Introduction

Broadband photodetectors with blackbody detection ability are crucial for their practical application; however, bulk material-based infrared detectors such as HgCdTe1, InSb2, and blocked impurity band detector3 often suffer from strict growth procedures and work at low temperatures at high cost. Bolometers such as polycrystalline VOx and amorphous silicon have shown uncooled broadband blackbody detection ability, but they suffer relatively slow photoresponse times4. Moreover, complicated thermal-bridge supporting structures are indispensable for improving the thermal insulation and bolometric response of thermal detectors4,5. As the device size decreases, the impact of defects on the growth process and final device yield becomes more prominent. With the increasing demand for wide-spectrum detection and intelligent sensing in recent years, reducing the size, weight, power consumption, and cost, and improving the performance of broadband photodetectors are urgently needed.

2D materials without dangling bonds and lattice mismatch problems represent great opportunities for next-generation low-cost infrared detection6,7,8,9,10,11,12,13,14. Among these materials, black phosphorus, black arsenic phosphorus, and their related heterostructures stand out for their remarkable blackbody response within the MWIR region at room temperature; nevertheless, they have a limited working wavelength region8,9,11,13,15,16. Dual-working mechanism based on photoconductive and bolometric effects has been applied to achieve broadband photodetection17,18,19,20,21. The devices demonstrate relatively superior detection capabilities under long-wave coherent light sources. Since the long-wave photoresponse mechanism of these devices is a bolometric effect, if the heat localization is further enhanced, the corresponding long-wave bolometric detection performance would be better. In addition, it is also essential to further investigate the long-wave blackbody response, since it characterizes the detector’s ability for weak target detection22.

In this work, inspired by the sensory systems of moths for detecting blackbody radiation23,24, we design a self-adapted ultrabroadband detector with a dual-working mechanism (photoconductive and bolometric). Through an electronic/phononic band engineering strategy elaborated by density function theory (DFT) calculations, we report an ultrabroadband blackbody-sensitive photodetector based on the novel 2D flat-band quantum material Nb3I8. The proposed photodetector with flat-band features shows an excellent blackbody response up to λ = 20 μm. To our knowledge, our work is the first to uncover such ultrabroadband infrared detection performance at room temperature among 2D materials, thus paving the way for exploiting 2D topological flat-band quantum materials for optoelectronic applications.

Results

Design of blackbody-sensitive ultrabroadband detectors

The moths have a full-day self-adapted sensory system, which allows them to detect high-temperature blackbody (sunlight) radiation during the day and low-temperature blackbody (thermal) radiation at night. There are short-wave photosensitive pigments coating the retina of their compound eye23; therefore, they observe the objects (e.g., the cat in Fig. 1a) through active imaging based on the photoconductive effect. Since the pigments are insensitive to infrared radiation in photoconductive mode, they perceive the thermal radiation of objects through antenna spines on the basis of the bolometric effect. To emulate the full-day blackbody detection ability of moths, we develop a dual-working mechanism photodetector based on a semiconductor with flat bands (Fig. 1d). As illustrated in Fig. 1e, via an electronic band engineering strategy for SWIR detection, a semiconductor with electronic flat-band feature would produce a high density of states (DOS) and dipole transition probability (the detailed analysis will be shown later), enabling high absorption above the bandgap; meanwhile, the flat band characterized by electron-correlation strength should be moderate by considering the material’s mobility for effectively transmitting photoexcited carriers. With a phononic band engineering strategy for MWIR–VLWIR detection, a semiconductor with a phononic flat-band feature would cause a limited phonon mean free path (MFP) where strong phonon scattering occurs, thus producing low thermal conductivity and enabling high thermal collection efficiency below the bandgap. In addition, the bandgap size should be moderate for a large temperature coefficient of resistance (TCR) and low dark current. Overall, suitable materials with intrinsic flat-band features are highly needed to implement ultrabroadband blackbody-sensitive photodetectors.

Fig. 1: Bioinspired ultrabroadband photodetector based on 2D flat-band quantum material.
Fig. 1: Bioinspired ultrabroadband photodetector based on 2D flat-band quantum material.
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a Schematic of an object observed by a moth. b Images of the photoreceptor (compound eye) and thermoreceptor (antenna) of a moth. The compound eye image is reproduced from ref. 54, and is used under the terms of the Creative Commons CC BY license; The antenna photograph is taken by Bastian Alexander-Coleman, sourced from Unsplash (https://unsplash.com/photos/a-close-up-of-a-large-insect-on-a-wall-w73vtLVqtRw), and is used under the Unsplash License. c Active image captured at daytime and the thermal target perceived at nighttime by a moth. d Proposed blackbody-sensitive ultrabroadband photodetector based on the 2D flat-band quantum material Nb3I8. e Electronic and phononic band engineering strategy for ultrabroadband photodetection. f Mechanisms of SWIR and MWIR–VLWIR detection.

