A bioinspired Au-Cu1.97S/Cu2S film with efficient low-angle-dependent and thermal-assisted photodetection properties.

Inspired by the geological processes, this study develops an innovative low-concentration-ratio H2 reduction method to reduce the stoichiometric Au-CuS nanoparticles to produce completely reduced stoichiometric Cu2S with "invisible" Au achieved for solid solution Au enhancement. A stable Au-Cu1.97S/Cu2S micro/nano-composite is then formed by spontaneous oxidation. From this composite, in combination with biomimetic technology, an omnidirectional photoabsorption and thermoregulated film (Au-Cu1.97S/Cu2S-C-T_FW) is designed and fabricated as a photothermal-assisted and temperature-autoregulated photodetector for broadband and low-angle-dependent photodetection that presents good performance with high responsivity (26.37 mA/W), detectivity (1.25×108 Jones), and good stability at low bias (0.5 V). Solid solution Au exhibits significantly enhanced photodetection (1,000 times). This study offers a new concept for improving the stability and photoelectric properties of copper chalcogenides. Moreover, it opens up a new avenue toward enhancing the performance of optoelectronic and photovoltaic devices using solid solution metal atoms and thermal-assisted, anti-overheating temperature autoregulation.

Introduction

Copper chalcogenide nanomaterials (CuxE with E = S, Se, or Te and x from 1 to 2) are regarded as a promising material for potential applications in energy resource (Wong et al., 2015; Jiang et al., 2015; Wu et al., 2008, 2019; Tian et al., 2015a; Joo et al., 2019; Zhuang et al., 2016; Yu et al., 2018), environmental (Manzi et al., 2015; Cui et al., 2015, 2016; Saranya et al., 2015) and medical therapy (Deng et al., 2017; Gao et al., 2018; Chang et al., 2018), and electron fields (Kriegel et al., 2012; Riha et al., 2011; Johari and Shenoy, 2011) that have drawn increasing attention due to their important semiconductor characteristics, with band gap energies of 1.0–1.5 eV (Wang et al., 2015) and plasmonic features (Luther et al., 2011) as well as tunable optical (Luther et al., 2011; Comin and Manna, 2014; Scotognella et al., 2011) and electronic properties (Riha et al., 2011). In particular, with its merits of being abundant, nontoxic, environment-friendly, and recyclable and having appropriate band gaps of ∼1.2 eV (Kriegel et al., 2012), Cu2S nanomaterials and their derived copper-deficient compounds have attracted a great deal of attention and have been highlighted as potential electronic (Riha et al., 2011; Liu et al., 2015), optoelectronic (Wong et al., 2015; Wu et al., 2008; Pan et al., 2012), photothermal (Wu et al., 2019; Tian et al., 2011, 2015a; Deng et al., 2017; Chang et al., 2018), photoelectrochemical (Yu et al., 2018; Han et al., 2016), and biosensor (Jia et al., 2019) materials. Unfortunately, except for the aforementioned merits, there are several hurdles to using Cu2S nanomaterials that would extensively hinder their performance in electronic and photoelectronic applications. A significant limitation is that the highly reactive surface sites of Cu2S nanomaterials are often prone to oxidation and are thermodynamically unstable when exposed to oxygen, because the surfaces of Cu2S nanomaterials have large numbers of unpassivated surface sites (Kriegel et al., 2012; Riha et al., 2011). Recently, to address these stability issues, protective layers (such as the Al2O3 protective layer [Martinson et al., 2013] and CdS protective layer [Wong et al., 2015]) were deposited on the surface of Cu2S, leading to progress in the stabilization of Cu2S and implying future uses for Cu2S nanomaterials in more efficient and stable electronic and photoelectronic devices. However, there exists a mismatch between the protective layer and the Cu2S nanomaterials (Wong et al., 2015). Furthermore, the deficiencies in the copper vacancies are responsible for low free carrier (holes) concentrations in Cu2S, which results in Cu2S that exhibits relatively low conductivity and suppressed localized surface plasmons (LSPs) (Kriegel et al., 2012; Riha et al., 2011; Luther et al., 2011). Currently, numerous efforts have been devoted to circumventing these hurdles using the following approaches: (1) introducing defects into the Cu2S semiconductor to form nonstoichiometric Cu2-xS, which boosts the free carrier density for enhancing the LSP effect and advancing the conductance (Riha et al., 2011; Wang et al., 2015; Ren et al., 2017); and (2) coupled plasmonic metal and semiconductors provided with a high free carrier density with the Cu2S semiconductor to form hybrid hetero junctions, in which the integration of noble metal and plasmonic semiconductors with Cu2S could not only enhance the LSP field but also would be able to change and improve the electron transitions in the semiconductor domain (Zhuang et al., 2016; Deng et al., 2017; Han et al., 2016). Accordingly, in addition to the abovementioned composition optimization, Cu2S/CdS coaxial nanowires (Pan et al., 2012), core-shell CdS-Cu2S nanorod arrays (Wong et al., 2015), hierarchically assembled ITO@Cu2S nanowire arrays (Jiang et al., 2015), and hollow-structured CuS@Cu2S@Au nanohybrids (Deng et al., 2017) were designed to enhance the charge collection and light trapping as well as to improve the charge transport by structural optimization, which further boosts performance and promotes application in electronic and photoelectronic devices (Wong et al., 2015; Jiang et al., 2015; Pan et al., 2012). Significantly, the current research on low-angle-dependent or thermal-assisted photoelectronic effects are rare, which further limits advances in performance and hinders their use in practical applications (Saran and Curry, 2016). The reason is that it is a great challenge to fabricate low-angle-dependent optical and photoelectronic devices with omnidirectional light absorption properties by artificial preparation, despite findings showing that various Cu2S nanostructures offer many advantages as candidates for electronic and photoelectronic applications with the development of good preparation techniques (Wong et al., 2015; Jiang et al., 2015; Pan et al., 2012; Liu et al., 2010). In addition, a superheating phenomenon arises during high dark current and excessive electron-hole recombination, thereby hindering thermal-assisted applications in electronic and photoelectronic devices.

