Lixiang
Han
a,
Mengmeng
Yang
a,
Peiting
Wen
b,
Wei
Gao
*b,
Nengjie
Huo
*b and
Jingbo
Li
b
aSchool of Materials and Energy, Guangdong University of Technology, Guangzhou, 510006, China
bInstitute of Semiconductors, South China Normal University, Guangzhou 510631, P.R. China. E-mail: gaowei317040@m.scnu.edu.cn; njhuo@m.scnu.edu.cn
First published on 15th March 2021
One-dimensional (1D)–two-dimensional (2D) van der Waals (vdWs) mixed-dimensional heterostructures with advantages of an atomically sharp interface, high quality and good compatibility have attracted tremendous attention in recent years. Herein, a mixed-dimensional vertical heterostructure is constructed by transferring mechanically exfoliated 2D WS2 nanosheets on epitaxially grown 1D tellurium (Te) microwires. According to the theoretical type-II band alignment, the device exhibits a photovoltaic effect and serves as an excellent self-powered photodetector with a maximum open-circuit voltage (Voc) up to ∼0.2 V. Upon 635 nm light illumination, the photoresponsivity, external quantum efficiency and detectivity of the self-powered photodetector (SPPD) are calculated to be 471 mA W−1, 91% and 1.24 × 1012 Jones, respectively. Moreover, the dark current of the SPPD is highly suppressed to the sub-pA level due to the large lateral built-in electric field, which leads to a high Ilight/Idark ratio of 104 with a rise time of 25 ms and decay time of 14.7 ms. The abovementioned properties can be further enhanced under a negative bias of −2 V. In brief, the 1D Te–2D WS2 mixed-dimensional heterostructures have great application potential in high performance photodetectors and photovoltaics.
Recently, mixed-dimensional heterostructures like zero-dimensional (0D)–2D, 1D–2D and three-dimensional (3D)–2D have drawn interest from researchers due to their unique properties via integrating nanomaterials with different dimensionalities.18–28 In 2019, Shang et al. demonstrated a p-Se nanotube and n-InSe nanosheet mixed-dimensional vdW heterostructure, which shows a high photocurrent on/off ratio of 103 and a responsivity of 110 mA W−1 under zero bias with 460 nm irradiation.29 Meanwhile, Li et al. reported a strongly coupled mixed-dimensional heterostructure via epitaxially grown Te nanowires on MoS2. The heterostructure based phototransistors displayed obvious anti-ambipolar transport and rectification behavior as well as a high photoresponsivity of 103 A W−1 and a fast response time of 15 ms under 1550 nm communication wavelength.30 Above all, a number of research groups have focused on the spintronic, electronic and photo-response properties of Te nanosheets and nanowires,31–38 while the photodetection properties of Te microwire based mixed-dimensional heterostructures have rarely been reported. Noticeably, the large dark current and ultrafast electron–hole recombination rate of Te are the main disadvantages for further application because of the narrow band gap of Te in bulk. Fortunately, as a typical TMD material, WS2 shows merits of moderate bandgap (1.4–2.0 eV), high optical quality and broadband light absorption coefficiency, which make it an ideal candidate in type-II heterostructure based devices.39
In this paper, we demonstrate a mixed-dimensional heterostructure of 1D Te microwires covered by 2D WS2 nanosheets via a polyvinyl alcohol (PVA) dry transfer method. A built-in electric field forms at the heterojunction interfaces, which can efficiently accelerate the separation of the photogenerated electron–hole pairs under light illumination and deeply suppresses the dark current as well. The photodetection properties are investigated systematically with and without bias. The high responsivity, high detectivity, fast response time and high Ilight/Idark ratio of the 1D p-type Te microwire–2D n-type WS2 nanosheet mixed-dimensional heterostructure can promote the development of novel monoelemental materials for optoelectronic applications.