The means for constructing the flat band of 2D materials include applying a stress/magnetic field25,26,27, twisting28,29,30,31, and intrinsic localization of electrons caused by quantum destructive interference32,33,34,35,36,37. Among them, the last method is the simplest because of the intrinsic properties of the material. Breathing kagome topological semiconductor Nb3X8 (X = Cl, I) with a frustrated lattice has been demonstrated with electronic/phononic flat bands in recent studies34,35,36,38, and it also has a 2D layered structure that can be thinned down to atomic-thickness34,39, providing an opportunity to meet the demand. Among them, Nb3I8 is more suitable for ultrabroadband photodetection, since its electron-correlation strength is relatively smaller than that of Nb3Cl8, which results in higher mobility and a smaller bandgap36; therefore, this work focuses on exploiting Nb3I8 for ultrabroadband infrared detection.

Electronic band engineering and SWIR photodetection

Nb3I8 belongs to a triangular system with a rhombohedral space group \(R\overline{3}m\)(No. 166) with lattice parameters of a = b = 7.60 Å and c = 41.715 Å. Figure 2a depicts a side view of the crystal structure, where each layer along the c direction is connected by weak van der Waals bonds. As shown in Fig. 2b, the origin of electronic flat-band formation is electron-destructive quantum interference in the kagome lattice36, meaning that when an electron is confined to a frustrated Kagome lattice with only nearest-neighbor transitions, completely coherent and destructive quantum interference occurs, and the electron is correlated in the lattice; thus, the flat bands form. A breathing cage lattice is formed by Nb atoms where there are two different Nb‒Nb distances in Nb triangles, the inversion symmetry is absent, which leads to a semiconducting ground state. The DFT band structure result reveals its obvious flat-band characteristic that is below the Fermi level by approximately 0.5 eV, as illustrated in Fig. 2c. Light absorption is closely associated with the joint density of states (JDOS) and dipole transition matrix element according to Fermi golden rule40, where the JDOS is related to the process in which photoexcited electrons transit from the valence bands to conduction bands. The calculated DOS clearly increases in the flat-band region, as shown in the right panel of Fig. 2c, indicating a large electron DOS for light excitation. In addition, the transition matrix element \({M}_{cv}=\langle c|{H}_{op}|v\rangle\), where Hop is the Hamiltonian for light–matter interaction. Figure 2d, e shows the calculated square of the dipole transition matrix element of the optical transition from the flat valence bands to the different conduction bands at different K points. The combined high DOS and dipole transition probability contributed to the large photocurrent generation and slow decay of the absorption trend towards the bandgap.

Fig. 2: Electronic band engineering and SWIR photodetection performance.
Fig. 2: Electronic band engineering and SWIR photodetection performance.
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a Side view of the crystal structure of Nb3I8. b Schematic (left) and AC-HAADF-STEM image (right) of the breathing kagome lattice formed by Nb atoms. c Electronic band structure and DOS calculation. d Dipole transition matrix element calculation from valence band (VB1/2/3) to conduction band (CB1/2/3). e Dipole transition matrix element calculation from valence band (VB1/2/3) to conduction band (CB4/5/6). f and g Second derivative plot of band dispersion map along Γ-M-Γ and Γ-K-M obtained from ARPES measurements, respectively. h Measured absorption spectrum of a Nb3I8 nanoplate. Inset: the Tauc-plot curve. i Measured SWIR response spectrum of the device. Inset: time-resolved photoresponse under λ = 2 μm laser modulation.