One way to overcome the above-mentioned limitations is to boost the free carrier density of Cu2S film to enhance the LSP effect and advance the conductance while constructing a hierarchically micro-nano structure with omnidirectional light absorption and temperature auto-regulation characteristics. Recently, these Cu2S films have been prepared using gas-state reaction (Martinson et al., 2013; Carbone et al., 2011), solid-state reaction (Manzi et al., 2015; Liu et al., 2010), and solution processes (Jiang et al., 2015; Zhuang et al., 2016; Yu et al., 2018; Deng et al., 2017). However, most of these preparation methods have poor control of the stoichiometry and construction of their extraordinarily fine functional structures (Yu et al., 2018; Yao et al., 2011). Moreover, especially for most gas-state and solid-state reactions, these reactions are energy-intensive and are performed using ultra-high-vacuum-solid-state techniques and valuable equipment, resulting in high fabrication cost (Riha et al., 2011). Encouragingly, with the recent development of nanotechnology, research on biomaterials, bio-inspired materials, and biomimetic materials has expanded to the micro- and nano-scales, which has led to a big breakthrough in the design of advanced functional materials (Yao et al., 2011; Zhang et al., 2015; Yu et al., 2013a; Liu et al., 2020; Tian et al., 2015b, 2021). Herein, inspired by the geological processes, this study presents an effective method for preparing a type of Cu1.97S/Cu2S micro/nano-structure film containing a solid solution Au, which is invisible in the X-ray diffraction (XRD) pattern (“invisible” Au: a solid solution Au whose diffraction peaks are invisible in the XRD pattern), with bio-inspired omnidirectional light absorption and temperature auto-regulation characteristics (Au-Cu1.97S/Cu2S-C-T_FW) for multifunctional photoelectric applications. Here nanotechnology is combined with biomimetic technology to circumvent the limitations of the current preparation methods. To the best of our knowledge, the thermal-assisted photoelectric detection properties of these Cu2S nanoparticles (NPs) systems have rarely been explored. Moreover, the solid solution metal atom enhancement has seldom been applied to photodetection. This study introduces the possibility of producing the thermal-assisted photoelectric application, and it suggests that the Au-Cu1.97S/Cu2S-C-T_FW can be a new class of low-cost, room-temperature photoelectric detectors. This effort shows that the concept of photo-heating can be used to enhance photoelectric detection by passive radiative balance. Furthermore, this strategy offers a new direction for preparing thermally stable solid solution metal atoms and copper chalcogenide systems with enhanced LSP effects and advanced conductance properties for photoelectric applications.

Results and discussion

Preparation of Au-Cu1.97S/Cu2S-C-T_FW

The primary procedure for fabricating Au-Cu1.97S/Cu2S-C-T_FW is shown in Figure 1. First, the black forewing of the T. helena butterfly (T_FW) was chosen as a biomimetic template; it has a sophisticated micro-nano functional structure (SMNFS) (Figures 2A, 2B, and S1) with omnidirectional light absorption and temperature auto-regulation characteristics (Tian et al., 2015a; Zhao et al., 2011; Wang et al., 2014; Berthier, 2005; Herman et al., 2011). Then, the T_FW are pretreated and aminated before the Au NPs are deposited on the surface of the sophisticated micro-nano functional structure of T_FW (Au-T_FW) by electroless deposition. Next, the CuS NPs are deposited on the surface of the Au-T_FW (Au-CuS-T_FW) by the hydrothermal method (Tian et al., 2015a). During this step, stable stoichiometric CuS NPs are prepared using a low-cost solution process, in which the poorly controlled stoichiometry for Cu2S synthesis is circumvented because the preparation of stable stoichiometric CuS NPs is easier and costs less compared with the preparation process used for stable stoichiometric Cu2S NPs (Tian et al., 2015a). Because of the efficient reducibility of H2, an innovative low-concentration-ratio H2 reduction method is used successfully to reduce the stoichiometric CuS NPs into completely reduced stoichiometric Cu2S. Here, Au-CuS-T_FW is placed in the hot center of the tube furnace vented with 5:95 H2/Ar mixture gases at 50 sccm. The furnace is gradually heated from room temperature to 450°C at a rate of 7.5°C/min, and then the chamber is maintained at 450°C for 120 min. During this procedure, first, with inspiration from the geological processes leading to the formation of sulfide ores that often result in the precipitation of gold-bearing sulfide, which can contain high concentrations of this metal in an “invisible” state, the Au starts to become “invisible” by the formation of isomorphous solid solution when the temperature rises to 120°C, and the “invisible” Au concentration grows with increasing temperatures from 120°C to 450°C (Tagirov et al., 2016). Then the CuS begins to be reduced by H2 when the temperature is greater than ∼433°C, and the following reaction takes place: 2CuS + H→CuS + HS (Habashi and Dugdale, 1973), which forms the “invisible” Au-Cu2S system. Eventually, the “invisible” Au either substitutes for Cu in the Cu2S lattice, or it is incorporated into the Cu2S matrix without distortions of the cationic sublattice that Au exists in the form of the isomorphous solid solution and is invisible in the XRD pattern-formed “invisible” state, which boosts the internal electron concentration of the system and forms the positively charged Au centers to elevate the performance of the Cu2S film (Tagirov et al., 2014, 2016; Yang et al., 2013; Jiang et al., 2019; Qiao et al., 2011; Wei et al., 2014). Correspondingly, the easier diffusion characteristics of the H2/Ar mixture gases not only provide a more efficient reduction performance at both the surface and interior domains of CuS film but also effectively prevent the injection of the extra component when compared with the reported reduction methods by injecting a strong reducing agent to drive the reduction of Cu2+ to Cu+ (Deng et al., 2017; Kriegel et al., 2012). Significantly, this strategy offers a new direction for preparing the thermally stable solid solution metal atoms on photoelectric supports, thereby overcoming the faults of single metal atoms that can be mobile and aggregate into NPs on supports (Jiang et al., 2019; Jones et al., 2016). Meanwhile, the chitin-matrix template (T_FW) has been carbonized to enhance the infrared absorption during this sintering process (Tian et al., 2015c). Here, the carbon-matrix Cu2S films containing “invisible” Au with bio-inspired omnidirectional light absorption and temperature auto-regulation characteristics are ultimately achieved (Au-Cu2S-C-T_FW). Last, because the stoichiometric Cu2S NPs are prone to oxidation into more thermodynamically stable and nonstoichiometric copper chalcogenide phases containing both Cu+ and Cu2+ ions, when the Au-Cu2S-C-T_FW film is exposed to air under ambient conditions, the Cu2S NPs located at the surface of the Au-Cu2S-C-T_FW film are transformed in the thermodynamically stable nonstoichiometric Cu1.97S phase (Au-Cu1.97S/Cu2S-C-T_FW) spontaneously, which forms a protective layer and boosts the free carrier density of Cu2S film to enhance the LSP effect and advance conductance (Kriegel et al., 2012; Riha et al., 2011). The detailed experimental procedure is presented in the Experimental section of Supplemental information. With benefits from the unique features of Au-Cu1.97S/Cu2S nano-composites and the bio-inspired characteristics of T_FW, the Au-Cu1.97S/Cu2S-C-T_FW achieves the enhancement resulting from solid solution metal atoms and photothermal-assist in photoelectric application that exhibits an excellent low-angle-dependent photodetection property.