Raman scattering measurement was used to characterize the phonon vibrations and interlayer coupling of the Te microwire–WS2 mixed-dimensional heterostructure. Fig. 1b shows the Raman spectrum of the pristine Te microwire, WS2 and the heterostructure. The Te microwire shows three vibration peaks located at 85.6 cm−1, 115.2 cm−1 and 134.3 cm−1, which correspond to the in-plane E1, E2 and A1 (out-of-plane) vibrations, respectively.13 The in-plane E12g and out-of-plane A1g modes of the multilayered WS2 nanosheet are observed at 348.1 cm−1 and 418.7 cm−1, respectively.39 The Raman spectrum of the Te–WS2 vdW heterostructure exhibits the combination of phonon modes of both Te wire and WS2. Noticeably, the vibration modes of the overlapped Te are weakened. Interestingly, both vibration modes of WS2 are enhanced compared to those in the single part, which may be attributed to the strain effect.40Fig. 1c shows the PL of the WS2 and WS2–Te heterostructure with 532 nm laser excitation. In general, the exfoliated multilayered WS2 nanosheet shows two clear PL peaks at 660 nm and 861 nm, corresponding to a red-shifted direct optical band gap of 1.88 eV and an indirect band gap of ∼1.44 eV.39 A PL quenching effect is observed in the overlapped region for both PL peaks, which is ascribed to a strong interlayer coupling effect between Te and WS2. The PL quenching effect indicates that the photo-generated carrier separation process can be significantly accelerated under the designed type-II band alignment. Intuitively, Fig. 1d also displays the corresponding PL mapping image of the WS2 on the Te microwire from the rectangular area in Fig. 1a at 660 nm light excitation. The PL intensity of the WS2 nanosheet on top of the Te microwire (in the white area) becomes much weaker than that of WS2 without Te underneath (in green regions). A similar PL quenching effect is also observed from the PL mapping image under 860 nm light excitation shown in Fig. S3.†
The AFM image of the heterostructure is shown in Fig. 2a. The thickness of the WS2 nanosheet is estimated to be 50 nm shown in Fig. 2b. A Kelvin Probe Force Microscope (KPFM) was used to measure the built-in contact potential difference at the interface between the Te microwire and WS2. The surface potential distribution (SPD) along the area of the Te microwire, WS2 and the AFM tip can be expressed as the following equations:41
eSPDWS2 = Wtip − WWS2 | (1) |
eSPDTe = Wtip − WTe | (2) |
ΔEf = WTe − WWS2 = eSPDWS2 − eSPDTe | (3) |
Fig. 2c shows the topological image of the SPD of the heterostructure interface. The ΔEf and the depletion width along the yellow line are about 56.7 meV and 1 μm from Fig. 2d, which unveils a strong built-in electric field across the WS2–Te microwire interface.42–44 Moreover, the energy band alignments of the Te microwire and WS2 before and after contact are shown in Fig. 2e and f. In general, the indirect bandgaps of multilayered WS2 and Te are 1.4 eV and 0.35 eV, respectively. Before contact, the conduction band minima (CBMs) of the WS2 and Te microwire are approximately −4.24 eV and −4.02 eV, respectively, and the corresponding valence band maxima (VBMs) of the WS2 and Te microwire are approximately −5.64 eV and −4.37 eV, respectively.39,45 ΔEf is 56.7 meV from the KPFM measurement. Thus, the fabricated Te–WS2 heterostructure theoretically has a type-II (staggered gap) band arrangement attributing to the PL quenching effect, which can facilitate the photo-generated carrier generation and separation at the heterointerface.46 After contact, the band alignment becomes bent and the electrons and holes can transfer within interlayers via a built-in electric field pointing from n-WS2 to the p-Te microwire.47
The 3D diagram of the 635 nm laser-illuminated Te microwire–WS2 heterostructure is shown in Fig. 3a. In Fig. 3b, the mixed-dimensional heterostructure device exhibits n-type (electron dominated) transport behavior, demonstrating that the transport properties of the heterostructure mainly depend on the multilayered WS2 channel. Output characteristic curves of the device show that the drain current at a forward bias of 2 V monotonously increases as the Vg increases, which further confirms the n-type behavior and moderate gate modulation shown in Fig. 3c. The maximum rectification ratio of the device is ∼61 shown in Fig. S4c (ESI†). As a control, the transfer properties of the pristine Te microwire and multilayered WS2 nanosheet are shown in Fig. S4a and b (ESI†), where the Te microwire exhibits a strong p-type behavior with a current on/off ratio of ∼1.1 because of the ultra-narrow band gap of 0.35 eV, high conductivity in bulk and strong capacitance screening effect. Meanwhile, the multilayered WS2 nanosheet demonstrates a typical moderate n-type behavior with current on/off ratio of ∼103. Fig. 3d demonstrates the I–V curves of the mixed-dimensional heterostructure device in the range of −2 V to 0.3 V under dark conditions and 635 nm light illumination with various light power intensities. The significantly enhanced current under reverse bias compared to that under forward bias (majority carriers) is shown under light illumination because of the obviously increased minority carriers in the P–N junction. Under higher light power intensity, more photogenerated electron–hole pairs are separated by a built-in electric field and driven by the external reverse bias voltage resulting in the increment of photocurrent. The photovoltaic effect can be seen in Fig. 3d with obvious Voc and Isc, which indicates a well built-in electric field at the interface and will be discussed later.
