., we discuss the phys. properties of metallic transition metal dichalcogenides such as periodic lattice distortion, magnetoresistance, supercond., topol. insulator, and Weyl semimetal. Approaches to overcome current challenges related to these materials are also proposed.
    32. 32
      Wang, B.; Xia, W.; Li, S.; Wang, K.; Yang, S. A.; Guo, Y.; Xue, J. One-Dimensional Metal Embedded in Two-Dimensional Semiconductor in Nb2Six-1Te4. ACS Nano 2021, 15, 7149– 7154,  DOI: 10.1021/acsnano.1c00320
      32
      One-Dimensional Metal Embedded in Two-Dimensional Semiconductor in Nb2Six-1Te4
      Wang, Binbin; Xia, Wei; Li, Si; Wang, Kang; Yang, Shengyuan A.; Guo, Yanfeng; Xue, Jiamin
      ACS Nano (2021), 15 (4), 7149-7154CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      The ternary van der Waals material Nb2Six-1Te4 demonstrates many interesting properties as the content of Si is changed, ranging from metallic Nb3SiTe6 (x = 5/3) to narrow-gap semiconductor Nb2SiTe4 (x = 2) and with the emergence of 1-dimensional Dirac fermion excitations in between. An in-depth understanding of their properties with different stoichiometry is important. Here the authors use scanning tunneling microscopy and spectroscopy to reveal that Nb2Six-1Te4 is a system with spontaneously developed and self-aligned 1-dimensional metallic chains embedded in a 2-dimensional semiconductor. Electron quasiparticles form 1- and 2-dimensional standing waves side by side. This special microscopic structure results in strong transport anisotropy. Along the chain direction the material behaves like a metal, while perpendicular to the chain direction, it behaves like a semiconductor. These findings provide an important basis for further study of this intriguing system.
    33. 33
      Lebègue, S.; Björkman, T.; Klintenberg, M.; Nieminen, R. M.; Eriksson, O. Two-Dimensional Materials from Data Filtering and Ab Initio Calculations. Phys. Rev. X 2013, 3, 031002,  DOI: 10.1103/PhysRevX.3.031002
      33
      Two-dimensional materials from data filtering and Ab initio calculations
      Lebegue, S.; Bjorkman, T.; Klintenberg, M.; Nieminen, R. M.; Eriksson, O.
      Physical Review X (2013), 3 (3), 031002CODEN: PRXHAE; ISSN:2160-3308. (American Physical Society)
      Progress in materials science depends on the ability to discover new materials and to obtain and understand their properties. This has recently become particularly apparent for compds. with reduced dimensionality, which often display unexpected phys. and chem. properties, making them very attractive for applications in electronics, graphene being so far the most noteworthy example. Here, we report some previously unknown two-dimensional materials and their electronic structure by data mining among crystal structures listed in the International Crystallog. Structural Database, combined with d.-functional-theory calcns. As a result, we propose to explore the synthesis of a large group of two-dimensional materials, with properties suggestive of applications in nanoscale devices and anticipate further studies of electronic and magnetic phenomena in low-dimensional systems.
    34. 34
      Zhang, C.; Gong, C.; Nie, Y.; Min, K.-A.; Liang, C.; Oh, Y. J.; Zhang, H.; Wang, W.; Hong, S.; Colombo, L.; Wallace, R. M.; Cho, K. Systematic Study of Electronic Structure and Band Alignment of Monolayer Transition Metal Dichalcogenides in van der Waals Heterostructures. 2D Mater. 2017, 4, 015026,  DOI: 10.1088/2053-1583/4/1/015026
      34
      Systematic study of electronic structure and band alignment of monolayer transition metal dichalcogenides in Van der Waals heterostructures
      Zhang, Chenxi; Gong, Cheng; Nie, Yifan; Min, Kyung-Ah; Liang, Chaoping; Oh, Young Jun; Zhang, Hengji; Wang, Weihua; Hong, Suklyun; Colombo, Luigi; Wallace, Robert M.; Cho, Kyeongjae
      2D Materials (2017), 4 (1), 015026/1-015026/10CODEN: DMATB7; ISSN:2053-1583. (IOP Publishing Ltd.)