Figure 1

Schematics of the preparation process

Schematic of the preparation process for Au-Cu1.97S/Cu2S-C-T_FW and Au-Cu1.97S/Cu2S-C-T_FW photodetectors.

Figure 2

Characterization of Au-Cu1.97S/Cu2S-C-T_FW

(A) Digital photograph of T. helena.

(B) SEM image of T_FW. The inset is the TEM image of the cross-section of T_FW.

(C) SEM image of Au-Cu1.97S/Cu2S-C-T_FW.

(D–F) TEM and high-resolution TEM images of Au-Cu1.97S/Cu2S-C-T_FW, respectively. The inset of (E) is the SAED pattern of Au-Cu1.97S/Cu2S-C-T_FW.

(G) XRD results of Au-CuS-T_FW and Au-C1.97S/Cu2S-C-T_FW.

(H–J) XPS spectra of (H) Au 4f, (I) Cu 2p, and (J) S 2P regions for Au-C1.97S/Cu2S-C-T_FW.

According to the couple effect between the melanin/chitin composite and the light trapping structure, the forewing of T. helena (T_FW) appears black and exhibits light absorption characteristics (Figure 2A), and it has attracted extensive interest (Tian et al., 2015a, 2015c; Zhao et al., 2011). The light trapping structure of T_FW has been characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) (Figure 2B and its inset). The omnidirectional, broadband absorption and temperature auto-regulation characteristics of T_FW are discussed in the Supplemental information (Figures S1–S3 and Table S1). These results demonstrate that T_FW with SMNFS has both good absorption (omnidirectional and broadband absorption) and effective emissivity to achieve temperature auto-regulation (Herman et al., 2011). The typical morphologies of Au-Cu1.97S/Cu2S-C-T_FW are shown in Figures 2C–2F, showing that the surface of SMNFS of T_FW were coated uniformly with Au-Cu1.97S/Cu2S NPs. Evidently, the SMNFS of T_FW are well retained. The diameter measurements of Au-Cu1.97S/Cu2S NPs are shown in Figure S4 and Table S2. In addition, the high-resolution TEM images of Au-Cu1.97S/Cu2S-C-T_FW are displayed in Figures 2E and 2F. In the inset of Figure 2E, the selected area electron diffraction (SAED) pattern indicates that the combined Au-Cu1.97S/Cu2S NP system is a polycrystal-line specimen. These diffraction rings are indexed to the (004) and (804) planes of the Cu1.97S (JCPDS no. 20-0365) and the (200), (220), and (311) planes of the Cu2S (JCPDS no. 02-1284) respectively, which also corresponds to the XRD result shown in Figure 2G. The distances of 0.31 and 0.33 nm are consistent with the d-spacing of the (111) crystallographic plane of Cu2S and the (004) crystallographic plane of Cu1.97S, respectively (Figure 2F).

