Recent progress in photodetectors based on low-dimensional nanomaterials

2.1 Graphene

Recently, graphene has emerged as an attractive material in PDs due to its high carrier mobility, broadband absorption, and mechanical flexibility. Due to graphene’s typical plasmonic nanostructures, photocurrent is no longer limited to photon energies greater than the bandgap of the semiconductor [9]. Moreover, it can be integrated into virtually any waveguide material, including Si, SiN, or AlN. Early data interpreted in the study have revealed an ultrafast photoresponse in graphene with an intrinsic bandwidth of 260 GHz, showing the promise for high-speed photodetection applications. Since then, several graphene-based PDs on Si waveguides have been realized [12].

Schuler et al. [13] presented an ultrafast graphene-based PD on an Si slot waveguide, where the Si strips serve as local back gate electrodes to create a p-n junction for efficient photodetection. A responsivity of 35 mA/W at zero-bias conditions was achieved, and while under a moderate drain-source bias of 300 mV, the responsivity increased to 76 mA/W. This PD has shown a record high 3 dB bandwidth of 65 GHz. To further improve the responsivity in terms of V/W, the electrical gating could be extended closer to the contacts, for example, using thin Si slabs to both sides of the waveguide.

However, the realization of broadband PDs has been hindered by graphene’s low absorption in the visible and near-IR (NIR) regions. Kang et al. [14] demonstrated a stretchable PD based exclusively on crumpled graphene, which solved this barrier perfectly. As shown in Figure 1, the crumpled graphene, which has been intentionally deformed to have a continuously undulating 3D surface, induced an areal density increase to yield higher optical absorption per unit area, thereby improving photoresponsivity. As a result, this textured graphene PD exhibited a 370% enhancement in photoresponsivity compared to a flat graphene PD. More importantly, the stretching capability could reach up to 200% of its original length and no limit on detection wavelength. This surface engineering approach can be used to enhance photoabsorption and develop a highly responsive PD based on a single photoactive material of other emerging 2D materials such as TMDCs, including MoS₂, WS₂, and WSe₂ monolayers.

Figure 1:

Mechanism of enhanced and strain-tunable photoresponsivity and photographs of textured graphene PD arrays. (A) Schematic illustrations of the physical mechanism enabling enhanced and strain-tunable photoresponsivity. Bottom schematic illustrations show close-up at the interface and illustrate the modulation of crumple areal density, height, and pitch in the illuminated region with an applied strain. (B) Photograph of a stretchable PD array (3*2 array). Bottom image shows an optical microscope image of the textured graphene PD. Reproduced with permission from Ref. [14].

Fang et al. [15] demonstrated a broadband graphene PD exhibiting enhancement factors of 8 to 13 over the visible spectrum by reforming the source and drain contacts into a fractal metasurface and a ring encircling it. The metasurface was realized through a fractal tree to mimic the snowflake geometry, as shown in Figure 2. The application of this fractal metasurface was not bound to graphene PDs but could also be integrated with photovoltaic/photothermoelectric effect PDs made of other photodetection materials. The great flexibility of our fractal metasurface design enabled broadband enhancement at other portions of the electromagnetic spectrum and for various spot sizes. Additionally, the photoresponse enhancement was insensitive to the incident light polarization, a quintessential feature of a practical photodetecting/photoharvesting system. These attributes, combined with dynamic tunability through source-drain bias, make this fractal metasurface an advancement toward the incorporation of graphene into modern photodetecting and photoharvesting applications.

Figure 2:

Construction of the branching generated by recursive iterations. (A) Construction of the fractal design with “snowflake” geometry from levels 1–4. (B) Total structure comprising four-level (blue) and three-level (red) fractals used in the study. (C) Finite-difference time-domain simulated in-plane electric field (of the incident electromagnetic wave) distribution just underneath the Au fractal metasurface on a glass substrate under the excitation wavelength of 530 nm. The electric field is linearly polarized along the y-direction. Reproduced with permission from Ref. [15].