To further evaluate the photodetection ability of the heterostructure, we calculated the photoresponsivity (Rλ), external quantum efficiency (EQE), detectivity (D*), response time and Ilight/Idark ratio of the device. In general, Rλ is used to evaluate the sensitivity of a photodetector, which is defined by the formula48
(4) |
EQE is the ratio of the number of effective photogenerated carriers to the number of incident photons, which can be expressed as48
(5) |
Specific detectivity (D*) is an important figure of merit of a photodetector, which shows the ability of a photodetector to detect a weak light signal, as calculated by the following equation:41
(6) |
Fig. 3e displays the photoresponsivity and net photocurrent as a function of light power density. The photocurrent increases with increased light power intensity. From the fitting curve which follows a power law of photocurrent and light power intensity (Iph ∼ Pα), the exponent (α) value of 1.07 is obtained shown in Fig. S5 (ESI†). The super-linear behavior may be ascribed to the decrement of Auger recombination sites leading to more photocurrent being transmitted through the pn junction without the trapping effect.49 Furthermore, due to the limited trap states, the captured carriers are saturated or reduced under high light power intensity resulting in a significant decrease of the photoresponsivity and increased Auger recombination process.26 The maximum value of photoresponsivity reaches 3.6 A W−1 at a reverse bias of −2 V shown in Fig. 3e. Fig. 3f demonstrates the dependence of EQE and D* of the mixed-dimensional photodetector on the incident light power intensity. The maximum EQE and D* are 720% and 1.34 × 1012 Jones, respectively. Fig. 3g illustrates the time-resolved response behaviors of the mixed-dimensional heterostructure with varying light power intensities at biases of 0 V (in black) and −2 V (in red). With higher light power intensity, more photogenerated electron–hole pairs can contribute to the photocurrent. Under a Vds of −2 V, the width of the depletion region is broadened and the corresponding built-in electric field of the heterostructure is enhanced.44 Therefore, the separation and collection of photogenerated electron–hole pairs are accelerated resulting in the increment of photocurrent and faster response time. Moreover, response time is one of the important parameters for the photodetector, which is defined as the time obtained from 10–90% (τrise is called rise time) to 90–10% (τdecay is called decay time) of the net photocurrent.48Fig. 3h illustrates the rise time of 9.5 ms and decay time of 9.1 ms at a bias of −2 V, which are comparable to or faster than that in previously reported 1D–2D mixed-dimensional heterostructures. The high electrical conductance of the Te microwire can contribute to the fast response speed and high responsivity as well in Fig. S4a (ESI†). As shown in Fig. 3i, the Ilight/Idark ratio of the device is as high as 103 at a bias of −2 V under light power intensity because of the low dark current in the PN junction. Furthermore, the deviation is less than 5% within 200 cycles for switching on–off behavior. In comparison, the Ilight/Idark ratio of pristine WS2 only reaches ∼20 due to a large dark current of around 10−9 A under −2 V bias. Meanwhile, the switching on–off curve of the WS2 nanosheet based photodetector exhibits poor stability shown in Fig. S4d (ESI†), which is mainly ascribed to the persistent photoconductive (PPC) effect in multilayered WS2.50 The photoresponse properties of the pristine WS2 nanosheet are shown in Fig. S6 and S7 (ESI†), which is worse than that in the heterostructure.