The “invisible” Au present in Cu2S in the form of an isomorphous solid solution and the stabilization of Au-Cu1.97S/Cu2S-C-T_FW are investigated here by means of XRD analysis. As displayed in Figure 2G, the XRD pattern of Au-CuS-T_FW confirms that the stoichiometric Au-CuS combination NP system has been prepared (trigonal-structured CuS: JCPDS no. 24-0060, cubic phase of Au: JCPDS no. 02-1095) (Tian et al., 2015a). Moreover, as exhibited in the XRD pattern of Au-Cu1.97S/Cu2S-C-T_FW, after the Au-Cu2S-C-T_FW was initially exposed to air for 5 months, the diffraction peaks with 2-theta values of 26.2°, 37.6°, and 48.6° were assigned to the (004), (804), and (1204) planes of the orthorhombic-structured Cu1.97S, respectively (JCPDS no. 20-0365), and diffraction peaks with 2-theta values of 27.8°, 32.1°, 46.2°, and 54.5° were assigned to the (111), (200), (220), and (311) planes of the cubic-structured Cu2S, respectively (JCPDS no. 02-1284) (Liu et al., 2013; Kriegel et al., 2011). As a result, the XRD pattern confirmed that the Cu2S NPs located at the surface of the Au-Cu2S-C-T_FW are transformed to the thermodynamically stable nonstoichiometric Cu1.97S phase and form a protective layer for stabilizing the Au-Cu1.97S/Cu2S-C-T_FW. Furthermore, the initial exposure of Au-Cu2S-C-T_FW to air for 5 months demonstrates that Au-Cu1.97S/Cu2S-C-T_FW achieves remarkable stabilization. Herein, the oxidation of Cu2S and the stabilization of Cu1.97S/Cu2S can be ascribed to the following process. First, upon exposure to the air, the surface Cu+ oxidizes to Cu2+. Meanwhile, it establishes a chemical potential gradient, thereby resulting in the diffusion of Cu+ from the NP core to the surface. Then, a more thermodynamically stable nonstoichiometric Cu1.97S is obtained, and equilibrium is achieved (Riha et al., 2011). In addition, during oxidation process, the decreased Cu combines with O, which forms the Cu2-O-S phase.16-18 However, as shown in electron energy loss spectroscopy (EELS) analysis (Figure S5), the quantity of O element is very small, which demonstrates that there is very little Cu2-O-S phase in Au-Cu1.97S/Cu2S-C-T_FW. Thus, the Cu2-O-S phase is reasonably ignored in our work and the reported researches about Cu2-xS nanocrystals.16, 19-21 Of interest, as shown in EELS analysis of Au-Cu1.97S/Cu2S-C-T_FW (Figure S5), there are evident Au distributed in the Au-Cu1.97S/Cu2S-T_FW, but the Au diffraction peaks are invisible in the XRD pattern of Au-Cu1.97S/Cu2S-C-T_FW (Figure 2G). In addition, compared with the XRD pattern of Cu1.97S/Cu2S-C-T_FW (Figure S6) that was fabricated using the same preparation process as that of Au-Cu1.97S/Cu2S-C-T_FW except for the step in which Au NPs are deposited on the surface of the sophisticated micro/nano functional structure of T_FW, the XRD patterns of Au-Cu1.97S/Cu2S-C-T_FW do not exhibit obvious differences. This result demonstrates that the Au is “invisible” and the crystalline structure of Cu2S is changeless, which indicates that the “invisible” Au is present in the form of the isomorphous solid solution formed by substitution with Cu atoms (Tagirov et al., 2016). Additionally, according to the XRD pattern of Cu1.97S/Cu2S-C-T_FW, we can also conclude that the Cu2S NPs located at the surface of Cu2S-C-T_FW are transformed during the thermodynamically stable nonstoichiometric Cu1.97S phase and form a protective layer for stabilizing the Cu1.97S/Cu2S-C-T_FW.

Furthermore, a systematic X-ray photoelectron spectroscopy (XPS) study was performed to investigate the “invisible” Au as well as the chemical composition and the valence states of both the Cu and S atoms in the Au-Cu1.97S/Cu2S-C-T_FW. As shown in Figure 2H, the XPS spectra of the Au 4f regions are evident, and the gold peaks are identified clearly, although the Au is invisible in the XRD pattern. Moreover, compared with the XPS spectra of Au NPs reported by Ding et al (Ding et al., 2014), the XPS spectra of the Au 4f for Au-CuS-T_FW are left-shifted (Figure S7A), which indicates a possible electron transfer from Au NPs to CuS NPs in Au-CuS-T_FW (Tian et al., 2015a; Ding et al., 2014). The similar left shift also suggests a possible electron transfer from Au atoms to Cu2S or Cu1.97S matrixes in Au-Cu1.97S/Cu2S-C-T_FW, which confirms the presence of Au atoms with a partially positive charge (Tian et al., 2015a; Tagirov et al., 2016; Jiang et al., 2019; Qiao et al., 2011; Ding et al., 2014). As depicted in Figure 2I, the Cu 2p XPS spectra contain a mixture of Cu+ and Cu2+, which shows oxidation from Cu+ to a mixture of Cu+ and Cu2+ when exposing the initial Au-Cu2S-C-T_FW to air (Riha et al., 2011; Conley et al., 2014). Furthermore, in contrast with the Cu 2p and Cu 2p peaks of the Au-CuS-T_FW that are narrow and devoid of satellite peaks, the Cu 2p and Cu 2p peaks of the Au-Cu1.97S/Cu2S-C-T_FW are right-shifted and broadened (Figure S7C). Additionally, after undergoing splitting, the pronounced Cu+ satellite peaks are formed (Figure S7C). These results indicate a possible electron transfer from Cu1.97S to Cu2S. Here, the Cu+ satellite peaks can be attributed to the factor in which the surface Cu+ oxidizes to Cu2+, forming a surface oxide because XPS is primarily a surface characterization technique (Riha et al., 2011). For S, the XPS spectra of S 2P regions are described in detail by deconvolution in subcomponents (Figure 2J). Here, there are two primary S 2p component peaks positioned at 161.7 and 162.6 eV, and they are ascribed to sulfides and disulfides, respectively, indicating the partial oxidation of the S species (Han et al., 2016). In addition, the other two peaks positioned at 163.4 and 164.9 eV can be referenced as complexes containing elemental sulfur or Cu-deficient nonstoichiometric sulfides (Tian et al., 2015a; Han et al., 2016). Moreover, compared with the sulfide peak of Au-CuS-T_FW positioned at 161.0 eV (Figure S7B), the sulfide peak of Au-Cu1.97S/Cu2S-C-T_FW emerges at 161.7 eV (Figure S7D). The shift in the sulfide peak can be attributed to the decrease in the average length of the Cu-S bond by oxidation (Han et al., 2016).