To exploit the SWIR detection performance by the electronic band engineering strategy, large-area Nb3I8 single crystals were synthesized via the chemical vapor transport (CVT) method (see synthesis details in “Methods” section). X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), and Raman spectrum measurements confirmed the structure and high quality of the grown single crystals (Fig. S1). The aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM) image demonstrates the breathing kagome lattice characteristic (right panel of Fig. 2b). Figure 2f, g demonstrates its flat-band characteristics by angle-resolved photoemission spectroscopy (ARPES) measurements, which is in agreement with our DFT calculation. The absorption spectrum in Fig. 2h shows two peaks induced by the flat bands and slow decay behavior towards the bandgap, which is consistent with the DFT calculation. Possible photoexcited carrier transitions related to the two peaks are marked with pink and red arrows in Fig. 2c. The Tauc-plot curve indicates an indirect bandgap of 0.49 eV (Inset of Fig. 2h). Two-terminal devices were fabricated to explore the SWIR response (see fabrication details in “Methods” section). Figure S3 shows a typical optical image of a device, where the thickness of the Nb3I8 nanoplate is approximately 28 nm. The measured I–V curves shown in Fig. S4a have a symmetrical linear characteristic, indicating Ohmic contact between the material and gold electrodes, which facilitates the transport of photogenerated carriers. Figure 2i presents the measured SWIR response spectrum of the device under a 1V bias voltage, revealing the expected flat-band enhanced photoresponse with two distinct peaks. Figure S5a demonstrates the high-temperature blackbody response ability of the device with peak detectivity of ~5 × 1010 cm Hz1/2 W−1. To characterize the SWIR response time, the time-resolved photoresponse was measured via a commercial oscilloscope under λ = 2 μm laser modulation. The rising time (τr) and falling time (τf) are approximately 204 µs and 704 µs, respectively. Additional photoelectric characteristics under the illumination of the SWIR lasers and blackbody are summarized in Figs. S4 and S5, respectively. The response time can be further decreased to several tens of microseconds by designing a photovoltaic-type detector with black phosphorus (Fig. S6).

Phonon band engineering and MWIR–VLWIR photodetection

To realize high thermal collection efficiency and responsivity, low thermal conductivity and high TCR values are essential. According to the Wiedemann–Franz law, the calculated electronic thermal conductivity of the flat-band material Nb3I8 is approximately 7.32 × 10−5 W/(m·K); therefore, the phonon thermal conductivity majorly contributes to the total thermal conductivity. The hot carriers generation and phonon propagation process under MWIR–VLWIR thermal radiation is illustrated in Fig. S7. Four distinct stages exist in thermal transport41,42: (1) hot carriers generation at the interface of Nb3I8 and metal electrodes; (2) carrier-phonon scattering via the Fröhlich interaction; (3) optical phonon conversion to acoustic phonon via phonon scattering; and (4) acoustic phonon propagation to the far-field region in the material. The transient photocurrent result reveals a slow relaxation time (~2.15 ns) of non-equilibrium hot carriers (Fig. S8), indicating that a long phonon scattering process occurs. This slow energy dissipation process promotes efficient heat localization and increases the lattice temperature, which is beneficial for realizing high bolometric response. In addition, the calculated electron localization function (ELF) section map (inset of Fig. 3a) shows a weak bonding characteristic near Nb atoms, where strong anharmonicity exists. The calculated average Grüneisen parameter is approximately 1.2 (Fig. S9), which is comparable to that of some reported thermoelectric materials43,44. Therefore, multiple optical mode bands strongly intersect with acoustic mode bands, which was demonstrated by the theoretical phonon band calculation, as shown in Fig. 3b, indicating that strong phonon scattering occurs. In addition, the calculated phonon band result also shows a flat-band characteristic in the optical and acoustic modes. Overall, these characteristics lead to a low phonon group velocity, which is verified by the phonon group velocity calculation, as presented in Fig. 3c.

Fig. 3: Phonon band engineering and MWIR–VLWIR photodetection performance.
Fig. 3: Phonon band engineering and MWIR–VLWIR photodetection performance.
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a Anharmonicity and calculated ELF map (Inset). b Calculated phonon band structure. Flat bands are marked by red boxes. c Calculated phonon group velocity as a function of frequency. d Measured thermal conductivity as a function of temperature. Inset: illustration of the measurement principle. e Detectivity as a function of wavelength under low-temperature blackbody radiation of 500 K and laser radiation. Inset: current noise power spectra. f Thermal response times extraction by blackbody and laser radiation of λ = 4.6 μm (Inset).

The low phonon group velocity results in a limited phonon mean free path and contributes to the low thermal conductivity, as shown in Fig. 3d. We also measured the thermal conductivity of a Nb3I8 nanoplate via the thermal-bridge method45. The inset figure in Fig. 3d illustrates the heat flow of the thermal bridge across the sample; the result shows it actually has a low thermal conductivity value of 1.12 W/(m·K) at room temperature (see Supplementary Note 1 and Fig. S10 for more preparation and measurement details). Furthermore, the measured TCR value of −3%/K shown in Fig. S11 at room temperature is almost the leading level among metals and reported 2D materials20,46,47,48, which is also comparable to that of commercial polycrystalline VOx and amorphous silicon with much thicker thickness.