2.2 TMDCs

Great improvement in the responsivity has been realized for PDs based on graphene; however, there is an intractable issue for graphene-based devices: the high dark current arising from its gapless nature, which has a strong impact on the sensitivity of photodetection [16], [17]. Alternatively, another kind of layered material, TMDCs with a natural gap, has captured much more attention in recent years. Up to now, TMDCs, including MoS₂, WS₂, WSe₂, InSe, GaS, GaTe, GaSe, and In₂Se₃[18], have been explored to develop plenty of new PDs with favorable photoelectronic properties. To illustrate this kind of PD vividly, MoS₂-based PDs are taken as an example.

As a typical TMDC material, MoS₂ is composed of a vertical stack in which the weakly interacting layers are held together by van der Waals forces. It is noteworthy that bulk-form MoS₂ is a semiconductor with an indirect bandgap of 1.2 eV, whereas single-layer MoS₂ is another kind of semiconductor with a direct bandgap of 1.8 eV [19]. This unique property of MoS₂ stems from the effect of quantum confinement on its electronic structure.

Ye et al. [20] demonstrated an advanced highly polarization-sensitive IR PD based on BP-on-WSe₂ van der Waals vertical heterostructure. This PD used a vertical photogate heterostructure of BP-on-WSe₂ in which BP serves as the photogate and WSe₂ as the conductive channel. Ultrahigh visible and IR photoresponsivity at room temperature can reach up to 1000 and 0.5 A/W, respectively, and ultrasensitive visible and IR specific detectivity is obtained up to 10¹⁴ and 10¹⁰ Jones, respectively, at room temperature. Moreover, the high sensitivity to IR polarization is about 40 mA/W with incident light polarized along the horizontal axis (defined as 0° polarization). It shows that the BP photogate structure of the BP-on-WSe₂ detector has higher sensitivity to polarized IR illumination photodetection and lower specific detectivity at room temperature. This detector provides a new strategy to integrate 2D layered materials to enable the corresponding PDs and may open up a new dimension for applications to IR imaging systems operating at room temperature.

Gomathi et al. [21] fabricated a novel paper-based broadband PD using hybrids of ZnS-MoS₂. The PD was fabricated using a simple two-step hydrothermal method. The spectral selectivity of MoS₂ has been extended to UV wavelength region by combining MoS₂ with ZnS having high sensitivity toward UV light. The fabricated PD displays high sensitivity toward visible light when compared to UV and IR. The PD exhibits a responsivity of 4.5 μA/W for IR, 9.4 μA/W for UV light, and 17.85 μA/W for visible light. In addition, ZnS-MoS₂ exhibited increased responsivity due to the reduced electron-hole recombination, which was a result of the straddling-type band alignment observed at the interface of ZnS-MoS₂. This is the first report on paper-based broadband PD with ZnS-MoS₂ hybrids as active sensing materials and it provides a promising route for the development of large-scale paper-based broadband PDs using TMDC hybrids at low cost, having diverse applications in the field of wearable electronics, environmental monitoring, and surveillance.

2.3 BP

In addition to TMDC layered materials, BP is the latest developed layered material [22], [23]. After the groundbreaking research on BP in 2004 by Manchester scientists, this layered material opened a new branch of science and provided a vast number of unlimited applications ranging from printable electronics to prototype devices [3].

In contact to other 2D nanomaterials, BP has intrinsic and tunable direct bandgap from 2.0 eV (monolayer) to 0.3 eV (bulk), which covers the optical spectrum range from visible to NIR. In addition, BP has also shown remarkable electrical properties, including a high hole mobility up to 1000 cm² V⁻¹ s⁻¹ at room temperature, an on-off ratio up to 105, and good current saturation in field-effect devices [24]. BP-based field-effect transistors (FETs) and their performances are shown in Table 1. Furthermore, it has unique in-plane anisotropic physical properties unlike most of other 2D materials. The anisotropy of any physical property refers to the change of that property in different directions. It is not new in materials but is quite exciting in new 2D material BP due to its large magnitude in anisotropy compared to other 2D materials [3]. These characteristics make BP as the most promising candidate for broadband photodetection.