Absorption and thermal radiation

To further explore the possibility of using the as-fabricated Au-Cu1.97S/Cu2S-C-T_FW as a broadband photodetection material, the broadband, omnidirectional, high-efficiency absorbance and thermal radiation of the Au-Cu1.97S/Cu2S-C-T_FW are discussed in Figure 3. As depicted in Figure 3A, the carbon matrix T_FW exhibits enhanced near-infrared (NIR) absorption compared with T_FW, which is consistent with previous reports (Zhao et al., 2011; Tian et al., 2015c). Moreover, in Figure 3A, Au-Cu1.97S/Cu2S-C-T_FW exhibits a more significant broadband photoabsorption performance (300–2,500 nm). Additionally, the absorption pattern of Au-Cu1.97S/Cu2S-C-T_FW shows another typical behavior of the plasmonic feature in Cu1.97S NPs that shows a pronounced LSP band centered at approximately 1,830 nm (inset of Figure 3A), which is highly consistent with the report by Kriegel et al. (2012). The absorption peak of plasmonic feature Au-Cu1.97S/Cu2S-C-T_FW demonstrates that the number of free carriers (copper vacancies) on Cu1.97S are increased, which gives rise to a higher free carrier absorption, wherein the high concentration of free carriers in Cu1.97S NPs similar to electrons in plasmonic metal NPs leads to LSPs (Kriegel et al., 2012). Furthermore, to expand the discussion on efficient broadband photoabsorption, a finite difference time domain (FDTD) simulation method is carried out. The detailed descriptions and schematic diagrams for the FDTD simulation are provided in the Supplemental information and in Figure S8. On the one hand, compared with the absorption of Cu2S-NPs (without HASAS), the Cu2S-C-T_FW (with SMNFS) shows an enhanced photoabsorption clearly, as depicted in the simulation results (Figure S9). Additionally, as shown in Figures 3F and 3G, the intensive electromagnetic field energy flux density (EFEFD) is present between every two ridges and in the windows, which demonstrates that the triangular roof-type ridges are good at focusing incident light into the scale interior, and the windows elongate the effective light path and the energy density distribution interspace efficiently (Tian et al., 2015a, 2015c). These results indicates that the SMNFS of C-T_FW contributes an important factor for broadband photoabsorption (Tian et al., 2015a, 2015b, 2015c; Zhao et al., 2011). On the other hand, from Figure 3E, we observe that the substantially enhanced EFEFDs are located in the adjacent region between two Cu2S NPs and the adjacent region between two layers of Cu2S NPs, which indicate that the coherent coupling between adjacent systems can enhance the interplay between the incident light and the material promoting the photoabsorption. Moreover, on comparing Figure 3F with Figure 3G, except for the intensive EFEFDs that are present between every two ridges and in the windows, there are obvious electromagnetic field hotspots situated on the surface of the ridges on the Cu2S-C-T_FW coated by Cu2S NPs, which are attributed to coherent couplings between adjacent systems. Consequently, from the measurement and simulation results, we can conclude that the efficient broadband photoabsorption performance of Au-Cu1.97S/Cu2S-C-T_FW is attributed to the plasmon and the coherent coupling between adjacent resonant systems of Au-Cu1.97S/Cu2S that synergistically combine with the SMNFS (Tian et al., 2015a). Additionally, given the nano hole window structure with antireflection ridges that produce an omnidirectional light absorption feature (Wang et al., 2014), the Au-Cu1.97S/Cu2S-C-T_FW exhibits an omnidirectional broadband (from 450 to 810 nm) and high-efficiency absorption performance (Figures 3B and 3C). As shown in Figures 3B and 3C, as the incidence angle is increased from 0° to 50°, the average absorption of the Au-Cu1.97S/Cu2S-C-T_FW gradually decreases from 96.93% to 95.01%, which exhibits an approximate linear relationship between the absorption and the angle (Figure S10A). Compared with the average absorptance at the vertical incidence (0°), the reduction rate of the average absorption at the incident angles of up to 50° is only 1.929% (Figure S10B). Such a low reduction rate and high absorbance demonstrate that Au-Cu1.97S/Cu2S-C-T_FW achieves a high-efficiency, low-angle-dependent and broadband photoabsorption that profits from the SMNFS of T_FW. Valuably, as depicted in Figure 3D, the Au-Cu1.97S/Cu2S-C-T_FW also reveals several strong absorbance peaks at 4.26, 6.39, and 7.47 μm, which predicts the spectral emissivity of Au-Cu1.97S/Cu2S-C-T_FW, because Kirchhoff's law states that emission is equal to absorption (Arpin et al., 2013). Herein, the thermal radiation peaks centered on the 6.39 and 6.48 μm positions at the edge of the blackbody radiation for 40°C, in which the emissivity stays at a relatively constant value up to 40°C and increases beyond this, thereby playing a key role as the thermal regulator (Berthier, 2005; Herman et al., 2011). Accordingly, the radiation regulation plays a key role as a thermal regulator that modulates the energetic balance to prevent overheating and achieves temperature auto-regulation. Consequently, except for the low-angle-dependent and omnidirectional broadband high-efficiency absorption feature inherited from T_FW, the Au-Cu1.97S/Cu2S-C-T_FW also inherits the temperature auto-regulation by radiative balance, which offers a significant advantage to overcome the hurdles to then promote the Cu2S nanostructures as candidates for electronic and photoelectronic applications, especially for thermal-assisted photoelectronic applications.

Figure 3

Absorption and thermal radiation analysis

(A–G) (A) The absorption spectra of T_FW, C-T_FW, and Au-Cu1.97S/Cu2S-C-T_FW over a wavelength range from 300 to 2,500 nm. The insert is the absorption spectra of Au-Cu1.97S/Cu2S-C-T_FW over a wavelength range from 1,795 nm to 1,870 nm; (B) angle-dependent absorption of Au-Cu1.97S/Cu2S-C-T_FW over a wavelength range from 450 to 810 nm. The incident angles increase from 0° to 50°. The insert is a schematic illustration of the incidence at different angles of the photodetector; (C) filled contour plot of angular dependence versus wavelength; (D) the absorption spectra and blackbody radiation at temperatures of 30°C and 40°C for Au-Cu1.97S/Cu2S-C-T_FW over a wavelength range from 2.5 to 25 μm; (E–G) the FDTD simulation of intensity distribution maps of (E) Cu2S-NPs, (F) C-T_FW, and (G) Cu2S-C-T_FW. The wavelength of the incident light in the FDTD simulation is fixed at 808 nm.