Bolometric responses of the device were demonstrated under radiation with laser energy lower than the bandgap of Nb3I8, as shown in Fig. S12. The inset of Fig. 3e shows the measured noise current spectrum of the device under a 1 V bias voltage; since the photoresponses were measured at a modulation frequency of 40 Hz, the corresponding noise current was approximately 6.4 × 10−13 A/Hz1/2. To confirm its low-temperature blackbody response ability, the relative response spectrum under blackbody source radiation with temperature of 500 K is plotted in Fig. 3e. The detectivity increases with increasing wavelength because of the limitation of the FTIR setup, where the slowest modulation frequency of the light source is beyond the bandwidth of the device (see Supplementary Note 7 and Fig. S18 for more details of spectral response characterization for thermal detectors). This circumstance results in an underestimation of the photoresponse at relatively shorter wavelengths, which is similar to the reported VOx49. Instead, we have conducted photoresponse measurements using a wavelength-tunable laser, as this allows direct control over the modulation frequency to accommodate the relatively slow response. The spectral response trend aligns well with the intrinsically flat spectral responsivity characteristic expected for thermal detectors, yielding an average detectivity of ~1.07 × 108 cm·Hz1/2 W−1. This value is comparable to the blackbody detectivity (~2 × 108 cm·Hz1/2·W⁻¹) measured at 20 μm, where the influence of modulation frequency is minimized. Figure 3f presents the response time characterization, and the thermal response time measured is approximately 19 ms under the blackbody radiation of T = 500 K. The inset shows the photoresponse under λ = 4.6 μm laser modulation, the rising time (τr) and falling time (τf) are approximately 30 ms and 63 ms, respectively, maintaining the same order of blackbody radiation.

Demonstration of broadband photodetection applications

Last but not least, we demonstrated practical broadband photodetection applications with the flat-band material Nb3I8. Similar to moths, our device also enables self-adapted broadband detection. As illustrated in the upper panel of Fig. 4a, we simulate its photon detection capability at daytime by employing reflective imaging of a “barricade cone” under incandescent lamp illumination. At nighttime, we simulate its thermal imaging capability by heating a “spiral wire” and a “soldering iron”. The relatively clear active and passive imaging results validate the self-adapted detection capability of the device, as shown in the down panel of Fig. 4a. The signal-to-noise ratio of the obtained image can be further enhanced by using the high-power radiation source, as presented in Fig. S13.

Fig. 4: Broadband photodetection applications and performance comparison with reported infrared detectors with blackbody radiation detection ability.
Fig. 4: Broadband photodetection applications and performance comparison with reported infrared detectors with blackbody radiation detection ability.
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a Self-adapted active and passive imaging application. b Infrared detection performance compared with that of traditional bulk materials and 2D materials with room-temperature blackbody detection ability. The dotted line “corrected value” indicates the detectivity value obtained by a wavelength-tunable laser to achieve more accurate spectral response characterization.

Moreover, low-power human hand radiation responses were explored for the typical LWIR application, as shown in Fig. S14. The established setup is illustrated in the left panel of Fig. S14, where a germanium window was used to eliminate the influence of short-wave light. Obvious human hand (T = 310 K) thermal radiation responses can be observed as shown in the upper panel of Fig. 4b, whereas any other reported 2D material of equal size cannot show such low-power radiation detection ability. Benefiting from the advantage of 2D materials for flexible integration, we also fabricated a flexible device based on a Nb3I8 nanoplate. It can be easily attached to a hand as a wearable device for temperature prewarning situations. Figure S15 depicts the recorded net photocurrent variations when the thermal source approached and then receded from the device. There will be a prewarning when a thermal source approaches a threshold distance, which can be used in touchless thermosensation and prevent burns. Furthermore, the device is also demonstrated for non-dispersive infrared (NDIR) gas detection application. Comprehensive details of the NDIR gas detection characterization can be found in Fig. S16 and Supplementary Note 2.

Discussion

To further illustrate the performance of the proposed photodetector based on the 2D flat-band quantum material, we make a comparison with other reported infrared detectors. Figure 4b shows the detectivity of our photodetector as a function of wavelength compared with that of traditional bulk materials and 2D materials with room-temperature blackbody detection ability. Our photodetector covers the widest infrared band (from the SWIR to the VLWIR); in particular, the thermal detection performance is comparable to that of a commercial bolometer, while it doesn’t need a special thermal isolation structure design. In addition, our devices demonstrate superior low-power radiation detection capability (e.g., human hand radiation) compared to reported two-dimensional material-based infrared detectors (see a detailed comparison in Table S1). The thermal detection performance of our device can be further enhanced by an infrared absorption layer coating, e.g., adding a SiNx layer5. For the SWIR band, the blackbody detectivity is comparable to that of the reported 2D Te10. The performance can be further enhanced by designing a momentum-matching and band-alignment heterostructure with black phosphorus (see Supplementary Note 3 and Fig. S6 for details) and cavity structures50,51. Recently, flat-band metallic λ-Ti3O5 powder has shown effective solar-to-heat conversion ability in solar steam generation52; however, the origin of its low thermal conductivity characteristic has not been clarified, and its low-temperature blackbody radiation detection ability has not been explored. A VO2 tubular bolometer rolled with a VO2 membrane also has an enhanced thermal collection efficiency without a supporting-structure design; however, it has a relatively low TCR (~−1.7%/K) and does not have blackbody responses53.