However, the current performance status of BP PDs in terms of broadband spectrum range, high photoresponsivity, and fast response time is still unsatisfactory. Furthermore, the impurities and the dielectric environment have an important impact on the electrical and optical properties of BP devices. In this section, we attempt to give a comprehensive overview and analysis of the published results of BP PD and show some new pathways to overcome these challenges.

Miao et al. [25] fabricated an NIR camera based on a single BP PD using the compressive sensing algorithm, which took advantage of the outstanding photoresponse of BP and overcame the difficulties of BP film growth and scalable PD array fabrication. The imaging was achieved by combining the PD with a digital micromirror device to encode and subsequently reconstruct the image based on compressive sensing algorithm. Stationary images of an NIR laser spot (λ=830 nm) with up to 64×64 pixels were captured using this single-pixel BP camera with 2000 times of measurements, which was only half of the total number of pixels. The imaging platform demonstrated in this work circumvents the grand challenges of scalable BP material growth for PD array fabrication and shows the efficacy of using the outstanding performance of BP PD for future high-speed IR camera applications.

Ren et al. [2] synthesized few-layer BP nanosheets with large size by a facile liquid exfoliation method. Photoelectrochemical (PEC) tests demonstrated that the current density of BP nanosheets can reach up to 265 nA cm⁻² under light irradiation, whereas the dark current densities fluctuated near 1 nA cm⁻² in 0.1 m KOH, as shown in Figure 3. In addition, the cycle stability measurement exhibited no detectable distinction after processing 50 and 100 cycles, whereas an excellent on-off behavior was still preserved even after 1 month. Furthermore, the PEC performance of BP nanosheet-based PD was evaluated in various KOH concentrations, which demonstrated that the as-prepared BP nanosheets might have a great potential application in self-powered PDs. The present work provides fundamental acknowledgment of the performance of a PEC-type BP nanosheet-based PD, offering extendable availabilities for 2D BP-based heterostructures to construct high-performance PEC devices.

Figure 3:

Investigation of BP’s optical property. (A) Schematic illustration of the proposed solution-phase exfoliation route for BP nanosheets. (B) Scanning electron microscopy (SEM) image. (C) EDS image. (D) Transmission electron microscopy (TEM) image. (E) High-resolution TEM (HRTEM) image. (F) Selected area electron diffraction (SAED) pattern of exfoliated BP. (G) UV-visible spectra of BP/NMP (inset: optical image of BP/NMP solution). (H) Raman spectra of bulk BP and exfoliated BP nanosheets on SiO₂/Si substrate. (I) Photoluminescence spectra of exfoliated BP nanosheets on SiO₂/Si substrate. Reproduced with permission from Ref. [26].

Youngblood et al. [26] demonstrated a waveguide-integrated and gate-tunable BP PD integrated on an Si photonic waveguide operating in the NIR telecom band. In a significant advantage over graphene devices, these BP PDs could operate under bias with very low dark current and attained an intrinsic responsivity up to 135 and 657 mA/W in 11.5- and 100-nm-thick devices, respectively, at room temperature. The photocurrent was dominated by the photovoltaic effect with a high response bandwidth exceeding 3 GHz.

Deng et al. [27] demonstrated a gate-tunable p-n diode based on a p-type BP/n-type monolayer MoS₂ van der Waals p-n heterojunction. Upon illumination, these ultrathin p-n diodes showed a maximum photodetection responsivity of 418 mA/W at the wavelength of 633 nm and photovoltaic energy conversion with an external quantum efficiency of 0.3%. These p-n diodes show promise for broadband photodetection.

Miao et al. [28] demonstrated a truly vertically stacked and self-encapsulated van der Waals heterojunction diodes built from 2D semiconductors (MoS₂ and BP) and compared its performance to conventional lateral 2D heterojunction devices (partially overlapped 2D heterostructures), as shown in Figure 4. Both vertical and lateral p-n heterostructure diodes exhibited a strong rectification ratio without gate voltage applied. Moreover, the current density under forward bias vertical diode delivered 70 times higher than a conventional lateral device design; it is attributed to the complete elimination of series resistance. This work demonstrates the potential of using the vertically stacked 2D semiconductors for future nanoelectronic and optoelectronic devices with optimal performance.

