In the past few decades, flexible near-infrared (NIR) photodetectors have attracted significant attention in imaging, data communication, environmental monitoring, and bioimaging applications^[1-4]. However, scalability, sensitivity and mechanical stability challenge the selection of channel semiconductors and device fabrication of flexible NIR photodetectors^[5]. Due to the excellent mechanical flexibility in comparison to the typical thin film or bulk counterparts, low-dimensional organic and inorganic semiconductors, such as one-dimensional (1D) nanowires (NWs) and two-dimensional nanosheets, have recently been attempted to realize flexible NIR photodetectors^[6-11]. Apart from the excellent mechanical flexibility, 1D NWs also have great advantages in large surface-to-volume ratio, high carrier mobility, and good light absorption ability, displaying competitive application in high-sensitive, high-durable and versatile flexible NIR photodetectors^[12-17]. For example, flexible CH₃NH₃PbI₃ NW NIR photodetector has been fabricated by Taoet al., displaying a responsivity of 0.227 A/W at the low light intensity of 1 × 10⁸ W/cm² and maintaining a good photoresponse up to 1500 folding cycles^[10]. In the case of inorganic NWs, Bi₂O₂Se NWs have been adopted to fabricate flexible NIR photodetector by Liet al., demonstrating a remarkable photoresponse and stable characteristics with 1200 bending cycles^[8].

Narrow bandgap semiconductors NWs of III−Sb have been considered as promising candidates for next-generation electronic and photoelectronic nanodevices. With the narrow direct bandgap of 0.726 eV and high hole mobility of 1000 cm²/(V·s), GaSb NW is certainly an active channel semiconductor for flexible NIR photodetectors. Up to now, vapor growth methods of molecular beam epitaxy, metal-oxide vapor phase epitaxy, metal-oxide chemical vapor deposition, and chemical vapor deposition (CVD), and so on have been universally developed for the growth of GaSb NWs, followed by the famous vapor–solid–solid or vapor–liquid–solid (VLS) growth mechanism^[18-21]. It is worth pointing out that a surfactant-assisted CVD method recently has been developed to prevent the sidewall growth of GaSb NWs, resulting in the successful control of diameter, length, growth orientation, crystallinity, hole concentration and so on^[22-26]. During the developed surfactant-assisted CVD process, complementary metal oxide semiconductor (CMOS)-incompatible and -compatible metals of Au, Sn, Pd and Cu have been deposited on substrates to serve as the growth catalysts^[22,25-27].

Although the NIR photodetection behavior of GaSb NWs has been studied, there is still very limited work on their flexible photodetectors^[25,27,28]. In this work, CMOS-compatible metal of Ag is first adopted as the growth catalyst for GaSb NWs during a surfactant-assisted CVD process. The as-prepared GaSb NWs show high density, smooth surface, balance stoichiometry, uniform diameter and good crystalline. When configuring into the famous back-gated field-effect-transistors (FETs), GaSb NWs exhibit excellent electrical performance with a highI_on/I_off ratio (10⁴) and high peak hole mobility (400 cm²/(V·s)). Additionally, the as-prepared NWs exhibit good NIR photodetection performance, such as high responsivity of 881 A/W, detectivity of 8.5 × 10¹⁰ Jones and fast response speed under 1550 nm illumination. Furthermore, the as-fabricated flexible photodetectors also hold the excellent photoresponse to NIR. After the parallel and perpendicular folding with 1200 cycles, the characteristics of flexible photodetectors exhibit no obvious decline, strongly revealing the promising advantage in robust mechanical flexibility, electrical and photoelectrical stability and bending endurance.