Broadband photodetection

To analyze the broadband photoresponse characterizations of Au-Cu1.97S/Cu2S-C-T_FW covering the visible (VIS) and NIR, the time-dependent response curves were measured at room temperature under ambient conditions. Herein, the as-fabricated devices with Au-Cu1.97S/Cu2S-C-T_FW as the channel and Ag paste as contact are used for the broadband photodetector. As shown in Figures 4A–4C, the photocurrent of the Au-Cu1.97S/Cu2S-C-T_FW photodetector at a 0.5 V bias potential exhibits a steady increase with increasing illumination power from 0.06 to 1.67 mWmm−2. At the relatively higher power densities (>0.56 mWmm−2), when the light is turned on, the photocurrent increases sharply at the beginning, and the photocurrents exhibit lower growth at the later stages. The opposite findings occur when the light is turned off. Moreover, compared with the slowly increasing process of photocurrent illuminated by 660 nm light, the slowly increasing processes of photocurrent illuminated by 808 and 980 nm light are much slower. However, at low power densities (<0.11 mWmm−2), the photocurrent only exhibits the sharply increasing/decreasing process when the light is turned on/off, and there is a lack of the slowly increasing/decreasing process at the end. The reasons are that the sharp photocurrent increases/decreases arise from the photoelectric effect generated photoinduced carriers, and the slowly increasing/decreasing process profits from the photothermal effect contributed by the LSPs of Au-Cu1.97S/Cu2S and the electron transitions in the semiconductor domain (Deng et al., 2017; Kriegel et al., 2012; Luther et al., 2011). For further comparison, as revealed in Figures 4A–4C and their insets, under increased illumination power, the photocurrent increased. Additionally, there is an approximately linear relationship between the photocurrents and the illumination power density at the low power densities (<0.11 mWmm−2), which suggests that the photocurrent is dominated by the conversion of single photons to singe electrons over this range of incident light intensities (Knight et al., 2011), and it also demonstrates that the photocurrent primarily arises from the photoelectric effect. When further increasing the incident power, the photocurrents deviate from the linear relationship. In particular, compared with the photocurrent illuminated by 660 nm light, the photocurrents illuminated by 808 and 980 nm incident light displayed a greater deviation from the linear dependence (the insets of Figures 4A–4C). These results further demonstrate that there are photothermal effect-assisted photoelectric effects which boost the carrier concentration and promote the carrier transmission to improve the photoelectric effects. Furthermore, as shown in Figure S11, there was a clear temperature increasing process when the light was turned on at the same laser power density (1.67 mWmm−2) as a temporal photo response test, which evidently demonstrates that the photoelectric effect on the photo response is accompanied by photothermal effects. Significantly, with incident laser power as low as 0.06 mW mm−2, the Au-Cu1.97S/Cu2S-C-T_FW photodetector still exhibits a clear photoresponse for the incident wavelengths of 660, 808, and 980nm (Figures 4A–4C). Moreover, to quantify the photoresponse performance of the devices, the spectral responsivity (R) and detectivity (D∗) are discussed to evaluate the ability to convert the light signals into electrical signals and measure the detector sensitivity under a certain wavelength at a 0.5 V bias. Here, R and D∗ are defined as (Xie et al., 2017)where P refers to the incident light intensity and S is the effective area under illumination (here S is approximately 0.5 mm2), I is the dark current, and q is the absolute value of the electron charge (1.6 × 10−19 C) (Deng and Li, 2014). For 660, 808, and 980 nm light illumination under a light intensity of 1.67 mW mm−2 at a 0.5 V low bias voltage, the R and D∗ of the Au-Cu1.97S/Cu2S-C-T_FW detectors are estimated to be 17.36, 26.37, and 13.40 mA W−1, corresponding to D∗ of 7.79×107, 1.25×108, and 7.71×107 Jones, respectively. All the obtained parameters are listed in Table S3. These parameters are comparable and even higher than those for most reported photodetectors, such as the reported few-layer group-VIB transition-metal dichalcogenides p-n photodiode (WSe2 p-n junction) which recently emerged as an interesting candidate for optoelectronic application with a responsivity of 16 mA W−1 (<750 nm) (Koppens et al., 2014). Reasonably, according to Equations 1 and 2, if replacing the Ag paste contacts process with photolithography and oxygen plasma etching processes, the S diminishes from 0.5 mm × 1 mm to 8 μm × 30 μm, as reported (Yu et al., 2013b). Consequently, the R and D∗ increase by four and two orders of magnitudes, respectively, which are very competitive for the R and D∗ values of current high-performance photodetectors (Xie et al., 2017; Koppens et al., 2014; Yu et al., 2013b; Baugher et al., 2014; Youngblood et al., 2015). The response speed is also one of the critical parameters of photodetectors. In this study, the response time (τ) represents the time needed from the dark current increase to 1-1/e ≈ 63% of the maximum photocurrent, and the recovery time (τ) is defined as the time needed for recovery to 1/e ≈ 37% of the maximum photocurrent (Cao et al., 2014; Lopez-Sanchez et al., 2013). As shown in Figures 4D–4F, the fast τ/τ of the Au-Cu1.97S/Cu2S-C-T_FW photodetector for 660, 808, and 980 nm incident light wavelengths are 1.71/1.04 s, 1.37/0.60 s, and 1.42/0.93 s, respectively. Here, the τ/τ for the NIR (808, 980 nm) incident light is quickly attributed to the more efficient NIR photothermal effect, which boosts the carrier transmission. In addition, the stability is another critical factor for evaluating a practical photodetector. Here, time-resolved photoresponses are measured for multiple illumination cycles to evaluate the stability of the Au-Cu1.97S/Cu2S-C-T_FW photodetector. As depicted in Figures 4G–4I, for 660, 808, and 980 nm light illumination with 1.67 mWmm−2, the current density and response speed after 17 cycles exhibit no distinct decay, and the error analysis of the photoresponse cycle stabilities are shown in Figure S12. The ultra-low errors show that the Au-Cu1.97S/Cu2S-C-T_FW photodetector have excellent stability (Figure S12). Moreover, the good stability of Cu1.97S/Cu2S-C-T_FW and Au-Cu1.97S/Cu2S-C-T_FW(20hr) photodetectors are also exhibited in Figures S13 and S14. These results demonstrate the robustness and reproducibility of our photodetector for broadband photoresponses.