Figure 4:

Fabrication procedure and characterization of vertically and laterally stacked BP/MoS₂ heterojunction p-n diodes. (A) Schematic diagrams illustrating the fabrication procedure of vertically stacked BP/MoS₂ p-n diodes. A 50 nm-thick Au film was used to fill the square trench followed by lift-off to form the bottom electrode (A- i). Fewlayer p-type BP flake was then transferred onto the as-fabricated Au/p⁺-Si bottom electrode (A-ii). Next, the MoS₂ flake fully covers the BP (A-iii). Finally, the top electrode was patterned onto the MoS₂ flake by EBL and thermal evaporation of 50 nm-thick Au film (A-iv). The current can flow between the bottom Au/p⁺-Si electrode and the top Au electrode through the semiconducting BP/MoS₂ heterostructure (A-v). (B) Optical microscope images showing a representative vertical BP/MoS₂ p-n junction diode after each fabrication step including Au(B-i), BP(b-ii), MoS₂ (B-iii) and top Au electrode (B-iv). Scale bar, 10 μm. (C and D) Ambient stability study of (C) vertically and (D) laterally stacked BP/MoS₂ diodes. Scale bar, 10 μm (C-i and D-i) and 5 μm (C-ii, C-iii, D-ii, and D-iii). (E) Raman spectra of MoS₂, BP, and BP/MoS₂ heterostructure. Reproduced with permission from Ref. [29].

Li et al. [29] first fabricated a bipolar phototransistor based on WSe₂-BP-MoS₂ van der Waals heterostructure. Broadband photoresponsivities for visible (532 nm) and IR (1550 nm) light waves reach up to 6.32 and 1.12 A/W, respectively, which are both improved by tens of times in comparison to similar photodiode devices composed of WSe₂-BP. The phototransistor also exhibited ultrasensitive shot noise limit specific detectivities, which were 1.25×10¹¹ Jones (1 Jones=1 cm Hz^1/2 W⁻¹) for visible light at wavelength λ=532 nm and 2.21×10¹⁰ Jones for the NIR light at wavelength λ=1550 nm at room temperature. This generic strategy by combining 2D layered materials into heterostructures to enable multifunctional devices and components may open a new avenue for high-density monolithic integration of functional devices in a limited area.

Ding et al. [30] constructed a novel 2D van der Waals heterostructure made by InSe and BP monolayer with high hole mobility (10³ cm² V⁻¹ s⁻¹) and investigated its structural and electronic properties using first-principles calculations. The InSe/BP heterostructure could be formed easily due to its negative binding energy. Its heterostructure exhibited an intrinsic type II band alignment with a direct bandgap of 1.39 eV, in which electrons were located in the InSe layer and hole localized in the BP layer. The band offsets of InSe and BP are 0.78 eV for CBM and 0.86 eV for VBM, respectively, which greatly enhance the performance of InSe-based photovoltaic devices and PDs. The electron mobility of the InSe/BP heterostructure is increased to 3×10³ cm² V⁻¹ s⁻¹ and hole mobility is increased from 40 cm² V⁻¹ s⁻¹ in InSe monolayer to 10⁴ cm² V⁻¹ s⁻¹, which is three orders of magnitude larger than that in InSe monolayer. Thus, the InSe/BP heterostructure is considered to have a huge potential application in PDs.

Ye et al. [31] demonstrated a PD with visible to NIR detection range based on the heterojunction fabricated by van der Waals assembly between few-layers BP and few-layers MoS₂. The heterojunction with electrical characteristics, which could be electrically tuned by a gate voltage, achieves a wide range of current-rectifying behavior with a forward-to-reverse bias current ratio exceeding 10³. The photoresponsivity of the PD was about 22.3 A/W measured at λ=532 nm and 153.4 mA/W at λ=1.55 μm with a microsecond response speed (15 μs). In addition, its specific detectivity was calculated to have the maximum values of 3.1×10¹¹ Jones at λ=532 nm and 2.13×10⁹ Jones at λ=1550 nm at room temperature. This generic strategy to integrate 2D layered materials in the vertical direction to enable functional devices and circuits may open up a new dimension for high-density integration of functional devices in the limited circuit area.