In this work, CMOS-compatible metal of Ag is deposited on amorphous glass as the growth catalyst during the developed surfactant-assisted CVD process. The detailed growth process can be found in supplementary material. From the scanning electron microscopy (SEM) image inFig. 1(a) and Fig. S1, all the as-prepared samples are uniform NWs with high density and smooth surfaces. As shown inFig. 1(b), as the thickness of Ag catalyst films increases from 0.1, 1.0, to 2.5 nm, the diameter of as-prepared NWs increases from 35.7 ± 9.2, 49.0 ± 10.6, to 56.8 ± 10.8 nm, while the corresponding length increases from 9.0 ± 2.6, 11.6 ± 2.7 to 13.3 ± 2.5μm. All of the diameters and lengths are estimated from the corresponding SEM images, as shown in Fig. S1. As verified by X-ray diffraction (XRD) inFig. 1(c), the as-prepared NWs are the pure zinc blende crystal structure (JCPDS Card No. 07–0215) of GaSb. Furthermore, high-resolution transmission electron microscope (HRTEM) is adopted to characterize the microstructure of Ag-catalyzed GaSb NWs. The clear lattice spacings of 2.2 Å are assigned to ( 022¯) and ( 2¯02¯) planes, indicating the good crystallinity of as-prepared GaSb NW grows along 〈211〉 direction. This result is further confirmed by the corresponding reciprocal lattice space extracted by fast Fourier transform (FFT). At the same time, a spherical catalytic seed is observed on the tip of NW (as shown inFig. 1(e)), inferring a VLS growth mechanism^[22,26]. The composition of catalytic seed and NW body are then checked by energy dispersive X-ray spectroscopy (EDS) inFig. 1(f). The spherical catalytic seed contains Ag, Ga and S elements, whereas the NW body shows a balanced stoichiometry of Ga : Sb ~ 1 : 1. In other words, high-quality GaSb NWs with controlled diameters and lengths are prepared successfully by using CMOS-compatible metal of Ag as the growth catalyst during a surfactant-assisted CVD process.

After the successful growth of high-quality GaSb NWs, back-gated FETs are constructed to investigate the corresponding electrical properties, as shown inFig. 2. The linear output characteristic ofFig. 2(a) confirms the ohmic-like contact of as-constructed GaSb NWFET with Ni electrodes. As depicted inFig. 2(b) and Fig. S2, the typical transfer curve exhibits the p-type conduction behavior of as-prepared Ag-catalyzed GaSb NW with a highI_on/I_off ratio of 10⁴, which is similar to the previous reports^[21,25]. The hysteresis observed in the transfer curve may be caused by the surface states or dielectric interfaces, which could be weakened further by surface decoration or passivation, construction of core-shell structure and introduction of stress, and so on^[29-33]. The field-effect mobility of as-constructed back-gated NWFET is then calculated by the square law model. The field-effect mobilityμ can be calculated fromμ =g_m × (L²/C_OX) × (1/V_DS), whereg_m is the low-bias (i.e.,V_DS = 0.1 V) transconductance extracted from the transfer characteristicsg_m = (dI_DS)/(dV_GS )|V_DS, in whichL is the channel length, andC_OX is the gate capacitance obtained from the finite element analysis software COMSOL for different NW diameters^{[25,29,34,35]}. In this case, as shown inFig. 2(c), the corresponding peak hole mobility reaches up to 400 cm²/(V·s), with NW diameter of 30 nm and channel length of 5μm, approaching to the theoretical mobility limitation with a hole concentration of 10¹⁸ cm^–3[36]. In addition, it is worth pointing out that a wire-to-wire variation is observed in peak hole mobility statistic ofFig. 2(d), contributing from the dependence of electrical properties on NW diameters, orientations, surface roughness and others. Nevertheless, the as-prepared Ag-catalyzed GaSb NWs exhibit impressive electrical performance, promising the next-generation electronic devices.

Benefiting from the narrow bandgap, the NIR photodetection behaviors of as-prepared Ag-catalyzed GaSb NWs are then systematically investigated by constructing the famous metal-semiconductor-metal devices on a hard substrate. TheI–t curves of GaSb NW photodetectors under different illuminations of λ = 850 nm to λ = 1550 nm are presented inFig. 3(a) and Figs. S3(a)–S3(c). Although fluctuation is observed, it is clear that the photocurrent (defined asI_ph = |I_light| − |I_dark|) is effectively modulated by periodic monochromatic laser illumination, which reveals that the as-fabricated photodetector shows excellent photoresponse to NIR bands and reproducible dynamic stability.Fig. 3(b) depicts the dependent relationship of the photocurrent on the incident light power intensity which obeys the power lawI_ph ~P^β, whereI_ph is the photocurrent,P is the light intensity andβ is the empirical value^[37-39]. Therefore,β = 0.23, 0.24, and 0.24 are fitted by the relationships according to the data of 850, 1310, and 1550 nm, respectively. Because the wavelength of incident lights is much larger than the diameter of NWs, the absorption difference of NWs under the illuminations of three incident lights is neglectable, resulting in the similar values ofβ. It is worth pointing out that the value ofβ reflects the complex process of electron-hole generation, trapping, and recombination within the NW^[39,40]. Meanwhile, the GaSb NW photodetectors also display good static detection performance, as shown in Fig. S3(d). Therefore, further efforts can be focused on the current fluctuation suppressing by surface passivation or the construction of core-shell NWs^[30,41,42].