Figure 4

Photodetection analysis

(A–C) Time-dependent photocurrent response of Au-Cu1.97S/Cu2S-C-T_FW photodetector under switched-on/off light (660 nm, 808 nm, and 980nm) with different power levels (0.06–1.67 mWmm−2) at a 0.5 V bias voltage. The insets are the power-dependent peculiarity of the photocurrent under different incident light wavelengths (660, 808, and 980 nm).

(D–F) The response speed under different incident light wavelengths (660, 808, and 980 nm).

(G–I) The photoresponse cycle stabilities under different incident light wavelengths (660, 808, and 980 nm). These tests were performed at room temperature (20°C), and the bias was 0.5 V.

To investigate the competitive photoresponse mechanism of Au-Cu1.97S/Cu2S-C-T_FW further, here, the photodetection performances of Cu1.97S/Cu2S-C-T_FW and Au-Cu1.97S/Cu2S-C-T_FW(20 h) are compared. In contrast with the photodetection performances of Cu1.97S/Cu2S-C-T_FW in which the isomorphous solid solution Au is non-existent (Figure S13 and Table S4), the Au-Cu1.97S/Cu2S-C-T_FW exhibits an approximate 1,000-fold, 1,000-fold, and an order of magnitude increase in the photocurrent, R and D∗, respectively. To examine how the isomorphous solid solution Au atoms enhance the photoelectric detection, density functional theory (DFT) calculations were further performed (Xu et al., 2012). The charge density difference distribution images (Figures 5A and 5B) reveal a stronger charge distribution at the Au-bonding region compared with the charge distribution at the Cu-bonding region, which demonstrate that the Au atom substitutes for the Cu atom, promoting a significant increase in the internal electron concentration of the system. In addition, the possible interaction is demonstrated by DFT calculations. As shown in Figures 5C and 5E, the band gap decreases when the Au atom substitutes for the Cu atom. The densities of state results reveal that the Au atom substitutes for the Cu atom arising from the hybridization between Au (d orbitals) and S (s orbitals). These experimental and DFT calculational results indicate that the “invisible” Au in Cu1.97S/Cu2S boosts the internal electron concentration of the system and forms the positively charged Au centers to elevate the conductivity, carrier transport, and electron-hole separation that contribute to the elevated photodetection performance of Au-Cu1.97S/Cu2S-C-T_FW (Tagirov et al., 2014, 2016; Yang et al., 2013; Jiang et al., 2019; Qiao et al., 2011; Wei et al., 2014). Moreover, compared with the photodetection performance of Au-Cu1.97S/Cu2S-C-T_FW(20 h) (Figure S14 and Table S5) that does not successfully retain the SMNFS of T_FW (Figure S15), the Au-Cu1.97S/Cu2S-C-T_FW exhibits approximate 1,000-fold, 1,000-fold, and an order of magnitude increases in the photocurrent, R and D∗, respectively. This result further indicates that the SMNFS of T_FW contributes an important factor to the broadband photoresponse.

Figure 5

Density functional theory (DFT) calculations

(A–F) (A) The equilibrium crystal structure of Cu2S in which one Cu atom is substituted by an Au atom, and the charge density difference distribution around Au, Cu, and S; (B) two-dimensional plane view of the charge density difference distribution in the cutting plane highlighted in (A); (C and E) calculated energy band structure of Cu2S over a range from −0.5 to 1.5 eV along high symmetry paths X-Y-Γ-Z-R-Γ-T-U in the first Brillouin zones, where X (0.5, 0, 0), Y (0, 0.5, 0), Γ(0, 0, 0), Z (0, 0, 0.5), R (0.5, 0, 0), T (0, 0.5, 0.5), and U (0.5, 0.5, 0) represent the reciprocal space coordinates of special points in high symmetry directions; and (D and F) the total density of states (TDOS) and the partial density of states for the Cu, S, and Au atoms for Cu2S in which one Cu atom is substituted by an Au atom, and the vertical dash line illustrates the Fermi energy level.