      Two-dimensional transition metal dichalcogenides (TMDs) are promising low-dimensional materials which can produce diverse electronic properties and band alignment in van der Waals heterostructures. Systematic d. functional theory (DFT) calcns. are performed for 24 different TMDmonolayers and their bilayer heterostacks. DFT calcns. show that monolayer TMDs can behave as semiconducting, metallic or semimetallic depending on their structures; we also calcd. the band alignment of the TMDs to predict their alignment in van der Waals heterostacks. We have applied the charge equilibration model (CEM) to obtain a quant. formula predicting the highest occupied state of any type of bilayerTMDheterostacks (552 pairs for 24 TMDs). The CEM predicted values agree quite well with the selected DFT simulation results. The quant. prediction of the band alignment in the TMD heterostructures can provide an insightful guidance to the development of TMD-based devices.
    35. 35
      Wu, R.; Tao, Q.; Dang, W.; Liu, Y.; Li, B.; Li, J.; Zhao, B.; Zhang, Z.; Ma, H.; Sun, G.; Duan, X.; Duan, X. van der Waals Epitaxial Growth of Atomically Thin 2D Metals on Dangling-Bond-Free WSe2 and WS2. Adv. Funct. Mater. 2019, 29, 1806611,  DOI: 10.1002/adfm.201806611
      There is no corresponding record for this reference.
    36. 36
      Li, J.; Zhao, B.; Chen, P.; Wu, R.; Li, B.; Xia, Q.; Guo, G.; Luo, J.; Zang, K.; Zhang, Z.; Ma, H.; Sun, G.; Duan, X.; Duan, X. Synthesis of Ultrathin Metallic MTe2 (M = V, Nb, Ta) Single-Crystalline Nanoplates. Adv. Mater. 2018, 30, 1801043,  DOI: 10.1002/adma.201801043
      There is no corresponding record for this reference.
    37. 37
      Zhang, Z.; Niu, J.; Yang, P.; Gong, Y.; Ji, Q.; Shi, J.; Fang, Q.; Jiang, S.; Li, H.; Zhou, X.; Gu, L.; Wu, X.; Zhang, Y. van der Waals Epitaxial Growth of 2D Metallic Vanadium Diselenide Single Crystals and Their Extra-High Electrical Conductivity. Adv. Mater. 2017, 29, 1702359,  DOI: 10.1002/adma.201702359
      There is no corresponding record for this reference.
    38. 38
      Zhao, S.; Hotta, T.; Koretsune, T.; Watanabe, K.; Taniguchi, T.; Sugawara, K.; Takahashi, T.; Shinohara, H.; Kitaura, R. Two-Dimensional Metallic NbS2: Growth, Optical Identification and Transport Properties. 2D Mater. 2016, 3, 025027,  DOI: 10.1088/2053-1583/3/2/025027
      38
      Two-dimensional metallic NbS2: growth, optical identification and transport properties
      Zhao, Sihan; Hotta, Takato; Koretsune, Takashi; Watanabe, Kenji; Taniguchi, Takashi; Sugawara, Katsuaki; Takahashi, Takashi; Shinohara, Hisanori; Kitaura, Ryo
      2D Materials (2016), 3 (2), 025027/1-025027/9CODEN: DMATB7; ISSN:2053-1583. (IOP Publishing Ltd.)
      Progress on researches of two-dimensional (2D) metals strongly relies on development of the growth technique. Studies on prepn. of 2D metals have so far been limited, and this is in stark contrast to the situation of 2D semiconductors, where various layered semiconductors, including MoS2,WS2, MoSe2, WSe2, have been isolated in its monolayer form. In this work, we have developed a facile method to prep. 2D metallic transition metal dichalcogenides (TMDCs) by chem. vapor deposition (CVD) method, where direct growth of few-layered NbS2 (3R phase) on atomically flat hexagonal boron nitride (hBN) has been demonstrated. Structural characterization of the so-grown NbS2 was performed with at. force microscopy, optical microscopy, electron microscopy and optical spectroscopy, revealing that the utilization of hBN as growth substrates is a key factor for the first successful CVD growth of 2D metallic TMDCs with large single-domain size (several μm). Elec. transport measurements have clearly shown that NbS2 at. layers down to few-layer-thickness are metal. The current study opens up a new synthetic route for controllable growth of 2D layered metallic materials, which is of great importance in study of rich physics in 2D metals, as well as in search for novel 2D superconductors.