As we know, self-powered photodetectors are extensively desired in the field of wearable electronics and Internet of Things featuring lower power consumption or a self-sustaining wireless sensing network.51 Here, the self-powered photo-response properties of the mixed-dimensional heterostructure photodetector are intensively investigated to highlight the contact quality of the PN junction. The open-circuit voltage (Voc) of the device is induced by the built-in electric field because of the photogenerated holes accumulating at the n-WS2 side and electrons accumulating at the p-Te microwire side. Fig. 4a displays the Voc and Isc as a function of incident light power intensity. With higher light power intensity, the built-in electric field is strengthened leading to the nonlinear increase of Voc and the linear enhancement of Isc for the device. Fig. 4b shows the dependence of photocurrent and photoresponsivity on incident light power density. The maximum photoresponsivity of the self-powered heterostructure is as high as 471 mA W−1 under 0.74 mW cm−2. The exponent (α) values of 1.62 (under weak light) and 0.76 (under strong light) are obtained by fitting the measured data because of the complex transfer and recombination process of photo-carriers. Fig. 4c shows the incident light power intensity related EQE and D* of the device; the maximum EQE (91%) and D* (1.24 × 1012 Jones) are obtained under 0.74 mW cm−2, respectively.
The self-driven heterostructure device also exhibits a fast photoresponse time with a rise time of 25 ms and a decay time of 14.7 ms shown in Fig. 4d. Due to the faster speed of recombination of carriers than the generation and transport processes, the decay time is shorter than the rise time.52 As shown in Fig. 4e, the Ilight/Idark ratio is up to 104 at the dark current level of 3.1 × 10−13 A. Last but not least, the switching performance of the as-fabricated self-powered photodetector also shows negligible degradation after 300 cycles without obvious deviation. The photodetection parameters of the 1D Te–2D WS2 device compared with the previously reported 1D–2D mixed-dimensional heterostructure are shown in Table 1.
Sample | Wavelength [nm] | V ds/Vg [V] | I light/Idark | Rise/decay time [ms] | R λ [mA W−1] | EQE [%] | D* [Jones] | Ref. |
---|---|---|---|---|---|---|---|---|
ZnO–WSe2 | 520 | −5/— | — | 50 | 670 | 160 | — | 25 |
CuO–MoS2 | 570 | −2/0 | 103 | 34.6/51.9 | 157.6 | 157.6 × 103 | — | 26 and 27 |
Se–InSe | 460 | 0/0 | 103 | 30/37 | 110 | 51 | — | 29 |
Te–MoS2 | 1550 | 2/80 | 103 | 15/32 | 106 | — | 1012 | 30 |
ZnO–MoS2 | 532 | 5/0 | — | 140/8320 | 350 | 80.9 | — | 28 |
Te–WS2 | 635 | −2/0 | 1480 | 9.5/9.1 | 3690 | 720 | 1.34 × 1012 | This work |
Te–WS2 | 635 | 0/0 | 104 | 25/14.7 | 471 | 91 | 1.24 × 1012 | This work |
In addition, the photoresponsivity spectra of the mixed-dimensional heterostructure device at biases of −2 V and 0 V were also recorded and are shown in Fig. 5a. Notably, the photodetector displays a broadband photo-response ranging from 400 nm to 750 nm wavelength. Obviously, the strongest responsivity peaks are located at an approximately sharp edge of 620 nm under both conditions. The corresponding light power–wavelength diagram is shown in Fig. S8 (ESI†). The broadband photoresponse of the heterostructure device can be attributed to the highly efficient broadband optical absorption spectrum of the WS2 nanosheet.39 The photodetection properties of the mixed-dimensional heterostructure under 532 nm laser illumination were also investigated shown in Fig. S9 (ESI†). The different photoresponsivity between 532 nm and 635 nm incident light is related to the wavelength-dependence of light absorption and semiconductor energy gap.48 The photo-generated carrier transport dynamics mechanism under light illumination is illustrated in Fig. 5b. Under illumination, the photogenerated electron–hole pairs are induced in the depletion between WS2 and Te microwire interface. Meanwhile, photocurrent is generated through the separation of photo-generated electron–hole pairs in opposite directions to the metal electrode driven by the built-in electric field with and without external reverse bias voltage.47 Meanwhile, the non-radiative recombination rate is reduced within the band structure of the Te microwire.
Footnote |
† Electronic supplementary information (ESI) available: A schematic diagram of the fabrication process of the mixed-dimensional heterostructure device; a SEM image of the device; a PL mapping image of the heterostructure; transfer properties and I–t characteristics; the photocurrent dependence on the light power intensity which follows a power law of the heterostructure; light power–wavelength diagram; photoresponse properties of the mixed-dimensional heterostructrue Te microwire and WS2 nanosheet based photodetector under an incident laser of 532 nm. See DOI: 10.1039/d1na00073j |
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