In addition, BP can also be configured as an excellent UV PD. Wu et al. [32] investigated a UV PD based on BP with a specific detectivity of 3×10¹³ Jones for the first time. The UV photoresponsivity could be significantly enhanced to 9×10⁴ A/W by applying a source-drain bias of 3 V, which was the highest ever measured in any 2D materials and 10⁷ times higher than previously reported value for BP. Such a colossal UV photoresponsivity was attributed to the resonant-interband transition between two specially nested valence and conduction bands. These nested bands provide an unusual high density of states for highly efficient UV absorption due to their singularity nature. This work shows that BP proves to be a strong viable candidate for future optoelectronic applications especially as state-of-the-art UV detectors.

3.1 ZnO nanowire film

ZnO is a group II to VI wide-bandgap semiconducting metal oxide with high electron mobility and exciton binding energy, which makes it an interesting material for a variety of applications [33], [34]. Owing to their wide bandgap (3.4 eV) [35], high exciton binding energy (60 MeV), and high photoconductive characteristic, ZnO nanostructures are frequently used to construct UV PDs [36], [37], [38], [39]. ZnO films with high specific area and physical flexibilities are easy and cheap to prepare and process with low costs, and the photoconductivity of ZnO film PDs changes notably upon on-off state switching, demonstrating their potential in practical applications [37], [40]. In particular, forming heterojunctions between ZnO and other materials [ZnS, poly(vinyl alcohol), poly(acrylonitrile), and poly(vinylcarbazole)] has significant effects on improving the photoelectric performance of ZnO film-based PDs [41], [42], [43]. For instance, ZnO-Ga₂O₃ core-shell microwire PD is highly suitable for practical applications of solar-blind photodetection [44].

A novel CdMoO₄-ZnO composite film has been prepared by spin-coating CdMoO₄ microplates on ZnO film and was constructed as a heterojunction PD. With the aid of CdMoO₄ microplates, the structure of the PD changed from MSM type to a heterojunction one, which largely benefited the photoelectric performance. Ouyang et al. [37] deposited Au nanoparticles (NPs) to modify the CdMoO₄-ZnO composite film, and compared to the CMO-ZnO PD, this PD achieves a higher photocurrent of 289.1 nA and a higher responsivity of 321.1 mA/W. At the same time, its decay time decreases to 9.2 s. This further improvement of photoelectric properties was due to the hot electron injection effect of AuNPs and the Schottky barriers formed between the AuNPs and CMO-ZnO composite film block the carrier transportation. However, an excess loading of CdMoO₄ microplates might shelter the ZnO film underneath and block the light absorption of the composite film, which would lead to the decrease of photocurrents. Thus, in practical applications, it should be avoided. The study on Au-CMO-ZnO composite film PD might be potentially useful for facile construction of ZnO-based PD with novel structure and high photoelectric performance.

Teng et al. [42] incorporated a BiOCl nanostructure into the ZnO film and constructed them as film PDs. With forming barriers to block the straight transmission of electrons between electrodes, the rise/decay time of BiOCl/ZnO hybrid film PD was 25.83/11.25 s, which was much shorter than that of pure ZnO film PD (69.34/>120 s). Due to the injection of photogenerated electrons into the ZnO film under UV light illumination, the responsivity of the BiOCl/ZnO hybrid film PD in the UV region could reach 182.87 mA/W, which was about 6.87 times that of pure ZnO film PD. This is different from those methods that enhanced the performance of PDs by adjusting the depletion layer on the surface of ZnO NPs. The BiOCl/ZnO hybrid structure film PD provides a new and simple route for the enhancement of optoelectronic performance based on the ZnO nanostructures.