    39. 39
      Zhang, Y.; Yin, L.; Chu, J.; Shifa, T. A.; Xia, J.; Wang, F.; Wen, Y.; Zhan, X.; Wang, Z.; He, J. Edge-Epitaxial Growth of 2D NbS2-WS2 Lateral Metal-Semiconductor Heterostructures. Adv. Mater. 2018, 30, 1803665,  DOI: 10.1002/adma.201803665
      There is no corresponding record for this reference.
    40. 40
      Man, M. K. L.; Margiolakis, A.; Deckoff-Jones, S.; Harada, T.; Wong, E. L.; Krishna, M. B. M.; Madéo, J.; Winchester, A.; Lei, S.; Vajtai, R.; Ajayan, P. M.; Dani, K. M. Imaging the Motion of Electrons across Semiconductor Heterojunctions. Nat. Nanotechnol. 2017, 12, 36– 40,  DOI: 10.1038/nnano.2016.183
      40
      Imaging the motion of electrons across semiconductor heterojunctions
      Man, Michael K. L.; Margiolakis, Athanasios; Deckoff-Jones, Skylar; Harada, Takaaki; Wong, E. Laine; Krishna, M. Bala Murali; Madeo, Julien; Winchester, Andrew; Lei, Sidong; Vajtai, Robert; Ajayan, Pulickel M.; Dani, Keshav M.
      Nature Nanotechnology (2017), 12 (1), 36-40CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
      Technol. progress since the late twentieth century has centered on semiconductor devices, such as transistors, diodes and solar cells. At the heart of these devices is the internal motion of electrons through semiconductor materials due to applied elec. fields or by the excitation of photocarriers. Imaging the motion of these electrons would provide unprecedented insight into this important phenomenon, but requires high spatial and temporal resoln. Current studies of electron dynamics in semiconductors are generally limited by the spatial resoln. of optical probes, or by the temporal resoln. of electronic probes. Here, by combining femtosecond pump-probe techniques with spectroscopic photoemission electron microscopy, we imaged the motion of photoexcited electrons from high-energy to low-energy states in a type-II 2D InSe/GaAs heterostructure. At the instant of photoexcitation, energy-resolved photoelectron images revealed a highly non-equil. distribution of photocarriers in space and energy. Thereafter, in response to the out-of-equil. photocarriers, we obsd. the spatial redistribution of charges, thus forming internal elec. fields, bending the semiconductor bands, and finally impeding further charge transfer. By assembling images taken at different time-delays, we produced a movie lasting a few trillionths of a second of the electron-transfer process in the photoexcited type-II heterostructure-a fundamental phenomenon in semiconductor devices such as solar cells. Quant. anal. and theor. modeling of spatial variations in the movie provide insight into future solar cells, 2D materials and other semiconductor devices.
    41. 41
      Majidi, L.; Yasaei, P.; Warburton, R. E.; Fuladi, S.; Cavin, J.; Hu, X.; Hemmat, Z.; Cho, S. B.; Abbasi, P.; Vörös, M.; Cheng, L.; Sayahpour, B.; Bolotin, I. L.; Zapol, P.; Greeley, J.; Klie, R. F.; Mishra, R.; Khalili-Araghi, F.; Curtiss, L. A.; Salehi-Khojin, A. New Class of Electrocatalysts Based on 2D Transition Metal Dichalcogenides in Ionic Liquid. Adv. Mater. 2019, 31, 1804453,  DOI: 10.1002/adma.201804453
      There is no corresponding record for this reference.
    42. 42
      Zhou, X.; Hu, X.; Yu, J.; Liu, S.; Shu, Z.; Zhang, Q.; Li, H.; Ma, Y.; Xu, H.; Zhai, T. 2D Layered Material-Based van der Waals Heterostructures for Optoelectronics. Adv. Funct. Mater. 2018, 28, 1706587,  DOI: 10.1002/adfm.201706587
      There is no corresponding record for this reference.