Photothermal-assisted photodetection

Furthermore, to study the influence of thermal effects on the Au-Cu1.97S/Cu2S-C-T_FW photodetector, we performed a photoresponse test at different test temperatures and mapped the heat source density map, temperature map, and thermal emission map by computational simulation (Tian et al., 2015a, 2015c, 2019). The detailed descriptions of the computational simulation are provided in the Supplemental information. Here, to weaken or boost the effect from the heat effect, the test temperatures of 10°C or 38°C were selected to compare with the test under room temperature (20°C) for the natural photothermal effect. From Figures 6A and 6B, the dark current increase with the test temperature increases, which indicates that the heat effect boosts the carrier transfer. A similar tendency and a greater enhancement appeared in the photocurrent under 808 nm light illumination. The R and D∗ of the photodetector were calculated to be 15.30 mA W−1, 23.18 mA W−1, and 47.74 mA W−1 corresponding to 8.31×107 Jones, 10.08×107 Jones, and 9.94×107 Jones under the test temperatures of 10°C, 20°C, and 38°C, respectively (the insert of Figure 6B). The R of the Au-Cu1.97S/Cu2S-C-T_FW photodetector demonstrates a steady increase when the temperature is increased. However, when the temperature rises to 38°C, the D∗ values decrease weakly, because the trend in the increasing photocurrent under 808 nm light illumination (NIR, with low photon energy and the favorable photothermal characteristics) is not enough to cover the growth rate of the dark current. Under 660 nm (VIS) light illumination, the dark current, R, and D∗ exhibit a steadily increasing trend with the increasing test temperature (Figure S16). These results demonstrate that the heat effect can promote the photoresponse by boosting electron-hole pair generation and improving the carrier transfer to advance the photocurrent, but the superheating will increase the dark current greatly and improve the electron-hole recombination, which leads to a negative effect on photodetection. To discuss the mechanism of the photothermal heat generation and thermoregulation in the Au-Cu1.97S/Cu2S-C-T_FW, the photothermal conversion and thermal radiation of the Au-Cu1.97S/Cu2S-C-T_FW can be demonstrated further by numerical simulation on the basis of the FDTD method, the Joule effect, and the thermal radiation principle (Tian et al., 2015a, 2019). To understand the photoinduced heat generation, we studied the heat source density distribution of Cu2S-C-T_FW on the basis of the intensity distribution maps (Figure 3G). As shown in Figure 6C, the heat source arises from the photothermal material (Cu2S) covering the surface of the HASAS of C-T_FW. In particular, the heat source density is more intense at the region of the top of ridge, as shown in the white dashed elliptical area of Figure 6C. Moreover, the intensity of the heat source density on the surface of the windows of the SMNFS is relatively low and decreases with the increased depth of the window. However, compared with the region of the top of the ridge with a more intense heat source density, the temperature at the middle of the ridge and the windows is higher, as shown in the black dashed elliptical area of Figure 6D. This finding is consistent with the relatively lower thermal radiation strength at the middle of the ridge and the windows (Figure 6E). As shown in Figure 6E, the top and bottom of the ridges (the white dashed elliptical areas of Figure 6E) with a lower radius of curvature display stronger thermal radiation compared with the middle of ridge and the windows (Tian et al., 2019). Consequently, Figures 6C–6E clearly exhibit the photothermal heat source, hot temperature, and thermal radiation characteristics of the SMNFS in Cu2S-C-T_FW, which indicates that the hot temperature features result from the balance between the photothermal conversion and the thermal radiation. This finding will provide guidance for designing an effective photothermal, thermal radiation temperature auto-regulation device using less material.

Figure 6

Thermal-assisted photodetection analysis

(A and B) (A) I-V characteristics and (B) photocurrent response of the Au-Cu1.97S/Cu2S-C-T_FW photodetector under optical irradiation (λ = 808 nm) with an incident laser power of 1.67 mW mm−2 under a series of testing temperatures (10°C, 20°C and 38°C, and 50°C) at 0.1 V bias voltage.

(C–E) (C) Simulated heat source intensity map, (D) simulated temperature map, and (E) simulated thermal radiation map of the Au-Cu1.97S/Cu2S-C-T_FW photodetector. The wavelength of the incident light is fixed at 808 nm.

Conclusion

Inspired from the geological processes, a new and innovative low-concentration-ratio H2 reduction method is used to reduce stoichiometric Au-CuS NPs to completely reduce stoichiometric Cu2S containing “invisible” Au, and then the thermodynamically stable Au-Cu1.97S/Cu2S nano-composite is formed by spontaneous oxidation. On the basis of this composite, an omnidirectional photoabsorption and thermoregulation Au-Cu1.97S/Cu2S-C-T_FW film with sophisticated micro/nano functional structure is designed and fabricated as an efficient photodetector for broadband and low-angle-dependent photodetection that exhibits good performance with high responsivity (26.37 mA W−1), detectivity (1.25×108), and good stability, which exhibit very competitive for the R and D∗ of the current high-performance photodetectors for which effective area is at the square-millimeter scale. The thermodynamically stable nonstoichiometric Cu1.97S phase forms a protective layer and boosts the free carrier density of Cu2S film for enhancing the LSP effect and advances the conductance, which enhances the photoresponse and contributes to the film stability. Moreover, the “invisible” Au in Cu1.97S/Cu2S achieves the solid solution-atom enhancement, which boosts the internal electron concentration of the system and forms positively charged Au centers to elevate the conductivity, carrier transport, and electron-hole separation, thereby contributing to the elevated photodetection performance of Au-Cu1.97S/Cu2S-C-T_FW. Furthermore, by benefiting from the unique features of Au-Cu1.97S/Cu2S nano-composite and the bio-inspired characteristics of T_FW, the Au-Cu1.97S/Cu2S-C-T_FW can be used in the low-angle-dependent and thermal-assisted photodetection application. Here, the negative effect from the superheating involved in the thermal-assisted application is overcome by radiative balance. This effort offers a new concept for improving the stability and the photoelectric performance of copper chalcogenides. Moreover, this study opens up a new avenue toward thermoregulation and thermal-assist-enhanced properties for applications in optoelectronic and photovoltaic devices.

Limitations of the study

In this work, we offer a new concept for improving the stability and the photoelectric properties of copper chalcogenides. Moreover, it opens up a new avenue toward enhancing the performance of optoelectronic and photovoltaic devices using solid solution metal atoms and thermal-assisted, anti-overheating temperature autoregulation. Although the Au-Cu1.97S/Cu2S-C-T_FW photodetector presents good performance, the effective area is square-millimeter scale, which is required to reduce down to square-micron scale.

Methods

All methods can be found in the accompanying Transparent methods supplemental file.