In summary, we have developed a new type of ultrabroadband blackbody-sensitive infrared detector through an electronic and phononic band structure engineering design strategy. The demonstrated flat-band quantum material Nb3I8-based photodetector shows wide-range temperature blackbody detection ability and covers the broadest wavelength band, which the previously reported infrared detectors based on low-dimensional materials are incapable of. More importantly, the proposed device achieves a sensitive bolometric response to blackbody radiation from λ = 2.5 μm to 20 μm. The proposed scheme provides an exciting opportunity for exploiting flat-band quantum materials for ultrabroadband infrared detection.

Methods

Materials synthesis

Single crystals of Nb3I8 were grown by the CVT method. High-purity Nb(4 N) and I(4 N) were first mixed in chemical stoichiometry, sealed in an evacuated quartz tube, and then the quartz tube was further put into a furnace with a small temperature gradient of 655–650 °C. After one week, surface-shiny plate-like single crystals with a typical size of ~5 mm × 5 mm were obtained in the cold zone.

Materials characterization

The XRD spectrum was obtained by a PANalytical X-ray diffractometer with a Cu Kα1 source. The SEM and EDX characterizations were performed with an Oxford SEM system. Thin flakes of Nb3I8 were prepared by mechanical exfoliation and then dry-transferred onto Cu grids for AC-HADFF-STEM image analysis (JEOL JEM-2100F). Raman spectrum was acquired on a freshly exfoliated Nb3I8 flake under a 100× objective lens (WITec alpha 300 R). Broadband optical absorption spectra were measured by a Fourier transform infrared (FTIR) spectrometer (Bruker Vertex 80 v). The ARPES measurements were performed at 305 K with 21.2 eV photon excitation.

Device fabrication

To fabricate the thin nanoplate devices, bottom metal electrodes (5 nm Cr/25 nm Au) were first prepared on a 285 nm SiO2/Si substrate by standard electron beam lithography, thermal evaporation, and lift-off processes. Thin flakes were prepared by mechanical exfoliation and then dry-transferred onto the electrodes with a home-made transfer stage in a high-purity nitrogen-filled glove box. Finally, the devices were protected by transferring a thin layer of h-BN or spin-coating a PMMA layer. Flexible wearable devices were fabricated by similar steps on a polyimide (PI) substrate instead of a SiO2/Si substrate.

Electrical, thermal, and photoresponse measurements

The I–V curves of the device were measured by a Keithley 2450 sourcemeter. Thermal conductivity measurements were performed with a closed-cycle cryostat (Oxford Instruments Inc.) via the suspended thermal-bridge method. For wavelength, bias voltage, and power-dependent photocurrent measurements, different SWIR band lasers (λ = 1064 nm and 2000 nm), MWIR quantum cascade lasers (λ = 3.3 μm and λ = 4.64 μm), and wavelength-tunable LWIR quantum cascade lasers (λ = 5–13 μm) were used as light sources. The incident light power illuminating the device was monitored by calibrated power meters. The noise current spectrum was measured by using a preamplifier (DL1211) and an FFT spectrum analyzer (SR770) in a shielding box. A blackbody source (HGH RCN1250) was used for varied-temperature blackbody response measurements at room temperature. The thermal response time is fitted by \(R={R}_{0}/\sqrt{1+{(2\pi f{\tau }_{{{{\rm{thermal}}}}})}^{2}}\), where f is the chopping frequency. The relative photoresponse spectrum was obtained at room temperature by using an FTIR spectrometer (Bruker Vertex 80v) without a focusing lens, and a deuterated triglycine sulfate (DTGS) detector was selected for background light source spectrum acquisition. See the detailed blackbody detection performance calculation in Supplementary Note 6. For self-adapted broadband single-pixel imaging, two linear motorized stages (OptoSigma TSDM60-20X-0B) were used for scanning the device. A N-BK7 lens and a ZnSe lens were used for active imaging and passive imaging, respectively.