    43. 43
      Yan, Y.; Li, S.; Du, J.; Yang, H.; Wang, X.; Song, X.; Li, L.; Li, X.; Xia, C.; Liu, Y.; Li, J.; Wei, Z. Reversible Half Wave Rectifier Based on 2D InSe/GeSe Heterostructure with near-Broken Band Alignment. Adv. Sci. 2021, 8, 1903252,  DOI: 10.1002/advs.201903252
      43
      Reversible Half Wave Rectifier Based on 2D InSe/GeSe Heterostructure with Near-Broken Band Alignment
      Yan, Yong; Li, Shasha; Du, Juan; Yang, Huai; Wang, Xiaoting; Song, Xiaohui; Li, Lixia; Li, Xueping; Xia, Congxin; Liu, Yufang; Li, Jingbo; Wei, Zhongming
      Advanced Science (Weinheim, Germany) (2021), 8 (4), 1903252CODEN: ASDCCF; ISSN:2198-3844. (Wiley-VCH Verlag GmbH & Co. KGaA)
      2D van der Waals heterostructures (vdWHs) offer tremendous opportunities in designing multifunctional electronic devices. Due to the ultrathin nature of 2D materials, the gate-induced change in charge d. makes amplitude control possible, creating a new programmable unilateral rectifier. The study of 2D vdWHs-based reversible unilateral rectifier is lacking, although it can give rise to a new degree of freedom for modulating the output state. Here, a InSe/GeSe vdWH-FET is constructed as a gate-controllable half wave rectifier. The device exhibits stepless adjustment from forward to backward rectifying performance, leading to multiple operation states of output level. Near-broken band alignment in the InSe/GeSe vdWH-FET is a crucial feature for high-performance reversible rectifier, which is shown to have backward and forward rectification ratio of 1:38 and 963:1, resp. Being further explored as a new bridge rectifier, the InSe/GeSe device has great potential in future gate-controllable a.c./d.c. converter. These results indicate that 2D vdWHs with near-broken band alignment can offer a pathway to simplify the commutating circuit and regulating speed circuit.
    44. 44
      Lei, S.; Ge, L.; Najmaei, S.; George, A.; Kappera, R.; Lou, J.; Chhowalla, M.; Yamaguchi, H.; Gupta, G.; Vajtai, R.; Mohite, A. D.; Ajayan, P. M. Evolution of the Electronic Band Structure and Efficient Photo-Detection in Atomic Layers of InSe. ACS Nano 2014, 8, 1263– 1272,  DOI: 10.1021/nn405036u
      44
      Evolution of the Electronic Band Structure and Efficient Photo-Detection in Atomic Layers of InSe
      Lei, Sidong; Ge, Liehui; Najmaei, Sina; George, Antony; Kappera, Rajesh; Lou, Jun; Chhowalla, Manish; Yamaguchi, Hisato; Gupta, Gautam; Vajtai, Robert; Mohite, Aditya D.; Ajayan, Pulickel M.
      ACS Nano (2014), 8 (2), 1263-1272CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Atomic layers of two-dimensional (2D) materials have recently been the focus of extensive research. This follows from the footsteps of graphene, which has shown great potential for ultrathin optoelectronic devices. In this paper, we present a comprehensive study on the synthesis, characterization, and thin film photodetector application of at. layers of InSe. Correlation between resonance Raman spectroscopy and photocond. measurements allows us to systematically track the evolution of the electronic band structure of 2D InSe as its thickness approaches few at. layers. Anal. of photocond. spectra suggests that few-layered InSe has an indirect band gap of 1.4 eV, which is 200 meV higher than bulk InSe due to the suppressed interlayer electron orbital coupling. Temp.-dependent photocurrent measurements reveal that the suppressed interlayer interaction also results in more localized pz-like orbitals, and these orbitals couple strongly with the in-plane E' and E'' phonons. Finally, we measured a strong photoresponse of 34.7 mA/W and fast response time of 488 μs for a few layered InSe, suggesting that it is a good material for thin film optoelectronic applications.
    45. 45
      Erdogan, H.; Kirby, R. D. Raman Spectrum and Lattice Dynamics of NbTe2. Solid State Commun. 1989, 70, 713– 715,  DOI: 10.1016/0038-1098(89)90987-3
      45
      Raman spectrum and lattice dynamics of niobium ditelluride
      Erdogan, Hasan; Kirby, Roger D.
      Solid State Communications (1989), 70 (7), 713-15CODEN: SSCOA4; ISSN:0038-1098.