Notably, responsivity (R) and detectivity (D*) are two essential parameters for a photodetector,R indicates the response efficiency of the photodetector to the optical signal andD* characterizes the minimum light signal that the photodetector can detect^[31,42].R can be defined asR =I_ph/(PA), in whichI_ph is the photocurrent,P is the incident power density, andA is the effective irradiated area of the corresponding photodetector.D* can be defined asD* =RA^1/2/(2eI_dark)^1/2, wheree is the electronic charge andI_dark is dark current^[38]. In this case, as shown inFig. 3(c), theR reaches up to 881 A/W and theD* reaches up to 8.5 × 10¹⁰ Jones under the illumination of 1550 nm laser with an intensity of 0.57 mW/mm², whileR andD* reach 1059 A/W, 1.1 × 10¹¹ Jones and 1661 A/W, 1.8 × 10¹¹ Jones for the NIRs of 1310 and 850 nm, respectively. Furthermore, another essential parameter for photodetector is the response time, which can be defined as the rising time (τ_r) and decay time (τ_d) for the photocurrent to increase/decrease from 10/90% to 90/10%, respectively^[25,42]. From the temporal photoresponse curve of GaSb NW photodetector under the illumination of 1550 nm laser inFig. 3(d), theτ_r can be calculated as 11 ms and theτ_d can be calculated as 16 ms. In short, the as-fabricated Ag-catalyzed GaSb NW NIR photodetector exhibits excellent photoresponse performance on a hard substrate, promising the next-generation photoelectric devices.

With the excellent photodetection performance, it is necessary to make use of as-prepared Ag-catalyzed GaSb NWs to fabricate flexible NIR photodetectors. In general, the surface roughness of flexible substrates is challenging the fabrication of high-performance NIR photodetectors^[2,6]. After the optimization of photolithography process, GaSb NW flexible photodetectors on polyimide (PI) substrates are successfully fabricated, as shown inFig. 4(a). The as-fabricated GaSb NW flexible photodetectors exhibit good bendability and flexibility, and even maintain good shape at bending radius of 3 mm as shown inFig. 4(a). As presented inFig. 4(b), the GaSb NW flexible photodetector shows good and stable photoresponse to NIR lasers of different wavelengths before bending. The R is as high as 618 A/W andD* is as high as 6.7 × 10¹⁰ Jones under the illumination of 1550 nm laser, as shown in Fig. S4(a), which is comparable to the performance of device on hard substrate. It is worthwhile to note that even under the small bias of 0.1 V, the as-fabricated GaSb NW photodetector still exhibits competitive NIR photodetection performances on both hard and flexible substrates, as shown inTable 1. The photodetection performance of flexible GaSb NWs photodetectors can be further optimized by using the smoother and softer flexible substrates^[6,28]. Apart from flexibility, the bending endurance is another indispensable factor restricting the development and practical application of the flexible devices^[12,45,46]. To more realistically test the bending endurance, the flexible photodetector is bent in parallel and perpendicular orientation alternatively, as shown inFig. 4(c). It is obviously that the photodetection performance of GaSb NW flexible photodetector shows no obvious decline after the multiple bending cycles. Even after bending with 1200 cycles, the GaSb NW flexible photodetector still maintains satisfactory detection performance for 850, 1310, and 1550 nm NIR lights as shown in Figs. S4(b) and S4(c). These results reveal that the as-fabricated flexible NIR photodetector enabled by Ag-catalyzed GaSb NW exhibits outstanding mechanical flexibility, photoelectronic stability and bending endurance, greatly promoting the development of NWs flexible photodetectors.

In summary, high-quality GaSb NWs have been prepared successfully by adopting the CMOS-compatible metal of Ag as the growth catalyst during a surfactant-assisted CVD process. The as-prepared NWs show high density, smooth surface, balance stoichiometry, uniform diameter and good crystallinity. They also exhibit the expected high peak hole mobility of 400 cm²/(V·s), high responsivity of 881 A/W, high detectivity of 8.5×10¹⁰ Jones and fast response speed under the illumination of 1550 nm. When configured into the flexible photodetectors, GaSb NWs have excellent NIR photodetection behaviors, along with high mechanical flexibility and bending endurance. All of these results promote the use of as-studied high-quality GaSb NWs catalyzed by Ag for the next-generation high-performance flexible NIR photodetectors.

Supplementary materials to this article can be found online athttps://doi.org/10.1088/1674-4926/43/11/112302.