Duan et al. [45] fabricated a ZnO:Al/Si heterojunction PD by a simple and low-cost CBD method. ZnO:Al/Si PD showed a sharp cutoff responsivity at 400 nm, a UV-to-visible rejection ratio (R_{386 nm}/R_{500 nm}) of 50, and fast rise and decay times of the photoresponse, which indicated that the hexagonal nanocrystal (NC) ZnO:Al film was the most promising materials for the fabrication of high-performance UV PDs. Self-powered UV PDs, which have vast applications in the military and for civilian purposes, have become particularly attractive in recent years due to their advantages of high sensitivity, ultrasmall size, and low power consumption. This should generate extensive interest in this field and encourage more researchers to engage in and tackle the scientific challenges [46], [47], [48]. In 2017, Yu et al. [49] fabricated three novel heterojunctions by depositing films through one-step in situ polymerization in acid solution, which provides a simple and efficient approach to prepare high-performance self-power UV PDs. Hu et al. [39] developed a high-performance UV PD based on an Se/ZnO p-n heterojunction. This device was theoretically equal to a parallel-connection circuit for its special structure, and due to its parallel connection of ZnO and the Se/ZnO hybrid structure, this UV PD realized a binary response of positive and negative output currents using an on-off light source at a small reverse bias. Additionally, the speed of the ZnO UV PD is greatly improved with a rise time of 0.69 ms and a decay time of 13.5 ms, which are much quicker than those of pure ZnO film PDs. All the results demonstrate that the high-performance UV PD based on an Se/ZnO p-n self-powered heterojunction provides a promising way to fabricate ZnO-based p-n junction UV PDs with high sensitivity, high SNR, high spectral selectivity, high speed, and high stability and thus may lay a solid ground for the future applications of this kind of high-performance multifunctional PD.

Purusothaman et al. [50] synthesized highly crystallized ZnO microarchitectures (ZnO microwire, ZnO coral-like microstrip, and ZnO fibril-like clustered microwire) through a simplified vapor transport technique (carrier gas/catalyst-free). As-fabricated PD is shown in Figure 5. Among them, ZnO fibril-like clustered microwire exhibited higher IPh290 μA at 17 mW cm⁻² and comparable responsivity (R_{405 nm}) of 716 mA/W. In contract to reported microarchitectures, ZnO fibril-like clustered microwire exhibited superior photosensing behavior with a detectivity range of 6.72×10¹⁰ Jones and broadened spectral activity extending from UV to the visible region (365, 405, and 535 nm). The results reported in this paper provide a facile route to develop flexible self-powered optical communication systems and sensors.

Figure 5:

Structure and photospectrum of this as-fabricated ZnO. (A) Rama spectrum of ZnO microarchitectures. (B) Schematic representation of as-fabricated PD. (C) Compressive strain-induced behavior of PD. (D) Schematic view of ZnO device architectures. (E) Optical images of as-fabricated PD devices using ZnO MW, ZnO CMS, and ZnO F-MW. Reproduced with permission from Ref. [50].

Chen et al. [51] developed a strain modulated solar-blinded avalanche PD based on ZnO-Ga₂O₃ core-shell heterojunction microwire. This PD is highly sensitive to deep UV light centered at 261 nm. Due to its ultrahigh sensitivity and spectral selectivity, this PD could respond to rare weak deep UV light with almost no response to visible light. Moreover, the detection could be enhanced by applying certain static strains on the device through the piezo-phototronic effect. The current response could be enhanced about three times under −0.042% strain. The strain-induced piezopotential modulates carrier transport mainly in the ZnO core. The photoresponse of this PD was enhanced by this effect and thus made it a more perspective PD in human-machine interaction areas, such as health monitoring, smart systems, and security communication.