      Raman scattering measurements on the layered-structure compd. NbTe2 are reported. No evidences for a phase transition is found for 80-420 K. A group-theor. anal. of the crystal structure predicts 51 optic phonon branches, with 24 of these being Raman-active. The obsd. Raman spectrum was interpreted in terms of the phonon branches of the simpler CdI2 structure.
    46. 46
      Qin, F.; Gao, F.; Dai, M.; Hu, Y.; Yu, M.; Wang, L.; Feng, W.; Li, B.; Hu, P. Multilayer InSe-Te van der Waals Heterostructures with an Ultrahigh Rectification Ratio and Ultrasensitive Photoresponse. ACS Appl. Mater. Interfaces 2020, 12, 37313– 37319,  DOI: 10.1021/acsami.0c08461
      46
      Multilayer InSe-Te van der Waals Heterostructures with an Ultrahigh Rectification Ratio and Ultrasensitive Photoresponse
      Qin, Fanglu; Gao, Feng; Dai, Mingjin; Hu, Yunxia; Yu, Miaomiao; Wang, Lifeng; Feng, Wei; Li, Bin; Hu, PingAn
      ACS Applied Materials & Interfaces (2020), 12 (33), 37313-37319CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      Multilayer van der Waals (vdWs) semiconductors have promising applications in high-performance optoelectronic devices. However, photoconductive photodetectors based on layered semiconductors often suffer from sizeable dark currents and high external driving bias voltages. Here, we report vertical van der Waals heterostructures (vdWHs) consisting of multilayer indium selenide (InSe) and tellurium (Te). The multilayer InSe-Te vdWH device shows a record high forward rectification ratio greater than 107 at room temp. The vdWH device achieves an ultrasensitive and broadband photoresponse photodetector with an ultrahigh photo/dark current ratio over 104 and a high detectivity of 1013 Jones under visible light illumination with weak incident power. Moreover, the vdWH device has a photovoltaic effect and can function as a self-powered photodetector (SPPD). The SPPD is also ultrasensitive to a broadband spectrum ranging from 300 to 1000 nm and is capable of detecting weak light signals. This work offers an opportunity to develop next-generation electronic and optoelectronic devices based on multilayer vdWs materials.
    47. 47
      Baugher, B. W. H.; Churchill, H. O. H.; Yang, Y.; Jarillo-Herrero, P. Optoelectronic Devices Based on Electrically Tunable p-n Diodes in a Monolayer Dichalcogenide. Nat. Nanotechnol. 2014, 9, 262– 267,  DOI: 10.1038/nnano.2014.25
      47
      Optoelectronic devices based on electrically tunable p-n diodes in a monolayer dichalcogenide
      Baugher, Britton W. H.; Churchill, Hugh O. H.; Yang, Yafang; Jarillo-Herrero, Pablo
      Nature Nanotechnology (2014), 9 (4), 262-267CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
      The p-n junction is the functional element of many electronic and optoelectronic devices, including diodes, bipolar transistors, photodetectors, light-emitting diodes, and solar cells. In conventional p-n junctions, the adjacent p- and n-type regions of a semiconductor are formed by chem. doping. Ambipolar semiconductors, such as C nanotubes, nanowires and org. mols., allow for p-n junctions to be configured and modified by electrostatic gating. This elec. control enables a single device to have multiple functionalities. Here, we report ambipolar monolayer WSe2 devices in which two local gates are used to define a p-n junction within the WSe2 sheet. With these elec. tunable p-n junctions, we demonstrate both p-n and n-p diodes with ideality factors better than 2. Under optical excitation, the diodes demonstrate a photodetection responsivity of 210 mA W-1 and photovoltaic power generation with a peak external quantum efficiency of 0.2%, promising values for a nearly transparent monolayer material in a lateral device geometry. Finally, we demonstrate a light-emitting diode based on monolayer WSe2. These devices provide a building block for ultrathin, flexible and nearly transparent optoelectronic and electronic applications based on ambipolar dichalcogenide materials.

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    • Materials characterization with Raman, PL, UPS, and absorption spectra, output Ids–Vds curves and transfer curves of additional devices, photocurrent mapping at negative gate voltage, short-circuit current at 100 μW illumination, measurement of response time, and comparison of various InSe-based photodetectors (PDF)


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