3.2 CNTs

1D semiconductor nanostructures have stimulated much attention in high-performance PDs, owing to the pronounced surface effects originating from the large surface-to-volume ratio [52], [53]. Since the discovery of multiwalled CNT by Lijima and Ichihashi and the related single-walled CNTs in 1993 [54], CNTs exhibited some strong advantages for applications in IR detectors in comparison to other 1D candidate materials, e.g. the naturally nanoscale size, simplified growth and device fabrication, chirality-dependent direct band structures that cover important communication wavebands, and room-temperature operation due to their moderate bandgap energies (26 meV) [55]. The very small size and good extendibility also provide great prospects for their flexible integrations into other photonic or hybrid optoelectronic systems [56], [57].

However, for CNT-based near- or shortwave-IR optoelectronics, the existing developments for enhanced absorption or signal detection are insufficient [58]. Besides, for signal sensing or information imaging applications, which involve the detection of a specific target signal among a broad background of signals or noise, the photoresponse of CNT aggregations usually covers a wide NIR scope with various characteristic peaks induced by chirality-dependent Van Hove singularity transitions [59], leading to random responses without any targets. That is to say, a specific response to a target signal requires a higher suppression ratio [60]. To further increase the signal outputs, the hybrid systems that enhance light absorption can also be considered. Compared to the fast progress in the fields of 2D materials [61], some concepts for enriching CNT PDs should be further investigated and developed. Here, we present a figure of merit and some enhanced light absorption PDs in this section.

In 2016, Liang et al. [55] demonstrated a high photoelectric conversion efficiency and signal recognition CNT PDs by integrating with a designed F-P cavity. A ~6-fold enhanced optical absorption could be achieved because of the confined effect of the designed optical mode, and the detectors exhibited a higher suppression ratio until a power density of 0.07 W cm⁻² and photocurrent of 5 pA, and the spectral full-width at half-maximum is ~33 nm at a signal wavelength of 1200 nm. More importantly, a concept of the “resonance and off-resonance” cavity was developed at signal detection at zero bias, which opened the possibility of chirality-specific CNT film-based signal recognition systems for information photonics on a chip and broadening the potential for the application of other nanomaterials in information photonics regardless of their doping polarity. The concept offered a promising and practical pathway for the development of stable, large-scale, multiwavelength signal capture, recognition, and sensing systems.

Pyo et al. [62] demonstrated a heterogeneous all-carbon PD composed of graphene electrodes and porphyrin-interfaced single-walled CNTs channel, exhibiting high photoresponse, flexibility, and full transparency across the device. The porphyrin molecules generate and transfer photoexcited holes to the single-walled CNTs even under weak white light, resulting in significant improvement of photoresponsivity from negligible to 1.6×10⁻² A W⁻¹. Simultaneously, the PD exhibited high flexibility, allowing stable light detection under ≈50% strain (i.e. a bending radius of ≈350 μm) and retaining a sufficient transparency of ≈80% at 550 nm. Notably, minimal loss in photoresponse even after heavy crumpling (a strain of ≈50%) and ability to detect sunlight gave the device great potential as an epidermal sunlight sensor. This PD could be used for low-cost, large-scale, and high-performance optoelectronic applications that require mechanical flexibility and optical invisibility. The hybridization of low-dimensional nanomaterials envisions a promising strategy for achieving flexible and transparent electronics as well as enabling further exploration into the unprecedented functionalities of nanomaterials.

However, the absorption restriction from single layer limits the effective utilization of incident light. In 2017, Huang et al. [63] demonstrated a plasmonic electrode structure in CNT thin-film PD based on randomly deposited high-purity semiconducting CNT, which could collect photoinduced carriers effectively and enhance light absorption at the same time. The plasmonic enhancement resonate around 1650 nm wavelength well corresponding to our simulation, which was a short-wave IR for plasmonic enhancement application. Furthermore, the best performance improvement of CNT detector with plasmonic structure could be enhanced as 13.7 times for photocurrent mode and 5.62 times for photovoltage mode than those devices without structure at 1650 nm resonant wavelength. At last, the plasmonic structures were applied on tandem PDs with nine virtual contacts, and both the photocurrent and the photovoltage were increased. By further improving the structure and the quality of CNT film, such as dense semiconducting CNTs array with thicker layer, the plasmonic detectors will show more powerful use in modern room temperature IR detection and even in optical integration circuit with compact construction.